i've been researching components for a solar project and have been struggling to find an answer to this general question:

how do you determine how many batteries, or series of batteries (lead acid in this case), in parallel a charge controller can safely charge?

i've read that for lead acid charge current should be 0.05C but that quality chargers can greatly exceed this in the bulk charging stage

  • \$\begingroup\$ The maximum chargingt current for a flooded lead-acid battery is usually given as 0.2 - 0.25C. AGM and Gel batteries can usually accept significantly higher charge currents. See batteryuniversity.com for lots more information on batteries. \$\endgroup\$ – Peter Bennett Mar 13 '18 at 23:40

The risks increase with charge rate as current mismatch is dependent on matched cells capacity and ESR.

So if one battery is weak (low C(Ah) or high ESR from sulphation) then the others carry the load.

If you have a common ground and heavy wire to each battery (-) all the same length, you can monitor the mV drop using the cable as a current shunt. using a rotary switch and DMM using the shared Gnd for Vdc in(-) on DMM.

  • \$\begingroup\$ A great point made here, and it's actually done in many very large battery configurations. They use a 1-2ft joiner cable from the battery to the busbars, and monitor the voltage across the joiners. Makes it really easy to identify a bad battery. \$\endgroup\$ – Jack Creasey Mar 13 '18 at 23:29
  • \$\begingroup\$ The more batteries in parallel, the more damage if a cell shorts out, but arrays of batteries should have row/column fuses. \$\endgroup\$ – Sparky256 Mar 13 '18 at 23:41

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