What does "NIMH" mean on batteries?

NiMH means "nickel-metal hydride." A little history might be interesting. One of the most successful exotic batteries is the nickel-hydrogen battery. It uses the same nickel-hydroxide positive electrode and KOH electrolyte as the Nickel-Cadmium battery, but it uses hydrogen gas to replace the cadmium in the negative electrode. It needs a pressure vessel to hold the hydrogen gas. The nickel-hydrogen battery is mostly used in low-orbit satellites, which charge and discharge the batteries on every pass around the earth, and so need a long cycle-life battery.

Another way to store hydrogen is in intermetallic compounds called metal-hydrides. Some metals have room in their atomic lattices to accomodate hydrogen atoms. The hydrogen can be induced to enter and exit the metal matrix by electrochemical means. So the nickel-metal-hydride battery is a variation on the nickel-hydrogen battery, with a new, low-pressure, method to store the hydrogen. Unfortunately, since the metal-hydrides corrode when exposed to KOH, they are not as long-lived as nickel-hydrogen batteries.
NiMH Charging

Basics

NiCad and NiMH batteries are amongst the hardest batteries to charge accurately. Whereas with lithium ion and lead acid batteries you can control overcharge by just setting a maximum charge voltage, the nickel based batteries don't have a "float charge" voltage. So the charging is based on forcing current through the battery. The voltage to do this is not fixed in stone like it is for the other batteries.

Parallel Charging: This makes these cells and batteries difficult to charge in parallel. This is because you can't be sure that each cell or pack is the same impedance (or resistance), and so some will take more current than others even when they are full. This means that you need to use a separate charging circuit for each string in a parallel pack, or balance the current in some other way, for example by using resistors of such a resistance that it will dominate the current control.

The coulometric charging efficiency of nickel metal hydride batteries is typically 66%, meaning that you must put 150 amp hours into the battery for every 100 amp hours you get out. The faster you charge the worse this gets.

The minus delta V bump that is indicative of end-of-charge is much less pronounced in NiMH than NiCad, and it is very temperature dependent. To make matters worse, new NiMH batteries can exhibit bumps in the curve early in the cycle, particularly when cold. Also, NiMH are sensitive to damage on overcharge when the charge rate is over C/10. Since the delta V bump is not always easy to see, slight overcharge is probable.

9.6V 0.4A nimh battery charger