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Techy question: charge management for NiMH batteries?

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Eric The Viking

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I've now got three cordless drills with u/s battery packs: all are NiCd packs with 'sub-C' cells. As far as I can tell, they all seem to use thermistors to detect end of charge, and they're all around 1.7AH cells originally.

One of them, the right-angled drill, I shall need shortly on a house DIY project, so I have to do something about re-celling the pack, and as the others are mechanically sound, they're worth refurbishing too if possible.

I've found the right size, tagged NiMH cells, 2.2AH, reasonably inexpensively (£2.74ea +VAT), but I'm slightly worried about the charge management systems of the chargers. They seem to have fast-then-trickle functionality.

So questions:

Given the difference in cell capacity, will I likely get away with using the old chargers unmodified? Or do I need to change the thermistors in the packs to increase the sensitivity to the end-of-charge δT?

If anyone else has been down this road, I'd appreciate any advice you can offer.

Thanks,

E.

PS: Why not NiCd like-for-like replacements? Simply because I can't find them at an economic price. People like RS components seem to have drastically cut back on the range they offer these days (and they never were cheap, either).
 

Pete Maddex

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Hi, Eric

I have only replaced like for like, mostly with cells from Maplins.

I thought the thermistor was for temperature not full charge, I seem to remember reading it in the manual .

Pete
 

CHJ

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A thermistor is a temperature sensitive resistor, it's included in battery packs to restrict the charging current if the cells overheat.

I think you will find final cell voltage/current flow (charge state) is monitored by the charger and characteristics for NMh are different to Ncads.


Long time since I put a charger together from basics so don't have circuits to hand, they come in little plastic boxes these days. :)
 

Pete Maddex

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Hi, Eric

I have only replaced like for like, mostly with cells from Maplins.

I thought the thermistor was for temperature not full charge, I seem to remember reading it in the manual .

Pete
 

Eric The Viking

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CHJ":kwxz8jnf said:
A thermistor is a temperature sensitive resistor, it's included in battery packs to restrict the charging current if the cells overheat.
Since it has a negative coefficient, I think it would have the opposite effect! They're used as thermometers.

I think you will find final cell voltage/current flow (charge state) is monitored by the charger and characteristics for NMh are different to Ncads.
Indeed they are, and it seems NiCd may be more tolerant chemistry than NiMH. The 'battery university' link that Deejay posted suggests that using δT isn't a good method (but that's because the manufacturers take chances with the values they use). The 'posh' ones use δV to detect EOC. I'm trying to avoid a complete rebuild of the charger electronics, but it might be the most practical approach. Another might simply be to limit the charge current more (series resistor?). The issue with δT is twofold: the charger will overshoot (this ought to be avoidable by picking the right thermistor), and repeated insertions in the charger will trigger bursts of boost charge (turned off by the δT measurement), which will quickly damage the battery.

Still mulling all this over, and have now found 1.7AH NiCd cells at an affordable price, so may go the cowardly route and just re-cell like-for-like. It seems to be that using older chargers in a cold place is a definite no-no -- room temperature or you risk overcharging.

E>
 

CHJ

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Eric The Viking":cgsx7cg7 said:
CHJ":cgsx7cg7 said:
A thermistor is a temperature sensitive resistor, it's included in battery packs to restrict the charging current if the cells overheat.
Since it has a negative coefficient, I think it would have the opposite effect! They're used as thermometers.

I think you will find final cell voltage/current flow (charge state) is monitored by the charger and characteristics for NMh are different to Ncads.

I did say it is used to restrict charging current, I did not say it increased in resistance. Perhaps I should have explained how.

On my unit it shorts out part of the voltage sensor divider simulating a high voltage battery.

A simple controller circuit can be found here although it does not show the thermistor connection.
 

Eric The Viking

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Sorry, Chas, I see what you mean now.

It's the same on the no-name charger for one of the packs I'm trying to sort out, but it's got an ST Micro IC I can't find at the moment. The number I found on the IC comes up as a BCD switch! I haven't dismantled the Bosch ones yet.

Managing NiMH batteries looks like a nightmare now! I'm leaning towards just replacing the cells like-for-like, but I'd be happier if I could nail that IC and be certain of the circuit...
 

CHJ

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If it is a programable device then it will probably be nigh on impossible to determine what it is actually doing other than monitoring it's output with a data logger on a new set of batteries.

