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I have a small 55 Ah AGM deep cycle battery and a larger 120 Ah AGM deep cycle battery in my car.

I know that wiring them in parallel to power a load is a bad idea.

What about connecting them in parallel only while the car is running and charging them? Both would be taking ~13.8V from the alternator / DC-DC charger.

After I stop the car I will disconnect them.

If parallel charging batteries of different capacities is still detrimental, why is this the case?

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  • \$\begingroup\$ Google for "DC-to-DC charger." It's basically a "smart" 12V battery charger that is meant to be powered by a 12V electrical system. It's the new way to charge a "house" battery in a boat or an RV from the engine's electrical system while keeping the engine starting battery isolated from the "house." Only catch is, because house batteries usually are larger than the engine-starting battery, it's hard to find a DC-to-DC charger that charges at less than 20A. ISTR, there is one on the market that can be configured down to 10A, but I forget just at the moment which one. \$\endgroup\$ May 16, 2023 at 13:12

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Assuming the two batteries voltages are matched at the time of parallel connection, there should be no issues.

Your generator will just see a 175 Ah battery from here and your charge time will increase accordingly.

However, if the batteries voltages are very mismatched, you will have a large current (and sparks) when the battery with higher voltage will force charge the other one.

As for running two batteries in parallel to power a load, this does come with some dangers and drawbacks, especial if one cell in one battery goes short circuit (rare, but happens), the situation is not good in the parallel configuration as the other battery will discharge with all its might into the other battery since the voltage just dropped. Apart from that, paralleling is commonly done in trucks, UPSes and the like.

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  • \$\begingroup\$ Yes, both are 12V AGM batteries. In my case, both are older AGM batteries, so less than their rated 55 Ah and 120 Ah capacities. Despite this, it's still fine to parallel them just for charging, correct? And using them in parallel to actually power things or even just to sit around means the old 55 Ah battery will deteriorate the 120 Ah one? \$\endgroup\$ May 16, 2023 at 13:39
  • \$\begingroup\$ @fuzzybabybunny I would just measure the voltage on both before connecting them in parallel. +-100 mV? No issues. +-1 V, charge the one with low voltage first. \$\endgroup\$
    – winny
    May 16, 2023 at 13:46
  • \$\begingroup\$ instead of measuring, what if I just turned on the car before connecting the batteries? The DC-DC charger would be providing ~14.8V, so the batteries won't be trying to equalize themselves, even with a +-V difference. \$\endgroup\$ May 18, 2023 at 1:00
  • \$\begingroup\$ @fuzzybabybunny I’d recommend against it. You’ll need a multimeter down the line anyway. \$\endgroup\$
    – winny
    May 18, 2023 at 7:30

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