Thread: Replacement batteries

I haven't had the problems that you mention, but forsee that it could be an issue.

Like all things, rechargeable cells come in various qualities, and can be specialised to do particular things. Tradie quality tools generally use specific high current rate charge/discharge cells and each cell in a pack is closely performance matched to the others. A battery pack is only as good as it's weakest cell, so matching cells is important. There are only a couple of companies producing the premium cells, and they are not generally found in retail stores. Budget tools use the cheapest cells that the manufacturer can source.

The chargers for the tradie quality kit generally have a lot of overcharge protection built in and carefully monitor the battery condition, the cheapies pack current in with little protection other than thermal sensing for the battery pack to end charging.

The last time I looked at the pricing of the genuine high rate cells, a set to repack a battery was considerably dearer that the cost of a genuine replacement battery pack.

Rechargeable batteries hate extreme temperatures, which vapourises the fluid in the electrolyte paste in the cell and cause it to vent through the pressure relief valve built into the cell to prevent cell rupture. Once vented, the fluid is no longer available to maintain the operation of the cell.

One prime cause of overheating is attempting to solder links between cells to assemble a pack. In commercial packs, the links are spot welded with a microwelder attaching both links in one hit with a current burst of about 10A for about 2 milliseconds, producing negligable heating of the cell. Soldering needs to raise the case or cap, and the link to about 700deg F for the solder to take, and the cell takes some time to cool, all the while vapourising and venting electrolyte. Then you solder the link to the other end of the cell and reheat it again.

Personally I figure that it is better to go with a replacement genuine pack, or have the pack repacked commercially with prime matched cells. In situations where this is overly expensive, it is possibly worthwhile replacing the entire kit. With many of the cheaper units, it costs less to buy a complete kit with 2 batteries than it is to buy a single replacement battery pack.

I've repacked the battery of an obsolete drill that was still OK.
I used cells with about double the original capacity, sourced from Hong Kong.
The price was less that an comparably sized new pack (same manufacturer) with approximately the same capacity as the originals - but the fittings were quite different - or I would have bought the 'new' model and unpacked it to pack mine.
Unfortunately, I didn't know about their heat sensitivity (until just then) and have soldered them together.
They take longer (predictably) to charge up, but also last a lot longer than the originals and the drill is/was significantly stronger then when new.
They have gone through maybe 250 cycles by now and are starting to fail by holding their charge for a much shorter time now.
They didn't get noticably hot.
Now the drill is starting to wear out and I won't repack them again.
Hope that helps.
By the way, you can make a spotwelder for battery terminals yourself - have a look on YouTube (I just did).
Cheers,
Joe

Thank you both for your very useful posts. I just typed out a lengthy reply and lost the lot. In short, I purchased a Lithium Ion batterypack from Hong Kong for a third of the original supplier cost but the thing heats up during both discharge and recharge. I am aware of the potential for spontaneous combustion but this is the first time I've seen it myself; they CAN get hot.

I've experimented with soldering discarded NiCad packs and failed miserably and wondered how they did it originally. Will explore the little welder.

I guess the main reason for my OP was to make people aware of the heat issue. This is the very first time I've bought a non-original pack and I'm beginning to think if it was all that wise.

Lithiums are a whole new kettle of fish specifically because they don't like hot charging. You haven't mentioned brands but again the tradie grade stuff uses a lot of monitoring, often including computer chips in the battery pack. Non genuine battery packs may have a similar chip set but the software etc cannot be copied within copyright provisions, so there has to be some difference, which probably affects pack performance.

There is also a possibly that the packs incorporate an inferior grade of cell, or smaller cells than used in genuine packs, or have a defective cell.

You can successfully solder to the caps of normal cells as long as you prep them well and work quickly to avoid overheating.

I abrade the top of the cell with a small file and then "tin" it with the soldering iron/solder. I also "tin" the wire which I use for the interconnects. I then join the two by placing the tinned wire on top of the tinned cap and apply heat to the top of the wire. Remove the heat once both the solder on the wire and cap have melted and fused - be quick.

I've done a number of repacks this way and have very good life from the resulting battery pack - the life was a least equivalent to the "factory" packs.

tradie grade stuff uses a lot of monitoring, often including computer chips in the battery pack. Non genuine battery packs may have a similar chip set but the software etc cannot be copied within copyright provisions

And that last sentence for me is reason enough to stick with the original supplier from now on.