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I've recently bought a 9 V, 3 W solar panel that I wanted to use to make a small project in my balcony, then I bought a 6 V, 4000 mAh sealed lead acid battery, I want the solar panel to always charge the battery through a PWM solar charger whenever possible.

I am a beginner in electronics, because I am just still young, in highschool, but electronics is my no 1 hobby and I need someone to answer my question.

My question is very simple, after dividing watt by voltage, I will get a maximum current of the solar panel which is around 300 mA, ironically I think in my area I would get just 100 mAh, but anyway I've recently read about how to charge a lead acid battery and there is a statement I read before, that a lead acid battery is charged at current of 30% of it's capacity if I applied this rule to my 4000 mAh battery I would need a 1.2 amp to charge it in (2.4 volts per cell), is this a must or any current less than 1.2 amp can charge a lead acid battery?

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  • \$\begingroup\$ 0.3C was probably a MAXIMUM recommended rate. Lower is fine. Very low it may not be able to "bulk charge" properly (see www.batteryuniversity.com advise on lead acid charging) but your I = P/4 = 3/p = 333 mA, which is a max rate of = mA/mAh = 333/4000 =~ 8% is very acceptable. [8% = 0.08C ~= C/12] \$\endgroup\$
    – Russell McMahon ♦
    Oct 14, 2019 at 6:41
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    \$\begingroup\$ Current is mA or amps not mAh. It does not have the "h" bit. The "h" bit makes it milli ampere hours and that's a measure of battery capacity and not what current it may deliver. \$\endgroup\$
    – Andy aka
    Oct 14, 2019 at 7:21
  • \$\begingroup\$ @Andyaka Thanks for your notice, I fixed it. I really sometimes mix amp and amp hours \$\endgroup\$
    – Shams M.Monem
    Oct 14, 2019 at 12:55

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The usual rule for charging a flooded lead-acid battery is that the charge current should be less than 20 - 25% of the Ah rating. for your 4 Ah (4000 mAh) battery,. that would mean a maximum charge rate of about 1 Amp. Gel and AGM batteries can accept a higher charge rate.

I don't think there is a minimum charge rate, but the lower the charging current, the longer it will take to charge the battery.

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  • \$\begingroup\$ in theory the charge current should be at least bigger than the self discharge current of the battery, In praxis the self discharge current should be much smaller than the charge current. \$\endgroup\$
    – Uwe
    Oct 14, 2019 at 19:43
  • \$\begingroup\$ @Uwe: yes. With very low charging currents, you will just be reducing the rate of self-discharge, rather than actually charging the battery. \$\endgroup\$
    – Peter Bennett
    Oct 14, 2019 at 20:55

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