Follow me down the optimization rabbit hole
I optimize software for a living. This blog is to share some personal projects which others may find interesting.
Powering your Arduino with batteries
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So, you’ve created an Arduino project and you want to power it with batteries to take it on the road. Your board’s components are designed to run on 5 Volts and you know you can’t feed 9V directly into the Vcc of a 5V board because it will damage it. Arduino has you covered. the pin marked RAW is for that purpose and according to the documentation, you can feed it between 6 and 12V and it will regulate that voltage down to the 5V needed by the board. Perfect, right? Well, not quite.
There are 2 main ways to regulate (aka control) the voltage. A linear regulator allows you to supply a higher voltage than desired (in our case 9V) and get a stable, lower voltage as output. It essentially does this by generating heat from the excess energy. Let’s say your board uses 100mA @5V while executing your code. If you’re powering it from a 9V battery through a linear regulator, then you’re creating 4V x 100mA = 400mW of waste heat (9V. 5V = 4V). If you measure the current coming out of your 9V battery, it will be very close to 100mA, so 9V @ 100mA is going into the regulator and 5V @ 100mA is coming out. The means that the linear regulator effectively has an energy efficiency of 56% in this case. For many many years, the 78xx series of linear regulators is what you would use for cases like this:
Aside from the simplicity, there isn’t much to like about linear regulators. When running on battery power, you’re throwing away a large percentage of your battery’s energy when you regulate the output this way. In circuits that use a lot of current, heat becomes a major concern too.
The older Arduino boards have a linear regulator built in to make it easier to power them from various energy sources. The assumption is usually that you’ll be running from a wall wart (A/C power brick) so wasted energy isn’t much of a concern and that you’ll be running a small current through your circuit, so waste heat isn’t a concern either.
If you’re using a 9V alkaline battery that has a typical energy capacity of 500mAh and you connect it to the Arduino’s linear regulator.
In effect, instead of having a 4.5Wh battery, you have a 2.5Wh battery because the rest of the energy is given off as heat.
Buck (or step down) converters are another way to convert a high voltage source to a lower voltage. Buck converters are a more complex circuit that relies on an oscillator and inductor (coil) to change the voltage. The advantage of the buck converter is that it doesn’t waste nearly as much energy as a linear regulator. Here are the efficiency curves for a typical buck converter:
With 5V output at low current, it approaches 97% efficiency. That means that your 9V battery could potentially provide closer to 4.5Wh of energy to your circuit instead of the 2.5Wh you get with the linear regulator. Electronut Labs sells a convenient buck converter specifically designed for easy use with 9V batteries:
What about going in the other direction? This is the option that’s usually not mentioned in Arduino project articles. It’s also possible to boost the voltage from a lower voltage to a higher voltage. DC-DC boost converters work similarly to buck converters and use a high frequency oscillator and a coil to generate a higher voltage. They also have typical efficiencies greater than 85%. This frees you to use other power sources such as a single AA battery. An Alkaline AA battery typically has a capacity of around 2500mAh. At 1.5V, this translates to about 3.75Wh of energy. Less total energy than a 9V battery, but used efficiently, it can save space and cost compared to a 9V. If we boost 1.5V to 5V and assume that the boost converter has an efficiency of 90%, we should be able to squeeze about 3.375Wh of energy out of it. Here’s a typical DC-DC boost converter sold by various vendors in China for 0.45-1.00 each:
I like to use these in my projects because they’re tiny and inexpensive. They operate down to about 0.8V as input and the output is clean enough (low noise) to use in most microcontroller projects.
The original set of Arduino boards were all based on AVR microcontrollers and all set to run at 5V. This made sense at the time because the AVR MCU can operate on any voltage between 1.8V and 5.5V, but at the higher voltages, the clock can run at up to 20Mhz (see chart below):
The reality is that your project probably doesn’t need the MCU to run at 20Mhz. If you’re reading a few sensors and updating a display, you could accomplish the same work at a lower clock rate. Another reason is that the amount of energy used by the MCU and peripherals does not perfectly follow a linear scale. Even so, you can potentially accomplish the same amount of work running at 8Mhz and 3.3V as you could at 16Mhz and 5V. Running the CPU slower or at a lower voltage uses less energy.
