There are two ways to connect batteries together, in parallel and in series. The following illustration shows how these wiring variations can produce different voltage and amp-hour outputs.
In the graphics we have used sealed lead acid batteries, but the concepts of how the units are connected are valid for all types of batteries.
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.
parallel connection only increases amp hour capacity
The basic concept is that when connected in parallel, the amp hour ratings of the batteries add up, but the voltage remains the same. for example:
- two 6 volt 4.5 ah batteries connected in parallel are capable of providing 6 volts and 9 amp hours (4.5 ah + 4.5 ah).
- four 1.2 volt 2000mah batteries connected in parallel can provide 1.2 volt 8000mah (2000mah x 4).
but what happens if you connect batteries of different voltages and amp-hour capacities together in parallel?
connecting batteries of different voltages in parallel
this is the great “forbidden zone”. the battery with the higher voltage will try to charge the battery with the lower voltage to create a balance in the circuit.
- Primary (disposable) batteries: are not designed to be charged, so the lower voltage battery is likely to overheat, leak or bulge, and in extreme circumstances where the voltages are very different, it can explode.
- Secondary (rechargeable) batteries – these are a bit 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 overheating, leaking, or bulging in the lower voltage battery and/or overheating in the higher voltage battery as it rapidly depletes. again, the greater the voltage difference, the greater the chance of fire or explosion.
It is worth noting that many people accidentally connect batteries of different voltages in parallel every day. for example:
- If you mix brands of even the same labeled voltage, you may experience problems. Due to different manufacturing processes, the exact voltages of batteries from different manufacturers may vary slightly. this means that a brand x 1.5 volt battery might actually be 1.6 volts, while a brand y 1.5 volt battery might actually be 1.55 volts. if these were connected in parallel you would be unlikely to see fireworks, but you would experience other problems.
- for primary (disposable) batteries: the stronger battery will still try to charge the weaker one, reducing the life of both.
- for secondary (rechargeable): the stronger battery would charge the weaker one, running out and wasting energy.
As such, the following guidelines are important:
- with primary (disposable) batteries: use only batteries of the same brand and age (ideally from the same pack). if this is not possible, double check the voltages of each unit with a voltmeter.
- with secondary (rechargeable) batteries – use only batteries of the same brand and age and do Make sure all units are fully charged before connecting them in parallel. if you are not sure of the state of charge, connect them individually to a charger until the charger confirms that they are fully charged, or check the voltage with a voltmeter.
connection of batteries of different amp-hour capacities in parallel
This is possible and won’t cause any major issues, but it’s important to be aware of a few potential issues:
- Check your battery chemistry: Sealed lead acid batteries, for example, have different charge points than flooded lead acid units. this means that if the two are recharged 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 voltage control – if you are using batteries with different capacities of amp hours, it is very likely that the voltages will be different (even if the voltage indicated on the labels matches). check this with a voltmeter or you will experience problems (covered in connecting batteries of different voltages in parallel above).
For these reasons, it is recommended to use batteries of the same brand, voltage and capacity. Failing to do so (if you don’t have the knowledge and tools to verify what you’re doing) could create a potentially dangerous circuit.