# 13 Travel-approved power banks that pass the TSA’s battery rules. 5v battery backup

## Battery Charge Time Calculator

Use our battery charge time calculator to easily estimate how long it’ll take to fully charge your battery.

## Battery Charge Time Calculator

Tip: If you’re solar charging your battery, you can estimate its charge time much more accurately with our solar battery charge time calculator.

### How to Use This Calculator

Enter your battery capacity and select its units from the list. The unit options are milliamp hours (mAh), amp hours (Ah), watt hours (Wh), and kilowatt hours (kWh).

Enter your battery charger’s charge current and select its units from the list. The unit options are milliamps (mA), amps (A), and watts (W).

If the calculator asks for it, enter your battery voltage or charge voltage. Depending on the combination of units you selected for your battery capacity and charge current, the calculator may ask you to input a voltage.

Select your battery type from the list.

Optional: Enter your battery state of charge as a percentage. For instance, if your battery is 20% charged, you’d enter the number 20. If your battery is dead, you’d enter 0.

Click Calculate Charge Time to get your results.

## Battery Charging Time Calculation Formulas

For those interested in the underlying math, here are 3 formulas to for calculating battery charging time. I start with the simplest and least accurate formula and end with the most complex but most accurate.

### Formula 1

Formula: charge time = battery capacity ÷ charge current

Accuracy: Lowest

Complexity: Lowest

The easiest but least accurate way to estimate charge time is to divide battery capacity by charge current.

Most often, your battery’s capacity will be given in amp hours (Ah), and your charger’s charge current will be given in amps (A). So you’ll often see this formula written with these units:

charge time = battery capacity (Ah) ÷ charge current (A)

However, battery capacity can also be expressed in milliamp hours (mAh), watt hours (Wh) and kilowatt hours (kWh). And your battery charger may tell you its power output in milliamps (mA) or watts (W) rather than amps. So you may also see the formula written with different unit combinations.

charge time = battery capacity (mAh) ÷ charge current (mA) charge time = battery capacity (Wh) ÷ charge rate (W)

And sometimes, your units are mismatched. Your battery capacity may be given in watt hours and your charge rate in amps. Or they may be given in milliamp hours and watts.

In these cases, you need to convert the units until you have a ‘matching’ pair.- such as amp hours and amps, watt hours and watts, or milliamp hours and milliamps.

For reference, here are the formulas you need to convert between the most common units for battery capacity and charge rate. Most of them link to our relevant conversion calculator.

Battery capacity unit conversions:

• watt hours = amp hours × volts
• amp hours = watt hours ÷ volts
• milliamp hours = amp hours × 1000
• amp hours = milliamp hours ÷ 1000
• watt hours = milliamp hours × volts ÷ 1000
• milliamp hours = watt hours ÷ volts × 1000
• kilowatt hours = amp hours × volts ÷ 1000
• amp hours = kilowatt hours ÷ volts × 1000
• watt hours = kilowatt hours × 1000
• kilowatt hours = watt hours ÷ 1000

Charge rate unit conversions:

The formula itself is simple, but taking into account all the possible conversions can get a little overwhelming. So let’s run through a few examples.

### Example 1: Battery Capacity in Amp Hours, Charging Current in Amps

Let’s say you have the following setup:

• Battery capacity: 100 amp hours
• Charging current: 10 amps

To calculate charging time using this formula, you simply divide battery capacity by charging current.

In this scenario, your estimated charge time is 10 hours.

### Example 2: Battery Capacity in Watt Hours, Charging Rate in Watts

Let’s now consider this scenario:

Because your units are again ‘matching’, to calculate charging time you again simply divide battery capacity by charging rate.

In this scenario, your estimated charge time is 8 hours.

### Example 3: Battery Capacity in Milliamp Hours, Charging Rate in Watts

Let’s consider the following scenario where the units are mismatched.

