Gaash lighting, have recently started to produce LED emergency lighting for commercial applications. The optical area of the lights are similar in shape to these of the american incandescent or halogen emergency lighting that associated with many green and red LED exit signs there.
In these emergency lights, Gaash lighting abandond the NiCd batteries, that are in use in their former BARAK fluorescent emergency lighting, and installed NiMH batteries instead.
While this move contributes to the environment, is NiMH batteries really suitable for commercial emergency lighting? (Charging control is electronic)

If the charger uses temperature compensation for the maintenance charge voltage regulation to match the cell dependence, it can work well (= keeping cells fully charged without overcharging them).
What complicate this is the way, how to get the temperature information from the core of the cell. As the charging (and more over the overcharging) reactions are exothermic in NiMh, there is very significant risk of thermal instability (if the cell is inside hotter then the controller think, the cell voltage get lower, what mean the charger voltage get too high, what mean overcgarge start, what mean dissipating heat in the cell, what mean further cell temperature rise and so on).
To solve this instability without the need of special cells with internal sensors is the main challenge in such systems.
What help is to set the maintenance current limit very low, so the thermal resistivity between the cells and the thermal sensor place is low enough to transfer the heat to make the system stable.
But such current would be too low for normal charging, so some system is necessary to reliably detect the end of charge and switch to the maintenance charging. Again complication.

So it is feasible to make such charging system, but it is not simple and it's parameters are tied to exact battery type. So you can not replace the dead battery with other then the exact original type. Using battery from another manufacturer is not possible at all, as the charger parameters would not match with those required by the battery ones. Matching the basic chemistry is not enough, as parameters like exact grain size, electrolyte density, separator properties,... play quite significant role in voltage levels and it's temperature dependence.

So NiCd are more suitable for commercial emergency lighting then NiMH. Right?
I'm aware that NiMH batteries, heats up considerably comparing to another types of rechargeable batteries which aren't heats at all. My GP Recyko+ AA NiMH batteries that i use in my camera, heats up considerably when charging, comparing to the lead-acid batteries in my portable emergency lighting/camping lantern and my UPS and the Li-ion batteries of celular phones.
Another question: is cooling the NiMH batteries during charging by a fan for example, good for them?

NiCd's are way easier to handle in such application, so i would say yes, technically they are better choice for such thing.

The temperature difference between NiCd and NiMh during charging is caused by difference in the chemistry behind.
Charging up NiCd is endothermic, so it consume the heat, so cells remain cold even when thermally insulated (e.g. by the cover) and charged by high current - the charge reaction is able to consume all heat generated by losses inside (e.g. electrolyte resistance), so cells do not require cooling. And it make the end of charge detection very easy, based on or directly (thermal sensor) or indirectly ("dV/dt" method) sensed temperature rise: As cells become fully charged, the endothermic reaction cease and the strongly exothermic overcharging take over - yielding very steep rise in cell temperature, what is easy to detect.
Then the continuing overcharge reaction only cause the cells being heated by the incoming power. When the temperature is kept low (so limit the power input to dissipate), there are no adverse effects, so they might be overcharged infinitely.

On the other hand NiMh charging reaction is exothermic, so it generate heat and that heat has to be carried away. This is the major problem in higher speed charging - it generate so much heat, the cell reach it's maximum temperature and gives no room for the overcharging (necessary to balance charge status between series connected cells) at the end of the charging process. Moreover it complicate the end-of-charge detection, what is difficult to tune, when the cell temperature is already close to it's maximum during charging. Few years ago it was tackled by using the cell internal pressure as the base for end of charge detection, but this require or special contact ("15minute charging batteries") or special sleeve with tensometer structure on them (metal meander, what change it's resistance when the cell expand due to internal pressure buildup) - so not usable on unmodified cells.
And there is anothe problem with NiMh cells: The hydrogen dissolved in the metal (so the hydride) become unstable and prone to thermal runaways, if the hydrogen concentration exceed some limit. This effect makes cell very sensitive to overcharge even at low current: he hydrogen concentration build up and if the runaway happen, it locally overheat the cell structure, what causes the separator pores to close (safety feature originally designed to protect the cell from overheating till explosion in case of too high currents during e.g. short circuit), so effectively disconnect the affected area.


Keeping anything from heating up is generally only beneficial.