[Coco] OT: Battery chargers

[gene heskett]

On Sunday, October 16, 2011 06:43:04 AM Stephen H. Fischer did opine:

> Hi,
> 
> Sorry, my mind works slowly these days.
> 
> 
> Gene, I second your comment!!!
> 
> >> http://www.newegg.com/Product/Product.aspx?Item=N82E16817355042
> > 
> > No where near smart enough to charge an NiMh battery correctly. No
> > thermal controls, possible a flat top detector though, meaning that
> > it expects the voltage to rise slowly, and shuts off when the voltage
> > no longer rises between sample periods.
> 
> It may charge 9V NiMH rechargeable that are the ones smaller than a
> match box with snap connectors, but being used for eight (8) 1400mAh
> NiMH cells, NOT!!!
> 
> The battery is being charged very little.
> 
Doing the math, this charger does 70 to 90 milliamps, so to fully charge a 
battery of 1400 mah would take 16 to 24 hours.  That low a charge rate 
encourages the growth of nickle whiskers, eventually causing the cell to 
short well before its rated 500 or so cycles.  For best battery life, most 
of these Nickle-whatever batteries need to be charged at the highest rate 
that the cell can tolerate without developing excessive internal gas 
pressure.  They WILL get warm at those rates.
> -------------------------------
> 
> http://www.all-battery.com/smartuniversalchargerfornimhnicdbatterypack-1
> 2withchargingcurrentselectiontemperaturesensor.aspx
> 
> Might work, but 1400 x 8 = 11200.
> 
Wrong logic.  They are in series, so the whole pack is 1400 mah at about 
9.6 volts.

This charger should, set at 1 amp, do the job nicely in under two hours.  
Its method resembles the Christy chargers, so I would assume the patent has 
now expired, or that its an under the radar patent violation.

"Use pulse and negative pulse technology to avoid battery overheating 
during fast charging, optimized for battery pack's cycle life" is straight 
out of the Christy brochure.

I might be tempted to set it at 2 amps unless the cell propaganda says no. 
You would get more cycles out of the pack before it needs re-celled due to 
a shorted cell.  The faster the charge, the slower the whisker growth.

"Automatically charging ending with negative delta V"

This is its temperature detection, also done by Christy.  When these 
batteries are fully charged, the overcharge turns into heat, and the heat 
decreases the terminal voltage.  This is how it shuts off, by detecting 
this millivolt or 2 drop in the batteries voltage.  This has 2 effects, one 
being that any one cell reaching full charge and starting to warm shuts it 
all off, and two, if the batteries are within 5% of each others capacity, 
that slight surplus will tend to bring the other cells pretty close to 
fully charged also, helping to maintain the cell matching.  It is also more 
sensitive to end of charge than letting the whole pack hit 135-140F before 
shutting down.  That shortens battery life.

Reads almost exactly like the even bigger Christy unit that is sold to us 
broadcasters for $3k and up, and which can charger a 12v, 6Ah news camera 
battery in 20 minutes.

The one thing a Christy could often do that this one can't, is detect a 
shorted cell, and give the pack a few fraction of a second bangs at 50 to 
100 amps in an attempt to blow the 'whisker' and return the cell to 
usability.

> .  Charging Current 1.8A for battery packs with capacity above 2000mAh.
> For 3000mAh pack, charging time is 1.5 hours.

But it is not an 11,200 mah pack.  Its a 1400 mah pack.

Secondarily, I don't think the temp cutoff is a good idea with that NiMH 
chemistry, the usual 135F cutoff is I believe too high.
> 
> 11200 is ~ 3.7 times 3000.

Again, wrong math.
 
> It does have a temperature sensor.
> 
By detecting the millivolt range drop in voltage as the battery starts to 
heat, a much better method than the thermal switch in the battery pack.
> ---------------------------------
> 
> MH-C777 Plus II Intelligent Charger & Analyzer
> 
> http://www.mahaenergy.com/download/pdf/mh-c777plus_manual.pdf
> 
> No longer being made and was $95.
> 
> (Used by blog author)
> 
> ----------------------------------------
> 
> SHF
> 
> 
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Cheers, Gene
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