First, you need to decide which set of matching units you want to convert to. You consider watt hours for battery capacity and watts for charge rate. But you’re unable to find the battery’s voltage, which you need to convert milliamp hours to watt hours.

You know the charger’s output voltage is 5 volts, so you settle on amp hours for battery capacity and amps for charge rate.

With that decided, you first divide watts by volts to get your charging current in amps.

Next, you convert battery capacity from milliamp hours to amp hours by dividing milliamp hours by 1000.

Now you have your battery capacity and charging current in ‘matching’ units. Finally, you divide battery capacity by charging current to get charge time.

In this example, your estimated battery charging time is 1.5 hours.

### Formula 2

Formula: charge time = battery capacity ÷ (charge current × charge efficiency)

Accuracy: Medium

Complexity: Medium

No battery charges and discharges with 100% efficiency. Some of the energy will be lost due to inefficiencies during the charging process.

This formula builds on the previous one by factoring in charge/discharge efficiency, which differs based on battery type.

Here are efficiency ranges of the main types of rechargeable batteries (source):

Note: Real-world charge efficiency is not fixed and varies throughout the charging process based on a number of factors, including charge rate and battery state of charge. The faster the charge, typically the less efficient it is.

### Example 1: Lead Acid Battery

Let’s assume you have the following setup:

To calculate charging time using Formula 2, first you must pick a charge efficiency value for your battery. Lead acid batteries typically have energy efficiencies of around 80-85%. You’re charging your battery at 0.1C rate, which isn’t that fast, so you assume the efficiency will be around 85%.

With an efficiency percentage picked, you just need to plug the values in to the formula.

100Ah ÷ (10A × 85%) = 100Ah ÷ 8.5A = 11.76 hrs

In this example, your estimated charge time is 11.76 hours.

Recall, that, using Formula 1, we estimated the charge time for this setup to be 10 hours. Just by taking into account charge efficiency our time estimate increased by nearly 2 hours.

### Example 2: LiFePO4 Battery

Let’s assume you again have the following setup:

Based on your battery being a lithium battery and the charge rate being relatively slow, you assume a charge efficiency of 95%. With that, you can plug your values into Formula 2.

1200Wh ÷ (150W × 95%) = 1200Wh ÷ 142.5W = 8.42 hrs

In this example, your estimated charge time is 8.42 hours.

Using Formula 1, we estimated this same setup to have a charge time of 8 hours. Because lithium batteries are more efficient, factoring in charge efficiency doesn’t affect our estimate as much as it did with a lead acid battery.

### Example 3: Lithium Ion Battery

Again, let’s revisit the same setup as before:

First, you need to assume a charge efficiency. Based on the battery being a lithium battery and the charge rate being relatively fast, you assume the charge efficiency is 90%.

As before, you need to ‘match’ units, so you first convert the charging current to amps.

Then you convert the battery’s capacity from milliamp hours to amp hours.

With similar units, you can now plug everything into the formula to calculate charge time.

3Ah ÷ (2A × 90%) = 3Ah ÷ 1.8A = 1.67 hours

In this example, your estimated charge time is 1.67 hours.

### Formula 3

Formula: charge time = (battery capacity × depth of discharge) ÷ (charge current × charge efficiency)

Accuracy: Highest

Complexity: Highest

The 2 formulas above assume that your battery is completely dead. In technical terms, this is expressed by saying the battery is at 100% depth of discharge (DoD). You can also describe it as 0% state of charge (SoC).

Formula 3 incorporates DoD to let you estimate charging time regardless of how charged your battery is.

### Example 1: 50% DoD

Let’s revisit this setup, but this time assume our lead acid battery has a 50% DoD. (Most lead acid batteries should only be discharged to 50% at most to preserve battery life.)

As before, let’s assume a charging efficiency of 85%.

We have all the info we need, so we just plug the numbers into Formula 3.

(100Ah × 50%) ÷ (10A × 85%) = 50Ah ÷ 8.5A = 5.88 hrs

In this example, your battery‘s estimated charge time is 5.88 hours.

