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. More detailed explanations and tutorials are shown below.

Ah battery bank cheat sheet
This cheat sheet outlines the best practice for building battery banks using amp hours. Layouts marked with a red cross may work but will shorten the lives of individual batteries.

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

So lets get started!

Connecting two amp hour batteries in parallel

Two ampere hour batteries connected in parallel
Two batteries connected 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:

2 ampere hour batteries connected in parallel incorrectly
2 batteries connected in parallel incorrectly. The battery closest to the appliance will wear out faster.

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

4 ampere hour batteries connected in parallel correctly
4 ampere hour batteries connected in parallel correctly

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:

4 ampere hour batteries connected in parallel incorrectly
4 ampere hour batteries connected in parallel incorrectly. The batteries closest to the appliance will wear out first.

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

Two ampere hour batteries connected in series
Two ampere hour batteries connected 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

Four ampere hour batteries connected in series
Four ampere hour batteries connected 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:

4 ampere hour batteries connected in series and parallel
Four batteries. Two pairs connected in parallel and then each pair connected in series

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:

  1. 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.
  2. 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?

Ampere hour batteries connected in series and parallel incorrectly
This layout will work in theory but in reality it will shorten the lives of the batteries
  • 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

A large ampere hour battery bank
A large battery bank. Image from solarpowerindonesia.com

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.

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33 Comments

  1. Kevin McQuoid

    Hi,
    Just wondering, is this statement correct?
    … and when you have an issue with the battery bank because of one faulty battery replace all the batteries, not just the faulty one.
    Regards
    Kevin.

    1. BatteryGuy

      Thanks for pointing this out. A further paragraph has been added to the end of the article to provide better clarity.

  2. BatteryGuy Editor

    KEVIN,
    There may be a few instances where this wouldn’t make sense. FOR EXAMPLE: If you just installed new batteries of the same brand, same voltages and same mAh of AH ratings and immediately or within maybe one day have a problem with one battery, then I would say this is an exception and it may be best to get a replacement for the one battery matching the specifications and brand of the others. During installation each battery should be checked to make sure it is good and within the same specifications of the others. Now beyond that one day I would be very concerned that the faulty battery would begin to effect the performance of the other batteries and eventually due damage to them.

  3. Yaacov

    The photo of the large solar battery bank. Could you imagine replacing all of these batts because one is bad? Obviously good advice for the wealthy. I had a bad batt and changed just one. System only ran for a little over five years. Whoops, that’s a lie, still using them. Since replacing that one batt would ruin the others, might as well run them until they are no longer useful then replace all. Lucky that my batt bank was oversized for the need.

    1. BatteryGuy

      Thanks for pointing this out. A further paragraph has been added to the end of the article to provide better clarity.

  4. John Monteforte

    Hi,
    In a bank with a 12v wired to a 6v in series, is it possible to use a circuit to prevent the 6v batterie from over discharging? Like a tool to cut it out off the circuit at a safe voltage so you can still use the 12v batterie. If so, what would you call that and how would you wire it in?

    1. BatteryGuy Editor

      FIRST – This could be dangerous if done in Parallel. I do not recommend anybody do this. If not done with the proper additional equipment, the 6 Volt battery would essentially be charged by the 12 Volt battery. This current would be limited by the internal resistance of both batteries. Depending on the battery chemistry and how it is constructed, there is a strong possibility of serious damage to one or both batteries. There is also a danger that this could possibly cause fire, toxic chemical venting or explosion.

      To do so in SERIES (series only) – the two battery voltages will add together to provide a combined voltage of 18 volts, make sure both batteries are rated with the same ampere hour level (AH). I however do not recommend this if the batteries have different current output, because it will cause inefficiencies.

      To do so in Parallel – Parallel would require a voltage splitter / isolator to prevent the 12-volt battery from overloading the 6-volt battery. In this situation, total voltage will end up being somewhere between 6 Volts – 12 Volts.

  5. Charles

    For a parallel setup, is there any drawback to connecting each battery to two busbars—one positive and one negatve, with charging and loads also connected to the busbars?

  6. Shtf333

    Having 6 batt 6v-232ah each, what would be the connection to have 12v batt bank? would that do 12v 696ah??

    How could I connect a charger to be able to when the 12v are not being used? 1 single charger or 2 x Genius 3?

  7. Peter keen

    I have 36x2v 1000ah batteries how do I hook these up to run A house hold for a12v/ 240 system using 3000/ 6000 inverter have 4×250 panels at the moment but can increase panels no worries would appreciate help thank you

  8. Orphiel

    I have six 5000 mAh, 3.7 v batteries and i need a bank to give me 11.1v and 10 000mAh. What’s the correct configuration?

  9. SP

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

    Then right below that you explain that “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.”
    So, which is it?

    1. BatteryGuy

      Thanks for pointing this out. A further paragraph has been added to the end of the article to provide better clarity.

  10. Curious George

    Hi Battery Guy, why do those big batteries shown in the last paragraph have more than one terminal instead of the two terminals in your examples above?

  11. Garret Neeld

    I have (4) 12 volt batteries wired in series for 48 volts at 140ah. Why can’t I simply add 1 more 12 volt battery at 140ah in parallel to achieve 280ah capacity ?

  12. Robert M

    I am new at this. I have sized my system to operate at 24 volts. The last items are batteries. I find 6 volt the most cost effective but the quantity required to to get 24v @ approx 1000 ah is expensive – 4 banks of 4 batteries each. Is it better to change the system to 12v? Or possibly using Lithium batteries to reduce my ah by 50%?

  13. Garry Sabraw

    How should I wire 4 12 volt ,deep cycle batteries in a rv to get 12 volt system g

  14. Wali Rashid

    Again, with six12v 35ah sla batteries, is 72v 35ah the Most that can be acheived using a series connection or can the AH be increased as well?

  15. Chuck

    Question about parallel configuration. You state that if using the leads of one battery for the load instead of using + on battery and – minus on the other battery will cause unbalanced charging and discharging. Is this because of resistance of the conductors?

  16. Petros doulos

    How much Batery power i need to run my ice cream machine 2.6kw for 10 hours run and what inverter should I use and what charger? Thanks in advance.

    1. BatteryGuy

      Unfortunately we do not deal in inverters. Try an electrical forum for advice on this.

  17. denis clarke

    what is better for 4 six volt batteries to 12 volt bank, series then parallel or parallel to series?

  18. Rob Smith

    I’m building a small power pack, two 12v 9ah batteries, is it best to wire in parallel or series? I’m after it being able to charge my gadgets for a longer period.

  19. dennis

    Dear battery guy, need your recommendation for potential battery bank layout. Request is an approximate 600V battery pack with 1000 AH, using a 12V 109 AH battery design. How would you configure this in a series parallel setup? (36V or 48V) Thank you for your help!

  20. Daniel Wheeler

    Hello, My question is, I have 3 batteries in series and I want to double the AH by adding in parallel another 3 batteries in series. Is that the correct way to accomplish my goal?

  21. David

    my query is to connect two 50,000 mah battery banks together, each having 20 v, 5 amp outlets to get 20 volts, 10 amp or 200 watt output. Is there an over the counter device for this?

    1. BatteryGuy

      That depends on the battery bank. I would suggest you contact the manufacturer of the battery bank to see if they offer such a product. Some do.

  22. Bob

    I have 4 100 watt solar panels going to a 12v 120ah battery if I add a second battery connected in parallel, same volts same ah. Do I need a second controller or just the one I am currently using? I have a 3000 watt invertor connected to my battery. I am not sure if both batteries would discharge at the same rate and then be charged at the same rate with just one controller.

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