Purpose of Tertiary Coil/Capacitor in Battery Charger

Hard to say from your description but it might be to reset the transformer core to reduce saturation by storing the energy in the tank circuit during reset.

Sounds reasonable...The secondary in all cases where I have encountered this is connected to either a half-wave or a full Wheatstone Bridge rectifier circuit. Generally, the circuits are designed for relatively high charging currents- 20 A or so, tapering off as the target battery (or batteries) come up to full charge. Saturation might be a problem only when the system is first initiated- which would explain why my o'scope shows no effect when the circuit is opened...

This is a form of filtering, . . . As a typical charger (without a L-C filter) will draw "current pulses", this filter will reduce peak current from the line - as some of the current will come from the capacitor.

Thank you, everyone. I have a pretty good feel for what this thing does. Seems like it is not expected to have much noticeable effect on the circuit under "normal" conditions which is why I measure no effect what so ever when I open the circuit...

I think it is not just battery charger; It may be an UPS system with built-in battery charger; please confirm. Any schematic?

I have two examples in my shop at present- one is most definitely a battery charger- intended for charging golf cart batteries at 36 Volts. This charger uses a half-wave rectifier and includes circuitry to shut the charger down when the battery reaches full charge. I have looked at several of these particular chargers, and in some cases, someone else has previously removed the control circuitry, and the chargers still function.

The other is an older "smart charger" that apparently provides multi-stage charging and appears to have been intended for marine use for maintaining battery charge for extended periods. This unit uses a full-wave rectifier (suggesting it may have been intended as a power supply as well), and has significant circuitry for controlling the charge rate, with a "manual" over-ride switch. This unit uses a full-wave rectifier and has a rated output of 20 A at 24 VDC- it may well have been intended to function as a power supply, but not a UPS.

Both consist of a stepdown transformer fed by 120 V 60 Hz feeding directly to the rectifiers. Pretty simple stuff, until one gets into the control circuitry.

Both seem to function with the tertiary coil/capacitor loop disconnected. No electrical connection from the tertiary coil to the other two windings in the transformer (inductive coupling only- all three windings are on the same iron core). No connection to ground on the tertiary circuit (at least, by the time they reach me). This sub-circuit appears to be nothing more than just a capacitor and inductor in an isolated loop.

No, no circuit diagrams are available- most of this stuff was built long before people were posting wiring diagrams on the Internet, and most of it is too old to expect any sort of manual to be available...

I think its an automatic "float" charger. One with tightly regulated voltage which can be permanently connected to a battery without overcharging it. I have heard this ferro-resonant system originated about 1935 with copper oxide or selenium plate rectifiers - The first "solid-state" battery float chargers! The transformer provides a good constant output voltage AC supply - provided the mains frequency is steady - I think frequency changes had proportional effect on voltage.

I think Mr.Parvathaneni is correct. I have seen one such set up in Toyota make pallet truck and the charger is fitted to the truck itself. The tertiary coil & cap if disconnected there will not any control on charging function. GA to Mr.Parvathaneni.