Hi everyone! I am new at this, so here it goes:
I am trying to build a solar powered smart battery charger for a lead acid battery. I believe that I am going to use a 20W, 12V panel, which at its maximum rating can supply 1.67A. The lead acid battery would most likely be a 12V 18Ah battery. My goal is to build a charger that would charge the battery, and turn off the charger when the battery is full (to prevent overcharging). How would I go about this?

Thanks!

 

Hi everyone! I am new at this, so here it goes:
I am trying to build a solar powered smart battery charger for a lead acid battery. I believe that I am going to use a 20W, 12V panel, which at its maximum rating can supply 1.67A. The lead acid battery would most likely be a 12V 18Ah battery. My goal is to build a charger that would charge the battery, and turn off the charger when the battery is full (to prevent overcharging). How would I go about this?

Thanks!

You can use a comparator and compare the battery's voltage by reference voltage of 13.4 for gelled type and 13.8 for liquid type batteries. When the batteries voltage will be more than the reference voltage the charging will stop.

 

I'd suggest you search for 'Solar Charge Controller'. Haven't looked in a while, but the few I saw were fairly complex, and kind of a high part count for me to build. Off the shelf controllers are in the $30 range. I figured it would be more of a hassle, and likely more expensive to build, doubtful it would be as good or better in the end. There are quite a few chips dedicated to the task. bu was never interested enough to look into them further. New stuff coming out rapidly for solar energy, should check out some of the alternate energy sites. The real ones, not the 'Joe Cells' and HHO guys...

 

Your 12v 20w solar panel will only generate that much voltage and current at high noon in a desert on the equator. It won't be enough voltage to charge your battery even then.

In order to charge a 12v lead-acid or SLA battery, you'll need to supply it with 14v to 15v. Once the battery is charged, you should supply it with a "float" voltage to reduce sulfation.

Float voltage varies with battery temperature. At 25°C (77°F) the recommended float voltage for lead-acid cells is 2.26V per battery cell. The temperature coefficient is -3mV/°C. So, if you have a 6-cell 12v battery, your float voltage at 25°C (77°F) is 13.56V. If your battery temperature is higher, you subtract 3mV*6=18mV°C; if lower, you add 18mV/°C. So, at 0°C, float= 14.01V; at 85°C, float= 12.48v.

But the float voltage is used when the battery is at or nearly charged. With only 1.67A available, you won't have much more than a trickle charger anyway.

 

Your 12v 20w solar panel will only generate that much voltage and current at high noon in a desert on the equator.

Depends on what the 12V spec refers to. Many "12V" panels actually have a "maximum power voltage" of over 17V which is measured at 1000W/m^2 irradiation (high noon in a desert on the equator). The spec sheet would be very helpful.

 

Depends on what the 12V spec refers to. Many "12V" panels actually have a "maximum power voltage" of over 17V which is measured at 1000W/m^2 irradiation (high noon in a desert on the equator). The spec sheet would be very helpful.

True. However, my real point is that the actual amount of time that the battery might be charging would be very small, particularly under less-than-optimal conditions. Using a pair of such cells wired in series would greatly extend the charging time.

Having the spec sheet would indeed be a huge help.

 

I wonder if DC to DC converters are used for commercial applications, those with variable input -say 6V to 20V in; 13.8 V out.
Would provide extended hours charge in low sunlight.

 

My 12 watt panel is 22 volts no load, and charges a 12 volt 10 Ah battery just fine. Pretty sure it could handle a larger battery. I get plenty of sunlight here in central Florida.

You don't want overcharge, and you don't want to let the battery discharge too far either, another reason to purchase a charge controller (cheaper than replacing the battery).

http://www.redrok.com/electron.htm#charger
This like might be of some use, the main site has a ton more circuits and information.

 

Thanks for all of the help! I am building this from scratch because it is for a project at school.. so I can't buy it from the store. But, my original idea was to have a simple comparator circuit with hysteresis and a MOSFET, that way I can set the low voltage that the battery would start charging at and I could set the higher voltage that the battery would stop charging at. Does this sound about right? I would also incorporate a diode so that the current from the battery would not flow back to the panels. I also realized that I meant to put a 20W, 18V solar panel, not a 12V. I don't think that I will need to regulate the voltage going to the battery because the battery will only start to charge once the voltage is over 12V. Am I understanding this correctly?

 

Your 12v 20w solar panel will only generate that much voltage and current at high noon in a desert on the equator.

And only when refrigerated!! I've recently learned that solar panels do better in Eastern Oregon than they do in Arizona. Hotter ambient temperatures mean less power output.

 

Thanks for all of the help! I am building this from scratch because it is for a project at school ... so I can't buy it from the store.

Ahh, OK. Usually we like to know up front if it's a school project. We can't do it for you, but we can offer suggestions and help you out if you're "stuck".

But, my original idea was to have a simple comparator circuit with hysteresis and a MOSFET, that way I can set the low voltage that the battery would start charging at and I could set the higher voltage that the battery would stop charging at.

