Proper voltage and amps to battery without a charge controller

[Kebast]

Hello all! I'm new to solar power and have a few of questions.

I bought some cheap cells off ebay to put a few solar panels together with. I also bought a 6 volt 4.5 amp hour SLA battery thinking I'd make a phone charging station. I pieced together several of the broken cells and ended up with a small panel of 8.5 volts and 2.7 amps (left the mostly whole ones alone until I have a good idea what I'm doing).

My question concerns charging the 4.5 amp hour battery. First off, voltage. From what I've read, bulk charge requires about 2.35 volts per cell and float charge is around 2.15 volts per cell (7.05 and 6.45 in my case). Is this correct? Will the battery limit the voltage it draws to charge and not be harmed if I disconnect it before overcharge state? I could easily remove 3 of the cells from the panel to hit near 7.05 if not.

Now for the amperage. SLA max charge rate is C/4 correct? So a current of 1.1 amps should be the max I use to charge the 4.5 amp hour battery? What is the simplest way to limit the amps without a charge controller? I had thought to put a resistor in parallel with the battery to soak up a good portion of the amps. That would be something between a 4 and 5 ohm resistor if my math is correct. Will this work?
I've found a few basic diagrams for building charge controllers. If necessary, I might try my hand at building one.

I'm not really worried about efficiency here, but I certainly don't want to cook the battery. This is a learning project, I'll buy a charge controller later when I step up to larger batteries.

[Sunking]

Now for the amperage. SLA max charge rate is C/4 correct?

Maybe, maybe not. SLA has a few different categories of lead acid batteries of AGM, and Gel. If yours is a Gel the max safe charge rate is C/20 and uses lower voltages.

[Sunking]

What is the simplest way to limit the amps without a charge controller? I had thought to put a resistor in parallel with the battery to soak up a good portion of the amps. That would be something between a 4 and 5 ohm resistor if my math is correct. Will this work?

No you will need a current and voltage regulator. Simplest method is to just use a 3-pin LM-XXX that limits current and set it to operate at float voltage.

[Kebast]

Thanks!

No you will need a current and voltage regulator. Simplest method is to just use a 3-pin LM-XXX that limits current and set it to operate at float voltage.

I had already grabbed an LM317 assuming I'd need it

[Sunking]

It has been a while since I worked with an LM-317 but you might need a higher voltage and current panel to overcome the LM-317 voltage drop.

Keep in mind a solar panel is a current source, not a voltage source, and the current is completely dependent on the iradiance input. So if the panel does not have enough light to generate enough current and voltage to drive the 317, it will not work. Lm-317 are designed to work from voltage sources, not current. A solar panel only operates like a voltage source in a very narrow range of Vmp to Voc. Anything below that and it operates as a true current source, voltage at that point is unknown and will be very low well below Vmp.

[RPKulik]

Simple Battery Charger

I am currently developing several applications for solar battery charging; MPPT and dynamically re-configurable PV arrays and battery piles. I would like to point out a very simple and robust charger you might find interesting. Since you are building your own PV array, you can configure it such that Isc is as close to your max charge current without going over (actually, you can exceed your charge rate since you will most likely never be charging a battery that has a float voltage of zero, but we are keeping this simple). Voc should be set close to, but not exceeding the max charging voltage plus 0.45V. Place a 1N5817 diode between the output of the PV array and the battery and viola! - a simple, robust and safe battery charger. The primary drawbacks of this design are that it requires full sun during the terminal stage of charging and it is less efficient than more complex designs.

[axis11]

12v system

I think it will be easier and more useful to make a 12v system. Construct an 18v panel, buy a 12v controller and battery, use a 12v charger for mobile phones. 12v accessories are easy to find and you can use the system for other purposes as well.

[Kebast]

+1 for 12 volt

I agree that 12 volt might be easier. I was thinking that using a 6 volt battery would be more efficient for charging 5 volt devices. I'm not really worried so much about efficiency though, and 12 volt seems easier to deal with. I've been reading up on the LM 317 and have come up with a simple design that I think might work:

18 volt max Solar Panel --> LM317 --> ------> DIODE,1N5408 --> Battery
.........................................|........ .......|..................................|
.........................................|---240R----|..................................|
.........................................|........ ..........................................|
.........................................|........ ..........................................|
.................................5k Trim Pot..............................................|
.........................................|........ ..........................................|
Ground <-----------------------------------------------------------

This certainly isn't the best format to draw this out in, but I think you get the idea. My only concern with this is having a charge rate that is too high. If I set the Pot so the output voltage at full SoC + Diode, will the circuit actually stop providing charge at that point? I've forgotten most everything from my Physics classes 15 years ago, and I have never had any training in circuit design.
RPKulik, is this something like what you had in mind?

[bcroe]

My only concern with this is having a charge rate that is too high. If I set the Pot so the output voltage at full SoC + Diode, will the circuit actually stop providing charge at that point? I've forgotten most everything from my Physics classes 15 years ago, and I have never had any training in circuit design. RPKulik, is this something like what you had in mind?

Its possible to add a simple current limiting feature to the 317, check the manual. Basically a low value resistor is placed
in series with the current, and fed back to reduce the output. Bruce Roe

[RPKulik]

Simple battery charging

My only concern with this is having a charge rate that is too high. If I set the Pot so the output voltage at full SoC + Diode, will the circuit actually stop providing charge at that point?

Yes, when the voltages are equal, no current will move. This only occurs AFTER the battery is charged fully. (In actual practice, the leakage current of the battery will be provided by the charger.) However, max current is supplied when the battery has a low state of charge. If your PV array has the potential to exceed safe current levels for your battery, some sort of current-limiting feature is required.

RPKulik, is this something like what you had in mind?

I was trying to point out that if you are configuring a PV array, you can do so in a manner that allows an extremely simple charger circuit. Your circuit looks fine, although I did not check the R values.

The original circuit I proposed:
------------->|-----------
|..............1N5817...........|
PV..............................Batt
|...................................|
gnd...............................gnd

Where Isc <= max charging current of batt, Voc <= terminal charging voltage of batt +0.45 V


For your application, you could use the LM317 in current-regulating mode, and use Zeners and/or diodes as shunt regulators to limit voltage. Again, the emphasis is on simplicity, not efficiency.

[Kebast]

I see!

I understand now. I actually tried to do something similar from the start, but I undershot the number of cells I needed to hit the proper voltage with the battery load. Also, I realized with that approach that I would only be getting a proper charge on the battery for 2-3 hours of the day. I went ahead and made an 18 volt panel at that point. I decided I would rather waste PV power during the high sun hours so that the panel would produce usable voltage for more hours.

Thanks for the help!

[Kebast]

Forgot

I forgot to say, the new panel should be right at 1 amp max. That is just under C/4 so it should be OK. Otherwise, I'll have to find a different circuit.

[Kebast]

Got it going!

So I finally got something put together. Might be the ugliest thing I've ever built!

Attached a couple of pics.

2012-05-23 16.17.24.jpg2012-05-23 16.17.16.jpg2012-05-23 16.18.21.jpg

[Mike90250]

Red-Green sez you need more duct tape, not masking tape

redgreen_480x360.jpg

[bcroe]

Current

[QUOTE=Kebast;46568]So I finally got something put together. Might be the ugliest thing I've ever built! QUOTE]

If you do get to 1A, that LM317 is going to be hard pressed without a heat sink. Turn it into a switcher,
and you can make more current instead of heat. Bruce Roe