There are still a lot of hand-held tools in use with Ni-Cd or Ni-MH batteries.
The simple charger has no switch-off criteria, and you need to measure and adjust the charge time, else you will badly overcharge the batteries. Or you need to add some charge electronics.
The battery pack life is limited because of overcharging, of crystallization due to long deep discharge, and especially because of charge instability due to being connected in series.
Assumed, that we have one weak cell in the chain. All others have 1.3V and this slightly weaker one has 1.2V. The charge current is assumed to be 1A. Then the good cells convert each an energy of 1.3V * 1A = 1.3W. The weak cell only 1.2W. After 4 hours of charging, the difference in converted energy is 4* (1.3-1.2) = 0.4Wh.
So the weak cell took less charge energy, will become even weaker, and during one of the next charges it might drop to 1.1V, with the neighbors having 1.3V. And so on. This instability continues as long as this cell is “dead”. Then it blocks the current and the whole battery pack does not work anymore. In this case it is best just to short-circuit the dead cell and continue with 11 instead of 12 cells (or 6 instead of 7 etc). In such a case, it is good to put 2 diodes into the charger cable to reduce the charge voltage accordingly.
The following balancer for Ni-Cd or Ni-MH keeps the battery pack in balance and additionally limits the charge voltage. It avoids both the mentioned instability and overcharging. The battery pack life is greatly extended.
The schematic is simple:
Each battery cell gets 2 cheap normal diodes in parallel, in conducting direction. While charging, the diodes duct the current around their cell at about 1.5V, and this happens to be about the full-charge voltage limit. A cell which is has not yet reached this voltage limit, is charged MORE instead of less and thus balanced.
Now, the other way round. If you leave the battery pack without charger but with connected balancer over a few days, you will find them all in 1.0V condition. This is because the diode characteristic starts already with ca 0.5V (the well-known 0.7 V -value is simplified, is an extrapolation of the linear range). This would be a balancing DISCHARGE, by which you can cure an already imbalanced battery pack.
The handwork is a bit tedious. You need to solder small thin flexible wires to all cells and duct them out of the casing to a connector. A good and small connector is made of IC-sockets (the high-quality round-pin model).
To avoid wrong connection, you can make color marks (red and black), or generate an intentional asymmetry or lock, that the connector can be engaged only one way.
The diodes have to be thermally connected well to a common heat sink. The idea is not so much that they would overheat, but they have to be EQUAL in temperature, else could become thermally instable. I would recommend 1N 5408 diodes with cells of over ca 2000mAh. For smaller cells, charged with less current, the cheaper 1N 4007 will also do.
The existing charger can be used as before, but you should switch it off when the diodes become warm, indicating that they now take the charge current instead of the cells. After having charged, the balancer is removed and put aside. The small connector remaining on the battery pack hardly causes any trouble when working with the device.