Can I low-rate charge (constant current) NiMH cells in parallel?

Question

I'm building a simple constant-current charger (intended to charge at C/10 or less) for consumer NiMH cells (AA batteries). This is intended to help extend the life of some of my older cells that no longer charge in my smart charger. (Whether this is just due to age or "memory" I'm not sure, but if I discharge them to a fairly low value and then trickle-charge them for a day they seem to work well again in low-current devices.)

For the purposes of this question, assume that I'm starting with fully discharged cells (or at least equally balanced cells) and will be removing the cells from the charger after an appropriate amount of time (120%-150% charge).

(If you have comments on continuous "trickle" charging, this question is probably a better place to talk about that. Note that an earlier version of this question mentioned trickle charging in the sense of non-terminated charging; that seemed to be distracting from my main question here; thus the separate question above.)

My circuit is basically the one below (excluding the red part), which I've prototyped on a breadboard and it seems to work ok. But I'm wondering if it makes sense to add a second battery holder in parallel with the first (the red part below) so I could charge two cells at a time, albeit at a slower rate.

Though not shown in the schematic, I'm also considering adding a pair of diodes (probably 1N5819 Schottky, as I've got some of those on order) in front of the cells to prevent back-feeding, if that's necessary and won't cause other problems.

The idea is that this will divide the current between the cells, presumably charging at about C/20 if the cells charge at an equal rate, and in any case never delivering more than about C/10 to a cell since the total current is limited to that.

Will this work? Are the diodes necessary? Is it safe to charge cells in parallel like this? Will it avoid any sort of weird positive-feedback loop where a cell with greater charge actually starts drawing more current as its charge difference increases as compared to the other cell?

Answer 1

This answer is intended to complement others: Edited January 19th 2023:

• Extended to emphasise the practicality of paralleling cells if done properly, and the fact that series diodes are a very poor solution:

Hard paralleling can be acceptable as long as the cells are balanced when joined.
This is common practice in many battery packs.
If the cells are separated to use then rebalancing would be necessary.

Balancing requires two nominally identical cells, ideally new ones, either discharged from well charged to a common endpoint, or charged from well discharged to a common termination point.
For maximum safety a commoning resistor between cells can be used to finalise balancing, and then replaced by a short circuit, but this step is not usually used or required. Having a starting point well away from the endpoint allows a similar common charge or discharge process in each case.

Use of diodes to prevent inter-battery currents is not a practical solution except in applications where achieving anything like full battery capacity or voltage is required. Even a Schottky diode will drop typically around 0.3V under moderate load. This is close to equal to the charged to discharged voltage swing of a NimH cell. A fully charged cell plus a diode will behave as if it is mostly discharged.

AND see additional addition at end.

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I've used over a million NiMH cells in products I designed. I've followed the history of NiMH cells and charging as they crossed a significan't boundary. Early low capacity NimH cells allowed trickle charging. Newer ones don't.

When cells are overcharged they generate hydrogen and oxygen due to electyrolysis. NimH cells initially included a mechanism to recombine these gases, allowing trickle charging at modest rates. When cell capacities exceeded about 1800 mAh AA packaged cells this mechanism was removed to allow more room for active material.

As a consequence modern NimH cells must not be trickle charged even at very very low rates. It is posisble that modern low capacity cells still have this mechanism inlcuded, but this is not certain and should not be relied on.

Most manufacturers indicate cells should not be trickle charged at all. A very few say a very low level of trickle charge (maybe C/100) may be applied for an extremely short period.

Individual or strings of cells charged from a voltage source substantially greater than the fully charged string voltage will be destroyed if charging voltage is not removed once charging is complete. The cells do not self-regulate to prevent further charging.
Fully charged cell voltage depends on charge rate and temperature. At 'room temperature' and C/10 rate end point voltage is about 1.45V/cell. At higher rates it is somewhat higher. Relying on end point voltage for charge termination is possible but 'getting it wrong' can lead to overcharging runaway.

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Added:

I was (apparently wrongly) assuming that the batteries are to be charged and used in parallel with no separation at any stage. My answer is directed mainly to that and is less useful if that's not the aim. Having seperate charging paths is desirable. And/but: (1) If the cells are not reasonably well balanced charging them in parallel even with diodes and with only single endpoint detection then the end result will be very uncertain. (2) Even if the cells are initially balanced the division of current is somewhat uncertain, and if they are not balanced the current division is even less certain. Adding a small amount of series resistance in each cell's circuit will tend to help current distribution. (Higher current in one leg increases resistive drop and causes more current to flow in the other leg. )

Answer 2

Two cells never have exactly the same equal voltage. Nor internal series resistance.

The problem thus is, if you put two batteries with unequal voltage in parallel. Say accidentally a fully discharged battery and fully charged battery.

That is a short circuit and the higher voltage battery pushes current into lower voltage battery.

Depending on internal resistances, the current could be many amps, and even shorting a single charged cell can pass so much current through a small wire that it can melt insulation and turn red hot.

It is far simpler, easier and safer to have separate constant current source for each battery.

Answer 3

The risk of high current between the batteries has been addressed in the Justme's answer and I'll not cover the problem of the chemistry of the battery not being tolerant to trickle charging (constant voltage, compensating its self-discharge rate). The circuit you posted is a constant current source, without voltage limitation:

If you manage to mitigate the risks and solve the other problems, TI's datasheet presents a possible solution to limit both current and voltage: