There are two ways to wire batteries together, parallel and series. The illustration below show how these wiring variations can produce different voltage and amp hour outputs.
In the graphics we’ve used sealed lead acid batteries but the concepts of how units are connected is true of all battery types.
This article deals with issues surrounding wiring in parallel (i.e. increasing amp hour capacity). For more information on wiring in series see Connecting batteries in series, or our article on building battery banks.
Connecting in parallel increases amp hour capacity only
The basic concept is that when connecting in parallel, you add the amp hour ratings of the batteries together, but the voltage remains the same. For example:
- two 6 volt 4.5 Ah batteries wired in parallel are capable of providing 6 volt 9 amp hours (4.5 Ah + 4.5 Ah).
- four 1.2 volt 2,000 mAh wired in parallel can provide 1.2 volt 8,000 mAh (2,000 mAh x 4).
But what happens if you wire batteries of different voltages and amp hour capacities together in parallel?
Connecting batteries of different voltages in parallel
This is the big “no go area”. The battery with the higher voltage will attempt to charge the battery with the lower voltage to create a balance in the circuit.
- primary (disposable) batteries – they are not designed to take a charge and so the lower voltage battery is likely to overheat, it may leak or bulge and in extreme circumstances where the voltages are very different, it may explode.
- secondary (rechargeable) batteries – these only fair a little better. The lower voltage battery is not designed to charge above a certain point, but the higher voltage battery will try anyway. The result can be over heating, leaking or bulging in the lower voltage battery and/or overheating in the higher voltage battery as it drains rapidly. Again, the larger the difference in voltage the greater the chance of fire or explosion.
It’s worth pointing out that many people accidentally connect batteries of different voltages in parallel every day. For example:
- If you mix brands even of the same labelled voltage – you can experience problems. Due to different manufacturing processes, the exact voltages of batteries from different producers can vary slightly. This means a 1.5 volt battery from brand X could actually be 1.6 volts, while a 1.5 volt battery from brand Y could be 1.55 volts. If these were connected in parallel, you are unlikely to see fireworks, but would experience other issues.
- for primary (disposable) batteries – the stronger battery would still try to charge the weaker one reducing the lifespan of both.
- for secondary (rechargeable) batteries – the stronger battery would charge the weaker one, draining itself and wasting energy.
- If you connect rechargeable batteries in parallel and one is discharged while the others are charged – the charged batteries will attempt to charge the discharged battery. With no resistance to slow this charging process, the charged units can overheat as they rapidly drain and the discharged battery can overheat as it attempts to charge at well above its design capabilities.
- If you mix batteries of different ages – the older batteries will always have a lower voltage as all batteries self-discharge over time. Even rechargeable batteries will not recharge to the same level as new ones.
As such, the following guidelines are important:
- With primary (disposable) batteries – only use batteries of the same brand and age (ideally from the same packet). If this isn’t possible, double check the voltages of each unit with a voltmeter.
- With secondary (rechargeable) batteries – only use batteries of the same brand and age and make sure all the units are fully charged before connecting them together in parallel. If you are uncertain about the state of charge, either connect them individually to a charger until the charger confirms they are fully charged, or check the voltage with a voltmeter.
Connecting batteries of different amp hour capacities in parallel
This is possible and won’t cause any major issues, but it is important to note some potential issues:
- Check your battery chemistries – Sealed Lead Acid batteries for example have different charge points than flooded lead acid units. This means that if recharging the two together, some batteries will never fully charge. The result here would be sulfation of those that never reach a full state of charge, reducing their lifespan.
- Double check voltages – if you are using batteries with different amp hour capacities, it is highly likely that the voltages will be different (even if the stated voltage on the labels match). Check this with a voltmeter or you will experience problems (covered in connecting batteries of different voltages in parallel above).
It is for these reasons that you are advised to use batteries of the same brand, voltage and capacity. Failing to do so (if you don’t have the knowledge and tools to check what you are doing) could create a potentially dangerous circuit.