If it’s a sealed battery pack you need a balance port on the pack and a charger that is able to balance charge the individual cells using the port. That solution is more expensive up front and depends how obsessive you are about making sure all the cells in the pack are balanced. I guess it also depends on how well the original pack was built as well i.e. was it built with the same cells in terms of make, age etc?
A good quality pack from the likes of MTB Batteries for instance will almost certainly be built this way. I don’t know if they do packs with balance ports but probably would if you asked but would cost more. A pack for a Magicshine light would probably be more likely to degrade quicker. It’s the usual trade-off of price vs quality.
I have built packs with balance ports and quite frankly I wouldn’t bother any more. Another option if you are concerned about balancing the cells is to have an open pack where you can remove the cells and charge them individually. Personally I don’t think balancing the cells is a big problem but others might say different.
All the packs I have made have lasted several years before one or more cells goes duff. When that happens I just chuck the whole pack and build a new one.
