Use NiMH batteries instead of NiCd in outdoor solar lights?

After recently purchasing and installing a set of outdoor solar lights, I found that the brightness of the lights was very weak, even after plenty of sunlight to charge the batteries. It appeared that some of the batteries were border-line dead.

I have replaced 2 of the NiCd batteries (600 mAh) with rechargeable NiMH batteries (2500 mAh) and the difference is astounding! They are not only brighter than the others, but after an overcast day that left 8 of the lights out of charge early in the evening, the other 2 stayed bright all night.

My understanding is NiMH batteries are more powerful, can hold a charge longer (explaining the higher cost) and also do not contain cadmium (a highly toxic metal), but is the internal solar charger capable of recharging the NiMH batteries I put in them? It has only been a few days since I put them in, so I think it may be too early to tell if they are being recharged, and I am concerned about potential damage to the light.

Appreciate some feedback.

Different battery technologies require different charging techniques. As the source linked below notes, "NiMH batteries should be rapid charged rather than slow charged. The amount of trickle charge applied to maintain full charge is especially critical. Because NiMH does not absorb overcharge well, the trickle charge must be set lower than that of the NiCd. The recommended trickle charge for the NiMH battery is a low 0.05C. This is why the original NiCd charger cannot be used to charge NiMH batteries. The lower trickle charge rate is acceptable for the NiCd. It is difficult, if not impossible, to slow-charge a NiMH battery."

Realize that not only is the coefficient of capacity used to determine the charging rate different for NiMH batteries different than for NiCd batteries, but the capacities are typically very different (as you note — 600 mAh vs. 2500 mAh). Thus, the actual charging current requirements can be vastly different. Your hack might work for a few weeks, but it’s unlikely to be reliable over the long term unless the solar lights were built with a smart charging circuit that automatically recognizes the unique characteristics of the NiMH batteries.

While it’s always possible incompatible NiMH cells could damage a NiCd charging circuit in the solar lights, it’s more likely the inadequate charging of the NiMH cells by the NiCd charging circuit would damage the NiMH cells. As cells discharge, they do not all run out of charge at the same time. Rather, the first cell to run out of charge is reverse biased (i.e., positive voltage at its negative terminal and negative voltage at its positive terminal) by the remaining voltage of the cells that have not yet run out of charge. That reverse-biased condition can easily damage the cell.

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