So I got a "good deal" on some Harbor Freight Centech battery chargers... I picked them up for $4.99 ea.
http://www.harborfreight.com/automatic-battery-float-charger-42292.html
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/article/charging_the_lead_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.
(https://i133.photobucket.com/albums/q46/nm_longshot/misc/IMG_0570.jpg)
Once you have scribed around a couple of times, take a sharp, small screwdriver. With as much surgical precision as possible, pound that sucker into the crease you just opened up ;D and pry the bottom off.
(https://i133.photobucket.com/albums/q46/nm_longshot/misc/IMG_0571.jpg)
Pull off the cap, then pull / pry out the circuit board. Bend the heatsink up to reveal the components beneath.
You are looking for a Resistor titled VR1. No doubt for voltage regulator 1, but i'm speculating.
(https://i133.photobucket.com/albums/q46/nm_longshot/misc/IMG_0573.jpg)
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...
http://www.rfcafe.com/references/electrical/resistor-values.htm
Thanks! Now that really helps.
Bruce
I have one on my battery now. The initial charge is keeping the module very warm, which is depressing the float voltage a little. It will be interesting to see what this settles out to once the battery starts to float.
WOW!! He lost me when I got to "inserted a 1k pot " that would be like 2.2 pounds of weed............. ???
Okay I'll retract that statement.....................
I'm satisfied... after trickle charging overnight, and a full 24 hours later, the battery is now floating at 13.46V, and I suspect it will come up just a tad more.
Rick you crack me up. Sorry, I meant "potentiometer". 1000 Ohm variable resistor at Radio Shack, in parallel with a 100 Ohm resistor. I wanted a 100 Ohm variable resistor, but the lowest they had was 1k so I cheated.
This got me to thinking about designing my own charger, that I could run off of my heavy 10A bulk charger that I use to get an initial recharge on my batteries. I'm not real happy with the technology that is available at the low end range. Even my 6A SunSaver by Morningstar allows the battery to over charge, which is nonsense. I think that with a cheap PIC or Arduino processor and power transistor, I could come up with a smart charger for about $15 in parts.
I should not gripe too much considering that I paid $4.99 for the Harbor Freight float charger. I couldn't buy the wall wart for that.
After having used these for a week, I suggest picking the 1% value lower than measured at a cold state. When warm, the voltage is suppressed a bit.
None of the ones that I am using are floating at higher than 13.51V
Hello and thanks for this info NM_Shooter and to all at this site.
Nice little article mate, thanks for the "heads-up"
I used a 100 ohm miniature 25 turn trim pot (and no, not of the horticultural variety), I cut out a small bit of the case....now with a turn of a screwdriver, I can now precisely set the float voltage, compensating for the warm up temperature etc and even better is the the ability to compensate for the small load I have connected to the battery at all times.
In any case what you suggested worked to absolute perfection.
Thanks.
P.S.. mine was slightly different from yours, however VR1 (which was 55.2 ohm on mine) was a similar value, it looks as though it may be for a variable resistor as there are 3 holes on the circuit board in this position and the electronic symbol on the PCB actually shows a pot "should" be located in that position?, maybe they used a pot for the factory trimming of the float voltage then inserted the nearest fixed resistor or maybe there is a de-luxe version out there somewhere.
NM
check this out -- these microcontrollers are amazing
http://arduino.cc/en/
I have a GPS running on a UNO -- investment $45
I have been wanting to fiddle with an arduino for awhile, but have not gotten a demo board yet. My programming skills are limited to simple assembly and some Verilog / VHDL. How do you code for that platform? Does it have a free / cheap compiler?
You can find cheap UNO's on ebay -- I bought 5 Megas from Hong Kong for $109 free shipping
Check out the Arduino website for the free compiler and free downloads
the programming language is C++ (more or less)
Cool stuff Frank and others. Thanks for posting. Reminds me of my old electronics class days though I don't fool with it much anymore.
Quote from: NM_Shooter on April 07, 2012, 12:39:19 PM
So I got a "good deal" on some Harbor Freight Centech battery chargers... I picked them up for $4.99 ea.
http://www.harborfreight.com/automatic-battery-float-charger-42292.html
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/article/charging_the_lead_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.
(https://i133.photobucket.com/albums/q46/nm_longshot/misc/IMG_0570.jpg)
Once you have scribed around a couple of times, take a sharp, small screwdriver. With as much surgical precision as possible, pound that sucker into the crease you just opened up ;D and pry the bottom off.
(https://i133.photobucket.com/albums/q46/nm_longshot/misc/IMG_0571.jpg)
Pull off the cap, then pull / pry out the circuit board. Bend the heatsink up to reveal the components beneath.
You are looking for a Resistor titled VR1. No doubt for voltage regulator 1, but i'm speculating.
(https://i133.photobucket.com/albums/q46/nm_longshot/misc/IMG_0573.jpg)
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?"
NM_shooter do you mind to repost the pics?
I have one of these floater charger and it indeed output little higher than I want and would to tune it down a little.
Thank you in advance!
Will do... sorry! Give me a bit.
Couldn't find them d*
I went and opened up one of the ones I have to show you the resistor location. Placing a variable resistor in that location and fishing around will get you close to the proper float voltage. Note in the picture below the odd resistor.... this is the one that you need to change.
(https://i133.photobucket.com/albums/q46/nm_longshot/IMG_1054.jpg)
Thank you so much NM shooter.
For some reason I did not get the reply notification until I login I think there's something gotto do with the user setting.
Is that the one marked R1 on the left on the dark brown resistor(VR1?) in the pic?
Thanks again you are a genius and save me a lot of money coz I can mod these cheap little charger without worrying it will boil my batteries for winter storage.
You are the man! [cool] [cool] [cool]
Nope... the one you want to change is the one that the dark brown resistor now occupies. It is labeled VR1.
In this instance, I think I had to change it to the 73.2Ohm value to get the value correct.
Note that you want to pick the 1% resistor value that is one step lower than the value that you measure at room temperature. When the float charger has been working a little, it warms up and the voltage drifts upwards a little.
I've been using the ones I have for months now and my batteries are all drifting along happily at ~13.5V
Good luck!
Got it understood!
Thanks again for your detail instruction and pics.
Will head to my local electronic parts store and see if I can find 100 ohms variable pot with 1% value lower when measure in cold.
Thanks again will see if I have time to tackle thus during the holidays.
Wish you and your love one have a happy holiday season!
Cheers
They may not have a 100 Ohm pot.... I could not find one locally. Buy a 1k pot and a 100 Ohm resistor to put in parallel with one another if not. You can then get decent resolution using the 1k pot that way.
Sure will do!
Thanks again for your tips and all the help will post result after its done.............most like after the holiday coz still got some last minute shopping to do!
Cheers and have a wonderful day! [cool]
Thanks for this. My charger quit working. I'd already stuck an xacto in the groove as you describe when I thought I should check if someone had written up how to get into them. It's good to know I was on the right track. One comment: The VR designation is for a pot. So it was designed to be adjustable before it was Muntzed to be able to sell for $5. (see https://en.wikipedia.org/wiki/Muntzing). I've got a bunch of mini multi turn trimpots lying around, so if I can resurrect the unit I'll pop one in.