Basically a Lithium battery requires a BMS providing:
- Overcharge protection with cell balancing
- Overdischarge protection
- Overcurrent protection
- Temperature protection
Without 1) you can't do fast charge properly, as the cell with the lowest capacity will overcharge and either explode or have a short life, depending on chemistry. This requires monitoring voltage on each cell.
Without 2) the pack would be destroyed by overdischarge. This also requires measuring voltage on each cell to cut off current when the lowest capacity cell reaches the lowest allowed voltage.
Both 1 and 2 have to be done in the pack (although the MOSFET switch may not be in the pack, just the sense circuit and logic), otherwise the connector would need many more pins which are points of failure. This is practical for RC Lipo packs where the priority is lowest weight. It is not practical for power tools where the priority is toughness (no flimsy multi pin connector).
3 has to be done in the battery pack because the tool doesn't know the maximum current the pack can deliver, but the pack does.
4 also has to be done in the battery pack because that's where the temperature probe is. Charging Li batteries is dangerous if they're frozen or too hot.
The question I'm trying to ask is are cordless tool battery packs indistinguishable from a makeshift DC battery pack of the same voltage? Are the battery contact points on tools always available to receive current?
I will only answer for the brands for which I know the information.
Makita batteries don't check if the tool is Makita. You can use them on anything that has the proper connector. There are several tools and adapters online for these batteries. This is quite safe because the battery has its own protection, so if whatever is plugged into it fails and shorts, it will disconnect itself. The batteries have an extra connector to talk to the charger. The charger allows LiIon and NiMH from 7.2V to 18V, so the battery has to tell it what voltage and current to use. The onboard micro also counts charge cycles, which they use for warranty.
I changed the cells in LIDL/Parkside batteries. So for this brand (and probably many others) it is possible to replace worn cells and get a "new" battery. You need tabs and a spot welder. For this brand, the 14.4V battery fits on the 18V charger. I have not tried to charge the 14.4V battery on the 18V charger though, for obvious reasons. Please do not stick the hot end of a soldering iron on a Lithium battery. It's a bad idea:
@TooTea: soldering cells directly will likely partially melt the (plastic) internals of the battery keeping the electrodes apart, so the cell could short out internally at any point. In the case of lithiums this is likely going to make the battery go up in flames (perhaps while you're soldering it, perhaps two years later), but it's just as much of a bad idea for nickel batteries
It is very likely that many other brands will let you change the cells in the pack. If this works, the microcontroller on the pcb will probably use its previous settings, including max charge/discharge current, so the new cells must be chosen to be compatible with the old ones (same chemistry). I wouldn't be surprised if some brands keep the setting in RAM, to make sure the micro forgets them if it is unpowered during cell replacement, which would screw the pack and make it useless.