10amp 6/12v charger has low voltage output in 12v range

atlarge54 · Sep 28, 2018

I've had this charger quite some time and it just started putting out 11.6v when on 12 setting, 7.8v on 6v setting.

I removed the cover to check connections and nothing looked too bad that I could see. Thinking the range switch might be bad I bypassed and connected direct with jumpers. Output voltages remained the same.

Probably not worth fixing but thought I check here for a possible easy fix.

MarkB_MI · Sep 28, 2018

Most likely failure mode is a blown diode. The other diode will continue to work, but you'll get current out only half the cycle. Eventually the good diode will blow as well.

Janicholson · Sep 28, 2018

If a transformer type, the secondary probably has a short between several windings. Each short reduces the voltage at the output,
and probably only affects 1/2 of the total output because there are center tapped secondary with one side failing. I would take
the transformer out and scrap separate from the tin. Using it can cause flame and smoke to be emitted. Jim

circus · Sep 28, 2018

atlarge54 "Most likely failure mode is a blown diode."

I have an old Schumacher with one of the two unobtainable diodes blown. Couldn't I just replace it with a larger diode from a newer alternator? Battery lead come directly from the diodes.

Russ from MN · Sep 28, 2018

Yes I have replaced the rectifier with a diode, but then they don't seem to taper off, full charge all the time. Maybe someone can explain why.

Dean · Sep 28, 2018

I agree with MarkB, below.

Carefully check the connections to the diodes. Some older chargers have puck type diodes clamped between fiberglass insulators with metal contacts. The fiberglass takes a bent set over time and the connections loosen. Inspect.

Dean

MarkB_MI · Sep 29, 2018

> Yes I have replaced the rectifier with a diode, but then they don't seem to taper off, full charge all the time. Maybe someone can explain why.

If you're replacing a selenium rectifier in an older (1960s) charger with a silicon diode, they have different electrical characteristics. The selenium diode has greater forward resistance and voltage drop.

CCer · Sep 30, 2018

Once I retrieved a quality brand charger, from a $crap pile. I opened it up, Cleaned and tightened all the bolt/screw
connections inside. Works FINE since! YMMV.

Geo-TH,In · Sep 30, 2018

Asking what's wrong with a charger is like calling Dr and asking
him what's wrong with me, I'm sick.
I've seen bad brain boxes on auto shutoff chargers. Bad circuit
breakers, bad circuit boards, Blown diodes, bad switches, bad
transformers, bad power cords, burnt off connectors inside or
rusty connectors because Mr Wilson leaves them outside in the
rain and wants me to repair them.

Some chargers can be fixed and some are throwaways.

LarrySTN · Oct 2, 2018

A selenium rectifier typically has 1.2 volt drop where as a typical silicone rectifier (diode) has .6 volt drop. This will make your charger want to over charge your battery.

10amp 6/12v charger has low voltage output in 12v range

atlarge54

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MarkB_MI

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Janicholson

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circus

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Russ from MN

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Dean

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MarkB_MI

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CCer

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Geo-TH,In

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LarrySTN

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