I suspect the modern chargers have propriety IC's in them to cut down production costs, not pulling mine apart.
I do know that a universal cell charger I have can detect if you have mixed and matched Nicad and NMh and refuses to play.
 

Jonzjob

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A problem with NMh is that they will discharge over a fairly short period, so if you don't use them for a couple of weeks and don't charge them before you try to then you can well end up with little or no power from them?

NiCads hold their charge very much better and in my experience are much tougher.
 

John Brown

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A good charger of either NiMH or NiCad cells will use a combination of deltaV, deltaT and possibly total charge time and terminal voltage.
Some battery packs have a poly fuse incorporated for saftety reasons.
There are recommended rates for fast charging, trickle charging and float charging.
You can read up on this stuff for days, and waste half your life seeking the perfect system. DAMHIKT.
As you guessed, the reason replacing the batteries with larger capacity ones might be a problem is because the deltaT may not be enough to trigger the end-of-charge. Similarly, if negative or zero deltaV is used, the effect may not be strong enough. NiMH often use a zero deltaV as the negative deltaV is not as pronounced as with NiCads.
The Battery University link posted above is fairly comprehensive, if memory serves. I'd suck it and see - I presume it's a fairly quick charger, so place it somewhere safe and keep our eye on it.
 

dickm

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(Not quite the same problem, but still battery related.)
By mistake, bought the wrong size of battery in a B&Poo sale, then realised that the cells were the same size as those in another dying battery. Tried to reconfigure the cells to fit the dying one's case, and hit a problem. The cells have silver coloured tags, but unlike the originals, they won't "take" solder - it just sits on the surface and doesn't wet. Tried Baker's fluid as flux, which usually works for steel, but no joy. Looks as if the tags must be some sort of stainless steel, spot welded to the cells.
Any suggestions how to get these to take solder?
 

Noel

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dickm":19xw8ha4 said:
(Not quite the same problem, but still battery related.)
By mistake, bought the wrong size of battery in a B&Poo sale, then realised that the cells were the same size as those in another dying battery. Tried to reconfigure the cells to fit the dying one's case, and hit a problem. The cells have silver coloured tags, but unlike the originals, they won't "take" solder - it just sits on the surface and doesn't wet. Tried Baker's fluid as flux, which usually works for steel, but no joy. Looks as if the tags must be some sort of stainless steel, spot welded to the cells.
Any suggestions how to get these to take solder?
I've had that problem in the past. And it remained a problem. Be interesting to see what the answer is.
 

Eric The Viking

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Chas obviously has the correct solution, but, depending on the tag thickness, you might try a Lucar (spade) connector:

Generally speaking, the two curled-over sides of the female ones make a pretty good low resistance connection, as long as they grip tightly enough. It's when they're loose (lots of disconnect/reconnect operations) that they give problems. The width of the "male" part isn't critical, so you could shape part of the strap into a 'tag', and you can gently tighten the sides of the female connectors with snub-nosed pliers so as to get a really good friction fit. The tags of the ready-tagged cells you can buy individually fit straight into the appropriate sized Lucar connector (in my limited experience).

Maplins used to sell rolls of thin white heatshrink, I think intended for sealing medicine bottles, etc. it's got far more 'shrink' than the normal kind, and isn't very flexible, but it's ideal for this application, for both insulating and holding everything together without adding much bulk. I haven't bought any for years (still have enough for occasional use), and if they don't sell it any more (they sell very little that's useful/affordable these days!) you might get some on eBay.

It's a fiddle, and might be tricky to arrange alongside the cells, but it would work.

E.
 

dickm

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CHJ":ldysi70t said:
It is because of the chrome content of the corrosion resistant steel.
You need a special flux for soft soldering.

Here is an example from CuP Alloys

Specialist fluxes are also required for Silver Soldering.
Thanks Chas - that confirms my materials scientist daughter's diagnosis. But that flux will cost considerably more than I paid for the battery :( .

Just remembered I've got some EasyFlo which I use for silver soldering. Possibly at least worth a try.
 

CHJ

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dickm":2alq78gr said:
....Just remembered I've got some EasyFlo which I use for silver soldering. Possibly at least worth a try.

No it won't work with soft solder. 1. temperature will be too low to melt it 2. it won't wet the chrome anyway, need a flux with borates in.
 
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