NiCd to NiMh problem.

[birdtrek]

Hi,
I've changed the 20 D cells in a battery pack. Took out the NiCd and switched to NiMh, soldering everything in same sequence.
The original NiCd charger would not work at all on these so I have tried another 24v NiMh Charger.
It will charge for about 15 mins and then thinks the battery is full.
I think the problem is the resistor. It is fitted with one marked 6K8J.
Any ideas please as to a replacement?
This is the current one .......

Many thanks ............ Chris

 

[Alan Quay]

That looks like a thermistor, although I don't know what value you need, depends on the charger.

 

[flecc]

Yes, it's an NTC thermistor. Those most commonly used in e-bike NiMh batteries to match their charger cutoff circuits are 10 kOhms with a B (beta) value between 3400 and 3950.

You really need the original charger makers specification for it when used with NiMh cells, but as they are cheap you could take a stab in the dark with two or three 10k ones with varied betas in that range. Farnell have a large range of suitable bead thermistors.

As a temporary measure you can substitute the thermistor with an ordinary 10k resistor and it should charge ok. You'll need to cut off the charge manually when the battery voltage reaches about full charge level or when the battery casing starts to get warm.

 

[birdtrek]

Thank you.
I've researched the charger and found:
All of these models use a NTC(R=10K, B=3960) sensor.
Now to try and buy one and solder in

 

[flecc]

Well done on finding the information. This one on the link below is close at B4080, the beta values are usually not very critical and at 54 pence each maybe worth trying:

10k B4080 NTC bead thermistor from Farnell

Or there's 10 of the exactly right value from Taiwan via ebay on this link, though inline leads rather than the usual bead type.

 

[birdtrek]

Thank you.
Ordered off Farnell, so should be here tomorrow. (Though I had to make the order up to £20 before they accept electronic payment.)
Next question; is the possibility of upgrading the original NiCd charger feasible?

 

[flecc]

I doubt it's practical, the cutoff circuit has been designed for a specific NTC thermistor value, so one would have to have knowledge of the circuit and component values to alter. You could have approached this differently by experimenting with a few different thermistor beta values in the battery to find one which works with the NiCad charger, instead of buying an NiMh charger.

That would still be possible if you use a four-pin charger connector on the battery and chargers. Using different connector pins for the thermistor positive on each charger, you could have the two thermistors in the battery. That would make both chargers usable.

I still couldn't guarantee the NiCad charger will work well with the NiMh cells though, the cell resistance characteristics are very different.

 

[birdtrek]

Update on thermistor:
Arrived this morning and soldered in.
Unfortunately, it only charged for about 30 mins and the charge light went green. It's a 1.8 amp charger and assuming a flat battery, a charge lasting about 6 hrs would be expected on a 10 ah battery.
The charger is borrowed from another ebike with 24v NiMh and seems fine.
I can only test the battery I rebuilt on the ebike it came from. (Mercedes Hybrid). It shows full when installed but after 5 mins the charge indicator on the ebike starts flashing indicating that it is almost flat.
Any ideas on what to try next?
Chris

 

[flecc]

Best way to prove the charger and battery are ok Chris is to temporarily replace the thermistor with any normal 10k resistor. That should then allow the battery to be fully charged in about 4 to 5 hours. If it does we can look at changing the thermistor, but I suspect there may be a deeper problem with either battery or charger.

 

[birdtrek]

Between the showers we did manage a 'test' run on the bike. We would conclude that a little more charge was put in but nothing like it should.
My electronic knowledge is minimal.
Can you send me a link to a 'normal 10k resistor' on eBay?
Are you able to explain how the 'beta' values work?
I would also add that whenever I have tried to charge the battery pack they have never become warm.
I will email the charger manufacturer as well to check that my particular charger is exactly the same as the one I found the spec for.
Thanks. Chris

 

[flecc]

NTC and PTC thermistors are non-linear resistors that change their resistance with temperature. The beta value is calculated by measuring the temperature at two points in their range and then is derived from this formula:

b = Ln(R t1/ Rt2) / (1/T1 – 1/T2)

where
Rt1 = the resistance at temperature 1
Rt2 = the resistance at temperature 2
T1 = Temperature one in °K
T2= Temperature two in °K

NTC is negative temperature coefficient, meaning resistance declines with temperature rise.

Not something for us to bother with, it's just the way the thermistor industry gives characteristics for circuit designers. It's also given in a more accurate graph curve of temperature versus resistance changes

You can pick up a 10k resistor from your local Maplin store if you have one. There are also plenty online from them, ebay and others, but if you like mail me an address to the below and I'll pop one in the post. These are penny items and I have plenty:

tinyworld@flecc.co.uk

If you do buy a cheap batch of mixed ones online, to identify a 10K one, read the colour starting with the colour band closest to an end. The band colours from that end are Brown-Black-Orange for 10k. Ignore any subsequent colours.

 

[birdtrek]

Thank you for the explanation.
When I tried the new thermistor, the pack was opened and I left the thermistor sticking up in the air. It is an unheated shed, so really it's temperature would not have changed during charging, but the charger still went green quite quickly. (30 mins).
Incidentally, I have two of these chargers from a matching pair of folding eBikes. After rebuilding the case and putting thermistor in correct place, and testing I tried other charger but the same short charge occurred.
Voltage readings:
27.3v apparently flat.
29.5v charger says charged.
Charger unplugged from battery 39.7v

 

[flecc]

It does sound very much like there is a fault somewhere, but the 10k resistor will prove if that is the case.