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I've read here that it is not recommended to connect two SLA batteries of different capacities in series.

What if you have two SLA batteries that are of close capacity such as one 12V 9AH and one 6V 7.5AH connected in series. Both batteries will be individually charged. Is the risk still present for damage to the 6V battery during discharge? If so, what would be the recommended cut off voltage for the 6V to mitigate against this damage? If the 6V is left to continue to discharge without a cutoff voltage could the SLA start reverse charging and possible leak/explode?

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Yes, this is still dangerous and will damage/destroy the lower-capacity battery by reverse-charging if it's discharged too far. As long as the voltage of the lower-capacity battery doesn't go below its normal end-of-discharge voltage (about 5.2V or so), it'll work fine.

However, if you just let it discharge without a low-voltage cutoff (or manual supervision), bad things will happen.

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