Determine the amp hour rating of a home made battery pack

Question

I'm thinking of making my own electric bicycle, and I need help determining what the amp hour rating of my battery pack would be. I'm going for 36v and 8 or 9 amp hours. So, with 10 batteries in series, I can get a single, low capacity pack with 36v. If I were to make three of these, and put those in parallel, I'd have three times the capacity. But, with each battery providing 2500mah, that's 75 amp hours, and I know that's not correct. What am I forgetting? image according to my knowledge and to the picture, series doubles the voltage and the capacity stays the same

Answer 1

If you series connect the batteries the amp-hour rating stays the same. The voltage has increased though, so the power available has increased. That's why using watt-hours is more useful than amp-hours when comparing battery packs of different voltages.

So ...

• If you're using 10 x 3.6 V batteries in a string you have a 36 V, 2.5 Ah battery.
• That's a 36 V x 2.5 Ah = 90 Wh (watt-hour) battery.
• Put three of those packs in parallel and you get a 36 V, 7.5 Ah battery.
• That's a 36 V x 7.5 A = 270 Wh battery.

Now here's where watt-hours are so much more useful. Let's say you're going to need 135 W average power for your motor. Your battery duration will be 270 Wh / 135 W = 2 hours duration.

My electric bike uses about 150 W in normal cycling mode at almost 30 kph. I reckon my legs are putting in another 150 W.

Little technical point: In the SI system of units it's capital letters for abbreviated units named after someone but lower-case when spelled out: V for volt, A for amp, W for watt, etc. Yes, there will be exceptions ...

Answer 2

Showing my work:

2.5 amp hours (one cell) at 36 volts (10 cells in series) is 90 watt-hours.

Times 3 of these strings in parallel is still 36 volts (you knew that) and 7.5 amp hours or 270 watt hours.

So I think you forgot a decimal point. :-)

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Side note about connecting batteries in parallel: If you can avoid it, don't! Use higher capacity batteries instead. The reason appears when one cell gets weak or fails short-circuit while the others are still good:

If you have one series string and that happens, the pack voltage drops and you have to replace that one cell. Inconvenient, but still the best of all scenarios.

If you have multiple strings in parallel, current can flow backwards through the string that contains the bad cell, potentially damaging the entire string.

If you tie across all the strings at each level, making one series string where each "cell" is actually a parallel combination of cells, then you're pretty much guaranteed to lose an entire "rung" at once because the bad cell will drag the rest down with it.

And there's the issue of how much current actually flows in each scenario. For example, when you first connect the batteries in parallel, whether single cells or strings of cells, they will be charged to slightly different voltages (nothing is ever equal, including end-of-charge from the same charger at different times), which will be imposed across the wires that connect the strings. How much current is that?

Answer 3

The same current goes through batteries in series, therefore the capacity in Ah is not the sum of the individual batteries, it's the same. The overall energy in Wh is increased because the voltage of the battery pack increases. If you want more capacity in Ah, you need to put several 36V stacks in parallel; 3 of them in parallel will give you 7.5Ah.

That's purely to answer your question, though you should watch out for cell failure modes and charge balancing.