NiMh cell testing

[Trubble]

Search has turned up some neat threads from several years ago, but haven't found my answer yet.

Been rebuilding some of my older battery packs, and need to know some stuff. I've pulled out the dead shorted cells from a couple packs. They were easy enough to find with the VOM, as they were at absolutely no volts. Now, I'm going back through and trying to weed out cells that are not shorted, but just weaker. Previously the method I used was to discharge the packs and note which cells got the hottest, then after cooling, charge the packs up and see if those marked cells were still getting hot durring charge. That worked well enough for seriously flawed cells, but now I'm down to a last couple packs giving me fits. I've got some 3800mAh packs that I cannot charge at more than 2 amps. Regular 'factory assembled' packs are taking 3.8 amps no problem, so I'm certain there's a dud cell in the mix somewhere. On my suspect packs, temps are within 20° of each other, VOM has the resting volts looking extremely similar, resistance (set to Rx1) reads very similar. Packs are all built side-by-side w/ bars, so I don't suspect the foil tabs being a problem, and the solder jobs on the bars look to be fine (both mine and the factory solders).
So, is there anything more us shadetree mechanics can look for? Is there any method you guys use for weeding out cells, or is it 'guess-n-go' time? My discharge rig has a voltmeter and ammeter, and my VOM is decades old, but still looks to be working fine. At this point, I am not willing to spend any money on new equipment, just want to know if there's a technique I can employ with what I have, or if I'm back to just pulling a cell, rebuilding the pack and see if that fixes it.

[Charlie-III]

Gee, I'd have to back and check really old notes.
Basically, put a 5A load on a fully charged pack and check each cell voltage as they go down.
Failing cells will drop faster relative to others.

Way back when, I used a rig to cycle the packs & record voltages/capacity. This was on individual cells. I would then use "like" cells to make a pack so they would all dump at the same time, this was more for NiCD packs though.

You should be able to do the same, cycling the packs fully (charge to full, dump to "empty") a few times & then checking would be best.

If you need more info, post back.

[floogldood]

Charlie gave you good method by checking cells' voltage during the discharge. They can be checked for capacity and/or voltage. Here's something that doesn't make sense.

"I cannot charge at more than 2 amps. Regular 'factory assembled' packs are taking 3.8 amps no problem"

Why can't you charge at more than 2 amps? The battery has a choice in the matter? There is a reason that a particular charger will not reach it's set-point for current and very well could be the battery's fault , but any pack can be charged at any practical rate. It may not be pretty , but it can be done. Perhaps the voltage rises up high?

[Charlie-III]

Originally Posted by floogldood

Charlie gave you good method by checking cells' voltage during the discharge. They can be checked for capacity and/or voltage. Here's something that doesn't make sense.

"I cannot charge at more than 2 amps. Regular 'factory assembled' packs are taking 3.8 amps no problem"

Why can't you charge at more than 2 amps? The battery has a choice in the matter? There is a reason that a particular charger will not reach it's set-point for current and very well could be the battery's fault , but any pack can be charged at any practical rate. It may not be pretty , but it can be done. Perhaps the voltage rises up high?

A sulfating or shorting cell will not allow a higher charge rate, it just can't handle it. Generally a cell like this will get warmer before the others since it is a resistor in a case like this (same thing happens with lead acid batteries......that is the business I'm in).......
Since the pack is a series connection, the whole pack is limited in charge/discharge by the worst connection or cell.
Hope this sheds some light on it.

[Trubble]

Yup, helps out quite a bit. I was checking cells' voltage after complete discharge, didn't remember/think to check durring discharge to see when they're dumping. Thanks Charlie.

[Manning]

Yep, what Charlie said. Put a load on an individual cell and pick a few times during the discharge and record the voltage. Say, 1 minute intervals.

Voltage under load is everything. Open circuit voltage doesn't mean much. Some of those cells with 0.00volts might have been OK, they just had a higher self discharge rate than other cells. The last few generations of Intellect cells (the blue 3800 and 4200's) were bad about this. If you let them sit for a week or two, the voltage would plummet to zero, but they would charge and run just fine.

But unless you just like tinkering with this stuff, you probably should just get new batteries as you may spend hours and hours to end up with one pack that is usable, but may only have adequate performance. Batteries do not last forever, they wear out just like anything else does. Time is a big enemy, batteries go bad just sitting around doing nothing. That's a tough concept for most people to grasp.

[Trubble]

Originally Posted by Manning

But unless you just like tinkering with this stuff, you probably should just get new batteries as you may spend hours and hours to end up with one pack that is usable, but may only have adequate performance. Batteries do not last forever, they wear out just like anything else does. Time is a big enemy, batteries go bad just sitting around doing nothing. That's a tough concept for most people to grasp.

Sadly, yes, I do enjoy just tinkering with these things. And these packs were labeled as "Old/Low Voltage" when I purchased them, but for $9 each (x6), I figured if I got two workable packs I'd be fine. So I do realize these things were gonna peter out fairly soon. Five of the six worked pretty good that first year, though.
And I do have some newer packs (and LiPos), for the go-fasts. Those I leave in their stick configuration, because those chassis' seem to like it that way.
But another reason for tearing these things apart is to get them re-configured for my AX-10. Last month I did my first comp, and had the stick pack up on top pulling me over onto my lid. I built plates for my axles, figure I might as well rebuild these into brick packs, so they'll fit down nice and low. Just figured if I'm tearing 'em apart anyway, might as well weed out some of the chaff.

[Charlie-III]

Glad to have helped out with info.
I still have my old Tekin discharger (10A draw) I use for some of my NiCD & NiMH packs.

[Manning]

Originally Posted by Trubble

Sadly, yes, I do enjoy just tinkering with these things. And these packs were labeled as "Old/Low Voltage" when I purchased them, but for $9 each (x6), I figured if I got two workable packs I'd be fine. So I do realize these things were gonna peter out fairly soon. Five of the six worked pretty good that first year, though.
And I do have some newer packs (and LiPos), for the go-fasts. Those I leave in their stick configuration, because those chassis' seem to like it that way.
But another reason for tearing these things apart is to get them re-configured for my AX-10. Last month I did my first comp, and had the stick pack up on top pulling me over onto my lid. I built plates for my axles, figure I might as well rebuild these into brick packs, so they'll fit down nice and low. Just figured if I'm tearing 'em apart anyway, might as well weed out some of the chaff.

Hey, that's cool. I did the exact same thing when I first started the crawler thing with the 2/3 A cells. As one cell went bad, it came out of the pack.

I still have my Victor Engineering Super Hi-IQ2 (circa 1992) that does all kinds of cool cycling, motor testing, etc. Still use it for many things. I'll cry a river if it ever goes bad.

[Charlie-III]

Originally Posted by Manning

Hey, that's cool. I did the exact same thing when I first started the crawler thing with the 2/3 A cells. As one cell went bad, it came out of the pack.

I still have my Victor Engineering Super Hi-IQ2 (circa 1992) that does all kinds of cool cycling, motor testing, etc. Still use it for many things. I'll cry a river if it ever goes bad.

1992??!!, sheesh, my stuff was OLD when that came out!

[Trubble]

I think the oldest thing I have in the basement is my TQ-10 (2PB, Novak ESC). As far as electronics and battery maint. equipment, all I have left is an Astro-Flight Model 111 charger, and a handful of CMP (Cougar Mountain Products, maybe?) Accu-spec precision dischargers. Supposedly they would discharge down to a certain voltage and cut off. I haven't been able to dig up much info on them, so I don't know what the cutoff is. No matter really, if I remember correctly, they had a nasty habit of just not turning off every now and again.