If you decide to solder together your own packs from individual cells, be careful to not rupture the positive seal. This is easily done if you get to much heat on the positive end of either NiCad or NiMh. If the seal is broken it may not be apparent immediately. soon you will see some white powder like substance appearing near the positive terminals and note that the batteries no longer hold a charge as long as they should. Most times, it is better to just buy one put together by the experts.