# Help calculating battery charging rate off Solar

#### fridgerova

Jul 17, 2015
8
Hi there,

Can you please confirm/correct my calculation?
I have a battery bank/charger with 4x 18650 Lithium batteries (3400mAh, 3.7V)
I am looking at possibly getting a 20W Solar pannel charger

I am trying to understand How fast I'll be able to charge that battery bank using that solar panel in ideal sunny conditions.

Power stored in 4 above mentioned batteries:
4(batteries) * 3.4A * 3.7V * 3600s (hour) = 181152J

Solar panel generation:
20W = 20J per s

181152J / 20J / 60s = 150 minutes

It will take 2.5 hours to charge the 4 batteries in ideal conditions? Am I correct or totally off?
Sorry, haven't done these type of calucations in some time heh.

#### Arouse1973

Dec 18, 2013
5,178
Yes you are correct. I did it this way and got the same answer. 4*3.4*3.7 = 50.32 Wh. 50.32 Wh / 20 W = 2.516 hours. Remember nothing is 100% efficient so this would be for a perfect system. You do understand that you will need a certain charger also for those cells?

#### fridgerova

Jul 17, 2015
8
I believe the bank battery/charger i linked above IS the charger. It says it takes 5V in though.

#### BobK

Jan 5, 2010
7,682
Yes, the solar charger seems to put out 5V for USB connector charging and the battery bank accepts this voltage so you have everything you need to charge the batteries. I would expect 3H to be the typical charge time.

Bob

#### fridgerova

Jul 17, 2015
8
Excellent, thank you! Exciting camping times ahead.

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