Trickle charge resistor for battery when replacing 3.6v NiCD with NiMH?

[Per Hansson]

Hi, I replaced a 21 year old 3.6v 60mA NiCD battery that had leaked over a PCB belonging to a PLC that keeps a GM76C8128CLL SRAM powered when it is off.
I used a 3.6v 80mA NiMH battery for the replacement that I found locally: Nedis BANM160SC3
It works fine but I am worried if the trickle charge circuit might be too aggressive for a NiMH battery?
I have drawn a schematic and attached of how it seems setup to me.
The battery gets 5VDC from the machine supply when it is on, this trickle charges the battery through a 470ohm resistor.
If my math in not off this means a charge current of 10mA
Now I did not find any datasheets for this specific battery but Varta has similar ones and they recommend only 2.1mA.
So should I replace the resistor with a 4.7Kohm to be on the safe side or is my math off somewhere?

P.S: The resistor was dropping 0.436v when the battery voltage was 3.935v and this equates to only 0.93mA which should be safe.
I arrived at 10mA (8.94 to be exact) by calculating with a voltage of 4.2v (one diode drop from 5v) through the 470ohm resistor but maybe this is wrong thinking?

[wraper]

Fully charged voltage of NiMH is about 1.45V (1.45*3 = 4.35V), so trickle current will be minimal.

[bdunham7]

P.S: The resistor was dropping 0.436v when the battery voltage was 3.935v and this equates to only 0.93mA which should be safe.
I arrived at 10mA (8.94 to be exact) by calculating with a voltage of 4.2v (one diode drop from 5v) through the 470ohm resistor but maybe this is wrong thinking?

C/40 to C/100 will work OK for a standard NiMH in an application like this.  Your calculation is off because you haven't included the battery voltage.  ((5V - diode drop - battery voltage)/470), so as an estimate for a 1.35V/cell fully charged battery, (5 - 0.6 - 4.05)/470 = .74mA or ~C/100.  Your battery may not even get fully charged at that rate, at least not for a very long time (weeks).   Often NiMH can be dropped in to replace NiCd without changing the trickle charge because the cell capacities are higher.  If your original NiCd cell was as small as the 80mAh NiMH, then its capacity was probably quite a bit less.

[Per Hansson]

Thank you very much for the explanation of where my math was off, much appreciated!
Also the tidbit of using a NiMH with a higher capacity to offset the charging current was good to know too.
The original was 60mA and the replacement 80mA as I wrote in the OP, so that should be good then
I attached two photos of the before/after

[magic]

P.S: The resistor was dropping 0.436v when the battery voltage was 3.935v and this equates to only 0.93mA which should be safe.
I arrived at 10mA (8.94 to be exact) by calculating with a voltage of 4.2v (one diode drop from 5v) through the 470ohm resistor but maybe this is wrong thinking?

The current is ~10mA into an empty battery
Later it decreases.

[mariush]

You probably could have used a rechargeable CR2032 battery, for example : https://www.digikey.com/en/products/detail/illinois-capacitor/RJD2032C1ST1/6596416

Find a 5v line, add a couple diodes to drop the voltage to around 3.6-4v and maybe limit current to around 5-10mA and should be safe to leave it like that.

[tunk]

The current is ~10mA into an empty battery

An empty battery is around 3V, so it would be ~3mA.

[magic]

You should stop discharging when it goes below ~0.9V per cell or so, depending on whom you ask.
But it doesn't mean that you can't go lower, or that an old battery can't self-discharge lower than that.

So there are various grades of "empty"

[Per Hansson]

Yep, and it is also a useful value to get an idea what will happen when the battery shorts out, since then the "charger" will deliver all it can through that resistor
It might not be so obvious in the other pictures so here you can see the split open whale as I had desoldered it

[Gyro]

You should stop discharging when it goes below ~0.9V per cell or so, depending on whom you ask.
But it doesn't mean that you can't go lower, or that an old battery can't self-discharge lower than that.

So there are various grades of "empty"

Yes, it's cell reversal that causes the damage - something that can never happen under cell battery self-discharge (or probably even an ultra low current load like cmos backup). It's things like drill batteries where catastrophic reverse charging of the weakest cell under load kills the pack. The 0.9V figure is really an imposed limitation to minimise the chance of a cell reversal when multiplied up by the number of cells - the larger the number of cells, the higher the imposed limit voltage (per cell).

Large capacity single NiCd cells for aircraft use actually used to be supplied with a shorting clip across their terminals.

[magic]

Depends on whom you ask. Some NiMH manufacturers discourage deep discharging. Example here on page 5.
https://eu.industrial.panasonic.com/sites/default/pidseu/files/downloads/files/id_ni-mh_1104_e.pdf

I have some old Varta cells which were unused for years and self-discharged to zero, one even leaked a bit.
They still work and hold quite reasonable charge (don't remember how much exactly) so dunno if it's a big deal.

[Gyro]

Yes, the answer seems less clear-cut for NiMh than it was with NiCd. [Edit: It's a shame Panasonic don't make a clearer distinction between cells and batteries, but they do make mention of both deep discharge and reversal].

[james_s]

The current is ~10mA into an empty battery

An empty battery is around 3V, so it would be ~3mA.

That's when it's considered "discharged" and ideally you should stop drawing from it, but left alone a battery will eventually discharge all the way to 0V due to internal leakage. In practice this is not really a problem though, the voltage will rise fairly quickly when connected to charging current and Nixx batteries will tolerate high charge/discharge currents for short periods quite well.