Battery charger issue.....

[427LX]

Have my old Sears Heavy Duty charger/booster and it has worked well for over 25 years. However now when you hook it to a battery for a charge the voltage jumps
to 15-16 volts measured at the battery while charging. Of course I shut it down so as not to overcharge the battery. Battery was at 12.5 before any charge.
Output at the charger leads with no battery hooked up is 13.7 when in the slow charge selection. Charger never used to act this way.

[1964Fastback]

I don't have direct experience but maybe see if you can find a schematic for your particular model online.  I looked at a few but didn't know the model.  Anyway, doesn't seem to be too much to them in the way of electronics.  I didn't see a voltage regulator chip, for example.  Basically just switches for the charging rates (connecting to different taps on the transformer), volt or amp meters, transformer and wires.  It does look like there is a rectifier to convert from AC to DC, like on an alternator.  I don't know if these are a single component or built out of 4 diodes.  But that might be your problem.

Pat

[427LX]

Thanks Pat for your reply.  I found a site that has replacement parts for some of the Sears units and all the electrical parts like diodes and such are discontinued. Didn't see anything about a regulator or even transformer.   Most likely will just buy a new charger.

[RustyCrankshaft]

Unlikely that charger has any discrete components that are actually out of production with no cross reference available. I'm sure most/all of the Sears replacement part numbers show discontinued. You can figure out what components you need to fix it and just order them by size from someplace like DigiKey or Mouser. I've fixed several of my old chargers that way as the new import built stuff is garbage and normally only lasts a few years, if even that long.

[427LX]

Well brought the thing in the house where it is warm and took cover off. What little is in there looked fine and then plugged it in and took voltage checks at 6 Volt Fast
12 Volt Slow     12 Volt Fast and 12 Volt Boost.   This is a 40 Amp/ 225 Boost Start charger and the voltage output to my meter was   8.45     13.0     14.4  and 17.0 on the Boost Start all with
only the voltmeter as the load.
Then I set up a large electric cooling fan from a Ford vehicle that pulls around 30 amps. Checked the voltage with fan running on the 12 Volt Slow charge setting and got 10 volts and 12 on the 12 volt Fast setting and a reading of 32 Amps on the Amp meter. Voltage showing a bit low due to my tap wires not quite heavy enough guage.

So looks like the charger is working fine now.  The battery in my Mustang is at 12.62 right now so it is in fine shape.