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NiMH charging.


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#1
samplerdave

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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 ?

 

Dave.


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

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#2
admin

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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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#3
samplerdave

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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 ?

 

Dave.


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#4
TooNew2

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Suppose you had two 6v lead acid batteries, each with a nominal output of 30 amps. If you hook them in series, the set will have an output of 12v and 30 amps. If you hook them in parallel, the nominal output would be 60 amps at 6 volts. In the first case, the voltage is added, in the second, the currents are. The time element doesn't change that.

 

In your example/calculations above ("It is a DeWALT 18 Volt 2.6 Ah battery"), 14 cells are in series, producing 18 volts total and supplying 2.6 amp-hours of current. thus each cell should theoretically supply 2.6 amps at ~1.3v for an hour, or perhaps 1.3 amps for 2 hours, (dependent on cell age/history/condition, of course). Maximum possible/allowable current draw isn't stated here.

 

Overcharging NiMH cells seriously damages them so detecting and reacting to that voltage drop is necessary; each cell is an individual and will vary from the others a bit, which complicates things a bit. My understanding is that the same is true of discharge; don't want to run any cells backwards, a possibility when used in series as in flashlights.


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#5
samplerdave

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TooNew2,

 

Thanks for your response and sorry for the delay in answering.

 

What you write seems to agree with what I am thinking.

 

My problem is that of knowing if the Smartcharger is cutting off the charge before it is complete.

 

I have worked out that the charge is complete when the charge voltage just starts to drop. Whether or not the charger is using this as an indication of full charge, or whether it is just cutting off after a pre arranged time I don't know.

 

I am thinking I may have to leave one cell charging at 300mA for a few hours and simply log the voltage at, say, 1 minute intervals.

 

Then examine the results and find if there is a voltage drop, and how long from the start of charge it was.

 

If a voltage drop is detected it shouldn't be too difficult to knock up a suitable detector circuit with a relay to stop the charger when it occurs.

 

Do you concur ?

 

 

Dave.


Edited by samplerdave, 24 September 2018 - 01:27 PM.

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