### Example 2: 80% DoD

For this example, imagine you have the following setup:

As before, we’ll assume that the charging efficiency is 95%.

With that in mind, here’s the calculation you’d do to calculate charge time.

(1200Wh × 80%) ÷ (150W × 95%) = 960Wh ÷ 142.5W = 6.74 hrs

In this example, it will take about 6.7 hours to fully charge your battery from 80% DoD.

### Example 3: 95% DoD

Let’s say your phone battery is at 5%, meaning it’s at a 95% depth of discharge. And your phone battery and charger have the following specs:

As before, we need to convert capacity and charge rate to similar units. Let’s first convert battery capacity to amp hours.

Next, let’s convert charge current to amps.

Because the charge C-rate is relatively high, we’ll again assume a charging efficiency of 90% and then plug everything into Formula 3.

(3Ah × 95%) ÷ (2A × 90%) = 2.85Ah ÷ 1.8A = 1.58 hrs

Your phone battery will take about 1.6 hours to charge from 5% to full.

### Why None of These Formulas Is Perfectly Accurate

None of these battery charge time formulas captures the real-life complexity of battery charging. Here are some more factors that affect charging time:

• Your battery may be powering something. If it is, some of the charge current will be siphoned off to continue powering that device. The more power the device is using, the longer it will take for your battery to charge fully.
• Battery chargers aren’t always outputting their max charge rate. Many battery chargers employ charging algorithms that adjust the charging current and voltage based on how charged the battery is. For example, some battery chargers slow the charge rate down drastically once the battery reaches around 70-80% charged. These charging algorithms vary based on charger and battery type.
• Batteries lose capacity as they age. An older battery will have less capacity than an identical new battery. Your 100Ah LiFePO4 battery may have only have around 85Ah capacity after 1000 cycles. And the rates at which batteries age depend on a number of factors.
• Lithium batteries have a Battery Management System (BMS). Besides consuming a modest amount of power, the BMS can adjust the charging current to protect the battery and optimize its lifespan. iPhones have a feature called Optimized Battery Charging that delays charging the phone’s battery past 80% until you need to use it.
• Lead acid battery chargers usually have a timed absorption stage. After being charged to around 70-80%, many lead acid battery chargers (and solar charge controllers) enter a timed absorption stage for the remainder of the charge cycle that is necessary for the health of the battery. It’s usually a fixed 2-3 hours, regardless of how big your battery is, or how fast your charger.

In short, batteries are wildly complex, and accurately calculating battery charge time is no easy task. It goes without saying that any charge time you calculate using the above formulas.- or our battery charge time calculator.- should be viewed as an estimate.

## travel-approved power banks that pass the TSA’s battery rules

Travel often means long days for both you and your electronics. The best way to keep your devices charged throughout usage is by using a power bank. But, it can be tough to figure out what kind of mobile chargers fit within the Transportation Security Administration or Federal Aviation Administration’s rules so they won’t get taken away during check-in or at the security checkpoint.

All battery packs face very strict guidelines for air travel. Lithium-ion (rechargeable) batteries and portable batteries that contain lithium-ion can only be packed in carry-on baggage. They’re limited to a rating of 100 watt hours (Wh) per battery. With airline approval, you can bring two larger spare batteries (up to 160 Wh).

Here are our favorite battery packs that are within the FAA and TSA’s rules so you can keep your smartphone, tablet, laptop or headphones juiced up during long airplane rides. Whether you need a boost to keep binge-watching your favorite series or keep up with calls before boarding, these portable chargers will have you covered.

### Anker PowerCore 26800mAh PD 45W With 60-Watt PD Charger

This top-rated battery pack from Anker packs the most power you’re allowed to take on a flight. It features a 45-watt USB-C port to charge your laptop back to full battery and 15-watt USB ports to charge smartphones and other small devices. The extra battery itself charges quickly and will reach a full charge in less than three and a half hours. It comes with a 60-watt USB-C wall charger and USB-C cable as well so you’re equipped with everything you’d need. It’s on the pricier side, but the speed at which it can repower your devices and the included wall charger and charging cable make the battery worth the splurge.

