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How to connect batteries in series and parallel

If you have ever worked with batteries you have probably come across the terms series, parallel, and series-parallel, but what exactly do these terms mean?

Series, Series-Parallel, and Parallel is the act of connecting two batteries together, but why would you want to connect two or more batteries together in the first place?

By connecting two or more batteries in either series, series-parallel, or parallel, you can increase the voltage or amp-hour capacity, or even both; allowing for higher voltage applications or power hungry applications.


Connecting a battery in series is when you connect two or more batteries together to increase the battery systems overall voltage, connecting batteries in series does not increase the capacity only the voltage. For example if you connect four 12Volt 26Ah batteries you will have a battery voltage of 48Volts and battery capacity of 26Ah.

To configure batteries with a series connection each battery must have the same voltage and capacity rating, or you can potentially damage the batteries. For example you can connect two 6Volt 10Ah batteries together in series but you cannot connect one 6V 10Ah battery with one 12V 20Ah battery.

To connect a group of batteries in series you connect the negative terminal of one battery to the positive terminal of another and so on until all batteries are connected. You would then connect a link/cable to the negative terminal of the first battery in your string of batteries to your application, then another cable to the positive terminal of the last battery in your string to your application.

When charging batteries in series, you need to use a charger that matches the battery system voltage. We recommend you charge each battery individually to avoid battery imbalance.

Sealed lead acid batteries have been the battery of choice for long string, high voltage battery systems for many years, although lithium batteries can be configured in series, it requires attention to the BMS or PCM.


Connecting a battery in parallel is when you connect two or more batteries together to increase the amp-hour capacity. With a parallel battery connection the capacity will increase, however the battery voltage will remain the same.

Batteries connected in parallel must be of the same voltage, i.e. a 12V battery can not be connected in parallel with a 6V battery. It is best to also use batteries of the same capacity when using parallel connections.

For example, if you connect four 12V 100Ah batteries in parallel, you would get a 12V 400Ah battery system.

When connecting batteries in parallel, the negative terminal of one battery is connected to the negative terminal of the next and so on through the string of batteries. The same is done with positive terminals, i.e. the positive terminal of one battery to the positive terminal of the next.

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For example, let’s say you needed a 12V 300Ah battery system. You will need to connect three 12V 100Ah batteries together in parallel.

Parallel battery configuration helps increase the duration in which batteries can power equipment, but due to the increased amp-hour capacity they can take longer to charge than series connected batteries. This time can safely be reduced, without damaging the batteries, by charging faster. Now that the battery is larger, a higher current charge is still the same percentage of the total capacity, and each battery ‘feels’ a smaller current.

While it is often debated what the best way to connect in parallel is, the above method is common for low current applications. For high current applications, talk to one of our experts as your situation may need a special configuration to ensure all of the batteries age at as similar as possible rates.


Last but not least! There is series-parallel connected batteries. Series-parallel connection is when you connect a string of batteries to increase both the voltage and capacity of the battery system.

For example, you can connect six 6V 100Ah batteries together to give you a 12V 300Ah battery, this is achieved by configuring three strings of two batteries.

In this connection you will have two or more sets of batteries which will be configured in both series and parallel to increase the system capacity.

If you need any help with configuring batteries in series, parallel or series parallel please get in contact with one of our battery experts.


Many brands of lithium batteries can not be connected in series or parallel due to their PCM or BMS configuration. Power Sonic’s PSL-SC series of lithium batteries can be connected in series or parallel, ideal for higher voltage or capacity applications.

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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:

battery, charge, time, calculator

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.

Building a battery bank using amp hours batteries

In this article we’ll look at different ways to build a battery bank (and ways not to) for amp hour rated batteries (and ways not to). In the illustrations we use sealed lead acid batteries but the concepts are true for all battery chemistries.

The battery bank cheat sheet for amp hour rated batteries

If you know your batteries and you’re just looking for a memory jogger here’s the battery bank cheat sheet. detailed explanations and tutorials are shown below.

What are battery banks and why have them?

A battery bank is simply a set of batteries connected together in a certain way to provide the needed power. Sometimes battery banks are the preferred choice compared to just buying one large battery for reasons such as:

  • Cost – a number of small batteries can be cheaper to purchase, especially if they are popular and so there are several manufacturers or suppliers to chose from.
  • Space – several small batteries can be arranged in awkward spaces where a large rectangular block wouldn’t fit.
  • Flexibility – you can rearrange the layout of a battery bank to give you different voltages and ampere hours rather than being stuck with one battery that has one voltage and one ampere hour output.

Building an amp hour battery bank

In this article we’ll show the different ways batteries can be wired together in order to get different capacities (voltage and amp hour outputs).

