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I’m new to batteries charging and discharging and I have some general questions:

If for example you have AA batteries being charged in parallel do they behave similar to how a load resistor would do (meaning that the battery with less charge (State of charge-SOC) would get the higher current)?

Also in the parallel section of this forum https://www.reddit.com/r/AskElectronics/wiki/batteries#wiki_parallel it suggests that baterries should all be balanced (have same SOC) before being charged but does not mention why is it dangerous, why is the reason? Since the batteries are in parallel wouldn’t they self balance anyway?

Thanks

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  • \$\begingroup\$ ”Since the batteries are in parallel wouldn’t they self balance anyway?” Eventually yes, but think about, calculate and/or simulate what will happen before that and how this can pose a danger. I believe this will be the path for you to figure out the answer to your own question. \$\endgroup\$
    – winny
    May 21, 2021 at 19:54
  • \$\begingroup\$ Which kind of AA rechargeables would that be? NiMH maybe? And in a fixed package, or random AAs so you can accidentally put a fully discharged battery in parallel with fully charged battery? \$\endgroup\$
    – Justme
    May 21, 2021 at 20:46

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Batteries can be viewed as an ideal voltage source and a small resistor. An alkaline AA battery has an internal resisitance of about 0.15Ω. You didn't specify the chemistry of your rechargeable batteries, but NiMH AA's are about 0.02Ω. Source: http://www.learningaboutelectronics.com/Articles/Battery-internal-resistance. If I recall correctly, NiCd are about the same.

If you're using NiMH, when you first connect the batteries in parallel to charge, you have a circuit with two opposing voltages, and two 0.02Ω resistors. If the voltages of the batteries are different by a mere 0.1V:

  • I = V/R
  • I = 0.1 / 0.04
  • I = 2.5A

The more powerful battery drives 2.5A through the circuit! And that's without the charger applying any additional power.

Related: always wear safety glasses as an absolute minimum if you're testing out charging circuits. Even better if the charger is behind a transparent shield. The last thing you want is a face full of super-heated battery chemicals.

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