### Anker Power Bank Power Core Slim

We like this pick from Anker because of the low price and slim design. It’s super lightweight and easy to slip into a purse, backpack or It features trickle charging mode so you’re able to safely charge smaller devices like your earbuds. You’ll need to get your own charging cables, as the only one included with the battery is a Micro USB to recharge the battery (it’s also rechargeable via the USB-C input port, but you’ll need your own cable). It’ll take 5.5 hours to recharge if you use a 10-watt wall charger. Overall, the slim design makes this a great pick for super-convenient portability.

### Anker PowerCore 13000

After testing, this was our pick as the best overall portable charger, and it’s also perfect for travel. It boasts an impressive 13,000mAh, which is enough to charge an iPhone 11 two and a half times. In our testing, this small but mighty — and affordable — charger packed a ton of value in its three ports.

### Belkin Boost Charge Power 5K

In our testing, we dubbed this device the best portable charger for iPhones. That’s because it has a Lightning port included in the device as well as a USB Type A port, meaning you can use the same cord to charge your phone and refill the battery. We were impressed by its fast charging and just how much it was able to charge, which made it one of the best options we tested in terms of living up to its 5,000mAh-promise.

### Anker PowerCore Fusion 5000 Portable Charger

The PowerCore Fusion doubles as both a wall charger and a portable battery. It’s a perfect option if you’re looking to travel light, as you’ll have power on the go and an easy wall plug once you arrive at your destination. It’s pretty small, measuring 2.7 inches by 1.2 inches, which makes it easy to toss in your carry-on. The portable battery charges via the wall plug, which also helps to reduce your cable clutter. It’ll charge your phone up to three times, so it’s perfect if you’re looking for quick boosts to your battery.

### Luxtude Portable Charger

This portable charger has 4.4 out of 5 stars in reviews on Amazon. It features a 5,000mAh battery with a built-in Apple MFi-certified Lightning charging cable so you won’t have to worry about carrying extra wires with you to charge your Apple device on the go. The design is super thin (0.31 inches, to be exact), so it’s perfect for quick and easy storage and holding while in use. It has a 5V max output for safe charging and can get your iPhone back to 50% battery in 30 minutes. We like it a lot for the super-slim profile and built-in Lightning cable for a super-convenient charge.

### Belkin Power 10K Power Bank

This power bank is aptly named, as it makes power on the go a breeze in a.size device. It features two universal USB ports for charging up to two devices at once. It features a 10,000mAh battery to provide up to 36 additional hours of video playback, so it’s a solid option if you love to stream your favorite shows on the go. It will deliver up to a 5-volt charge to smartwatches and fitness bands, earbuds, action cameras and other Bluetooth-enabled devices. It has a quick recharge using the included Micro USB cable. It also has a smaller version in the Belkin Power 5K, which we dubbed the most portable power pack in our testing.

### Belkin Portable Power Bank Charger 10K

This pick from Belkin has a 10,000mAh battery to give you an extra 36 hours of additional battery life for your smartphone. It features two USB-A ports, a USB-C port and a Micro USB input port. It can deliver up to a 12-watt charge from the USB-A port when a single device is plugged in for a quick battery boost. A total of 15 watts is shared when all ports are being used. It’s available in three colors — rose gold, white and black —so you can choose a color to match your other accessories.

### Mophie Powerstation

This 6,000mAh battery pack can charge your smartphone up to two times. It has two dual charging ports so you can power more than one device at a time while on the go. You can use it to charge your smartphone, headphones, speakers and tablets. The LED indicator on the side of the power bank will let you know how much power is left in the battery so you know when it’s time to plug in the power brick. The bank itself charges via an included Micro USB cable.

