Automotive Battery Charger Diagnostics & Problems Help

Discussion in 'The Projects Forum' started by a._grofield, Feb 23, 2014.

  1. a._grofield

    Thread Starter New Member

    Aug 7, 2013
    I have an older roll around Sears Automotive Battery Charger Engine Starter 6/12volt, 40amp with 225amp start feature. It is model 934718460 The settings are 6volt slow setting, 6volt fast setting which shares its position with 12 volt slow, then there is a 12 volt mediuim, and a 12 volt engine start. And, of course a timer, and the ammeter.

    This one was made by Hitachi as best I can tell, for branding as a Craftsman/Sears.

    If you hook up the leads to a battery, the ammeter on the machine registers absolutely nothing. I figured I'd check to see if the ammeter had just given up the ghost.

    So, I hooked up my trusty Extech 430 multimeter to the leads to see that there is absolutely no 6volt or 12 volt DC(actually extremely tiny and inconsistent bouncing .0000 mv readings). However, accidently hitting the range button produced a strange discovery.

    *6 volt low reads- 7 volts AC output
    *6 Volt start-12 volt low(this is a combination setting) low puts out 12.9 volts AC output
    *12 volt medium reads 14volt AC output
    *12 volt start reads 17-18.x AC Output

    What would cause a battery charger that takes in 120v AC and transforms it to 6 and 12 volt DC, to put out AC at the battery charger leads ?

    What kind of numbers should I be seeing at the rectifier on this machine- ie voltage and AC or DC ?

    Can anyone offer me any diagnostic procedures to execute from here ?

    Finally, if it turns out to be the rectifier, the OEM rectifier is long discontinued. I am not afraid of soldering and reconnoitering- is there a chance I could source a substitute from digikey or the like ?

    Thank you all.
  2. tcmtech

    Well-Known Member

    Nov 4, 2013
    You can replace the stock diodes with most any stud or block diode set without problems. Just make sure the new set adds up at least 2X the chargers peak amp rating. I have done it to many battery chargers just like yours and it's an easy conversion.

    The other thing to look at is your DC side circuit breakers. They tend to go bad and cause odd problems as well.
    a._grofield likes this.
  3. burger2227


    Feb 3, 2014
    I would check the rectifiers before just replacing them.
    a._grofield likes this.
  4. tcmtech

    Well-Known Member

    Nov 4, 2013
    I really would not be surprised if the whole rectifier assy is made up of cheap button diodes held in by metal clips or a disk attached to plastic anchor points all on a flimsy aluminum strap or sheet for a heatsink and half or more are blown to bits or have fallen out.

    When I rebuild those types of battery chargers I tend to replace the whole rectifier assy with a pair of welder diodes mounted to a real and adequately sized heat sink. ;)
    a._grofield likes this.
  5. t06afre

    AAC Fanatic!

    May 11, 2009
    If you have the specs for your current rectifier. I am sure you can find something with the same specs that will fit in your charger. At least as long as it is not a selenium rectifier. They were quite common in older auto battery chargers.
    Last edited: Feb 23, 2014
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  6. MaxHeadRoom


    Jul 18, 2013
    I'm suspecting you are seeing a normal AC component maybe, I would be very surprised if there were a nice steady DC out.
    250amp semi's can be expensive.
    a._grofield likes this.
  7. crutschow


    Mar 14, 2008
    I believe battery chargers use little or no filtering on the rectifier outputs so, without a battery connected, the meter will see the rectified sine-wave, which will register as an AC voltage.
    a._grofield likes this.
  8. wayneh


    Sep 9, 2010
    Ditto the previous two replies. I don't see solid any evidence that anything is wrong, except for the unreliable button switch issue noted by the OP.

    You should see a DC voltage when things are working properly, although it might require a battery to be attached. (My charger leads are dead until it sees a voltage on the leads.) You will also see an AC voltage because of the unfiltered ripple that will more-or-less disappear once a battery is attached.

    If you want to play around, you could put a diode in series with your meter probe, and an electrolytic capacitor between your probe leads. This forms a peak detector. It should give a DC voltage reading somewhat higher than you see without it. I say "somewhat" because I can't recall the exact value (I guess it's 1.41X minus the diode drop), and it will depend a bit on what diode you use. If the output is true AC, you will see a much larger increase in DC voltage on your peak detector.
    a._grofield likes this.
  9. a._grofield

    Thread Starter New Member

    Aug 7, 2013
    After doing some more checking, the charger ceased to produce any output to a battery. The transformer was still putting out, but nothing beyond the rectifier.

    This one has just the disc type diodes with the aluminum sheet for heat sink for a "rectifier".

    I have checked the rectifier assembly, and it is gone.

    This charger is rated at 40 amps charging , 225 amp start at 7.2vdc for 7 seconds on, then 100seconds off.

    Can anyone suggest an appropriate rectifier(or pair) that I can get from an electronics supply ? I don't know what kind of PIV rating or current rating I should be looking at for ordering a replacement. If I could use a silicon bridge rectifier, or pair of them, that would be absolutely fantastic.

    I have checked over the rectifier I took off and there is NO writing still visible. You can see faint etching where there has been a P/N or spec, but it is gone. Sears has no idea what the specs are, their PN for it is 865-366-000, and they discontinued it with no replacement in the mid-90's.

    Thank you for all your help.
    Last edited: Feb 26, 2014
  10. inwo

    Well-Known Member

    Nov 7, 2013