I've been riding a Merida 550 back and forth to work for about 4 years (9 mile round trip). I've hit a unique problem after purchasing a NIMH battery pack to replace the SLA packs I'd been using. I bought a 26.4V 13Ah pack (22 1.2V F cells) from batteryspace.com. When I plug the new battery in, it powers up okay, and it will ride okay for short distances over level terrain. However, when I try to tackle a medium grade hill (one that requires switching to speed 2), the power mysteriously cuts off. A few seconds later it will power back on again as if I just turned it on, and I can ride around on level terrain again, but trying to tackle the hill will cause the power to cut off again. The fuse doesn't blow, and there are no sparks; I'm just losing power. There don't appear to be any loose connections on the battery pack itself, and the consistency with which it happens make me think it is something else. I also have a second battery pack (25.2V 10Ah, 21 1.2V NIMH D-cells) that worked okay before and still works okay after, so there aren't any loose connections on the bike itself. I experimented with removing one of the F-cells (down to 25.2V) and then a second (24V) and I still get the same result. The only thing I can think of is that there is a loose connection in the battery pack that I missed or that something in the bike doesn't like the amperage, although in the past I wired up a 24V 20Ah SLA pack successfully.
Also, for anyone else thinking of trying one of these packs, the Powerizer 26.4V 13Ah battery pack from batteryspace.com is about 1/2" too wide to fit in the original Merida battery case. I managed to barely sqeeze it in by drilling out portions of the case, but I wouldn't recommend it.