### Satechi Quatro Wireless Power Bank

This is a wireless power bank that’s great to use while traveling since it eliminates the need for extra wires if you’re using Qi-enabled devices. As a bonus, there’s a designated charger for your Apple Watch. You can fast-charge your iPad or another tablet that isn’t Qi-enabled using the USB-C PD port that maxes output at 18 watts for compatible devices. The 10,000mAh capacity can charge an iPhone 11 up to two times to keep you powered throughout your travels.

### Zendure 10,000mAh Portable Charger

Zendure’s portable charger has a sleek and high-tech look while packing a 10,000mAh battery to charge your smartphone up to three times. It’s made of a crushproof composite material, so it’s safe from bumps and knocks if you toss it in your carry-on bag. It has USB-A and USB-C ports, and the battery pack itself charges through a Micro USB cable. Plus, we like the eye-catching design and color options.

### Aukey USB-C Power Bank

Available in black or white, Aukey’s power bank has a 10,000mAh battery to keep your devices charged through the two USB-A ports and single USB-C input/output port. It’s slim in design and about the same size as your phone, so it’ll easily fit in s and bags. You can use the two USB-A ports to charge multiple devices at once, and the output maxes out at 12 watts for compatible devices.

### INIU 10,000mAh Portable Charger

This device has more than 20,000 5-star reviews on Amazon — and for good reason. It’s dubbed as the thinnest 10,000mAh power bank on the market, making it ideal for travelers who want to stay charged on the go. It can charge an iPhone 8 more than three times. Plus, it’s got a USB-C in/out port for added convenience. Perhaps best of all, it comes with a three-year warranty.

Looking for a travel credit card? Find out which cards CNN Underscored chose as our best travel credit cards currently available.

Note: The above reflect the retailers’ listed price at the time of publication.

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## UP – simple 5V UPS

Raspberry pi, routers, wireless nodes, NAS, some lights, all of mine have one thing in common: they need 5V power supply and could use one that is backed up. Now, I am not looking for hours of backup since i find the local electricity very reliable, I cannot remember when power was out for more than 10 minutes, but 1h should be ok for extreme cases. Since everything is already powered from a 5V supply with ample power reserve, i thought that a 5V in, 5V out version is best.

### First fail

At first I tried using my 4×18650 batteries portable phone charger with both charger in and output on. It turns out that it will only work for some hours: as the step up converter is drawing power from the battery, even though it gets charged, the charger has a timeout. No matter what, the charger gives up and you are left consuming the battery and then it’s over. This takes more than a day if I start with a full battery, so it might fool you at the beginning. I tried looking for options to power the step up converter from the 5V input directly, but it turned out the compact PCB was making things worse.

### Parts bin

I turned to my parts bin and searched for ingredients that would be needed to make one: A battery charger, a battery, and a step up converter. A 2.5Ah 18650 battery was just right, but i did not have a case for it so I 3d printed this one. The step up is an unknown DC/DC converter from ebay, it claims to be able to supply 5V at 1.2A, from a lithium battery, but that is about it. The charger is a 1A charger, which would charge the battery fast, while not overloading my 5V 2.4A supply.

Important: LiIon batteries can be very dangerous if mistreated, because of this I have used a protected cell: the battery contains over voltage, over current and over temperature protection circuits inside it, so in case something happens from the outside, it is safe. Please use a protected cell if you are replicating this design. I am not responsible for any consequences resulting from somebody using the information presented here, you are at your own risk!

Next up, let’s build the schematic:

I have not been able to find this design online, hence the article. It is pretty straightforward: a step up DC/DC converter gets its input either from the battery or from the original 5V supply. When 5V_IN is available, because the battery is a lower voltage than this, D2 conducts and provides power to the converter while D1 blocks the battery from discharging. There is about 0.3V dropped on the diode and some on the cable, in my tests the input of the converter dropped till 4.5V. If power fails, there is no 5V_IN available and D1 conducts and powers the converter.

