Continuously charging NiMH - best approach?

[tooki]

In the coming weeks I will be rebuilding a couple NiCd packs with NiMh cells and my plan was to leave them permanently trickle charging. I do not quite understand how can that be a problem. How can it be a problem to supply a trickle current equivalent to the self-discharge? Or even a bit less?

They're not optimized for that, and you're basically constantly overcharging them.

[jirij]

Now that we know what your "X" is, I think it would be a better idea to design a relatively well-engineered modern-day fast charger, with the twist that once done, it goes into very low leakage mode, so that you can be confident that leaving the cells on the charger does not discharge them even in long term; basically combining the pros of your initial proposal with the pros of the usual fast chargers.

Yeah, I might eventually do that. I looked around at some charger ICs like MC33342 or BQ24401, and some of them are SPI/i2c programmable, so I could program in my desired top-up charge period (in minutes), and "duty cycle" (in seconds). But many are old (2001 to 2010) and no longer available, and what remains is pretty expensive, given that I'd need ~15 of them. And, as it happens, lcsc.com doesn't have filtering based on battery chemistry, so I'll have to dig through the results to find something cheap (and programmable). ... I guess I could always roll my own solution with adjustable linear regulators, given that I'd already need MCU(s) anyway.

I have several NiCD/NiMH chargers, but they all seem to suck at terminating the charge. They all over-charge to >1.5V while the batteries are rather hot to touch. So I'd like to go with a SMD thermistor as close to the cell as I can get it (fractions of a mm, cutting a hole in each battery holder, maybe adding a thermal pad or a formed thermal conductive silicone plug, for perfect contact).

How can it be a problem to supply a trickle current equivalent to the self-discharge? Or even a bit less?

I don't pretend to understand all the herbs and spices in today's NiMH cells, but there can logically be a difference between applying a voltage potential to a chemical cell long-term (with little current), and for that to have a different effect than applying a higher current short-term.

Not a chemist either, but I know enough to know that the rate of some reactions needs to be controlled, or you get different resulting compounds (see ie. polymerization, or even crystalization).

So it's quite possible that a modern NiMH cell can't be modelled by a simple energy storage equation (X goes in, X minus self-loss comes out). The cell might benefit from a "top-up" charge once in a while, as opposed to constant current equal to self-discharge.