I'm an electrical noob (don't know the right formula to use for this calc) I have a 36V 4Ah battery and a mini solar panel that puts out 5V at 2.4A.

How long would it take to charge the battery?

Also, if someone can point me in the right direction for the formula I'd love to learn how to do this calc for myself. thanks!!

This is the solar panel I'm using.

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    \$\begingroup\$ Since that solar panel only produces 5 volts, it cannot charge a 36 volt battery. It appears to have some electronics to produce a regulated 5 volts - plain solar panels don't do that. The panel appears to be designed specifically to charge iPhones and other things designed to be charged via a USB port. \$\endgroup\$ – Peter Bennett May 22 '17 at 16:00

I don't know of any formula, but let's do some ballpark math.

36V 4Ah battery has nominally 144Wh (36V * 4Ah).

Now, 42W at 5V 2.4A doesn't make any sense (5V and 2.4Ah makes less sense for a solar panel).

So I'd guess it was actually 5V * 2.4A = 12W.

144Wh / 12W = 12h of charging considering 100% efficiency, etc.

If you consider 80% efficiency for the charger*, then 12h/0.8 = 15h of full output from the solar panel.

So maybe two days for 12W solar panel?

And probably 1/3 or 1/4 that for a 42W solar panel.

*Peter correctly pointed out that you can't charge a 36V battery with a 5V supply. Even if you had a 36V supply you shouldn't directly connect the battery either. I'm optimistically accounting 80% efficiency with whatever devices you put between the 5V supply and the 36V battery (possibly a step up converter and a charger? idk :P).

  • \$\begingroup\$ thanks very much for helping me understand this! I've added a link to the Solar Panel in the question for clarification. The panel captures 42W but only puts out 5v @ 2.4Ah so you are 100% correct that it it's only 12W! :-) \$\endgroup\$ – nelsonic May 22 '17 at 15:32
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    \$\begingroup\$ You can't charge a 36 volt battery with a 5 volt power supply. \$\endgroup\$ – Peter Bennett May 22 '17 at 16:01
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    \$\begingroup\$ @PeterBennett -- Yes I forgot to point that out. I was mostly focused on sharing steps for top-of-the-head calculations and arbitrarily put 80% efficiency to account for conversion losses. That might be a bit optimistic though. \$\endgroup\$ – Wesley Lee May 22 '17 at 16:07
  • \$\begingroup\$ In addition you will also have to take the battery chemistry in account. This, and nothing else, determines the charge cycle. Which isn't linear. So this whole formula is just nonsense. \$\endgroup\$ – Lundin May 24 '17 at 11:20

I know very little about solar at the time being but Wait a minute...the comments about it being 12W are wrong!

The solar panels have 4 USB 5V 2.4A ports. So it is 4 * 12W = 48W just as stated. You could remove those four ports and interconnect the panels together to an MPPT (maximum power point tracking) regulator. If that regulator supports higher voltage output than input then you should set it to your battery voltage and your nominal solar cell voltage (which you can measure after having removed the USB circuitry (unless there is information printed on the panels).

Just as an example, here is an MPPT which can handle 36V batteries: https://www.aliexpress.com/item/MPPT-Solar-Panel-Battery-Regulator-Charge-Controller-with-LCD-Color-Display-24-36-48-60-72V/32653059039.html?spm=2114.01010208.8.4.bwsOM7

And here Julian Ilett has a review so you can see how to use an (this) MPPT:

Part 1: https://www.youtube.com/watch?v=mM6JTquNSZU

Part 2: https://www.youtube.com/watch?v=vzhwR0mAshs

Part 3: https://www.youtube.com/watch?v=rC4rF62CHKY

Hope this helps!

  • \$\begingroup\$ thanks for sharing these links! very interesting/useful! :-) \$\endgroup\$ – nelsonic May 24 '17 at 15:09
  • \$\begingroup\$ @nelsonic You're welcome! :) \$\endgroup\$ – 10100111001 May 27 '17 at 23:10

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