Properly charging NiCd or NiMH cells is rather tricky since it is rather difficult to tell when they are fully charged unless they are watched VERY closely.
Here are a few GENERAL observations that I have made over the years concerning NiCds and NiMH cells:
- "Quick charge" circuits (e.g. anything that will charge a NiCd/NiMH in less than 6 hours) will likely reduce its lifetime significantly
- A "normal" charge circuit
(e.g. at 1/10th "C") at 14-16 hours for a DEAD cell is the best and safest way to go.
- When doing a "normal" charge, if the cell has perceptively warmed above ambient
(often only a few degrees - and one should be able to differentiate this warming from that caused by heat produced by the charger) it should be removed from the charger IMMEDIATELY!
- If you get above 1.6-1.65 volts for any length of time
(more than 4-6 hours) then the cell in question probably well-charged.
- NiMH cells will tolerate overcharge/reverse charge abuse far better than NiCds.
- If not abused, NiCds will far outlast NiMH cells
(e.g. strict avoidance of gross overcharging and charge reversal).
In an application such as a yard light where a SINGLE cell is used, a NiCd is arguably more suitable than an NiMH since the former can tolerate more frequent, deep discharge cycles than the latter, and with a only single cell, there is zero chance of cell reversal, perhaps the single thing that kills more NiCds than anything else! Unfortunately
(or fortunately, depending on your point of view) NiCds are getting more difficult to find
(and banned outright in some locales for all but certain critical applications such as medical and aerospace - which tells you something!)
Fortunately, from the viewpoint of the NiMH cell in a yard light, they typically use a buck-type converter, often something akin to a "Joule Thief"
(Google it if you are unfamiliar) using either discrete components or a COB
(Chip-On-Board) IC that is under a black epoxy blob, which is likely a similar, simple circuit. Typically, these circuits go inert by the time you get down to 0.4-0.8 volts and the battery discharge stops there, but this is likely too deep discharge to keep the longevity of NiMH cells which, for long life, should probably not be discharged below 50%-70% if repeatedly discharged.
(I believe that hybrid/electric cars that use NiMHs never discharge below "mostly charged" to assure longevity.)
In dancing around an answer to your original question:
"How do I prevent from overcharging my cells" - which I presume are NiMH, only because NiCds are difficult to find these days? I would recommend that they be charged MUCH more slowly to minimize the damage. If you were to charge them at no more than 1/10th C for the maximum of 14-16 hours
(for a "dead" cell) then you are likely to get pretty good life out of them. If you can drop this even more and tolerate even longer charge time
(e.g. 1/20th C for approx. twice as long) then the chemistry of the NiMH can more readily reabsorb excess hydrogen when overcharge
(produced at a lower rate at the lower charge current) and be much less likely to vent the cell and lose capacity.
There is yet another problem that I've run across: Cell quality. Having equipment to measure the internal resistance of cells and their actual amp-hour capacity, I find the name brands (e.g. Energizer, Duracell, etc.) to be generally pretty good and consistent, but the "off" brand names to be all over the map. What is worse, the cheap AA cells are sometimes really AAA cells in AA cases - something immediately apparent by weight
(if you have a "calibrated" hand - particularly if you compare the weight of "X" number AA cells versus "X" number AAA cells and find their heft to be very similar) or by inspection of the amp-hour rating if it is printed on the label: If it is an off-brand and there is no sign of amp-hour rating, don't even think about buying them! Even so, I find that the "off" brands rarely live up to their amp-hour rating.
(Never buy an AA NiMH if its stated Amp-hour rating is much below 1.8 amp-hours as it is probably an AAA in disguise!)
Once I have used these NiMH cells, particularly after a year or so, their self-discharge rates tend to increase, so it is hard to know if they are fully charged if they have just been laying around: It's not a good idea to just plop them in a "fast" charger since there is a good chance that they will be overcharged if they didn't run themselves down very far - and measuring the terminal voltage isn't really much help with this type of chemistry in determining if they are "90%" charged or "25%" charged either since there is likely to be little difference in that voltage.
What to do?
I thought about a float charge, so along these lines I ran across a web page a while ago that described a device that "float charged" NiMH cells
(based on manufacturers' recommendations) to keep them topped off, but as a side effect it also allows one to spot those that have gone bad. It took me a while to find this web page via a Google search
(there seems to be a "robots.txt" that "breaks" searching at the web site for some reason) but here is the link:
https://www.ka7oei.com/floaty_thingie.html
Note that this is NOT a cell charger, but a "maintainer" as the text says.
Best of luck.