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Old 3rd Jun 2020, 3:04 pm   #1
elanman99
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Default PAG Charger info wanted

This is a probably a long shot but just wondering if anyone here has any experience with PAG products?


I have had this charger for about 20 years and have used it occasionally to charge two 12 volt NiCad packs (10 'D' cells ea) in series. I wondered whether I might be able to modify its circuit so that it could natively charge a single 12V pack? Its microprocessor controlled etc but I was vaguely hoping it might be possible to modify its analogue reference or whatever but I have been unable to find a circuit diagram or anything else for that matter.


Ian
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Old 3rd Jun 2020, 3:11 pm   #2
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Default Re: PAG Charger info wanted

With all those cooling slots it looks a bit analogue to me, probably not in that case.
 
Old 3rd Jun 2020, 4:55 pm   #3
broadgage
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Default Re: PAG Charger info wanted

Fast chargers for NiCad batteries are surprisingly sophisticated and often can not be readily modified.

If you need to fast charge a single 12 volt NiCad pack, then I would purchase a dedicated charger for this purpose.

If slow charging is acceptable, then a crude improvised circuit is fine.
All common NiCad batteries can be slow charged at about 10% of their rated capacity for about 14 hours.
For a 12 volt, 4 ampere hour battery I would use a smoothed DC supply of about 24/25 volts.
Connect a 12 volt lamp in series with the battery so as to limit the charging current to 400ma, continue for 14 hours.
Several pilot lamps in parallel may be used to get a charging current of 400ma.

A smoothed DC supply is not truly essential but does simplify matters.
If rectified AC is used, then the brilliance of the lamp and the reading of most ammeters will be in proportion to the RMS current, but the degree of charging will be related to the average current.
With smooth DC average and RMS are the same.
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Old 3rd Jun 2020, 5:01 pm   #4
elanman99
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Default Re: PAG Charger info wanted

It has a switchmode type of PSU and a 68705 processor, what I meant by analogue circuitry was really whether it had a voltage reference or other device/circuit so that I could modify its operating points to suit a battery that was half of the original design target. I cannot spot anything on the board that looks like a Zener or Vref IC.

Picture of board attached.

I thought it was worth asking here in case someone might have worked on PAG equipment in the past.

Ian

Thanks Broadgage, That is pretty much what I expected and I knew it was a longshot.
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Old 3rd Jun 2020, 10:48 pm   #5
broadgage
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Default Re: PAG Charger info wanted

Fast chargers for NiCad batteries are usually constant current, not constant voltage. Many types can charge batteries of different voltages, within certain limits.
The charger pictured in the O/P would seem to be designed for batteries from 20 cells/24 volts, up to 26 cells/31.2 volts.
As a NiCad battery charges, the voltage increases initially but when nearly full there is a brief and fairly small decrease in the battery voltage.
This drop in voltage is detected by the charger which then terminates the fast charge, often then continuing a slow or trickle charge, either for a set time or indefinitely.

Known as "negative delta V" control.
Some fairly clever electronics is involved, not readily modified. A brief dip in the mains voltage will briefly stop the charging current, and the battery voltage will then fall a bit due to the temporary absence of voltage drop in the connections.
A good charger will "know" the difference between a drop in battery voltage caused by full charge, and one caused by mains voltage dips.

An alternative and earlier way of detecting full charge was by monitoring the battery temperature. Popular in first generation cordless power tools.
Worked fairly well at room temperature but tended to overcharge in very cold conditions and to not fully charge in very warm conditions.
Also required that each battery be equipped with a compatible temperature sensor, OR that the battery be physically inserted into the charger in order that the charger could directly measure the battery temperature.
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