# Old Auto Battery Charger

#### vey

Dec 6, 2012
1
I have this old ('80's era?) simple, 2 button type diode, manual auto battery charger that was working fine, but then not so hot. It is supposed to be a 6/12 volt (there is a switch), 6 AMP charger.

I tested the voltage output and saw 11.6VDC, so I knew something was wrong. I opened it up and there was a lot of rust inside. The most rusty section was the plate that had a couple of button diodes attached to it.

Before I investigated everything, I jumped right in and I assumed that a diode was bad and replaced the plate with a silicon bridge rectifier rated with the same voltage and amperage. I just used the + and not the - connection, since that was connected to the center tap.

Same results, exactly 11.6VDC output with a recently calibrated voltmeter.

This thing is about as simple as it gets. Here is a similar diagram of the circuit that is in it: www.circuitstoday.com/simple-battery-charger-circuit except there is no cap.

When the switch is on 6 volts, the output is 7.6VDC volts with no load and I think that is right.

But when the switch is on 12 volts, the reading is 11.6 VDC and inside 26VAC between the hot and neutral on the transformer outputs and 13VAC from the hot to the center tap. That doesn't seem high enough to me, so when the voltage is rectified and voltage loss is considered, it drops below the required amount to charge a battery.

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,180
Without the capacitor your voltmeter will not give correct readings. The output is a recitified sine where the average value is approx. 0.6*Vpeak.
If you connect a battery (or a capacitor), you should see a higher voltage on your voltmeter because the peak voltage is buffered by the battery or capacitor.

13V AC from hot to center is the RMS value. That translates to 13V*1.4=18V peak (all values rounded). That is sure enough to charge a 12 V battery.

Did you measure the current into the battery in the 12V position?

#### Woodman

Dec 3, 2012
47
For what it's worth, when I test transformer secondary AC output voltages, both my chargers show the following:

1. On the 6 volt 6 amp setting 8.6 volts center tapped, and 17.6 not centered tapped.
2. On 12 volt 2 amp setting it is 12.2 and 24.8
3. On 12 volt 6 amp AC voltages are 14.2 and 28.2

A voltmeter was attached to battery and charger was set to the 12 volt 2 amp setting. DC voltages slowly rose from about 12.8 all the way up to 13.6 before I disconnected 1/2 hour later.

Last edited:

#### duke37

Jan 9, 2011
5,364
Looks about right to me. Car alternators are set between 13.8 and 14.4V. A simple charger will overcharge a battery if left to its own devices, just leave it to charge up to 14V.

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