Precisely for this voltage drop, I decided to use the step up converter after generating the uninterrupted supply at the common cathode of the diodes. I could have connected the schottky diode D2 between the output and input, and provide power to the output through it. This is seen in some other designs online. The drawback is that it requires the step up converter to be tuned to a voltage that is always lower than the supply reaching the UPS, otherwise the battery might supply the pi. With 4.5V from a nominal 5 reaching the UPS, I would have to tune the step up to about 4V which might not be enough for some attached peripherals, while running on backup battery.

### The build

I plugged everything on a prototype board and started doing a couple of tests. I loaded the circuit to about 1A mimicking a Pi and some things around it, with a 4.9Ω resistor(precise measured value). After a while it looks like the step up module reached a stable temperature of about 75°C maximum. This is rather high, so i recommend using it in a well ventilated enclosure, possibly with forced cooling. This is an extreme case, the Pi will be the greatest consumer at about 0.7A. On the right you can see the output voltage, on the left the battery voltage, in this case it is being charged.

The LEDs on the modules provide a good impression of the status: the one on the step up converter signals power is available to the PI and any of the ones on the charger(charging/full) signal AC power.

Efficiency results: Normally, the pi draws 2.8W from the mains while being idle, with a HDMI screen connected and Ethernet. While using the ups, after the battery is charged the total power draw is 3.2W, which means an efficiency of 87%. The actual lost power means 3.5KWh/year.

Idle power: Just the UPS alone draws 0.2W from the mains, after the battery is charged. All power numbers include the first 5V power supply, they are measured directly as power drawn from the mains with my sensitive power meter.

Backup time: as mentioned in the above test, the pi with only display and Ethernet is backed up for about 2 hours with a single 2.5Ah battery. The precise number of my 2 experiments is always between 2 and 2:15 hours, as I checked the status every 15 minutes.

Trust: I really don’t trust these eBay modules for a long time operation, the thing here is just for a proof of concept. As soon as I will get the chance I will purchase some “brand name” ICs to rebuild this with proper quality components.

Note: at some point i have used some step up modules that did not have a common ground between input and output. I suggest avoiding those.

## DETAILED DESCRIPTION

A 5V, 2.5A Uninterruptible Power Supply (UPS) based on the Linear Technology (now Analog Devices) LTC4040 2.5A Battery Backup Power Manager.

The LTC4040 includes a step-up DC-DC converter to power your 5V equipment from a single cell Li-Ion or LiFePO4 Battery in the event of loss of external power. When external power is available, the step-up regulator operates in reverse as a step-down battery charger.

## Battery Chemistry and Charge Voltage

Battery chemistry and charge voltages can be set up via inputs F0, F1 F2. For Li-Ion cells, regulated charge voltages of 3.95V/4.0V/4.05V/4.1V are available. For LiFePO4 cells the thresholds change to 3.45V/3.5V/3.55V/3.6V. As the batteries are used for back-up purposes and as capacity is degraded when cells remain fully charged, it is recommended to trade off capacity (up-time) for battery lifetime.

## Charge Current

A fixed resistor on the PROG pin, sets the maximum charge current. The current design uses a 2K resistor to set the charge current to 1A. The LTC4040 includes a safety timeout or approximately 4.25h for Li-Ion batteries and 2.13h for LiFePO4.

The current shunt (R1) can be used to prioritise system load over charge rate, ensuring the input power supply is not overloaded. At present the 0.01 ohm shunt sets the maximum input load to 2.5A (0.025V/0.01). This means if the load is drawing a full 2.5A, the charger will be throttled back to almost nothing. You can change R1 to 0.007 ohms to set the current at 3.5A. This allows the battery to charge at 1A, while the load can consume 2.5A.

