Connecting in series increases voltage only
The basic concept when connecting in series is that you add the voltages of the batteries together, but the amp hour capacity remains the same. As in the diagram above,
two 6 volt 4.5 ah batteries wired in series are capable of providing 12 volts (6 volts + 6 volts) and 4.5 amp hours.
This is where most tutorials end, but what happens if you wire batteries of different voltages and amp hour capacities together? Most people simply answer by telling you “Don’t do it!” … but why not?
Connecting batteries of different voltages in series
In theory,
a 6 volt 5 Ah battery and a 12 volt 5 Ah battery connected in series will give a supply of 18 volts (6 volts + 12 volts) and 5 Ah. A 6 volt battery is often three 2 volt cells and a 12 volt battery is usually six 2 volt cells. Therefore, all you have done is connected nine 2 volt cells together to get 18 volts … so what’s the problem?
The reality is that no 6 volt battery is
exactly 6 volts and no 12 volt battery is
exactly 12 volts. Individual cell voltages differ, even with batteries of the same brand and manufacturer. A 6 volt battery might have a cell voltage of 2.2 volts and a 12 volt battery might have a cell voltage of 2.1 volts. This can however be fairly easy to read with a volt meter if one was to check.
Matching amp hour ratings is much more difficult. The 6 volt battery might really be a 5.2 Ah, while the 12 volt battery might be 5.5 Ah. Amp hour ratings are also much harder to test without accurately discharging both units at the same rate under the same conditions and accurately measuring the results.
You also need to check with the manufacturer on how they arrived at their amp hour rating, because different manufacturers use different methods – not all 5 Ah batteries are 5 Ah in the way you might think. Some manufacturers will claim their battery is 5 Ah using the “20 hour rating”, while others will say their battery is 5 Ah using the “100 hour rating”. For more on this subject see
Which deep cycle ah battery.
Furthermore, these ratings and behaviors can be different depending on the structure of the battery. A flooded lead acid battery may have different discharge and recharge patterns compared to a sealed lead acid battery.
What do these issues mean in practice?
The first practical outcome is that the amp hour capacity will be the lowest of the batteries connected together. In the example above, this would be the 5.2 Ah battery. Not a disaster if you were only expecting 5 Ah, at least not a problem right away. If you were to connect a device to the battery bank it is capable of powering (say a 0.5 amp bulb) then it would work.
The real problems arise during discharging and recharging cycles (if the batteries are rechargeable) .
