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

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?

• To learn about NIMH charging, reading the datasheets of some NIMH charger chips provides good insights. I recommend getting TI's datasheet for the BQ25172 and/or ADI's (ex-Maxim) datasheet for the DS2710. They tell you what the chip does and everything it incorporates to get the job done properly and safely. Jan 11, 2023 at 15:11
• @Smith I just looked at the data sheet for the TI chip, and it doesn't seem to be nearly as clear an explanation of NiMH battery behaviour as Linden and Reddy's Handbook of Batteries, 3rd ed. And note that, for my purposes here, the TI datasheet includes a lot of extraneous information since it's designed to do fast charging, which is exactly what doesn't work with the batteries in question.
– cjs
Jan 11, 2023 at 16:06
• Hard parallel can be OK as long as the cells are balanced when joined. ,|| I've used over a million NiMH cells in products I designed. NiMH under about 1800 mAh in AA pkg could be be trickle charged at very low rates indefinitely as they had internal gas recombination mechanism. Over that capacity it is not present - to allow more room for active material.. They must not be trickle charged even at very very low rates. || The book may be old? Jan 13, 2023 at 11:25
• Energizer and a Japanese battery company said 13 years ago to never trickle-charge a Ni-MH cell at higher than 1/40th its capacity. Now their Ni-MH batteries have Eneloop technology that holds a charge for 1 year then the trickle charge current might be less or not needed. Jan 14, 2023 at 20:56
• The Energizer Ni-MH Battery Manual is already posted at the other question. Jan 16, 2023 at 18:05

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.

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.

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. )

• This is a great answer, though I can see now that I should have asked separate "trickle charge" and "parallel charge" questions. (I simply didn't know about the changes to modern NiMH cells because it wasn't mentioned in the handbook I was using.)
– cjs
Jan 15, 2023 at 2:36
• So how does one tell if cells are balanced (which I presume means, "the two cells are of identical manufacture and capacity and contain very close to the same amount of charge")? Assuming that they're the same model of AA cell, is their voltage a good proxy for this? Should the voltage be measured open or under load?
– cjs
Jan 15, 2023 at 2:38
• FWIW, I've created a separate question to discuss trickle charging; it may be helpful to move that part of this answer over there. I've edited this question to remove the trickle charging part assumption and am now assuming a standard low-current, time-limited charge, in the hope that reduces distraction from the main thrust of this question.
– cjs
Jan 15, 2023 at 6:35
• @cjs I saw the new question. I'll try to get to amswer it. I see several sites including a recentish Panasonic manual mention possibility of low C rate trickle charging. This was very much not the case when I was dealing with this. Whether things have changed or people have got sloppy is tbd. Panasonic are close to gold-stabndard in most things, but not always. I've seen manufacturers, app notes give terrible advice - you have, alas, to check everything. It MAY be that experience has shown that self discharge rates matched by low trickling are OK - but I'd suspect not. Jan 15, 2023 at 8:02
• C/100 is a full charge in four days. C/1000 in a month or so. Self discharge is well below that these days. IF gas is generated it will leak through seals and safety valves and dry out the cell. It MAY be that a smidgeon of recomination system is now used, but I've not heard ot it. TBD ... Jan 15, 2023 at 8:07

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.

• Right; doh. For some reason I wasn't considering the sub-circuit of just the two batteries. Though, come to think of it, could putting a diode in front of each battery fix the issue? (As well as perhaps providing protection against a battery inserted backwards?)
– cjs
Jan 11, 2023 at 13:53

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:

• I am not clear on why I need external voltage limitation here. I've been working from Linden and Reddy's Handbook of Batteries, 3rd ed. §29.5. Fig 29.16 appears to indicate that "during charge at a moderate constant-current charge rate" the battery itself limits the voltage, reaching a peak of just over 1.5 V at 120% capacity, and drops after that. There's no discussion there of externally limiting voltage while charging.
– cjs
Jan 11, 2023 at 13:56
• I've moved the trickle charging concerns to a separate question. Regarding circuit you showed, that's the one from §9.3.6 of the datasheet, right? Am I misreading it, or is that a current-limited but not constant-current supply? Pretty much every discussion I've seen of NiMH charging has said to use constant current, and I've no idea what the implications of a non-constant-current charge would be for parallel batteries.
– cjs
Jan 15, 2023 at 6:39