Many Nimh cells have a high self discharge rate. 5 - 10 % in the first 24 hours then around 1 % per day, higher if stored in hot conditions, so after 3 weeks expect a 30% loss in charge. The higher capacity cells on offer are supposed to have higher self discharge rates.

There are some cells around with lower self discharge rates, often designated as camera cells. Just be sure which kind you have.

I like to be safe and KNOW that my battery packs are fully charged. Hence the full charge then daily top up from a timed charger set up.