Battery voltage??

Hello all! I have a fun project for my "mini RV" that I'm building up from a VW hatchback. I'd like to install a Li-Ion battery system that can run an a very compact AC via an inverter for about 5-8 hours at night. I plan on constructing this battery from 18650 cells. Now, if I understand battery charging correctly, simply put, the charging time has a lot to do with the amount of charging amps and the Ah rating of the battery. That being said, a 100Ah 60v battery will charge much faster at 60 amps than a 400Ah 12v battery at the same 60 amps (voltages different of course). Am I missing anything here? Why would I ever choose a low voltage battery?

This of course is not taking into account the challenges with charging a 60v battery, I'm more concerned about the time to charge.

If it helps for context: There will not be a solar system installed. The idea is to be able to charge the battery with a DC to DC charging system from my vehicle's (overkill) 125 (idle) amp high-output alternator during the day. This is simply a fun alternative to always having the vehicle engine on for an inverter to work, especially at night when we're sleeping in the vehicle. The idea would be able to have a "quick" charge of the battery the next morning after use and not have to run the car for 8 hours for a complete charge. 1-3 hours would not be so bad. We also drive a lot on trips, not just boon-docking around in one spot forever.

Anyway, thanks for any input!!

by VWproject
July 20, 2020

1 Answer

Answer by SionSR

I am confronted with a similar problem. In my case the 18650 batteries are prepacked 6 in series per pack, They are to be used in a scooter that requires about 60 Volts and lots of amps. So there are 5 lines in parallel each having 5 packs in series. The scooter has a built in charger originally used to charge lead acid batteries over night,

What could go wrong?

The failure of one or more of the hundred and fifty 18650 would lead to overloading the rest and possibly a sort domino effect. The scooter and batteries are not mine so my initial advice was don;t do it, (The current set of batteries were a sunk cost from another project.)

Without a sophisticated 60 Volt battery charger the system is vulnerable. I have posted a circuit that allows 60 volt series discharge and 12 volt parallel charging and am hoping someone will tell me why I should not use it.

My point is that protecting the batteries is probably more important than fast charging. Sorry if this doesn't help.

+1 vote
by SionSR
July 25, 2020

SionSR, Thanks for your input! I have a little update for my system. I've decided to do a 20p, 18 - 36s battery at 72V. So a little different. What I've done is found a 72V battery BMS that I hope will protect my battery sufficiently during charge and discharge. I have 5 DC-DC power/ voltage step-up regulators for the charging current and voltage that I can manually adjust. With the protection of the BMS system I'm hoping that I have everything I need. Cool to hear about your neat project!

by VWproject
July 27, 2020

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