It could be one cell is starting to go south. I have some old TP 2100 3 cells and a few of them started taking longer to balance. I have a better charger/balancer and right away the display showed one cell was off. Those still continue to work OK and still take a long time to balance and eventually will be retired.
As stated, if you are going to stay with electrics get a decent 50w charger. It's the next step-up charger with a display, good balance circuit , and preset options to charge and maintain the packs.
Here are a few choices:
www.valuehobby.com/power-systems/chargers.html (first three)

www.leaderhobby.com/product.asp

www.venom-group.com/Venom-Pro-Charger-w-Power-Supply-US.html