Battery Charging (Harbor Freight item 42292)

Part 2 of this project is here and part 3 is here.

I have a tractor and two older quads in my barn.  Yes, I have a barn.  The problem is that about half the time I go out to use one of them the battery is dead.  I drag out a charger and extension cord from the house, charge it up, and then get to use the vehicle; usually a day or so later than I wanted.  All too often, I find out the battery won't recharge and needs to be replaced.  This is not only annoying, it's darned expensive.  And eventually I want to get a nice big portable generator with electric start; I really don't want the battery dead on that when I need it.

Obviously, I need a float charger on the vehicles to keep the batteries charged.  One that doesn't boil the water out and can run unattended for months if necessary.  I can't find such a thing at a reasonable price.  Yes, there are devices like the BatteryMINDer, Battery Tender, and hundreds of float chargers.  These things can cost a heck of a lot if you need several of them for different machines that have a battery in them.  Sure, it's cheaper than replacing the battery, but you have to leave them plugged in all the time, and some of them just don't work as advertised; if at all.  Wouldn't it be nice if there was a cheap little float charger that could be adapted to a solar cell?  One that I could control the float voltage it puts out so I could use it on the AGM battery in one of the cars as well as the standard small lead acid battery in a quad?  One that wouldn't drain the battery if I kicked the cord out of the wall and didn't notice until a month later?  Such a device would run me at least $40 and I need several of them.  Crap.

Looking around, I found a coupon for the Harbor Freight float Charger, but it has some draw backs.  It will drain a battery if it is unplugged from the wall, it puts out too much voltage to be left alone essentially forever, and the stupid wires on it suck.  But, it also has some compelling positives, it's cheap, it has a DC wall wart for power, it's cheap, it has simple circuitry that can be modified to do what I want, and it's cheap.  The DC wall wart is expecially attractive since it can be cut off and a solar cell substituted to run it where there is no power.  So, credit card in hand, I went to Harbor Freight and bought several of them (I did mention they were cheap didn't I?).

First inspection showed me that the little devices are set to 14.1 volts constant voltage with a current limit of around .9 Amps, it will discharge a battery through a status LED, and the combination of thick clumsy wires with smaller power wires is somewhat hard to deal with.  Time to take one apart and see what's in it.  After prying on the cover for a while unsuccessfully, I gave up and cut it open with a Dremel.  Reverse engineering it gave me the schematic below:

Harbor Freight Charger 42292

So, it has a good old 7805 power regulator set up as a variable supply and a TIP41 transistor to regulate the current.  The S9013 transistor is set up to turn the device off around 9V, because a 9V lead acid cell is probably beyond recovery for a float charger.  The resistors in parallel are to get a specific value out of the various dividers in this thing.  My first question was, "Why do it this way instead of a variable resistor?"  The answer was obvious when I checked the prices of the various components; fixed resistors cost a fraction of a cent in quantity, variable resistors are much more expensive.  But, the question I don't have an answer to is why they didn't put in a diode to stop from discharging the battery if it's unplugged from the wall?  Oh well, I have a few (hundred) diodes laying around that can do this job for me.  

When I cut open a second one I found a different set of components.  The second one appeared to be an older version and did not have the parallel resistors.  It had one resistor in each position and was laid out slightly differently.  The circuitry was essentially the same, it just appeared to have been changed over time.  Same drawbacks and advantages as the first one.

So, I hooked one of them up to an AGM battery I have in my car and let it work for a few days.  Sure enough, it float charged the battery up to 14.1 volts and kept it there.  This is not a good thing.  The specs on the Optima AGM automobile cranking batteries warns against this and tells me to keep it between 13.2 and 13.8 volts.  A little rework should take care of this problem; especially if I add a diode to the output which will cut .7 volts off the top and get me right in range.  It also appears that I can set the voltage by adding a simple little potentiometer and setting it to be what's needed for a normal lead acid battery like my tractor.  This would be really useful since the instructions for float charging a lead acid battery vary by each author I look at.

I've seen instructions that insist I shouldn't charge a flooded lead acid battery to more than 2.1V per cell for a 12.6 volt float.  Then when I look at another site it says the float voltage should be 2.2V per cell which gets me up to 13.2 volts float.  Then, they start talking about surface charge, waiting 24 hours to check it, parasitic current draws from onboard devices, and temperature corrections.  Then some of them talk about how as little as one tenth of a volt will shorten the life of the battery considerably.  Obviously, many of these people are spouting off stuff they don't understand and others are just repeating things they read on some forum somewhere.  What to do?  I'm just going to start with the info from Wikipedia and see how the water in the battery holds up.  I'll set the AGM voltage at 13.5 and the flooded cell batteries at 13.9 for a month or so and see how much water goes away.  I'll adjust down if the flooded cells lose water.  Note that using a constant voltage supply will take care of parasitic devices like the little LED that blinks for the alarm system on my car.  That's why I prefer constant voltage with current limiting techniques.  

One of the interesting items is that various articles say that the float voltage has to be correct to 0.02 volts.  First, I've never seen anything that kept that close a tolerance, and second, how the heck is someone supposed to measure that?  Sure, there are meters that give that resolution, but how accurate are they?  It would be relatively easy to set up a little computer to measure the voltage and adjust the bias on a voltage regulator to keep it right on the money, but how expensive is a meter that is calibrated to be that accurate?  And, how can you trust it to be that accurate over time?  My method is going to be much more pragmatic.  I'm going to measure the voltage I set the devices to on several meters and just average them.  Then I'll set the voltage slightly high and adjust downward to minimize water loss.  I know for a fact (long years of experience) that it's better to overcharge a tiny amount than to undercharge.  Undercharging causes sulfation and one of those sulfate crystals can pierce the lead causing a short that cannot be corrected.  Been there, done that.

Here are the two devices I've messed with so far:

Notice that the top one has a smaller component count than the lower one, but essentially the same circuitry?  This is a result of the parallel resistors to get a particular value that I described above.  The schematic I show applies to both of them, just adjust for the difference in methods.  Also, the various values of the resistors involved are different for the two devices.  A friend of mine suggested that they change the various values based on a particular run of 7805 and TIP41 components involved.  Also, variable resistors can be bumped and change value; an important consideration in something that will be tossed around a garage by people like me.  I also want to point out what, to me, is another shortcoming: the wire sizes and types used.  Notice the wall wart has a nice long wire that is quite small and the part that clamps onto a battery has a heavy coiled wire.

Someone out there want to tell me the logic in this?  The coiled wire is strong, relatively inflexible, and drags the device around in the car.  This thing puts out less than one amp and just doesn't need that kind of cable.  They did use good strain relief techniques though so the cables won't just pull the thing apart when you stretch it out.  The heat sink gets warm, not hot, as does the wall wart, and after reaching the float point, cools right down.  On my car with the parasitic draw from various devices, the device stays a little warm because it is supplying current to overcome this instead of sucking the starting battery down to nothing.

Obviously, this project is going to take weeks, if not months, to complete.  I have to test for days at a time and monitor the voltage and current feed to be sure I have something that will maintain the batteries not destroy them.  Then I want to get a solar cell to power several of the devices at once (in my barn where there is no power) and install a nice permanent solution.  You'll be seeing more on this over time.