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:
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.