In our example we’ll use several 6 volt 4.5 amp hour batteries as follows:

Number of Batteries Wiring Output
2 Connected in Parallel 6 volts, 9 Ah
2 Connected in Series 12 volts, 4.5 Ah
4 Connected in Parallel 6 volts, 18 Ah
4 Connected in Series 24 volts, 4.5 Ah
4 Connected in Parallel and Series 12 volts, 9 Ah

Connecting two amp hour batteries in parallel

To calculate the output when wiring in parallel add the Ah ratings together. In this case 4.5 Ah 4.5 Ah = 9 Ah. The voltage does not change. Note the way the appliance is connected. Many sources explaining parallel wiring suggest the following instead:

This will work but a greater load is placed on the battery closest to the appliance which means the batteries will not wear out evenly. This is especially true of deep cycle batteries which are meant to discharge and recharge on a regular basis.

Connecting four amp hour batteries in parallel

To calculate the output when wiring in parallel add the Ah ratings together. In this case 4.5 Ah 4.5 Ah 4.5 Ah 4.5 Ah = 18 Ah. The voltage does not change. Again, note the way the battery bank is wired to the appliance so that the load is shared evenly across all the batteries. Some source suggest the following:

This layout will work but places greater loads on the batteries closer to the appliance causing them to wear out faster, especially if they are deep cycle batteries meant to discharge and recharge regularly.

Connecting two amp hour batteries in series

When connected in series the amp hour output does not change but the voltage becomes the sum of the batteries. In this case the voltage is calculated as 6 volts 6 volts = 12 volts. The ampere hour rating is unchanged at 4.5 Ah.

Connecting four amp hour batteries in series

Again to calculate the output voltage its just a case of adding the voltages of all the individual batteries together. Here it would be 6 volt 6 volt 6 volt 6 volt = 24 volt. The amperage is the same as for one battery – 4.5 Ah

Connecting batteries in series and parallel

When you wire batteries together in parallel you are essentially just making each battery a cell of a larger unit. So you could, for example, arrange each pair wired in parallel and then wire the two pairs together in series as follows:

To calculate the output we have:

  • Two pairs connected in parallel. Each pair has an amp hour output of 4.5 Ah 4.5 Ah = 9 Ah but because they are wired in parallel their voltage is unchanged at 6 volts.
  • The pairs are then wired in series so the voltage is the sum of each pair: 6 volts 6 volts = 12 volts.
  • Altogether then this creates a battery bank with an output of 9Ah and 12 volts.

You can continue to scale this up as needed. All you have to remember is that each set of batteries connected in parallel gives the same output.

Connections and wiring

To achieve the expected results with a battery bank and stay safe ensure the following:

  • Use the correct connectors which will be defined by the battery terminal (see [link to battery terminal types ] article). Make sure if you need to connect two wires to one terminal you have a connector designed to take two wires. Although clips can be used temporarily they are not recommended as they do not always provide solid connection and they could easily come loose raising the risk of short circuits which would damage the batteries and could cause electric shocks.
  • Use the correct gauge of wiring for the circuit you are creating (see [link to wiring calculator] ). If the wire is too thin it could overheat raising the risk of short circuits, electric shocks, battery damage or even fire.

Battery banks with different amp hour or voltage ratings

So far the examples all used identical batteries but what if you have different batteries that you want to wire together?

battery, charge, time, calculator
  • Connecting batteries with different voltages in series – on paper this is possible but in reality slightly batteries with different voltages often have slightly different cell voltages and the same is true of ampere ratings. The result is smaller batteries will over-discharge and overcharge while larger batteries will not fully recharge. In exceptional circumstances an over-discharged battery may leak or explode. For full details see Connecting batteries in series.
  • Connecting batteries with different ampere ratings in series – as with different voltages smaller ampere rated batteries will drain faster and deeper than they are designed to withstand. For details on this process and why it occurs see Connecting batteries in series.
  • Connecting batteries with different voltages in parallel – this is a “never, never” idea. The larger rated battery will attempt to charge the smaller leading to battery damage in the best case scenario or fires and explosions in extreme situations where voltages are substantially different or primary (disposable) batteries are in use. For more on this see Connecting batteries in parallel.
  • Connecting batteries with different ampere ratings in parallel – this is possible but again the reality is that batteries with different ampere ratings usually have different cell voltages (no matter what the label actually says) which can lead to problems as batteries try to charge each other and balance out voltages across the circuit. See Connecting batteries in parallel for full details.

As an example the layout pictured is theoretically correct because on paper each row has an output of 9Ah and 6 volts. However small differences in the manufacturing process between the two models can cause issues.

