Charging Solar Batteries Inexpensively




January 19, 2017

I have a friend that lives off grid in a beautiful country setting. In the winter when it rains there is just not enough sun to keep the batteries fully charged. The charger in his old inverter is rated 30 amps @ 24 volt system. And it is a bit cantankerous so will cut out if it doesn't feel that the generator is not providing the proper line voltage. So we started looking for a solution. First thing that comes up is finding a used forklift charger. They are around, as I recall for as little as $.20/watt. But those old chargers use a line wound iron core transformer. That makes then pretty heavy and not so efficient. Also, if it goes belly up, that is it unless you have two of them; lights out...

Then I thought about those inexpensive power supplies on Ebay used for CNC and everything else. But these supplies are constant voltage and won't run up to the 29 volts that is required to charge a battery bank. It just means they need to be modified. Great thing is that if you are running a bank of these things and one goes out, the rest of them just keep on working. We are running with ten of them. So 150A * 28V is 4200 watts of charging power.

Best part, if one supply fails it does not bring the whole thing down. You just loose 10% of capacity. And the price point, about 7 cents / watt for new equipment! Hard to beat this solution.

Here is the board pulled from case:




12V 3A supply




They are built reasonably well built, especially for the price. This one is rated 15 Amps @ 24 Volts and in the door, ~$25. The weakest point on these supplies is the input line filter marked LF2 on the board. I've changed these out as they burn up as we so far have lost like one a year. I use something like this, beefy. But in this application it is probably not even necessary and can be jumped out.






At first, I was not checking and adjusting the current limiter and had to replace other parts on these boards. But I think now we have a pretty solid system. Here is how it goes.

PS Regulator

    Pull R16. It is a bleeder and a phantom load. You don't need to drain the batteries unnecessarily.

    I added the red LED grounding it to the voltage adjuster pot and running a wire, via a 220 ohm resistor to R37. This is the running power supply indicator as the board's green led will always shine from the batteries.

    Solder a 10K resistor across R40. This will allow you to set the output voltage above 30V.

    If you have a puny looking filter you can just jumper it out.

    If the supply has a thermostat for the fan, usually floating inside the large output inductor, just jumper it out and have the fan run full time. There is no point in doing any other way in this application.

    And most important, trimming the 'J' jumpers. I've seen just one of these supplies that did not need trimming. Because the supplies will be running constant current this part of the circuit must be within specifications. I've damaged a few of these by not setting this early on as I assumed the current limiter was good to go. I'd lost the driver transistors and emitter resistors. So far I've been able to quickly repair these. So once the modifications are done, fire it up with a load that you can adjust. I just used a heater core and set the jumpers to pull the current I was looking for.

    Get a couple of small aluminum plates to put onto your switchers and the output diode(s). I've seen these come with a one or two diode configuration. As long as you don't run with a full load for more than 10 or 15 seconds at a time, they will work fine.



Here I used a 5 milli-volt / amp shunt. You can use any piece of wire, I just happen to have this. Set the supply output to some 28 volts and adjust your load to pull about 18 amps. If you can't reach the jumpers with a Dremel tool, unsolder one and put it on the bottom of the board. There is plenty of clearance for this. I use a cutoff wheel, it makes it easy to shave the jumper. So just remove a bit of material from the jumper turning the supply on between shaves until you see the current drop. From there you can get a feel of how much trimming it takes to bring it down to 15 amps. When done, set the output to about 30.5 volts. This will keep the supply from dropping out as the batteries charge. It is up to the client not to overcharge the batteries. But that would be hard to do on a cloudy winter day.

Yes, this is over 400 watts out. But I've tested at 450 watts out which requires 480 watts in. So the supply will dissipate 30 watts fully loaded. With the fan the output transistors and output diode just get warm at this power. The rating on the output transistors is more than adequate for this service. And the proof is in the pudding, the supplies I've modified are all holding up fine. They are used for 30 to 60 minutes at a time.

I'll get a picture of the bank of supplies and put it here:

future piture of supply bank

Each supply has a 30 amp fuse on the output in the case something shorts in the supply. So it is easy to see if there is a problem with one of them as all the green LEDs should be powered from the batteries when the generator is off. And the red LEDs will light up when the generator is running.

Thanks, Dan. (pubdan5 at lakeweb dot net)


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feb 17