Is it OK to connect several lead acid cells with different Ah capacities in series? I know it can be done in parallel as long as their nominal voltage is the same.
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1\$\begingroup\$ It's OK if you need to start an engine ONCE to fly a rebuilt wrecked aircraft out of a desert. Then you don't care the battery life is that of the smallest cell. Otherwise, your name isn't Jimmy Stewart, so just don't. \$\endgroup\$– user16324Commented Jul 11, 2021 at 12:06
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No, do not connect different capacity batteries in series, because after the lowest A-h capacity battery is discharged, it will be charged in reverse by the other batteries, quickly destroying that, and possibly outgassing dangerous hydrogen. You would also need to charge batteries individually, or the smaller batteries would be overcharged, again, releasing H2.
However, if the batteries are the same voltage and same construction, i.e., all wet plate, or all gel-cell, then put them in parallel and use a boost converter, such as this example. Of course, you need to choose a converter for your voltage and current needs, and remember that current drawn will also increase in proportion to voltage gain.
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\$\begingroup\$ If I change the discharge and charge voltages to match this, or if I monitor Ah of the pack this could be possible? The series string would have an total Ah rating of the lowest batterie? \$\endgroup\$– FritzCommented Apr 20, 2022 at 10:04
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1\$\begingroup\$ "Possible"? Sure. But not a good idea: under load, a larger battery could reverse charge a smaller battery, destroying it. Charging would also be problematic: you'd need to charge each battery separately, because fully charging the largest capacity one would overcharge a smaller one, again, destroying it, as well as releasing explosive hydrogen. If you only draw a tiny current, and never let the weakest battery get below perhaps 50%, and charge each separately, it might work... for a while. \$\endgroup\$ Commented Apr 20, 2022 at 15:04
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\$\begingroup\$ after some thinking I get that this is a stupid idea from my side, since you can't guarantee the smaller battery is within its limits even when you set the voltages. And if you reduce the range of SoC than you loose the same or more - so would have more capacity if you didn't add the smaller ones in the first place \$\endgroup\$– FritzCommented Apr 20, 2022 at 19:56