Friday, March 2, 2012

Battery Charging Part 2 (Harbor Freight item 42292)

Part 1 of this project is here:

I've worked out my modifications to the little charger. Here is the schematic I ended up with:

I used two 100 ohm resistors to keep from hitting the supply rails on each end.  These aren't strictly necessary, but I had them handy and it seemed a little safer.  The diode on the output is to prevent the battery from discharging if the power is down for a long time; like I kicked the plug out of the wall.  It really isn't a 1N4001, it's some generic 1A 400V power supply diode that I had around.  The pot is 1000 ohm 25 turn and cost about a buck or so off ebay.  It allows me to set the voltage precisely.

Here is a picture of the way I hooked it up:

This will give me the float voltage that I want and won't discharge a battery if the power dies.  That makes it reasonable to put on a solar cell for things that don't have power near them.  A tractor parked out in the back somewhere, a quad setting next to a tent on a hunting trip; things like that.  Notice I got rid of the wires with the clamps on them.  I put a plug on the end and can now just plug it into the vehicle.  Improvements could be made like adding a volt meter to it to tell how it's doing and another light that says everything's OK, but those are for the future.  Now all I have to do is put it back in the case and label it with the voltage I set it to and let it do its job.


  1. I had one of these on my emergency generator. I was a little afraid of it given some of the terrible reviews. But it seemed to work OK for several months. So I bought another with the idea of modifying it to be used on my antique car 6 volt battery. I opened it up and created a schematic yesterday.
    I decided to check on the original unit, and found the LED was out.
    We had a wicked lightening storm early this morning. Had a strike very near us and I heard a loud snap like a high voltage arc.
    I disconnected it from the battery and open circuit the unit now outputs 19 volts! This would surely cook the battery. I am glad I checked on it.
    Any thoughts on what/how it failed this way?
    I guess I will return it and get another.
    Any thoughts on how to modify it for a 6 volt battery?

  2. If you scout around this blog I came up with a better solution that uses the wall wart and little else of the original charger. is the link, but sometimes they don't work in comments.

    You can definitely adjust the voltage on this charger by adding the little variable resistor I show above, but the problems don't end there. The short-circuit protection will cause you troubles and I don't trust the regulator itself to handle the voltage properly if your battery ages and the internal resistance goes up. The little LM317 I used in the revision is a nice little device and will protect itself quite nicely. You can get them for a couple of bucks on the web and then juggle the resistors a little bit to get any charge voltage you need.

    As for the failure you observed there are a couple of possibilities: most likely, the LM7805 died. These things are good devices, but they are not designed to work optimally as a variable regulator. Floating it above ground causes it to have a number of problems that need special circuitry to overcome. That's why they made the LM317 device. The other possibility is that the wall wart itself went out. That has not happened to me, but it certainly is possible. A sharp spike on the incoming current can fry just about anything and could have taken the wall wart out. My bet is the the LM7805 is dead.

    I have four of the revised chargers I show in the link above. They haven't been in service very long, but they have worked really well so far. They don't have a problem with short circuits, reverse polarity or anything else. The only disadvantage so far is I didn't put an LED on them so I don't have a visual indication that it is working. I'll eventually get around to doing something about that.

    The really nice thing is they don't boil the water out of the battery. I haven't had to add a drop since I started using them.

  3. Awesome project this is really great when especially if you like DIY projects.