Can I solder 4 AA batteries together in series and make my own rechargeable battery pack?

Question

I've bought my young son a couple of remote control tanks for Christmas. They are supplied with two rechargeable 'battery packs' (4 x AA batteries heat shrunk together). The battery pack supplied is Ni-Cd 400mAh 4.8V

The charger says "Output 4.8VDC 250mA".

The reviews of these tanks say they are great but the batteries don't last very long.

I wondered if I could buy 4 x 2850mAH AA batteries and then solder them together to replicate the supplied battery pack and then charge them with the supplied charger and use these instead of the supplied ones?

I've been reading on the internet how to solder the batteries (i.e. don't apply heat for long and 'rough' the ends up first to help the solder stick). But I also read somewhere about 'balancing' the batteries so that they would charge??

The supplied batteries are just linked together (i.e. nothing in there that I could see that would 'balance' them) so thought I would get expert advice on the best way to proceed with this?

I'm fine at soldering but don't want to make a home-made battery pack and then have it blow up when I try and charge it!

I bought a plastic 4xAA battery holder that links the batteries together but it doesn't fit in the tank compartment so that won't work.

Help appreciated :-)

Answer 1

Edit: see also Is soldering wires directly on a NiMh battery safe?

nothing in there that I could see that would 'balance' them

This may be why they don't last very long.

Due to manufacturing variance, one of the batteries will go flat first while the others have more charge. This can result in current being driven through it by the others while it's flat, damaging the battery. Or the reverse happens while charging: one will fill up first, and then suffer overcharge damage. Does the charger automatically stop on full charge or it it timed?

It may be better to adapt the battery holder to the batteries with a saw, so you can use the holder and charge them individually.

Or look for cheap repurposeable packs, eg. http://www.dx.com/p/gd-509-2-4v-1200mah-rechargeable-2-x-aa-cordless-phone-replacement-battery-pack-white-292547#.VIXLYsmfjjU is clearly 2x AA heatshrunk together, you may be able to find 4 heatshrunk together in the right format then just change the connector.

Edit 2: by the way, I would put money on the tanks suffering mechanical failure or enemy action before the battery packs become an issue. Unless they're expensive 'models' rather than 'toys'.

Answer 2

It's possible to buy AA batteries with solder tags on each end. The tags are spot-welded to the cells during manufacture. At least, it used to be possible to get them, from industrial suppliers such as Farnell. I bought one in NiCad form several years ago.

Answer 3

Make sure that the AA batteries that you select are re-chargable. The commonly available Alkaline batteries are not re-chargable. With appropriate charger for each type you could select NiCD or NiMH batteries and have a viable solution.

Commerically available chargers for NiCD and NiMH batteries typically can charge four cells at one time but they charge each cell individually. For this reason it is advisable to not solder to the cells so they can be used in the charger.

One way to make a tubular cell holder for AA batteries is to slide them into a piece of 1/2" rigid copper tubing cut to the right length. Attach a copper end cap with a spring attached inside to make the negative end of the holder. The copper tube becomes the negative end of the pack provided you mount the batteries inside in the correct orientation. For the positive end use a screw type fitting so you can open and close the tube. You have to fabricate your own top + contact by epoxy mounting a brass screw into the axial center of the screw-on part of the pack. There are various choices of how to achieve the screw-on part of such a pack but not knowing the size constraint in your situation I leave it to your DIY. I used an union fitting one time when I made a tube to hold 45 AA cells.

Answer 4

You could try using a BMS4 module like the ones used on lithium batteries. These charge each pair of cells individually. I think the standard circuitry would not require changing except for the input voltage would be lower than for lithiums. Some experimenting needed and fit a fuse in case of overrun. Others may have reason not to but I intend to give it a try using a variable power supply with current limiting to see if the idea works. I need 6v for a vintage m/c.