B
bakalorz
Guest
There were a couple of threads about the Harbor Freight $5 chargers a
little while back.
I got one of the 42292 chargers myself a few weeks ago.
The first thing I noticed, was that the charge voltage was listed as 12.5 volts.
This is too low. A fully charged 12 volt battery will have a resting
voltage of 12.6 to 13 volts or so, depending on type, temp, and possibly
other factors. And you would want the float storage voltage to be a few
tenths above the resting voltage.
Fortunately, this appears to have been a typo; when I measured the
actual voltage with a digital multimeter it was 12.77
I still think this is too low too, but it is at least marginally
acceptable for some batteries. It is NOT high enough for Absorbed Glass
Mat (AGM) batteries though; on the box the charger is listed for only
flooded and maintenance free batteries.
Others reported higher voltages, up to 13.2 volts or so
So apparently QA in china is not quite up to par.
12.77 is marginally acceptable but it is not really enough either.
13.2 or so would be good. (thats what the battery tender is set at)
So, since it wasn't really as high as I would want and I like to
experiment, I took my charger appart. This was very easy to do, just
pry up on the plastic cover and it popped right out.
What I found was somewhat annoying. There were 2 resistors in series in
the spots marked R1 and VR1 (black and red arrows on fig1) Curiously
enough the spot marked VR1 had an unused third hole.
(by the time I took the picture for fig 1 I had already clipped out the
resistor at VR1)
The resistors were about 300 ohms at R1 and 50 ohms at VR1.
Clipping out VR1 and substituting various values for the series total
let me get output voltages of from 6.3 volts to 21.7 volts
For example: 220 ohms = 17 volts, 320 = 13.9, 380=12.6
Freaking bean counters ... What obviously happened is that VR1 was
designed to have been a pot to let the manufacturer set the output
accurately, but someone had decided that it was cheaper to just use a
plain resistor and hope the output was close enough. With the existing
R1 and a 100 ohm pot for VR1, the output could be adjusted from 12 to 14
volts or so.
So I looked in my junk box for a 100 ohm pot that would fit into the
existing holes.
Putting in a 100 Ohm pot would have made this into quite a nice little
charger for an extra 50 cents or so and 5 minutes of effort.
Unfortunately I didn't have any pots that were anywhere close to the
right value and had pins that would fit the little triangle.
I wasn't about to order a 50 cent pot and pay $5 shipping to fix it
either.
The closest I had was a physically MUCH bigger 15 turn pot at 200 ohms.
But looking at it, it would just barely fit on a diagonal and connect to
almost the right pads.
It needs to connect to the pad in the very bottom left of figs 2, 4, and
5. It couldn't quite reach if it was going to fit back in the
enclosure. So I moved it in slightly and soldered it to the wrong pad,
and then bent it over to the right pad, and tacked it down there.
I then took an xacto knife and cut a groove through the wrong pad to
isolate that part from the rest of the pad. See the arrow in fig 5.
The other pins in the pot were soldered to the rest of that pad
(compare figures 2 and 4)
I had to drill holes and scratch the green coating to solder them.
This would have given me an adjustment range of about 11 to 13.7 volts
or so. This would make it a fine float charger, useful for storing
batteries.
If I bumped up the high end a bit, it would make a charger which could
be set for either (semi)fast charging or float storage.
So I decided to also change R1 while I was at it. By making R1
equal to 220 ohms, the adjustment range changed to 12 through 17 volts
or so. Pretty much perfect.
I then drilled a hole in the case so I could access the pot with the
case sealed up (figs 6 and 7)
---
So what are my conclusions.
The harbor freight charger is a very reasonably priced little charger.
It is short circuit protected, and has an antispark circuit.
It is not diode isolated, and should not be left attached to a battery for very long if it is not plugged in.
If you get one which is at the right voltage for your application, it can effectively more or less replace a battery tender.
Unfortunately the voltage may be off from what you need, and not all of
them are the same either.
If you go to a local harbor freight, take a digital voltmeter along, and
plug in and check the output voltage of a couple of them until you find
one that is at the voltage you want, they will all be slightly different.
If you want to change one that you already have, there are 3 ways to go.
In order of increasing difficulty:
1) get a little 100 ohm pot with the leads in a triange that will fit
where the original one was supposed to go and solder it in.
2) replace R1 and VR1 with a series/parallel combination of resistors to
set the (non-adjustable) voltage you want. The holes are big enough to
easily fit the leads for 2 or 3 or 4 resistors in each one.
To increase the voltage, you want to reduce the resistance.
This is very easy to do by paralleling R1 or VR1 with other (probably
higher resistance) resistors. You can find out what you need easily.
Hook a voltmeter to the output clips and plug the charger in.
Then just touch various resistors to both sides of R1. I would start by
trying 1000 ohm, or 2200, or 3300, or 4700, or 6800, or 10,000
3) fit in a 100 ohm 15 turn pot.
For options 1 and 3 if you reduce R1 to 220 ohms and use a 200 ohm pot
you can get a wider adjustment range.
little while back.
