Building a charge controller with a BS2?

GICU812

So im finishing up a project, it uses a 48 volt battery bank (4 x 12v AGM 70ah) and will be chargable by either solar or a DC power source if needed.

I was just wondering, since the BS2 is allready in there, if I could somehow use it to control the charging instead of·buying an additional (and expensive) charge controller.

The solar pannels will be outputting 79.2v, and the DC power source I could set to any voltage, but for simplicity, lets just say it will be 80v unless there is a better voltage to run it at.

The batteries need to see 58.8 volts for primary charging, and can take·up to 7A, and stop when the batteries reach at 57.6 volts.· Then output needs to drop to 54.8·volts at no more than·700 ma for the float, or storage charge. The solar pannels will only be able to push 1.75A anyway.

So I would either need a way for the stamp to read the voltage of the pack, or a series of comparators to trip at certain voltages.

What is beyond me, and I know it shouldnt be, is how to regulate the voltage down, and how to limit the current. Im pretty sure this is some basic stuff involving resistors, and im going to feel real stupid when I see the responses. Maybe its just too late to think, I dunno.

So, what would be the easiest way to get the stamp to read a voltage in the 50-60v range? ADC?

What would I need to do to take 80v and regulate it down to 58.8v @<7A? Will this same circuitry work for a far lower power source, like the solar cells?

So any ideas or leads would be very welcome.

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GICU812

Okay, now that I am awake enough to think,

I guess there would be two ways to do it, not that im entirely clear on how to do either.

1 Would be with PWM, I could use a transistor of some type (the part I dont know) and use a pulsout command to pulse it on \ off rapidly, reading the output voltage (another part I dont know) and increasing\decreasing the on time to get the voltage I want. I've got a few blanks there I will work on filling in.

The other way would be with a resistance setup. Basically I could just hook the high end to a pot, give it ground, and turn the pot until I got the voltage I wanted. Probably couldnt actually use a pot, but a set of resistors of the right value. Seems to me there would be a ton of power wasted in heat this way though. I also dont know how to limit the current in either design. hmmm... thats probably in Ohms law somehow... V=IR = I=V/R So theoretically increasing resistance would limit the current, but wouldnt that change the voltage? Ehhh thats where I always forget or get lost.

Still dont know the best way to feed the stamp the voltage signal...

GICU812

Seems to me that the first way, PWM, would be the easier, smaller, and most efficient way. As would only need one control circuit, and the stamp could command any voltage it wanted. To do the resistors, I would need a couple of seperate circuits. Then again, I might be able to do this without the stamp involved at all, if I had more experience with comparators... Im not so concerned with efficiency here, just so long as it is charged in time for the next use, which I think it should do easily, so loosing a couple of percent of the power probably wouldnt matter anyway.

GICU812

Picked up a transistor today rated at 100V, so I guess I'll start playing with the PWM idea. Also got a quad comparator.

Dave E

I use an analog to digital converter to read the battery voltage and solar panel voltage. If my solar panel voltage is greater than battery voltage, then I use the PWM command to generate an analog voltage connected to the base of a transistor that controls the current from the solar panel. I also let the solar panel current pass through a 0.04 ohm resistor. I measure the voltage across the resistor and use that to calculate the current flow. If too much current is flowing to the battery or the battery voltage is at the full charge voltage, my BS2 will adjust the PWM to decrease the voltage on the base of the transistor which will reduce current to the batteries. Like most of my projects, I started out simple and added refinements as time permitted. The first revision of my charger was a relay that connected the output of the solar panel directly to the batteries for 30 seconds if the battery voltage was low. After 30 seconds, the relay would open and the battery voltage was measured. If it was still low, cycle the relay on again. One analog to digital input and one output for the relay control. And, of course, a few blinking LEDs. Got to have a few blinking lights. Have fun.

Hope this helps,

Dave