Charging Ni-Mh Batteries.

[George7941]

They are in my trusty old Fluke 98 scope. 3 Powerex C size 5000 mAH cells. I charge them with the original Fluke charger which outputs 150 mA at 4.8V, makig it a C/33 charger. After depleting the cells completely I hooked up the charger and monitored the battery compartment temperature rise, on the case of the scope with an infra red gun.

Hour 0 0 degreesC
12 5
24 5
30 7
35 8
41 9.5
45 9

Do these temperature readings indicate when the cells were fully charged? At 41 hrs when the temperature delta was the highest at 9.5C? Around 38 hrs?
I do not have voltage readings.

 

[Quattro Pete]

45 hours to charge nimh cells? You sure have the patience. I would have bought a better charger already.

As for your question, I dont think you can automatically assume that. Depending on the cherging algorithm being employed, a period of top off charge using very low current may be required, and you will see lower temps during that top off cycle.

 

[George7941]

I don't think the original Fluke charger is a smart charger. I think it is an unregulated charger.
The Fluke manual just instructs you to charge the cells for sixteen hours. They are referring to the much lower capacity NiCad cells which the scope originally came with.

 

[jhellwig]

NiMH cells have a quick little peak in temperature and voltage when they are fully charged. You would not be able to pick it up with an ir gun on the outside of the case. That slow of rate on that size of cell is not going to create a noticeable peak anyways. A 2.5 amp rate would give a nice indication at the end of charging.

If you want to properly charge and condition the cells you would need to get a hobby charger and make a setup to charge the pack. Most hobby chargers can in various schemes and temp can be included if you put a probe on a cell.

 

[jhellwig]

Did you shrink wrap the new cells or just using them loose? If loose you could use this charger https://www.mahaenergy.com/mh-c808m . It can condition the cells also.

 

[George7941]

The batteries are loose but it is a bit of work to open up the case and extract the cells. I would rather just plug in the charger to the charging port on the side of the case.

I do have a Maha charger but it only does AA and AAA http://www.mahaenergy.com/analyzer/

The Powerex cells are nine years old and they have only lost about 10% of their capacity, so my 40 hr charging regimen does not seem to have hurt them.

The instructions which came with my Maha charger states that for the quick drop in voltage, when Ni-Mh cells reach full charge, to occur reliably, the charge rate has to be at least C/3. My Fluke C/33 charge rate is far from that.

I made one mistake in my original post, it is 4 C cells, not 3.

 

[HorseThief]

I don't think you can really use dT as a full charge indication in such a low current application. That being said, @ an assumed .03C charge rate, and assuming cells where a full 5Ah and 66% charge efficiency (none of it's probably completely accurate) then it would take 50hrs to tpcharge from completely dead, so you're in the ballpark. Also, at such a low current you could leave it plugged in pretty much indefinitely with no real damage done - .03C is standby territory (emergency lights, etc).

 

[Wolf359]

I just use the Zanflare C4 charger. There's a few other ones out there, but I got mine when it only had 25 reviews on Amazon and now it's up to over 500. About $30 and charges both NiMh and Lithium ion batteries. I use them for both my AA NiMH and 18650 which I use for flashlights. Nice thing about it is that it tells you how many mah it charges and there's also a mode where you can get them fully discharged and then charged up so you can tell exactly how many mah it has. Lots of those fake ones claim one number and it's usually something lower.

https://www.amazon.com/Universal-Zanflare-C4-Rechargeable-Batteries/dp/B07428G1G2

 

[jhellwig]

Originally Posted by George7941
The batteries are loose but it is a bit of work to open up the case and extract the cells. I would rather just plug in the charger to the charging port on the side of the case.

I do have a Maha charger but it only does AA and AAA http://www.mahaenergy.com/analyzer/

The Powerex cells are nine years old and they have only lost about 10% of their capacity, so my 40 hr charging regimen does not seem to have hurt them.

The instructions which came with my Maha charger states that for the quick drop in voltage, when Ni-Mh cells reach full charge, to occur reliably, the charge rate has to be at least C/3. My Fluke C/33 charge rate is far from that.

I made one mistake in my original post, it is 4 C cells, not 3.

The way you are doing it is never going to be optimal for the batteries but isn't going to hurt anything. Nimh don't much care for a timed charge or overcharge. The life will suffer but for occasional use it would be fine. If you wanted reliability and to charge them in place a fresh set of nicads would be best.

You could rig up a c cell adapter to your c9000 and run the refresh cycle on the every now and then to keep them going a bit longer. The refresh on the c9000 is time based on the charge so temp or a voltage change doesn't matter. While it can only do a 2.5 amp charge per cell at most the refresh algarithm supports up to a 25000mAh battery.

 

[George7941]

I do cycle the batteries every so often by engaging the RECORD function (on all other functions the scope switches itself off in ten minutes if no key is pressed) and it now records for nine hours. When the Powerex cells were new, it would record for 9 1/2 hours, so that represents a 5% loss of capacity over nine years.

The old NiCads I replaced would only record for 4 hours.