The Small NiMh Battery, BMHS is the lightest weight option in the Nickel Metal Hydride battery cells and it’s able to work at high power like the other batteries. It's just smaller so it will run out of power sooner than the bigger batteries. It gets 50% more flashes than the NiCd Small Battery.
It's not unusual for photographers to prefer some smaller NiMh Batteries over a larger one, especially since these already have more flashes per charge than the same size NiCd and you can just swap them if one gets low. It’s only $50 more than the NiCd of the same size.
If you use 200ws we guaranty 150 flashes per charge. If you use 25ws, that same battery gives you 1200 flashes. Likewise, if you use only 400ws, you should expect 75 or more flashes from this little battery.
The NiMh Batteries all have a Four Year pro-rate warranty on the cells. For the Small Battery, that warranty loses value at the rate of: Full coverage in the first year, 25% in the second year, 50% by the third year and 75% for the fourth year and full price after that. In some cases, there may be a sale price that is a better value than the total pro-rate at the end of the warranty. There is a small labor fee to install the cells and new cells always start a new pro-rate warranty. The case and electronics have a Two Year warranty from when it's originally purchased.