Why do series combinations of Batteries not increase Capacity?

I've come to realize that batteries connected in series does not increase the capacity. But why is this so?

This question explains that it doesnt Adding mAh when wiring battery cells in series?, but does not get into why this is the case.

It seems like it would have something to do with the Current not increasing over a series combination either, but why would the total useable charge not add?

For example: Why when connecting two 12V batteries of 20Ah capacity do we not have a total usable capacity of 40Ah?

• Correct the capacity does not change when connecting batteries in series but what about the amount of stored ENERGY ? Which battery would contain more energy, a 10 V 1 Ah battery or a 100 V 1 Ah battery ? When batteries are connected in parallel you can indeed sum the capacity. Conclusion: the amount of mAhours does not tell you the complete story ! Commented Oct 20, 2015 at 14:37
• Because you will violate conservation of energy. Series connection is multiplying the output voltage. If it will multiply the capacity as well, the total energy will be more than just the sum of the energy stored in the batteries. Commented Oct 20, 2015 at 14:37
• The accepted answer to the linked question explains what happens quite well. What do you not understand from that explanation?
– JRE
Commented Oct 20, 2015 at 14:38
• Ahhhhhhh. 12V * 20Ah * 2 !== 24V * 40Ah. Thank you very much fellas. Commented Oct 20, 2015 at 14:41
• This is much easier to reason about when you think in watt hours, not amp hours. Commented Oct 20, 2015 at 14:52

It does! The capacity of a battery is correctly measured in watt hours (or equivalently, joules), not amp hours. A rough approximation of a battery's capacity in watt hours is its rating in amp hours multiplied by its nominal voltage.

Putting two 1V 1AH batteries in series results in a 2V 1AH battery - which has twice the nominal capacity. If you were to use the battery as an input to a buck regulator (known as a BEC in RC hobbyist terminology), you'd be able to draw 1V with 2AH, before accounting for losses in the voltage regulator.

• I am in the minority obviously, but I publicly hereby do declare that 99% of the time when the "capacity" of a battery is discussed, it is in units of "charge" i.e. A-h. NiMH batteries say right on the package e.g. "2200 mAh" NOT watts. Car battery? 54A-h. Ever shop for any kind of lead-acid battery? Not a W-h to be seen. Why would the world persist in publishing this value, yet call some other value the very very common term "capacity" ? Maybe it is different in different locales? Commented May 12, 2020 at 22:12

You are going to get 20A for 1 hour from a 24V supply (it is not 12V anymore ).if you want 40AH from a 12V supply ,you can connect your batteries in parallel.

I'm not sure why the explanations weren't made more simple. Say you have 12 V batteries with 50 amp hours.

When you connect the batteries in series, say, 12v+12v then you get 24 volts at 50 Ah When you connect the batteries in parallel, say 50 Ah + 50ah you get 100 Ah at 12 V.

If it isn't obvious as to what's happening yet, 24 V is pushing twice the amount of amperage "force". Therefore, it is why it "seems" only half of the amp hours are used, but indeed you are pushing the amperage with twice the force of 12 V.

When you connect two batteries in parallel, the amps add up. If you take 1A then 0.5A comes from battery A and 0.5A comes from battery B (if they're identical to each other). Since the batteries are 20Ah each, they each last for 40 hours. The overall system gives 1A for 40h, which is 40Ah.

When you connect two batteries in series, the same amps flow through both batteries. If you take 1A, that 1A goes through both batteries. Since 1A is going through each battery they last for 20 hours each. The overall system lasts for 20 hours. But, the voltage is twice as much.

Note that since the voltage is twice as much, you can cut the current in half and get the same amount of power as before. Then since you're using 0.5A, the system lasts for 40 hours.