Replies

Since your battery "bank" is a mismatched pair of batteries, you will end up killing the 8a/hr battery first. As voltage drops while powering a load, the smaller battery will have to work 3 times harder to maintain the same voltage as the larger battery. Also, as battery voltage drops during use, amperage will go up. For example, at 12v, a 12 watt lamp will pull one amp. As the battery discharges, let's pretend it'll discharge to 10v, that same 12 watt lamp will require 1.2 amps, Ohm's law. That might not seem like much but at 10v, your battery is beyond dead (as far as its state of charge is concerned).

You definitely need moe charging capacity, for AGM batteries like you have, figure on ~2% self discharge per day - you'll need to replace that even if you don't use that battery at all so the 18a/hr battery will need to be 'fed' about .36a/hr daily to maintain its charge. Also figure on only drawing down 50% of the battery capacity for decent battery life, the 18a/hr would be able to provide 9a/hr or 96w/hr before needing to be fully charged. Also, avoid "deficit" charging where you never fully charge the battery from the previous days' use. That will cause sulphation on the plates effectively ruining the battery, particularly AGM's since they can't be equalized like FLA.

Also, as battery voltage drops during use, amperage will go up.

Got that right - electrical circuits will try to maintain power [watts] at a constant level. If voltage drops, amperage increases to maintain the wattage level.

I used to work for NEC in the Fiber Optic Transmission division. The designer of the power module for one of the fiber optic multiplexer models forgot to include an under-voltage cut-off circuit in the power module.

Note: The under-voltage cut-off is a mandatory BellCore spec.

Anyway, New York Telephone [now Verizon] performed the BellCore under-voltage protection test when it installed 100 of these multiplexers in a new Central Office installation.

They dialed down the voltage and ALL 100 power modules FRIED !!!