26984 Total Charge III voltage has me stumped

[JohnnieB]

Let me explain the control board first. The only thing it controls is the relay that you have bypassed. Everything that looks, feels or smells like a battery charger is in the transformer, capacitor and diodes alone. The control board is simply a smart (semi-intelligent at most) on/off switch.

There are two aspects of ferroresonant transformer circuits, Constant Voltage and Current Current (Current limiting actually).

The capacitor boosts the secondary voltage of the transformer until the core saturates, but the battery pack voltage determines the output voltage, so the capacitor boosts the current in the secondary until the core saturates. As the pack voltage increases, the core saturates at a lower current flow.

If the right capacitor and transformer are picked, the max current flow will be some fixed amount, about 20A in the case of a typical 36V golf cart battery charger, but could be higher or lower. IE: You can find 25A and 30A and 50A chargers for industrial applications like forklifts.

Pick the right values and you get a charge profile like the on attached.
Basically, the output is currentlimited until the pack voltage reaches about 42.3V (2.35VPC) and then starts tapering off as the pack voltage climbs and reaches a minimum in the 2 to 5 amp range as the pack voltage climbs into the 44V to 50V range.

Here is where the smart switch comes in (aka Control board). It shuts of the AC power to the transformer when the pack voltage climbs into the 44V to 46V range. A FLA battery isn't fully charged until the voltage stops climbing, but the technology in that era wasn't capable of doing dV/dT (Change in Voltage over Change in Time) calculations cost effectively, so a fixed voltage that would almost fully charge most batteries without overcharging weak batteries was selected.

FWIW: The main reason the charger won't turn on if the pack voltage is too low is because it is the pack voltage that energizes the relay initially.

The third function of the control board is a safety timer. If the pack voltage does not reach the preselected shutoff voltage in a preselected number of hours (12 or 16 typically), the relay is de-energized.

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Try the charger with the 20 µfd capacitor connected to a battery pack.
The initial current should be upwards of 20A.
It should start tapering off when the pack voltage climbs above about 42V and drop down to about 5A or less as it goes above 44V.

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I think we have been spinning our wheels looking at the open circuit DC voltage.
The only spec I've been able to find is the OC VAC going to the diodes is supposed to be 2.5 times the nominal voltage of the battery pack (2.5 x 36 = 90) and we are getting that.

The light loading (125 ohms) appears to be is kicking the voltage above the 44V to 46V shutoff voltage, so it ought to work.

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P-Dave how did you get the Ohms symbol? Upper case Omega isn't in the ASCII code I use.

[radioman]

I think the best test is to connect charger to a golf cart and observe what happens when you try different capacitors if you are unsure of the correct value.

[Pirate Dave]

Thanks for the explanation; makes sense now. Guess I've been chasing my tail a bit. Wife divorced me once for that, then again she usually does at least once a day
I'm working on the sailboat all day tomorrow; will see if we can hook it up on Sunday with the 20uf cap and post results.
The ohm symbol was just luck; I put my readings in a word document for future reference and when I cut and pasted it the symbol came with. Sometimes it's better to be luck than smart. Let's see if this works: µF.

[radioman]

I was just thinking that charger would make a good boat anchor if the capacitors don't fix it.

[Pirate Dave]

I'm out of town on business for the next 10 days and won't be able to get with my friend. Will post when I have results with cart. Thanks again.

[radioman]

We needed a break anyway.

[JRaf]

Six years later but what a great thread!