Commercial Batteries in Parallel

[ptj]

For a project I want to double (or triple) the battery life of my Zen Vision:M. I know theoretically this can be done by wiring them up in parallel. The problem is there are three wires connecting the battery to the board, +, - and a white one. I've noticed that there are usually 3 or 4 contacts on commercial batteries. Does anybody know what that other wire is? Can I still work things out?

[tom61]

The other wire is to monitor the amount of power remaining in the battery. I'd try leaving that disconnected except on the first cell. Discharging them in parallel to run the device shouldn't be too much trouble, but charging all of them together would likely cause troubles. The internal charger is designed to charge one battery at a time, plus, most commercial multi-cell power packs have the cells impedance matched to have even charge across all cells.

[ptj]

ok, kind of a dirty diagram but I know there are some smart people around here.

When Switch 1 is to B, C, or D it should be able to charge and play battery 1, 2, or 3, respectively. When switch 1 is to A and Switch 2 is enabled i should be able to get 3x battery life and monitor remaining charge on battery 3. All correct?

Is it worth trying to charge all 3 in parallel or would that be dangerous?

Thanks for any help.

EDIT: image link--http://www.fileden.com/files/2006/12/28 ... agram2.jpg

[argelfraster]

I'm not seeing the image.

[tom61]

When Switch 1 is to B, C, or D it should be able to charge and play battery 1, 2, or 3, respectively. When switch 1 is to A and Switch 2 is enabled i should be able to get 3x battery life and monitor remaining charge on battery 3. All correct?

Looks correct. The main issue I see is that to switch that many lines with a mechanical switch you'd have to use a rotary, which is large compared to the average MP3 player.

Is it worth trying to charge all 3 in parallel or would that be dangerous?

It would be dangerous. The lithium inside the cells is highly reactive, and a charge gone bad could set them on fire.

[ptj]

ok, thank you. I'll work something out for the switch, size really isn't an issue.

[timmeh87]

cant you just carry them around separately? It also avoids having a huge heavy battery on your device.

[ptj]

I dug into these batteries a little more and there are protection circuits on each of them. Is it still unsafe to try and charge them in parallel?

[tom61]

I dug into these batteries a little more and there are protection circuits on each of them. Is it still unsafe to try and charge them in parallel?

Yes. The protection circuit is in there for normal faults, not parallel charging. There are issues with charging in parallel if they are not impedance matched, at the very least issues of one or more cells not being charged as fully as the others. There may even be troubles with parallel discharging, but those issues seem less likely to start fires.

Have you considered getting a bigger mAh cell that is charged separately instead?

[ptj]

I originally wanted to use the native batteries so that I could charge and play simultaneously without any problems, but now my dreams are fleeting.

EDIT: I think I have decided on have the 3 batteries inside and having a rotary switch so they can be used separately. Thanks tom for the info. I got more details at batteryuniversity.com.