A lot of newer MCUs in the Arduino lineup operate at 3.3v (e.g. ARM Cortex-M MCUs) and so do many add-on boards, so it makes a lot of sense to run your project at 3.3v. Without having to know too much about AVR fuses and hardware, it’s possible to run a board designed for 5V and 16Mhz at 3.3V and 8Mhz with a simple trick in software. The main CPU clock divider can be set in software. We can use this to cause a 16Mhz part to run at 8Mhz so that it can run reliably at 3.3V. The clock prescaler is normally set to 1 on Arduino boards. By setting it to 2, the CPU will run at a more stable 8Mhz:
Now with this new information, let’s look at cost and battery life of our original 5V project running on a 9V battery versus our new idea of running it at 3.3V from a single AA battery. For the cost, I’m making the assumption of buying a 4-pack of each battery type (Duracell) from Amazon.com.
v battery for arduino
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Please, before submitting a support request read carefully this README and check if an answer already exists among previously answered questions: do not abuse of the Github issue tracker.
This is a simple Arduino library to monitor battery consumption of your battery powered projects, being LiPo, LiIon, NiCd or any other battery type, single or multiple cells: if it can power your Arduino you can monitor it!
The principle is simple: we are going to measure our battery capacity by measuring the voltage across the battery terminals.
The big assumption here is that battery capacity is linearly correlated to its voltage: the assumption itself is wrong, but in most cases it’s close enough to reality, especially when it comes to the battery higher capacity side.
In reality, the relation between battery capacity and its voltage is better represented by a curve and there are many factors affecting it: current drawn, temperature, age, etc.
The library requires at least 1 analog pin (we will call this the sense pin ) and no less than 2 pieces of info on your battery: the voltage you will consider the minimum acceptable level, below which your project/product becomes unreliable and should be shut down, and the maximum voltage you can expect when the battery is fully charged.
Additionally, you can provide a second pin (either analog or digital) to activate the battery measurement circuit (we call it the activation pin ), useful in all those situations where you can sacrifice a pin to further increase your battery duration.
If you want your readings to be more accurate we strongly suggest to calibrate the library by providing your board reference voltage: most of the times you assume your board has exactly 5V between Vcc and GND. but this is rarely the case. To improve this we suggest using the VoltageReference library to obtain a better calibration value for all analog readings.
The sense pin wiring can vary depending on your battery configuration, but here are a few examples based on the assumption you are using a 5V board: in case of a 3.3V board you should be performing the necessary adjustments.
Lesser than 5V, with voltage booster
Voltage sources made of single cell LiPo or LiIon, along with some single or multi-cell NiCd configurations (like up to 3 AA or AAA), are not able to provide the suggested 5.0 volts input to your board and a voltage booster can solve your problem. What does that mean when it comes to measuring your battery level? We need to measure the battery voltage before it gets boosted, which means your sense pin must be connected between the battery positive terminal and the booster positive input and we don’t need any additional components as the voltage is already in the acceptable range:
SENSE | | |. | | | | | | | BAT.- IN | 5V | OUT. 5V | Arduino | | BOOSTER | | | BAT-. IN- | | OUT-.- GND | |. | |.
Higher than 5V, with internal voltage regulator
Voltage sources made of multiple cells LiPo or LiIon, along with some single or multi-cell NiCd configurations (like up the classic 9V battery or 4 AA or AAA), provide voltages above the 5.0 volts input : most of the Arduino boards are equipped with voltage regulators able to dissipate into heat all the excess. To measure such batteries we need to hook our sense pin before it gets regulated, between the battery positive terminal and the Arduino unregulated input VIN or RAW. but we require two resistors to reduce the voltage to acceptable values:
. BAT.- VIN | | | | | R1 | | | | |. SENSE | Arduino | | | | R2 | | | | | BAT-. GND | |.
The values of R1 and R2 determine the voltage ratio parameter for this library: for information about this value refer to the section below.
Because the resistors in this configuration will constantly draw power out of your battery, you shouldn’t pick values under 1k Ohm. or you’ll deplete your batteries much faster than normal. On the other end, going too high on the resistor values will impede the library from getting accurate readings.
Higher than 5V, with external voltage regulator
Whenever your battery maximum voltage exceeds the onboard regulator (if there is any) an external voltage regulator is required. Once again, to measure such batteries we need to hook our sense pin before it gets regulated, between the battery positive terminal and the voltage regulator positive input VIN or RAW and, as before, we require two resistors to reduce the voltage to acceptable values:
– |. |. | BAT.- IN | | SENSE | | | | | | | | | R1 | | | | | | | | | |. | REG | OUT.- 5V | Arduino | | | | | | R2 | | | | | | | | | BAT-.- IN- | | OUT-. GND | |
The values of R1 and R2 determine the voltage ratio parameter for this library: for information about this value refer to the section below.
