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Old Nov 29, 2004, 02:39 AM
hul
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modified Polyquest PCM guard

didn't like these "features" of my pcm guard:
- it automatically resumed charging after a high cell voltage shut down
- the guard's FET is rated for 30V, not high enough for the voltage spike my charger produces when the guard decides to interrupt the charge

I now use the pcm guard to de-energize a relay on cell overvoltage.
That solved both my problems. See pictures for details.

New components:
- 12V relay K1 with 2 normally open contacts
- switch S1 (push button, normally open)

Hans
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Old Nov 29, 2004, 02:46 AM
hul
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forgot diode D1 (1A) over the coil of the relay (just visible in the last picture).

Hans
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Old Nov 29, 2004, 07:14 AM
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Warwick, Qld, Australia
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I would suggest adding a transzorb across the PCM guard to protect against the initial spike if you unplug the battery while the charger is still running.
The Seiko S-8244AAFFN-CEF-T2 battery protection IC has a small intentional delay before tripping. The relay will also take a while to trip (especially with a diode across the coil).
The transzorb will need to be able to dissipate the current until the relay drops out.

The relay will also have to have a reasonably high coil resistance. My charger does not terminate the charge until the current has dropped to 100mA (charge rate set to 1.8A).
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Old Nov 29, 2004, 05:36 PM
hul
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the spike happens on the charger side after the relay contacts open, the pcm guard won't see it. Voltage should stay normal as long as the contacts are closed.
Should call the spike a voltage excursion, it's about 2.5 seconds long and up to 65V in my case, see picture (52.8V+12.6V pack voltage). I tried a 1W zener diode to suppress the spike; blew the diode. Never heard of a Transzorb, but I doubt if they can dissipate enough energy to survive, looking at their size.
The 40mA relay coil current is out of the lipo pack. You're correct, the charger won't terminate on small packs. The relay could be supplied by the car battery, just more wires to worry about.

This is clearly not a perfect solution, but it gets around the pcm guard's shortcomings (voltage rating too low for my charger and the philosophical point of me wanting to have some input in the process before charging continues) without having the start from scratch.

Hans
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Old Nov 29, 2004, 09:14 PM
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Warwick, Qld, Australia
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I said " if you unplug the battery while the charger is still running".
If this occurs the spike will occur before the relay opens.

As regards the transzorb, this is a proprietry name. "Transient voltage suppressor" is probably better.

A suitable part number would be 1.5KE18A. For this part to survive it will be relying on the relay tripping fairly quickly on overvoltage.
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Last edited by dkselw; Nov 29, 2004 at 10:10 PM. Reason: Typo
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Old Nov 29, 2004, 09:33 PM
hul
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sorry, I misread that.
I thought you meant unplugging the lipo pack and pcm guard as a unit from the charger as per the manufacturer's instructions.

I changed my procedures a while ago after reports of damage to chargers and PCM guards. I now remove the charger's supply if I need to interrupt a charge half way through. That definitely prevents voltage spikes.

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