## Backup / Power Failure Threshold

R16/R15 sets the power failure threshold at 4.6V. When the input exceeds 4.6V, the device will operate in pass-through mode (via external Q1 MOSFET) and the output voltage will track that of the input. When the input voltage drops below this threshold, the DC-DC converter will switch on and power the load from battery. In this mode, the output voltage is set via R3/R4 to 5.0V.

## Status

20/06/2019. Prelimary testing with DC Load and LiFePO4 returns good results. Waiting on extra parts. USB-C and Output Capacitors in order to complete design.

## How to Use a Single Supercapacitor as Backup Power for a 5-Volt Supply

Did you know how to design a simple and elegant solution to power a 5-volt rail using just a single supercapacitor ? This article written by Steven Keeping, published by DigiKey, explains how to use a single 2.7V supercapacitor on 5V rail combined with a reversible buck/boost voltage converter.

Once limited to mission-critical devices, backup power solutions are now in demand for a wide range of electronics applications in industrial, commercial, and consumer end-products. While there are several options, the supercapacitor offers the most compact and energy-dense solution as an energy reservoir when the main supply is interrupted. For example, when there’s a mains power outage or when batteries are being swapped out.

### Digi-Key Launches Supply Chain Transformed Season 2 Video Series

However, supercapacitors introduce design challenges because each device can only provide up to 2.7 volts. That potentially means multiple supercapacitors are needed—each with associated cell balancing and step-up (boost) or step-down (buck) voltage converters—to supply regulated power to a 5-volt power rail. The result is a complex and nuanced circuit that is relatively expensive and takes up excessive board space.

### Design considerations for supercapacitors

If an electronic product is to rely on a supercapacitor for backup power, it is vital that the designer understands how to select the best component for reliable energy storage and delivery, and long life.

One of the first things to check on the datasheet is the effect of temperature on capacitance and resistance. It is good design practice to select a device that exhibits very little change across the intended operating temperature range of the end-product such that if backup power is needed, the supplied voltage is stable, and energy is delivered efficiently.

Supercapacitor lifetime is largely determined by the combined effect of operating voltage and temperature (Figure 1). The supercapacitor rarely fails catastrophically. Instead, its capacitance and internal resistance change over time and gradually degrade performance until the component is no longer able to meet the end-product specification. The performance decline is typically greater at the beginning of the end-product’s life, tailing off as the end-product ages.

When used in a backup power application, the supercapacitor will be maintained at the working voltage for long periods, only very occasionally being called upon to discharge its stored energy. This will eventually impact performance. The datasheet will indicate the decline in capacitance over time for typical operating voltages and at different temperatures. For example, a 15% reduction in capacitance and a 40% increase in internal resistance may occur for a supercapacitor held at 2.5 volts for 88,000 hours (10 years) at 25˚C. Such performance decline should be considered when designing backup devices for end-products with long service lives.

The time constant for a capacitor is the time taken for the device to reach 63.2% of full charge or discharge to 36.8% of full charge. The time constant of a supercapacitor is around one second; this is much shorter than an electrolytic capacitor. Because of this short time constant, the designer should ensure that the backup power supercapacitor is not exposed to a continuous ripple current, as damage may result.

Supercapacitors can operate between 0 volts and their maximum rated capacity. While efficient utilization of the supercapacitor’s available energy and power storage is achieved when operating over the widest voltage range, most electronic components have a minimum voltage threshold. This minimum voltage requirement limits the amount of energy that can be drawn from the capacitor.

For example, the energy stored in the capacitor is E = ½CV 2. From this relationship, it can be calculated that approximately 75% of the available energy is accessible if the system operates at half the rated voltage of the capacitor (for example from 2.7 to 1.35 volts).

### Supercapacitor Balancing – Design challenges when using multiple supercapacitors

While the advantages of supercapacitors make them suitable for providing backup power to a wide range of electronic products, the designer must be wary of the design challenges they introduce. Implementing a backup power supply circuit can be a significant undertaking for the inexperienced engineer. The key complexity is that commercial supercapacitors are rated for around 2.7 volts, so to supply a typical 5-volt power rail, two supercapacitors must be used in series (Figure 2).