I got one of the 42292 chargers myself a few weeks ago.
The first thing I noticed, was that the charge voltage was listed as 12.5 volts.
This is too low. A fully charged 12 volt battery will have a resting
voltage of 12.6 to 13 volts or so, depending on type, temp, and possibly
other factors. And you would want the float storage voltage to be a few
tenths above the resting voltage.
Fortunately, this appears to have been a typo; when I measured the
actual voltage with a digital multimeter it was 12.77
I still think this is too low too, but it is at least marginally
acceptable for some batteries. It is NOT high enough for Absorbed Glass
Mat (AGM) batteries though; on the box the charger is listed for only
flooded and maintenance free batteries.
Others reported higher voltages, up to 13.2 volts or so
So apparently QA in china is not quite up to par.
12.77 is marginally acceptable but it is not really enough either.
13.2 or so would be good. (thats what the battery tender is set at)
So, since it wasn't really as high as I would want and I like to
experiment, I took my charger appart. This was very easy to do, just
pry up on the plastic cover and it popped right out.
What I found was somewhat annoying. There were 2 resistors in series in
the spots marked R1 and VR1 (black and red arrows on fig1) Curiously
enough the spot marked VR1 had an unused third hole.
(by the time I took the picture for fig 1 I had already clipped out the
resistor at VR1)
The resistors were about 300 ohms at R1 and 50 ohms at VR1.
Clipping out VR1 and substituting various values for the series total
let me get output voltages of from 6.3 volts to 21.7 volts
For example: 220 ohms = 17 volts, 320 = 13.9, 380=12.6
Freaking bean counters ... What obviously happened is that VR1 was
designed to have been a pot to let the manufacturer set the output
accurately, but someone had decided that it was cheaper to just use a
plain resistor and hope the output was close enough. With the existing
R1 and a 100 ohm pot for VR1, the output could be adjusted from 12 to 14
volts or so.
So I looked in my junk box for a 100 ohm pot that would fit into the
existing holes.
Putting in a 100 Ohm pot would have made this into quite a nice little
charger for an extra 50 cents or so and 5 minutes of effort.
Unfortunately I didn't have any pots that were anywhere close to the
right value and had pins that would fit the little triangle.
I wasn't about to order a 50 cent pot and pay $5 shipping to fix it
either.
The closest I had was a physically MUCH bigger 15 turn pot at 200 ohms.
But looking at it, it would just barely fit on a diagonal and connect to
almost the right pads.
It needs to connect to the pad in the very bottom left of figs 2, 4, and
5. It couldn't quite reach if it was going to fit back in the
enclosure. So I moved it in slightly and soldered it to the wrong pad,
and then bent it over to the right pad, and tacked it down there.
I then took an xacto knife and cut a groove through the wrong pad to
isolate that part from the rest of the pad. See the arrow in fig 5.
The other pins in the pot were soldered to the rest of that pad
(compare figures 2 and 4)
I had to drill holes and scratch the green coating to solder them.
This would have given me an adjustment range of about 11 to 13.7 volts
or so. This would make it a fine float charger, useful for storing
batteries.
If I bumped up the high end a bit, it would make a charger which could
be set for either (semi)fast charging or float storage.
So I decided to also change R1 while I was at it. By making R1
equal to 220 ohms, the adjustment range changed to 12 through 17 volts
or so. Pretty much perfect.
I then drilled a hole in the case so I could access the pot with the
case sealed up (figs 6 and 7)
---
So what are my conclusions.
The harbor freight charger is a very reasonably priced little charger.
It is short circuit protected, and has an antispark circuit.
It is not diode isolated, and should not be left attached to a battery for very long if it is not plugged in.
If you get one which is at the right voltage for your application, it can effectively more or less replace a battery tender.
Unfortunately the voltage may be off from what you need, and not all of
them are the same either.
If you go to a local harbor freight, take a digital voltmeter along, and
plug in and check the output voltage of a couple of them until you find
one that is at the voltage you want, they will all be slightly different.
If you want to change one that you already have, there are 3 ways to go.
In order of increasing difficulty:
1) get a little 100 ohm pot with the leads in a triange that will fit
where the original one was supposed to go and solder it in.
2) replace R1 and VR1 with a series/parallel combination of resistors to
set the (non-adjustable) voltage you want. The holes are big enough to
easily fit the leads for 2 or 3 or 4 resistors in each one.
To increase the voltage, you want to reduce the resistance.
This is very easy to do by paralleling R1 or VR1 with other (probably
higher resistance) resistors. You can find out what you need easily.
Hook a voltmeter to the output clips and plug the charger in.
Then just touch various resistors to both sides of R1. I would start by
trying 1000 ohm, or 2200, or 3300, or 4700, or 6800, or 10,000
3) fit in a 100 ohm 15 turn pot.
For options 1 and 3 if you reduce R1 to 220 ohms and use a 200 ohm pot
you can get a wider adjustment range.
