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I have been trying to understand if it is possible to connect two identical batteries in parallel (same: number of cells, manufacturer, capacity, CRate, usage).

I want to create a circuit to select between the battery pack (two batteries in parallel) and a DC input. With a relay and diodes I manage to select this.

However I'd also need to be able to charge the batteries. I don't know if I need to split them before charging and if they both have to be charged at the same time (so they both have the same voltage after charge). This part I could do with just a manual switch. And if I put them in parallel and introduce a diode in series with each battery for protection - this diode wouldn't let the battery to receive current (won't be charged).

Could anyone explain if it is possible to do this?

By the way, I'm planning to use this LiFe batteries or this Lead Acid batteries.

update: Ah ok. So in this schematic I don't need the ON/OFF charge button?

enter image description here

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It is very common to connect lead-acid batteries in parallel to increase capacity.

A common house battery bank on pleasure boats consists of four or more, 6 volt "golf cart" batteries connected in series/parallel to make a 12 volt battery bank. There is no need for diodes or other isolation between batteries.

Some people express concern that a shorted cell in one battery will cause overheating of the wireing, but I have had a couple of cases of a shorted cell with no problem other than being unable to fully charge the bank.

I have no experience with LiFe batteries.

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  • \$\begingroup\$ but you can connect them in parallel and charge them in that configuration with no precautions? \$\endgroup\$
    – Electronics_Guy
    Commented Oct 3, 2014 at 16:41
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    \$\begingroup\$ Yes - the battery bank is treated as a single battery as far as charging and use is concerned. Also, there is no need to do any switchng between charge/use - if the charger can supply more currrent than the load needs, the excess will be used to charge the battery. If the load wants more current than the charger can supply, the battery will supply what is needed. The charging/discharging "selection" happens automagically, depending on charger current and load requirements. \$\endgroup\$
    – Peter Bennett
    Commented Oct 3, 2014 at 16:48

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