Hacking a Harbor Freight Float Charger

However, they may not be so great after all. I noticed that after leaving one on a battery for a long period of time, it was actually floating the battery at 14.20V. (I have three more of these chargers, and they range in output up to 14.6V) The recommended float level for low pressure lead acid float charge is 2.25V to 2.27V per cell. So most of our lead acid batteries should be floated at 13.5V to 13.62V. http://batteryuniversity.com/learn/a...d_acid_battery

So I cracked open the case, drew out the schematic, and started fiddling. I was able to adjust the float voltage to 13.55V. If you want to do the same, here is how.

First, using an exacto knife, trace around the bottom of the enclosure cap while pressing down. Make a couple of loops around the base of the thing. You'll notice a different feel once you get down to the enclosure.

This resistor, along with 4 others, sets a bias voltage which controls the output voltage of the float charger.
Here is the tricky part. Being a high quality Chinese charger, they used some fairly low tolerance parts.

VR1 is a 39 Ohm resistor. It is in series with a 260 (ish) Ohm resistor. I am somewhat color blind and not able to read the colors on the resistor.

If you short out the 39 Ohm resistor, the output value of the charger approaches 15.25V. I yanked the 39Ohm resistor out. Not wanting to replace 5 resistors with 1% metal film, and being impatient, I inserted a 1k pot with a 100Ohm fixed resistor in parallel. I preset the thing at about 60 Ohms, put a voltmeter on the output, and started tweaking. I found that if I replaced VR1 with a 64.9 Ohm (standard value for 1%), I got 13.55V output. That's acceptable.

Also, I noticed that as the charger warms up, the output voltage slowly drifts lower. After 15 minutes of operation, I am at 13.53V

Much better.

P.S. I can't use the term "good deal" without thinking of my oldest daughter. When she was little, maybe 5 or so, a neighbor lady brought over a bunch of cantelopes that she had bought at a roadside stand for $0.10 each. They were a little over-ripe and tasted a little funky.

We ate them for dessert, and my daughter asked "why did Ms. Kathy bring over so many cantelopes?"..... I was in a conversation with my wife and answered quickly "Becasue she got a good deal on them". I went back to talking with my wife when I noticed that my daughter had a really puzzled look on her face, spoon aloft above the cantelope... I asked her what was wrong and she replied "How do you get a good deal on a bad cantelope?"

So resistance values varied quite a bit...

For my four chargers, I measured 59, 64.9, 73.2, 73.2 Ohms to get me to that 13.56V number.

Here is a link to standard resistor values