While this is a satisfactory working solution, it incurs additional cost and complexity because of the need for active or passive cell balancing. Due to capacitance tolerances, different leakage currents, and different ESRs, the voltage across two or more nominally identical and fully charged capacitors can be different. This voltage imbalance results in one supercapacitor in a backup circuit supplying a greater voltage than the other. As the temperature increases and/or the supercapacitors age, this voltage imbalance can increase to the point where the voltage across one supercapacitor exceeds that device’s rated threshold and impacts operational life.

Cell balancing in low-duty-cycle applications is typically achieved by placing a bypass resistor in parallel with each cell. The value of the resistor is chosen to be a value that allows any current flow to dominate the total supercapacitor leakage current. This technique effectively ensures that any variation in equivalent parallel resistance between the supercapacitors is negligible. For example, if the supercapacitors in the backup circuit have an average leakage current of 10 microamps (μA), a 1% resistor will allow a current bypass of 100 μA, boosting the average leakage current to 110 μA. In doing so, the resistor effectively decreases the variation in leakage current between the supercapacitors from tens of percent to just a few percent.

With all parallel resistances fairly well matched, any supercapacitors with higher voltages will discharge through their parallel resistance at a higher rate than the supercapacitors with lower voltages. This distributes the total voltage evenly across the entire series of supercapacitors. For high-duty applications, more sophisticated supercapacitor balancing is required.

## Using a single supercapacitor for a 5-volt supply

The backup power supply circuit could be made less complex and take up less space if a single supercapacitor is employed instead of two or more. Such an arrangement eliminates the need for supercapacitor balancing. However, the 2.7-volt output from a single device needs to be increased using a boost voltage regulator, creating a sufficient voltage to overcome the voltage drop across a diode and provide 5 volts to the system. The supercapacitor is charged by a charging device and discharges through the boost converter when needed. Diodes allow either the primary power source or the supercapacitor to power the system (Figure 3).

A more elegant solution is to use a single capacitor complemented by a specialized voltage converter, such as Maxim Integrated’s MAX38888 or MAX38889 reversible buck-boost voltage regulator. The former offers 2.5 volts to 5 volts and up to 2.5 amperes (A) output, while the latter is a 2.5-volt to 5.5-volt, 3 A output device (Figure 4).

The MAX38889 is a flexible storage capacitor or capacitor bank backup regulator for transferring power efficiently between the supercapacitor(s) and a system supply rail. When the main supply is present and its voltage is above the minimum threshold system supply voltage, the regulator operates in charging mode and charges the supercapacitor with a maximum 3 A peak, 1.5 A average inductor current. The supercapacitor needs to be fully charged to enable backup operation. Once the supercapacitor is charged, the circuit draws only 4 μA of current while maintaining the component in its ready state.

When the main supply is removed, the regulator prevents the system from dropping below the set system backup operating voltage by boosting the supercapacitor voltage to the required system voltage at a programmed peak inductor current, up to a maximum of 3 A. The reversible regulator can operate down to a supercapacitor supply voltage of just 0.5 volts, maximizing the stored energy use.

The duration of backup depends on the supercapacitor’s energy reserve and the system power draw. The features of the Maxim Integrated products allow for maximum backup power from a single 2.7-volt supercapacitor, while reducing the number of circuit components by eliminating the need for separate charger and boost devices, and diodes.

## Conclusion

Supercapacitors offer several advantages over secondary batteries for backup power in particular applications, such as those that demand frequent battery changes. Compared with rechargeable batteries, supercapacitors charge more quickly, can be cycled many more times, and offer much higher power density. However, their maximum 2.7-volt output introduces some design challenges when looking to back up a typical 5-volt supply.

As shown, reversible step-down/step-up voltage regulators offer an elegant solution by allowing a single supercapacitor to back up a 5-volt line while minimizing space and the number of required components.