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For example, if I have a charger that supplies 300 milliamps:

  1. Will it take a bit more than 3 hours to charge a 900 mAh AA battery?
  2. What if I have 4 AA batteries in series? Will it still take 3 hours?
  3. How does voltage play into all of this?
  4. I am planning to use a solar panel to charge 4 AA 2300 mAh batteries. The solar panel is 6 volts and 100 milliamps.
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  • \$\begingroup\$ Well if it supplies 300mA at any voltage then yes it could charge batteries in series. But then it would be a rather dangerous charger because it wouldn't cut off when your sin gle cell was fully charged. So I doubt it really works that way. As we know nothing about the charger I doubt you'll get much help. \$\endgroup\$
    – user16324
    May 7, 2016 at 19:36
  • \$\begingroup\$ With a simple charge controller, your solar cell will take over 23 hours of sunlight to charge the four AA cells. (Maybe slightly less if you can find a small enough MPPT charge controller) \$\endgroup\$
    – user16324
    May 7, 2016 at 19:47
  • \$\begingroup\$ There are special charging IC's on the market that select one battery at a time to monitor and charge, even if in series with many batteries. Use Google-fu (not an endorsement) and the search term "battery charger IC", then find one that fits your needs. \$\endgroup\$
    – user105652
    May 7, 2016 at 20:02

2 Answers 2

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  1. Will it take a bit more than 3 hours to charge a 900 mAh AA battery?

Yes, due to chemistry losses given out as heat.

  1. What if I have 4 AA batteries in series? Will it still take 3 hours?

Yes, provided the charger has enough "headroom" to drive 300 mA through the batteries as their voltage will rise during charge.

  1. How does voltage play into all of this?

The charger must be capable of a higher voltage than the batteries if current is to flow from the charger into the batteries. For example, your car alternator gives out about 14 V to charge the 12 V car battery. (Be careful though: your car is using a lead-acid battery which is charged at constant voltage whereas your AA cells require constant current but still with higher voltage.)

  1. I am planning to use a solar panel to charge 4 AA 2300 mAh batteries. The solar panel is 6 volts and 100 milliamps.

As discussed 6 V might not be enough. Even if it was, at 100 mA charger current and, say, eight hours good sunshine per day it will take at least three days to charge your cells. If you have to split them into two 3 V pairs on account of the voltage you might only get 50 mA per circuit. Charge time will be six days.

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You're asking 5 questions all at once:

  1. What is the difference between charging batteries in series and parallel?
  2. if I have a charger that supplies 300 milliamps, will it take a bit more than 3 hours to charge a 900 mah AA battery?
  3. What if I have 4 AA batteries in series?
  4. Will it still take 3 hours?
  5. How does voltage play into all of this?

According to the "rules" around here, you're only supposed to ask a single question at a time, but I'll help you out with number 2.

First, your charger has to be able to pump electrons into the cell/battery being charged, so the charger's output voltage must always be higher than the voltage the cell will rise to when the cell is fully charged.

Second, since there's no free lunch and work has to be done in order to transfer charge from the charger into the cell/battery, that work will manifest itself as heat and will radiate energy away which would otherwise be going into the cell/battery.

Consequently, if you've got a battery with a discharge rate of 900mAh and you charge it, constant current, at a rate of 300mA, it'll take longer than 3 hours to recharge it to its capacity.

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