Large transformer as charger

Batteries, Chargers, and Battery Management Systems.
Post Reply
vax   100 W

100 W
Posts: 214
Joined: Dec 14 2007 11:30am
Location: Keila, Estonia

Large transformer as charger

Post by vax » Sep 29 2014 8:11am

Hello folks
Recently I got a rather bulky transformer (around 5kg of metal), that has following markings:
Primary coil 220V
Secondary coil 67V
8 A

So, I thought, perfect candidate for undestroyable charger, just add diode rectifier and thats it
However, how to control current? I suspect, that empty battery draws significantly more than 8A, maybe even so much that transformer or battery is overheated? Couple of H4 bulbs in series?
Prototype fabrication

Matt Gruber   1 MW

1 MW
Posts: 1816
Joined: Feb 08 2007 11:02am
Location: New Smyrna Beach FL

Re: Large transformer as charger

Post by Matt Gruber » Sep 29 2014 5:02pm

i ordered one of these for such experiments ... EBIDX%3AIT

ARod1993   100 W

100 W
Posts: 190
Joined: Feb 25 2014 12:20am
Location: Cambridge, MA

Re: Large transformer as charger

Post by ARod1993 » Sep 29 2014 8:41pm

The easiest trick for that is to throw a motor start capacitor in series with the 220V primary winding; at 50Hz the capacitor will exhibit a reactance equal to (3183 Ohms/capacitance in uF). If you only want 8 amps coming in the primary side, then you want a reactance around 27.5 ohms; a 100uF motor start capacitor would get you an input current of 7-8A on the primary side; it's cleaner and easier to control than using lightbulbs.

User avatar
Schlafmutze   10 W

10 W
Posts: 71
Joined: Apr 23 2014 4:53pm

Re: Large transformer as charger

Post by Schlafmutze » Sep 30 2014 11:46am

You're going to need something to control the voltage, as rectifying it will make 105VDC.



Though the transformer likely has isolation, so its safer than directly using power from the wall.

Post Reply