That sounds like a plan - the hysteresis will prevent it from cycling on and off too quickly. Now, if you used a thermistor (a resistor that changes resistance depending upon temperature) you could implement float charging. Since your charge current is so low, you would not have to worry about "normal" charging phases. A typical charger has a "bulk" charge mode, where perhaps 10A current is fed to the battery at 10v to 15.5v until the battery is around 80% charged, and then a full charge mode is entered to slowly bring the battery to peak charge.

I would also incorporate a diode so that the current from the battery would not flow back to the panels.

That's important. You should research using low-voltage Shottky diodes. They have lower Vf (forward voltage) than standard silicon diodes, which will mean that your implementation will be more efficient.

I also realized that I meant to put a 20W, 18V solar panel, not a 12V. I don't think that I will need to regulate the voltage going to the battery because the battery will only start to charge once the voltage is over 12V. Am I understanding this correctly?

Well, the battery will start charging when the Vf of the diode + the battery voltage is less than the Vout of the solar panel.

You might consider using a P-channel power MOSFET in your circuit, and use a 10k resistor tying the gate to the drain. This will keep the MOSFET turned OFF by default. If you use an N-channel power MOSFET, you'll have to use both a pull-down and a pull-up resistor, and it'll get ugly in a hurry.

Make everything powered by the solar cell. If anything is powered by the battery, you will be draining it about 3/4 of the time. This is going to make testing the battery voltage a bit dicey, but it's definitely do-able.

Research using a "buffer amplifier". If you used a FET-input buffer amplifier that was powered by the solar cell... - think about that for a while.

 

Thanks SgtWookie! I greatly appreciate it and the posts are very helpful. I have never worked with solar panels before, so it's all new to me, and a little frustrating . I am starting from scratch, so all of the posts are helping me learn more about this stuff. Thanks again!

 

That sounds like a plan - the hysteresis will prevent it from cycling on and off too quickly. Now, if you used a thermistor (a resistor that changes resistance depending upon temperature) you could implement float charging. Since your charge current is so low, you would not have to worry about "normal" charging phases. A typical charger has a "bulk" charge mode, where perhaps 10A current is fed to the battery at 10v to 15.5v until the battery is around 80% charged, and then a full charge mode is entered to slowly bring the battery to peak charge.

You might consider using a P-channel power MOSFET in your circuit, and use a 10k resistor tying the gate to the drain. This will keep the MOSFET turned OFF by default. If you use an N-channel power MOSFET, you'll have to use both a pull-down and a pull-up resistor, and it'll get ugly in a hurry.

Make everything powered by the solar cell. If anything is powered by the battery, you will be draining it about 3/4 of the time. This is going to make testing the battery voltage a bit dicey, but it's definitely do-able.

Could you explain these a little more, I am a bit confused..

 

I have a "12V" 1.8W solar panel. Its output is supposed to be 150mA at 12V (at a desert on the equator at noon).
I am in Canada which is pretty far from the equator but in summer the sun is pretty high in the sky at noon.
In late spring its output was 18V at noon with no load and was 80mA with a load of 150 ohms so its output was 12V. In the afternoon the output current was only 30mA at 12V. Behind window glass the output current was halved.

Guess what?
When it was cloudy outside its output current was nothing.
At night its output current was nothing.

It is made to trickle-charge a battery, not to charge a battery.
A trickle charge is a very low current that replaces the small amout of charge that a battery loses when it is not doing anything.

 

Sorry I haven't responded; power has been off since the morning. Just came back on a bit ago.

I lost a great reference page when power went down, but I'll find it again soon.

Hang in there for a few.

 

in addition to what Salamander27 has just asked, i'd like to add that how to handle the input voltage of 10 to charge a 9V Rechargeable battery ,and then when it's full, it triggers the load subcircuit....i've been incubating this for days so far, please help me..thanks for answering

 

Well it's a 20W 18v panel, into a 18A/H battery?

Even for the couple of hours around noon it will barely make 1amp, so just connect it direct to the battery. That is charge of 1/18 C which is fine for direct connect.

If you have very low current draw from the battery you can prevent overcharging by using a 3pin voltage regulator set to about 14.5 volts. It's unlikely the battery will ever reach that from a 1A at 2-3 hours a day panel though...

Also you won't need the diode when the panel is an 18v type into a 12v battery. The reverse voltage breakdown of a 18v panel is never going to be less than 12v. You barely need the diode in a 12v-12v system.

 

Here is a plot of an 18V, 840mW solar panel, S. C. current is 108mA, O.C. V 18V. Power plotted on same axis as V. 1 PM Tucson AZ 100 deg. F. early May, panel wes too hot to hold in bare hand. Use is a float charger.

 

(at a desert on the equator at noon).

I think you meant to say "at 1kW/sq.m and 20°C." Noontime desert will exceed 20°C and output will drop accordingly.

 

" Build this simple Mini Maxiimiser" http://www.voltscommissar.net/minimax/minimax.hym
Interesting article re powering motor with solar panel; with slight modification of output could be used to charge a battery using optimum solar output. I might try it.