Higher than 5V, activated on demand
Batteries are a precious resource and you want to prolong their life as much as you can so, deplete your battery to determine its capacity is not desirable.
As a consequence of connecting the battery terminals through two resistors we are drawing some energy out of the battery: for a 9V battery and 1k Ohm for R1 and R2, you will be adding a constant 4.5mA current consumption to your circuit. Not a huge amount, but definitely not desirable.
If you have an unused pin on your Arduino it will be easy to limit this additional current consumption to be drawn only when needed: during battery measurement. We will be turning the activation pin HIGH during battery measurement so that the voltage divider will be disconnected most of the time:
. BAT.- VIN | | | | | SW- ACT | | | | | R1 | | | | Arduino |. SENSE | | | | | R2 | | | | | BAT-. GND | |.
In the above schematics SW is a circuit which can connect or disconnect the sensing circuit depending on the voltage on ACT : the most common and cheap circuit is made of a NPN transistor Q1, a p-channel MOSFET Q2, a 1k-4.7k Ohm resistor R3 and a 5k-20k Ohm resistor R4:
BAT |. | | R4 | |\ | ACT. R3.Q1 \ Q2 | | | | GND VDIV to R1/R2/SENSE
Feel free to refer to this circuit simulation to better understand how the circuit works and how much current draws when in operation.
Whenever your battery voltage is above your board voltage you need a voltage divider to constraint your readings within the 0-5V range allowed by your Arduino and you will have to provide this library with its ratio.
BAT. | R1 |. SENSE | R2 | BAT-.-
The voltage divider ratio is determined by the formula (R1 R2) / R2 : if you use two resistors of the same value the ratio will be 2, which can be interpreted as whatever value we read it will be half of the actual value. This allows us to sense batteries up to 10V. If you use a 22k Ohm resistor for R1 and a 10k Ohm for R2 than your voltage ratio will be 3.2 and you will be able to safely monitor a 12-15V battery.
You must select the resistors in order to get a ratio which will produce values between the 0-5V range (or 0-3.3V for 3.3V devices) at all the times and to obtain that the process is quite simple: divide your battery maximum voltage by 5V and you’ll get the absolute minimum value for the voltage ratio. then pick any two resistors values whose combination produce a ratio equal or higher than the absolute minimum. For a 12V battery the absolute minimum voltage ratio is 12/5=2.4, meaning you can’t use a split supply divider made of two equal resistors: you need R1 to be a higher value than R2! Get this wrong and you will probably burn your sense pin.
You can use this nice website to find some appropriate values for the resistors setting your battery maximum voltage as Voltage source and aiming at obtaining a Output voltage value lesser than your board voltage ( 5V or 3.3V ) but as close as possible.
The voltage divider total resistance, made of R1 R2. will determine the current drawn from your battery by the sensing circuit: lower is the total resistance and more accurate are your readings, higher the resistance and less current is drawn from your battery (Ohm’s law rulez!). My suggestion is to keep this value within 20k-22k Ohm when using an always-connected circuit and under 10k Ohm if you use an on-demand configuration.
When determining the ratio don’t stick with the resistors nominal values, instead, if possible, use a multimeter to actually measure their resistance so to improve your results: a 4.7kΩ resistor could easily be a 4.75kΩ in reality!
Remaining capacity approximation
The level available functions aim at providing an approximation of the remaining battery capacity in percentage. This is not an easy task when you want to achieve reliable values and it is something the industry of mobile devices invests a decent amount of resources. When an accurate estimate is desireable the battery voltage is not the sole parameter you want to take into consideration:
- cell chemistry has a very high influence, obviously
- cells based on the same chemistry might produce pretty different results depending on the production process
- each chemistry has a different ideal operating cell temperature
- the rate you draw current from the battery influences the remaining capacity
- batteries are not everlasting: as the cell ages, the battery capacity gets reduced
- and more
The library itself doesn’t aim at providing accurate estimates, but what I consider an improvable but good enough estimate.
The library can be configured to use a mapping function of your choice, given the function complies with the mapFn_t interface:
uint8_t mapFunction(uint16_t voltage, uint16_t minVoltage, uint16_t maxVoltage)
To configure your personalized function you only have to provide a pointer to it during initialization:
Battery batt = Battery(3000, 4200, SENSE_PIN); uint8_t myMapFunction(uint16_t voltage, uint16_t minVoltage, uint16_t maxVoltage) // your code here void setup batt.begin(3300, 1.47, myMapFunction);
You are not limited in considering only the parameters listed in the function interface, meaning you can take into consideration the cell(s) temperature, current consumption or age: that’s open to your requirements and circuitry.
