Charging LiPo batteries in series

Hi all

This might be a long shot, but I’ll take a punt…

I’m looking at building a portable USB device charger, based on some nice big LiPo batteries. My intention is to have two in series, for a nominal voltage of 7.4v. In the RC hobby world, you wouldn’t think of charging a series LiPo battery pack without using a balance charger, which makes sure all cells have balanced voltages. I was led to believe, that just applying a voltage to charge a series pack across the whole pack would lead to Bad Things.

But I’ve been looking at some of the LiPo charger/management ICs out there (in particular MAX1737) and they often say they support multiple cells in series. All the datasheets just show the cells connected in series, with no “balancing” going on. Can anyone shed any light on this? I guess it must be OK, I’m just a little wary of proceeding, due to risk of death, burning my house down, etc.


The RC hobby world tends to want to charge at rates well in excess of “normal” 0.7C.

You may be able to get away without balancing at lower rates, maybe.

In practice it works for 100’s of cycles. Maybe it’s more difficult towards the end of a batteries life. In theory, both cells are fully charged in series as the current decreases to 0 at 4.2v each. One of them is overcharged but at a very slow rate. This keeps them balanced when they are healthy. You could balance them every 10 cycles if you were extra careful. It is far more important to keep them from going below 3.0v individually, than it is to Balance charge them. A circuit built into some cells does exactly that. Hugo is correct, slower is better.

I was told by an expert that going over 4.20v is significantly bad for LiPo. LiFePo4 is safer and more tolerant. If you can sense 4.20v in series for LiPo that would be sufficient if you stop there.

Cool, thanks for the replies. I guess the batteries in the RC world do take a fair hammering, with discharging the entire battery in 2 to 3 minutes being completely within spec. Any USB charging is going to be at a much more sedate pace, so I suppose the batteries are able to behave themselves a bit more.

Anyway, I got a couple of bare 5000 mah 3.7V cells from HobbyKing for about £6 each, so there’s only one way to find out!

@tom wrote a guide to Lithium batteries with a few sample circuits - you can find it in our new doc center here!

Hear, hear… its really good stuff… as well as the document on building good devies… must read information, IMHO.