NiCad Battery - Internal Component ID

[SeanStevens]

Hi all

This is a cry for help.

I've got battery packs that now refuse to take (or possibly deliver) a decent charge. They are quite old. So I've popped one apart and intend to replace the NiCad cells with new ones.

There is a heat sensitive resistor within - that I believe detects if/when the battery gets hot and turns off the charger.

The component that I need help with is used to bridge two of the cells (they are all in series - this component is in that string). It is totally flat and the markings on it don't seem to appear in internet searches.

QKC9
? L26

I've measure it's resistance - it is close to perfect continuity.

What I'm worried about is that this component may be the issue with the packs not delivering a good charge. The one pack I have disassembled seems to contain cells that have a good charge and capacity - but this does not get to the application reliably.

Any ideas what this component is or how to test it?


SEAN

[SiriusHardware]

It seems highly likely to be a fuse. If you think it is in some way limiting the current to the load, temporarily bypass it (short it out).

You say you feel the cells themselves are in good condition / of good capacity but how exactly are you measuring that property?

Try powering a realistic load, such as a lamp bulb of appropriate wattage and voltage or a high power low value resistor, and see if the voltage output from the cell(s) holds up then. Measuring the cell voltages off-load isn't really an adequate check.

[The Philpott]

-Have you done any testing on individual cells within the pack? There could be a rogue one that has started to grow crystals inside prior to failing altogether.

I have known a cordless drill NiCad pack to continue delivering power despite a defective cell...the defective cell continued to pass current without donating any itself, the symptom which identified this was the terminal voltage at full charge was suspiciously low.

If there is any 'smart' processing taking place inside the charger and/or the appliance this complicates things further, of course.

Dave

[G6Tanuki]

The "thing" in series with your cells is, I suspect, a "Polyfuse" - if the battery-pack is shorted then the Polyfuse heats up and turns into a very high resistance; remove the short, let things cool down and the Polyfuse goes back to near-zero-Ohms.

A good way to both protect the battery-pack against shorts, and also save the drill-motor from burning-out due to excess durrent if it is stalled.

[SeanStevens]

Many thanks all

I'm happy that this isn't the culprit now. A fuse - makes perfect sense and would possibly stop a fire occurring!

As for how I tested the individual cells - well, it wasn't that scientific - but better than just measuring the voltage without load.

Just measuring each cell, I was getting 1.2v give or take 0.09 of a volt. I then attached a small electric motor to the battery an noted a small (0.1v) voltage drop. By pinching the commutator shaft I could reduce the motor speed (increasing load) and the voltage noted on the volt meter dropped to 0.8 volts on average.

Voltage bounced back to normal - when repeated, this continued to happen until I got bored. All cells performed in a similar manner.

Thanks again all

SEAN

[SiriusHardware]

I'll have to remember that one, nice idea, using a motor as an easily variable test load. The only thing I would say is that that little motor is likely to need far less current, even when loaded, than the real load.

But if it's not the cells and it's not the fuse and it's not the thermal resistor, what's left? Have you tried running the load (drill, or whatever it is) on a bench power supply - assuming you don't have any known good battery packs?

[SeanStevens]

Good question SiriusHardware

What has caused two of these packs to give up? Well, they would not take a charge, I'm guessing that they were already charged and turning off the charger.

The symptom was that a very low power delivery was evident, so it must have been the connection of the battery to the motor unit itself. New batteries will have new shiny surfaces for the motor pick-ups to get old of. This should sort the issue.

I'll report progress.

SEAN

[SiriusHardware]

Do your battery packs have an Amp-Hour rating on them?

If so, create a test load which will draw approximately that amount of current from 1.2V and connect the test load across each cell in turn. If the cell voltage holds up while it is supplying that sort of load then the batteries are good, but if the cell voltage collapses, they are not good.

If you think your problem is actually tarnished or corroded external pack contacts, prove it - bypass them by making connections directly from the cell pack's +ve and -ve to the power terminals of the unit which is normally powered by these.

If that doesn't work then you know your problem is elsewhere.

[G6Tanuki]

Tour original post says the battery-packs are "quite old" - just how old?

NiCd cells are usually rated for a specific number of charge/discharge cycles [the two-way-radio ones I worked with in the 80s and 80s this was typically 500, equating to less-than-two-years of service]. They also degrade with age even if they're not being subjected to regular charge/discharge - indeed, regular use seemed to keep them healthier than sitting on a shelf unused for a couple of years. NiCd cells have issues with 'self-discharge' and I had a rather draconian policy of discarding any packs that had sat unused for more than a year [if only to avoid annoyed clients whose battery-packs that were supposed to work for a day of 9:1 receive;transmit duty-cycle shut-down prematurely].

NiCd technology is now long-obsolete anyway; perhaps it's time you should be upgrading to Lithium-Ion stuff [lots more Milliamp-hours-per-gram-weight of battery-packs, much less self-discharge in infrequently-used devices]

[SeanStevens]

Yes, the cells are

GP SYLVA-CHARGE - 95AAS

1.2v 950mAh

As for their age - well they were all second-hand when I got them, history unknown. I guess they were made around 1998-2000.
That density, 950mAh is low, suggesting they are very old. It was hard to get anything at that rating for replacements - I got 1000mAh, closest I could find easily.

I guess it is an acceptable age for the cells to 'let go'. Oddly, it seems that this pack I have dissected has good NiCads in it! I blame dirty connections.

(Some would suggest operator error!)

SEAN

[Herald1360]

950mAH is pretty good for an NiCd AA cell. MiMH AA cells typically have much higher capacities.

RS current AA NiCd offerings range from 600-780mAH.