Building a simple charge controler for lead acid battery

[farrukhtalib]

Hi Every one, I am just tring to build a simple charge controller which is meant to charge a 12V 100 amp lead acid battery using a 120 watts PV panel having max output voltage around 17 volts. I will use a pic microcontroller to perform switching actions. I will
1. Connect the Panel with the battery via schotttky diode and a switch (relay).
2. Connect load to the battery via an ON off switch (Relay).
Now Microcontroller will monitor the voltage across the battery terminals and across the panel terminal simultaneously. If battery volts are equal to 14.5 volts then it will disconnect panel from the battery and if battery volts drop to 12 volts it will disconect the load from battery.
If panel volts will drop below 12 volts it will disconnect panels from battery.
There are few questions in my mind and I seek answers and expert opinion from all of you.
1 Will it work?
If it will work, then how I may measure panel voltage separately coz when I will connect the panel to the battery the voltage will become same as of battery terminals. Should I use periodic disconnection (1 sec after every 30 minutes) of panel from battery to monitor the panel voltage or else.

My request is, please let me know how I may improve my idea or you may guide me with batter option.

Regards

[daz]

1 Will it work?

That depends....do you know what you are doing? If you are taking a guess at what needs to be done, then chances are it will not work.
The other thing I noticed is that you want to use relays. For the load side that should be fine, but on the control side for charging, you may run into problems. I assume, as you are using a PIC, that you will setting this charge controller up as a PWM? If that is the case, relays will not be able to handle high frequencies. For that you normally use FETs.

Why dont you just buy a charge controller? They are not that expensive, and will at least work!

[andy93]

Hi farrukhtalib ... I think you mean your battery is 100AHr. not 100A, using 120w panel a 10A charge controller is more than enough, ($10 inc. delivery on eBay, also search 'adjustable dc-dc' on eBay) , but it's great you want to build it yourself, it's the best way to get an education. I think 14.5v disconnect is a bit high, will cause electrolysis and reduce battery life. ....'If panel volts will drop below 12 volts it will disconnect panels from battery" I don't think you need to do this as you have schotttky diode. As for the rest I don't know .....Good luck

[daz]

I think 14.5v disconnect is a bit high, will cause electrolysis and reduce battery life...

It depends on the type of battery he is using...
14.5V should be fine, for FLA, although it will depend if it is for AGM, as they are more sensitive to overcharging compared to FLA.

The op should use the search function on the top, as I think there are a few threads discussing charging voltages already. He will also get his answer much quicker that way.

[john8750]

Sounds like fun. You could build a simple PWM using MOSFETS. You only need to regulate voltage. Make sure you know how. And I agree it would be less trouble to buy one.

[Kebast]

It's all been done before

Try this:

http://www.mdpub.com/555Controller

[Sunking]

Now Microcontroller will monitor the voltage across the battery terminals and across the panel terminal simultaneously. If battery volts are equal to 14.5 volts then it will disconnect panel from the battery and if battery volts drop to 12 volts it will disconect the load from battery.
If panel volts will drop below 12 volts it will disconnect panels from battery.
There are few questions in my mind and I seek answers and expert opinion from all of you.
1 Will it work?

In theory it will work, but in practice will fry your battery. Applying 14.5 volts to a charged up battery will cook your battery if applied for any length of time. If you are looking for a simple charging algorithm just use a constant voltage of around 13.6 volts aka Float Charger.

Second issue is for a standard lead acid battery the minimum recommended charge rate is C/12 so for a 100 AH battery is 8.33 amp or let's just say 8 amps or as high as C/8 or 12.5 amps on a 100 AH battery. You stated you are going to use a 100 watt panel. I assume a typical battery panels and you are going to use PWM type of voltage regulator. As with any type of series regulator Input Current = Output current. What that means is th echarge current will be whatever current the panel produces, and for a 100 watt panel the max is around 4.8 to 5.5 amps. That would be at best a C/20 charge current on a 100 AH battery.

[john8750]

Built my own charger. Puts out a safe 14 volts, will handle 60 amps plus. No need for blocking diode, the mosfets will handle that function.
Opens gate circuit when panel voltage drops below 14 volts. Thats all there is to it. Alot of fun but--- not for a beginner.
BTW I changed to all digital meters, much better.

[PNjunction]

Built my own charger. Puts out a safe 14 volts, will handle 60 amps plus.

14.0 volts, unless it is a gel, is kind of a no-man's land so I'd be very careful. At the very least, put it on a timer.

In other words, that is too high of a voltage to be a float-charger, typically 13.2-13.8 volts tops. And at 14v, unless it is a gel, is too low to be an effective CV two-stage charger (after bulk, battery goes into absorb on it's own.)

Personally I'd make the decision as to being a 13.2 to 13.8v float, or a 2-stage CV charger in the from 14.2 to 14.5 or so, and not hang out in no-man's land with 14v even. Of course it all depends on what your battery specs are.

[john8750]

I see your point. The charger I was using went up to 13.1 volts. At this voltage the current was not enough for a floated battery condition. Checking my batt stats, the highest safe charging voltage was 14.6, so I figured 14 would be good. Seems to work well so far. I can fully adjust charger voltage, depending on the panel voltage of course which cuts off at 14 volts.
I agree with you......

[PNjunction]

Now Microcontroller will monitor the voltage across the battery terminals and across the panel terminal simultaneously. If battery volts are equal to 14.5 volts then it will disconnect panel from the battery and if battery volts drop to 12 volts it will disconect the load from battery.

Merely reaching an absorb voltage, and not holding it there for awhile to actually absorb, means compensating with a LONG period of float-charging, say 13.2 to 13.8 volts to finish the job. Since solar-insolation is usually short, you REALLY need some absorb time held at 14.5 volts which lets the battery do the regulating as it absorbs and current falls. If your pic controller can't detect about 2-5% of the battery capacity in current as a cutoff to switch to float voltages, then perhaps you can at least program a time period, say a minimum of 1-4 hours to hold it there at 14.5 volts.

This is all very simplistic, and much of it depends on your actual battery specifications for voltage and current.

I wouldn't spend a lot of time checking the panel voltage since panels are a current-source, and not a voltage source. Other than making sure the panel meets the OCV spec for proper operation, current monitoring of the panel would be more useful.

If your battery is an FLA, then make sure your panel current can supply at least C/8 to C/12, (8 to 12A for a 100ah FLA). Less than this will mean ineffective charging and stratification. AGM's can take more, usually up to .3C and specialty types much more current.

If your system has no way of regulating voltages other than on/off switching at voltage setpoints, then program it to merely switch off at 14.5v and back on again at 13.4 volts - but know that this kind of slow "ping-pong" regulating (as seen in very cheap charge controllers) which relies on the battery's own hysteresis is not kind to the battery. Do not confuse ping-pong regulating with PWM which operates much faster on a different principle.