SparkFun Forums

[wittend]

I have one of the small Sparkfun LiPo charger boards (with mini USB-b connector), and I like it very much. It works well with all of the single-cell rechargeable lithium batteries I have tried.

It is my understanding that a charger designed for a single cell LiPo will not work correctly for multiple cell (series) units because the charging current will not be distributed evenly across the cells. Is this true for cells connected in parallel, as well? Or do I have the entire matter backwards?

Thanks,

-- Dave

[bsodmike]

What you say does make sense, and if you do want to say use two of the 2000mAh SFE cells for example, what you could do is wire them up in parallel with each having the MAX1555 circuit on their branch.

You could wire in a relay to break the battery to system link when the charger is active, i.e. use a 5vdc relay to trigger on the USB line, so once this is connected, the coil magnetises and isolates the battery from the system.

This could be handy cause you can monitor both batteries individually (look at the SFE LIPO charger schematic!) - this can be extended to have say a bar-graph on each cell. You can then plug in the USB for the cell that's most depleted while the other continues to power the system.

If using an external voltage source, you could potentially use a single 5vdc input to charge n batteries, but you need to ensure the source and supply that much current...A typical bench PSU rated at say 3-5A could charge 2-3 cell in parallel...

...also, the SFE LIPO charger is too big imo, and you could make it quite tiny with the bare minimum of connectors (SMT components + mini 3way screw terminal) and you could put a run of PCBs through batchpcb

[ohararp]

If you run lipos in parallel you would not need a sceond charger. You may have a slight unbalance between the two so try to ensure that they are at nealy the same voltage before you parallel them up (+/-.1V max, .05 is better).

[bsodmike]

The problem I see here is that it would take twice the time to charge as I don't see the MAX1555 being able to supply twice the current...

You could use the charger to charge each indiividually and then connect them in parallel...once in parallel I guess they will attempt to balance each other out?