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Old 10-09-2013, 10:59 AM   #1
rlw
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Default Variac float charger?

This is probably a question for JohnnieB.

I've got an old Variac sitting in the basement (bequeathed to me by my dear departed father), a pile of old bridge rectifiers from pre-civil-war computer equipment, and 6 new (purchased a month-and-a-half ago) US1800CX batteries.

My charger is one of the old timer-based ones with a ferro-resonant transformer and a half-wave rectifier. I'm not too sure how well it will work this winter to keep a float charge up and running.

I'm thinking I can rig something up using the variac and rectifryers to set the correct voltage, and a wall timer to turn the thing on every day for some as-yet-unspecified amount of time.

Any suggestions on how to proceed? Should I go with a full-wave or half-wave? What's the correct voltage for a float charge for these batteries? Is there a rule of thumb I can use for the daily charge time?

If it all works out, I'll whip up a diagram for anyone else who has this kind of stuff sitting around.

There's a reason I never throw anything away (and my wife says I have "hoarder issues" - BAH!)

RLW
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Old 10-09-2013, 11:12 AM   #2
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Default Re: Variac float charger?

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Old 10-09-2013, 11:59 AM   #3
yurtle
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Default Re: Variac float charger?

I have a "few" of those lying around my garage, the biggest one weighs over 100 pounds.

I think a better way would be to utilize the sensing wire of the buggy on a timer to "unplug and replug" in the buggy every week. If you have a timer that has "dry contacts", you would simply unplug and reconnect the sense wire. If not, a relay could be used to do this.

EDIT: I'm assuming your charger senses the pack voltage.
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Old 10-09-2013, 03:49 PM   #4
rlw
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Default Re: Variac float charger?

Yurtle,

Thanks for the reply, but my charger *doesn't* have a sense line - it's an old ferro-resonant "dumb" charger.

I was thinking I could set the variac to 40-umpty volts (I need to know what "umpty" should be), monitor the DC past the rectifier and come up with a rule of thumb for how long each day to float charge. Actually, it occurs to me that I may have my terminology wrong (is "float charge" what I'm looking for?). What I want to do is automate keeping a maintenance charge on my batteries over the winter so they don't deteriorate.

RLW
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Old 10-09-2013, 04:00 PM   #5
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Default Re: Variac float charger?

Yes, float and trickle are different. I don't understand ferro-resonant circuits (prolly the third time I've said that this week). The only one I've played with besides chargers is a power conditioner, which allowed CRAZY +/- variations on the input, but would output rock solid 120 volts output. If I still had my old PowerWise, I'd go play with it, but my DPI is kinda a "hybrid", so I ain't gonna risk it.

You're right, JohnnyB is your best hope.
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Old 10-09-2013, 04:01 PM   #6
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Default Re: Variac float charger?

HEY! If you have a truly dumb charger, couldn't you just initiate a 6 - 12 hour charge cycle each week? I'm assuming you won't be riding it.
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Old 10-09-2013, 04:10 PM   #7
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Default Re: Variac float charger?

One thing you could do is find out the voltage your 36 volt pack charger "should" turn off when it achieves. I have 48 volt, so don't remember the 36 volt "number", but if your charger get's 'er done in 4 hours, just set the timer for that. Tweak monthly.

As I understand ferro-resonant chargers, they aren't likely to "overcharge" a pack. Their current drops off as voltage climbs, and an old school Powerwise charger will time-out somewhere between 12 and 16 hours, so someone decided that's a safe number.
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Old 10-09-2013, 04:12 PM   #8
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Default Re: Variac float charger?

Just in case, post the model number of your charger. As I understand it, some of the old "automatic" chargers would have a timer dial that showed you how many hour it took to charge, but would still turn off automatically. Worst case, the timer ran out.
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Old 10-09-2013, 06:28 PM   #9
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Default Re: Variac float charger?

A Float charger is a constant voltage source, meaning the output voltage stays the same as the output current varies or the AC line voltage varies. (Note: Some devices sold as Float chargers are voltage limiters, rather than true constant voltage devices.)

A Trickle charger is a constant current source, meaning the output current (Amps) stays the same when the output voltage varies or the AC line voltage varies. (Note: Some devices sold as Trickle chargers are current limiters, rather than true constant current devices.)

A properly selected float charger holds the pack voltage above the 100% SoC At-Rest voltage, but below the On-Charge voltages at which major gassing occurs.
My 36V DPI Accusense charger floats at 39.8V in the Us Battery mode, 39.6V in the Exide mode and 39.4V in the Trojan mode.

A properly selected Trickle charger holds or limits the Amp rate to a specific value, typically 1% to 3% of AH rating. This may allow the battery pack's On-Charge voltage to climb into the gassing range.

I don't believe the rectified output of a variac would make a very good battery maintainer since the output voltage will vary as in AC line voltage varies and the output current will vary pretty wildly is a small variance in output voltage. Some form of output voltage or output current regulation is needed.
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Old 10-10-2013, 12:16 PM   #10
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Default Re: Variac float charger?

JohnnieB,

Thanks for the response. Maybe what I'm looking for is a trickle charger schematic. The reason for the variac was to be able to set the voltage, but I guess that doesn't matter. For a constant voltage, I *do* have an old Sola laying in a pile somewhere ;^)

RLW

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