Daddylonglegs
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| posted on 22/3/13 at 03:43 PM |
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O/T - NiCad Thermistor
I have a Wickes 18V cordless drill which has performed excellently for years, but unfortunately the battery packed in. The thermistor is O/C and I do
not know what value I need to replace it with. For now, I have popped a 470 Ohm resistor in line and will just charge by my watch rather than letting
it automatically manage it.
Does anyone know roughly what thermistor is likely to be in there? I am guessing that most 18V NiCads for drills will be similar in value?
Many thanks.
JB
It looks like the Midget is winning at the moment......
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Bluemoon
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| posted on 22/3/13 at 04:47 PM |
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quote:
Originally posted by Daddylonglegs
I have a Wickes 18V cordless drill which has performed excellently for years, but unfortunately the battery packed in. The thermistor is O/C and I do
not know what value I need to replace it with. For now, I have popped a 470 Ohm resistor in line and will just charge by my watch rather than letting
it automatically manage it.
Does anyone know roughly what thermistor is likely to be in there? I am guessing that most 18V NiCads for drills will be similar in value?
Many thanks.
JB
No idea of value, but you could put a variable resistor in change value until the charger thinks it's charged. Then used a DVM to measure the
resistance of the variable resistor, this should give you a good idea of what value it is..
Dan
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Daddylonglegs
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| posted on 22/3/13 at 05:44 PM |
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I've just checked the battery after about 35 mins (I have a good battery which takes 30 mins to charge). It wa charged a little but not enough
to give the drill any torque.
I am guessing that the thermistor is the type where the resistance goes down as temperature goes up and the 470 Ohm one I have in is too low so only
trickle-charging (the good battery shows about 12K Ohm. I do not want to try and prise the thermistor out of the case because I will probably knack
the battery completely 
I might be talking complete gobledy gook but maybe someone on here has some better info 
It looks like the Midget is winning at the moment......
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MikeRJ
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| posted on 22/3/13 at 05:55 PM |
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Thermistors are characterised by both their resistance at a fixed temperature (usually 25C) and also by their 'Beta' value which which
describes the logarithmic temperature/resistance characteristic.
You can have numerous different thermistors that have the same resistance at 25C, but if they have different Beta values their resistance will not
match at other temperatures (and could be miles out at very low and very high temperatures).
This means that simply measuring the resistance at a single temperature point will not help you find a replacement that closely matches the original.
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Bluemoon
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| posted on 22/3/13 at 06:24 PM |
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quote:
Originally posted by MikeRJ
Thermistors are characterised by both their resistance at a fixed temperature (usually 25C) and also by their 'Beta' value which which
describes the logarithmic temperature/resistance characteristic.
You can have numerous different thermistors that have the same resistance at 25C, but if they have different Beta values their resistance will not
match at other temperatures (and could be miles out at very low and very high temperatures).
This means that simply measuring the resistance at a single temperature point will not help you find a replacement that closely matches the original.
True but I suspect it's not that critical, if my under standing is correct a threshold is used to say "stop charging" as the
temperature is "high". The increase in temperature is due to the cells no longer charging and so the energy is lost via heat; so I suspect
the beta value is less critical than then nominal value.. In any-case caution is needed as over charging can head to "explosive" release
of gases if you over do it!
Having just re-read the last post, if the thermistors is in series with the battery charger supply and hence is the "intelligence" and
thus in direct control of the charge current it's value and current rating might be more critical???
Dan
[Edited on 22/3/13 by Bluemoon]
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Daddylonglegs
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| posted on 22/3/13 at 06:42 PM |
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Thanks for the replies chaps.
There are 3 connections on the battery: +Ve and -Ve one either side and a centre connector. The thermistor connects between the -Ve and the centre so
I am assuming it controls the current circuit based on measured voltage and the temp of the pack?
I have checked the battery again after about 70 mins and it seems to have as good an output as the good battery on load. I am a little worried about
'cooking' the battery so will probably just settle for a charge time of around 50-60 mins and see how it goes.
[Edited on 22/3/13 by Daddylonglegs]
It looks like the Midget is winning at the moment......
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CNHSS1
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| posted on 22/3/13 at 06:55 PM |
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in the days when i sold high end sophistated r/c model batts and chargers, there were two main types of cut off, peak detection and therma
detection.
nicads (as opposed to more modern nimh, li-po's etc) have a voltage trait where given a fixed constant charge, the voltage across the pack wi
rise slowly for 90% charge capacity. in the last 10% the voltage wi start to rise sharply, and at full charge will start to drop back. if plotted on a
graph, the voltage shows a 'peak' before it drops back. This type of detection is more exensive to make accurately and reliably.
the 2nd method is simpler, and thats to monitor the heat of the cells. the temp rise is similar to the voltage, gentle at first and ramps up towards
full charge. the cleverer charger monitor this accurately but a thermistor type just checks that theres a marked temp rise and cuts off the
current.
crude but effective, albeit rarely accurate to a degree or so
HTH
CNH
"Racing is life, everything else, before or after, is just waiting"---Steve McQueen
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