Charge the new battery at about 1/10 of its capacity for 15 hours. So if it is a 600 mAh battery, charge it at 60 mA for 15 hours. This is not that critical. If you have a 50 mA charger (this is very common), this works OK. And the 15 hours is a minimum. You can charge it for 20 hours or 30 hours and it will still be OK. 15 hours is ideal, but at a low charge rate, you really cannot hurt it. I have left batteries on the charger for several weeks and they still work OK. This is not recommended, but it happens. This is with NiCd batteries which are the most bulletproof as far as charging. You need to be a little more careful with NiMh and very careful with LiPo.