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.
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
|Connected in Parallel
|6 volts, 9 Ah
|Connected in Series
|12 volts, 4.5 Ah
|Connected in Parallel
|6 volts, 18 Ah
|Connected in Series
|24 volts, 4.5 Ah
|Connected in Parallel and Series
|12 volts, 9 Ah
So lets get started!
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?
- 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.