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NiMH charging.

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Hello Geeks.


I'm not sure which section to present this, so any mod please feel free to move it if required.


While passing a skip I noticed a cordless battery that was looking for a new home.


Closer examination shower a hole had been burnt in the side where the battery attaches to the drill.


But I thought it would be interesting to play around with this thing.


It is a DeWALT 18 Volt 2.6 Ah battery and is comprised of 14 cells.


The next bit is supposition on my part, so don't hesitate to tell me if my reasoning is wrong.


I am assuming that each cell is 18/14 = 1.286 volt nominal.


And that each cell can supply 2600 mA/14 =185.7 mAhr.


The test so far.


One cell was scrap, burnt.


The others measured 0.083 Volt (no load).


I attached each cell across my variable power supply and slowly raised the voltage.


At about 1.5 volt the amp meter kicked into life and slowly fell back to around zero.


The cells then measured about 0.953 Volt.


I am now charging each cell via my SC-608 Smart Charger at 300 mA. On the NiMH/CD setting, of course.


The charger is indicating 1.4V at 0.3 Amp.


(waiting for this cell to complete)


Stopped charging after 37.39 minutes.


Indicates 182 mAh and 246 mWh.


Cell  now showing 1.337 Volt (no load)




Looking at the recommended charge rate it seems to be 0.4C. Where C is the capacity in mAh.


This would give me a charge rate of 74.28 mA if the capacity is as I have calculated.


My question is, am I approaching this right ?



Edited by samplerdave, 27 May 2018 - 01:42 PM.

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You lost me at the end. I haven't tore one of these apart, but I'm guessing they just use standard size AA or AAA cells?

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Sorry for the confusion.


The physical size of the cells is 40 mm long by 22mm diameter. Nothing written on the card wrapper.


I've been looking through the Wiki on cells and the nearest match I can find is Sub-C.


The info provided is 1.2 Volt  1800 - 5000 mAHr capacity.


I suspect my charger is cutting of the charge too soon. On a timer.


From what I can glean from the web the problem with charging NiMH cells (under a constant current) is detecting the very slight drop in charge voltage that tells you that the cell is full.


I have an idea to work around that..


What I am really wanting to know is am I correct in simply dividing the stated battery Voltage and AH capacities by the number of cells to derive the capacity of a single cell ?



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