After collecting a few data points on battery voltage vs. battery capacity, I’ve used the https://mycurvefit.com/ and https://www.desmos.com online tools to calculate the math functions best representing the data I’ve collected.
In the above plot I represent the battery percentage (Y axis) as a function of the difference between the current battery voltage and the minimum value (X axis): the graph represents a battery with a voltage swing of 1200mV from full to empty, but the functions scale accordingly to the minVoltage and maxVoltage parameters.
The library ships with three different implementations of mapping function:
- linear is the default one (dashed red), probably the least accurate but the easiest to understand. It’s main drawback is, for most chemistries, it will very quickly go from 25-20% to 0%, meaning you have to select the minVoltage parameter for your battery accordingly. As an example, a typical Li-Ion battery having a 3V to 4.2V range, you want to specify a 3.3V configuration value as minimum voltage.
- sigmoidal (in blue) is a good compromise between computational effort and approximation, modeled after the tipical discharge curve of Li-Ion and Li-Poly chemistries. It’s more representative of the remaining charge on the lower end of the spectrum, meaning you can set the minimum voltage accordingly to the battery safe discharge limit (typically 3V for a Li-Ion or Li-Poly).
- asymmetric sigmoidal (in green) is probably the best approximation when you only look at battery voltage, but it’s more computational expensive compared to sigmoidal function and, in most cases, it doesn’t provide a great advantage over it’s simmetric counterpart.
I strongly encourage you to determine the function that best matches your particular battery chemistry/producer when you want to use this library in your product.
Here follow a few real case scenarios which can guide you in using this library.
Single-cell Li-Ion on 3.3V MCU
As an example, for a single cell Li-Ion battery (4.2V. 3.7V) powering a 3.3V MCU. you’ll need to use a voltage divider with a ratio no less than 1.3. Considering only E6 resistors, you can use a 4.7kΩ (R1) and a 10kΩ (R2) to set a ratio of 1.47 : this allows to measure batteries with a maximum voltage of 4.85V. well within the swing of a Li-Ion. It’s a little too current hungry for my tastes in an always-connected configuration, but still ok. Considering the chemistry maps pretty well to our sigmoidal approximation function I’m going to set it accordingly along with the minimum voltage which lowest safe value clearly is 3.0V (if a Li-Ion is drained below 3.0V the risk of permanent damage is high), so your code should look like:
Battery batt = Battery(3000, 4200, SENSE_PIN); void setup // specify an activationPin activationMode for on-demand configurations //batt.onDemand(3, HIGH); batt.begin(3300, 1.47, sigmoidal);
Double cell Li-Ion (2S) on 5V MCU
For a double cell Li-Ion battery (8.4V. 7.4V) powering a 5V MCU. you’ll need to use a voltage divider with a ratio no less than 1.68 : you can use a 6.8kΩ (R1) and a 10kΩ (R2) to set the ratio precisely at 1.68. perfect for our 8.4V battery pack. The circuit will continuously draw 0.5mA in an always-connected configuration, if you can live with that. As we don’t want to ruin our battery pack and we don’t want to rush from 20% to empty in afew seconds, we’ll have to set the minimum voltage to 6.8V (with a linear mapping) to avoid the risk of permanent damage, meaning your code should look like:
Battery batt = Battery(6800, 8400, SENSE_PIN); void setup // specify an activationPin activationMode for on-demand configurations //batt.onDemand(3, HIGH); batt.begin(5000, 1.68);
NOTE: I could have used the sigmoidal approximation, as the chemistry fits pretty well on the curve, in which case a 6V minimum voltage would have been a better configuration value.
Another classic example might be a single 9V Alkaline battery (9V. 6V) powering a 5V MCU. In this case, you’ll need to use a voltage divider with a ratio no less than 1.8 and, for sake of simplicity, we’ll go for a nice round 2 ratio. Using a nice 10kΩ both for R1 and R2 we’ll be able to measure batteries with a maximum voltage of 10V consuming only 0.45mA. The trick here is to determine when our battery should be considered empty: a 9V Alkaline, being a non-rechargeable one, can potentially go down to 0V, but it’s hard our board can still be alive when this occurs. Assuming we are using a linear regulator to step down the battery voltage to power our board we’ll have to account for the regulator voltage drop: assuming it’s a 1.2V drop, we might safely consider our battery empty when it reaches 6.2V (5V 1.2V), leading to the following code:
Battery batt = Battery(6200, 9000, SENSE_PIN); void setup // specify an activationPin activationMode for on-demand configurations //batt.onDemand(3, HIGH); batt.begin(5000, 2.0);
NOTE: Most 5V MCU can actually continue to operate when receiving 4.8V or even less: if you want to squeeze out as much energy as you can you can fine tune the low end, but also consider there is not much juice left when a battery voltage drops that much.