Lets say the two larger 6 volt batteries are truly 6 volts but the three smaller 6 volt batteries are each actually 6.2 volts despite what is written on the label. Here we’ll end up with the larger batteries over charging and discharging which will shorten their lifespan.

It will work, but this battery bank won’t last as long as one made up of identical batteries.

The role of age and chemistry in voltage and ampere capacity

It should always be borne in mind that age and chemistry affect the voltage and ampere capacity of batteries, both disposable and rechargeable.

Age – All disposable batteries self discharge and all rechargeable ones slowly loose their ability to fully recharge as they get older. As such even if you have batteries of the same make and brand, if one is significantly older than the others this is the same as mixing batteries of different voltage and ampere capacity

Chemistry – Even batteries closely related (such as sealed lead acid batteries and flooded lead acid batteries) behave differently in the way they charge and discharge so it is important to ensure that all units in a battery bank are of the same chemistry in order to avoid some units over-discharging and overcharging.

Battery bank best practices

As discussed above building battery banks using different batteries with different voltages and ampere hour ratings can damage the batteries and in extreme circumstances lead to explosions or fires. Even batteries of identical voltage and ampere hour ratings can cause damage if old and new units are mixed. These issues can only be avoided with the correct tools and circuitry. Without these it is better practice to use:

  • batteries of the same voltage and ampere hour rating
  • batteries of the same age and chemistry type
  • ideally batteries of the same brand from the same company and if possible from the same production run

… and when you have an issue with the battery bank because of one faulty battery replace all the batteries, not just the faulty one.

Maximum size of a battery bank

There isn’t really any maximum. Some battery banks are huge like the one pictured here which is designed to store energy from solar panels.

In this type of application the battery bank needs to store vast amounts of energy and its a clear example of where many smaller batteries connected together is far more practical than large batteries.

Damaged or worn out units can be replaced easily and the size of the battery bank and be increased or decreased by small amounts as needed.

To imagine this lets say each long row of batteries in the room have an output of 240 volts and 500 amp hours and there are three rows connected in parallel so the total output of the battery bank is 240 volts and 1,500 amp hours (500 Ah x 3 rows).

The engineers decide they actually now need 2,000 amp hours so all they need to do is add another identical row wired in parallel to achieve their goal.

The replace one or replace all argument

Now you may have noticed there seems to be a contradiction above. Earlier we mentioned that if one battery fails in your battery bank then you should replace all the units but when we talk about very large battery banks we say this practice is not always followed.

The decision is more accurately based on a number of factors to balance cost and lifespan:

  • SCENARIO 1: If a battery bank if fairly new (say 6 months old) and one battery fails that failure is probably down to a manufacturing fault. In this case sourcing a new battery (ideally from the same manufacturer) to replace that single unit would be the best approach, especially if the battery bank is extremely large.
  • SCENARIO 2: If a battery bank if coming to the end of its lifespan then replacing each single unit as it fails will quickly damage these new batteries and shorten their lifespan leading to a constant vicious cycle of replacements that would not be economical in the medium or long term. As such in this situation all batteries in the bank should be replaced.
  • SCENARIO 3: If a battery bank is mid way through its lifespan and one unit fails then it is possible to replace it with a new unit provided the battery bank is fitted with the correct circuitry to balance charging and discharging to the new unit. The costs of such circuitry makes economic sense in large scale commercial battery banks.

How to wire 6V Batteries in series or parallel configuration

How do you create a multi-bank battery system for an RV, boat or other application? It’s rather simple but it requires you to know how to wire 6V batteries in series or parallel configuration.

There are several reasons why you may want to configure multiple batteries together; whether it be for cost savings, efficiency or increasing voltage or capacity. There are basically two ways to configure multiple battery systems, in Series or in Parallel. There is also a Series/Parallel combination as well. This article will help you to wire 6 volt batteries.

Wiring Batteries in a Series

In a Series Configuration the batteries are wired per the diagram below and the result would be a doubling of the voltage while the capacity remains the same. In our illustration we show two 6V batteries with 225AH wired together. The result would be a battery bank that produces 12V and 225AH.

Wiring Batteries in Parallel

In a Parallel Configuration the batteries are wired per the diagram below and the result would be a doubling of the capacity while the voltage remains the same. In our illustration we show two 6V batteries with 225AH wired together. The result would be a battery bank that produces 6V and 450AH.

Wiring Batteries in Series/Parallel Combination

In a Series/Parallel Combo Configuration the batteries are wired per the diagram below and the result would be a doubling of the voltage and doubling of the capacity. In our illustration we show four (4) 6V batteries with 225AH wired together. Each set is wired in series creating 2 banks, then the 2 banks are wired together in a parallel configuration. The result would be a battery bank that produces 12V and 450AH.

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