This tutorial demonstrates how to power your Arduino Uno with a solar cell. Solar cells can be a useful solution for powering projects that require portability or remote monitoring. This tutorial uses concepts drawn from the following resources:
This project requires the following components:
he following steps describe how to set up your Arduino Uno with solar power. As a note, components should be soldered together for stability.
Step 1: Solder M-M jumper wires to the positive and negative (-) terminals of the solar cell.
Step 2: Solder the other end of the M-M jumper wires to the input terminals of the TP4056 battery charge controller.
Step 3: Solder the output wires from the battery holder to the TP4056 battery charge controller B and B- terminals.
Step 4: Solder a second set of M-M jumper wires to the output terminals of the TP4056 battery charge controller.
Step 5: Solder the other end of the M-M jumper wires to the input terminal of the XL6009 – Voltage Adjustable DC-DC (5v-35v) Boost Converter. Use a voltmeter connected to the output terminals to determine the output voltage. Powering the Arduino Uno through the Vin port requires an input between 7 and 12 Volts, so the desired output from the Boost Converter is 9V. The voltage output can be adjusted by turning the knob located on the blue rectangle.
Step 6: Solder another set of M-M jumper wires to the output terminals of the Boost Converter. Insert the other end of the M-M jumper wires to the Arduino Uno with the positive terminal connected to the Vin pin and the negative terminal connected to the GND pin (-).
If working properly, the green light of your Arduino Uno should light up and it should now be ready to use!
Can I connect the solar cell directly to the Arduino Uno?
This is not a good idea for several reasons. Due to variability in sun This is not a good idea for several reasons. Due to variability in sun exposure, the solar cell may not provide a steady stream of power. The Arduino Uno may not be able to draw the maximum power at any given instant from the solar cell. Additionally, the power demands from the Arduino Uno may overload the solar cell. Using a rechargeable battery provides a constant, reliable energy source.
Are lithium-ion batteries safe to work with?
Lithium-ion batteries are extremely sensitive to charging characteristics and can easily catch fire or explode. It is necessary to take precautions when working with these batteries, considering they contain a high amount of energy and volatile chemical content.
The TP4056 battery charge controller works to mitigate the risks of working with lithium-ion batteries. The controller regulates the current produced by the solar cell to protect the batteries from overcharging. The controller detects when the battery is fully charged and can stop or limit the current received by the battery. Additionally, the controller also protects the solar cell by stopping reverse current flowing back from the batteries when there is no sunlight.
How do I choose a solar cell and battery?
The TP4056 battery charge controller has a maximum input of 6V, thus, the solar cell should be at maximum 6V. The voltage of the solar cell should be at least 1.5 times the voltage of the battery. So a 3.7V lithium-ion battery needs a solar cell of at least 5.55V. The current of the solar cell should have 1/10th of the capacity of the battery divided by 1 hour. So a lithium-ion battery of 2000 mAh, should be supported by a solar cell with around 200 mAh.
Why do I need a boost converter?
The power source that connects to the Vin pin on the Arduino Uno has to be 7 to 12 volts for the regulator to work reliably. The Vin pin converts unregulated input voltage to a stable 5V. The output voltage from the lithium-ion battery is 3.7V. A boost converter converter can step up the voltage from its input to its output to meet the desired input range of the Vin pin of between 7 and 12 volts.
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- Solar powered robot
- Small solar street lamp
- Solar power bank
- Solar Power Management IC: CN3165
- Solar Input Voltage (SOLAR IN): 4.5V~6V
- Battery Input (BAT IN): 3.7V Single cell Li-polymer/Li-ion Battery
- Charge Current(USB/SOLAR IN): 900mA Max trickle charging, constant current, constant voltage three phases charging
- Charging Cutoff Voltage (USB/SOLAR IN): 4.2V±1%
- Regulated Power Supply: 5V 1A
- Regulated Power Supply Efficiency (3.7V BAT IN): 86%@50%Load
- USB/Solar Charge Efficiency: email@example.comV 900mA BAT IN
- Quiescent Current: 2023-01-11 00:17:07