Battery Question for the "PROS"

[Radar67]

I'm working on setting up my first solar panel. I'm wondering if there will be a problem with different AMP capacities of batteries?

Here is what I have...

I'm connecting a 15 watt solar panel to a charge controller to keep a couple of batteries charged. I want to connect the controller to a positive and negative buse bar, then connect the two batteries to the buse bars in parallel. One battery is 12 volt, 7 amp hours (to ocassionally run a small 12 volt light in my shed) and the other battery is in my tractor (12v 300 amp hours). I want to keep the battery fresh in between uses for this. This is my idea for the set up.



Does anyone see any problems with the set up, or have any recommendations?

Stew

[scsmith42]

Stew, I would not label myself as a battery pro, but I do know that you're not supposed to mix and match different batteries in the same string.  As a matter of fact, you don't even want to mix batteries of the same type but different ages in the same string.

The charging rates and float voltages will fluctuate, and you'll have problems.  In an extreme instance, you can damage a battery and potentially end up with an explosion. 

In a previous job I worked with the cell site operations teams at a major wireless company.  We had thousands of batteries used in our network as back-up power at the cell sites.  During preventative maintenance, if we tested a string and found one bad battery, if we just replaced that battery chances are that within a few months the entire string would go bad.  I recall at least one cell site in a different market where the batteries within the string had been mixed (different types and capacites) and the battery bank later exploded - not a pretty sight and a quarter of a million dollars of electronic equipment destroyed.  So, we would replace the entire string, and then take the three good batteries and set them aside, waiting for more used batteries to come from different sites and strings.  We would then make up a new "used" string from batteries of the same age and type, and redeploy it.  We finally got to the point where we would not reuse the batteries at all, because uptime for our customers was more important than saving a few bucks by redeploying used batteries.  However, I know that you're in a different situation.

The best thing to do would be to have one or two batteries of the same age, type and model on your string.  Also, you want to get "deep cycle" batteries, (such as marine batteries), as they are designed to be fully discharged and recharged multiple times w/o failing.  I recall that the lead on the plates is a bit thicker to withstand the deep charges.

Hope this helps...

Scott

[DanG]

What Scott said is correct. However the application is so vastly different, that you could do what you're proposing without much risk.  The difference is, those cell-sites are running off the batteries, 24-7, and they are constantly pulling several hundred amps from them while charging them constantly.  In a small, low current situation, it should be just fine.

Radar, the drawing you made is just a simple parallel circuit.  The only potential problem I can see is that a bad battery could draw down the other battery.  You could avoid this by inserting some diodes between each battery and the buss to prevent back-flow of current.

Also, make sure the batteries are in a well ventilated area.  The gasses produced when charging batteries is not only poisonous, but explosive.

[Radar67]

Thanks for the reminder on ventilation Dang. I was also considering the blocking diode...should it be on the positive or negative lead to the battery?

As for the current situation, the panel only gives 1 AMP of output per hour, so that should not be a problem. The system was designed as a battery charging system. The idiot sheet says it will handle 2 car sized batteries that are in good condition, as long as they are wired in parallel.

Scott, I understand what you are saying about the strings of batteries. I don't plan to discharge my batteries that much, this is more of a float type charge situation. The main charging on the tractor battery will be done while the tractor is working, but sometimes, there is a break in time of "working" and I want to keep the battery fresh.

Stew

[Fla._Deadheader]

Might be updated info on Solar Panels, but, we never used a charge controller on one panel. A panel putting out 33 watts or more, MIGHT need a controller. It does take juice to run the controller, or, at least it did back in the 70's-80's.

  Also, we always put a Diode between the panel and batteries, to prevent slight discharge back into the panel.

[Radar67]

FDH, the controller is called a Battery Minder. It monitors the charge levels on the battery and reduces power after the battery gets to a certain level. It also desulphates the battery. The wife picked the system up at northern toolhttps://forestryforum.com/board/index.php?action=position"> Note:Please read the Forestry Forum's postion on this company as a surprise Christmas gift (item #167987).

Stew

[Fla._Deadheader]

Yeah Stew, same old-same old.  Ever hear of a battery "trickle charger " ???

  It's a wall plugged in charger, that puts 1 amp or slightly less in a battery, all the time. Batteries DO self discharge, a little at a time.

  Go ahead and use it. You might bypass it, if yer batteries are not fully charged, though.

[Radar67]

Yep, I'm familiar with the trickle chargers. No power on my land, so this will be a good learning experience.

Stew

[Fla._Deadheader]

Well then, this guy's site you NEED to visit.

off grid site

[scsmith42]

Stew, you'll like the Batteryminder products.  I have that same solar panel setup - haven't hooked it up yet but it's on the list to do soon.  I've got about 8 of the regular 120V Batteryminder desulphanator chargers and absolutely love them.  They've more than paid for themselves in terms of increased battery life span.

I'm planning to install my solar charger on my Brute trailer.  I've got two 12V batteries on it - one for the winch and the other for the hydraulics.  Although they charge off of the truck, I figure that the solar panel will keep them charged if I go a few months w/o using the trailer.

Let me know how you like your setup.

Scott

[Greg Cook]

Stew, I'm with Fl. Deadhead on this one. The charge contoller won't be necessary with a 15 watt panel, unless it contains a blocking diode and then you won't have to wire one in. The one amp charge rate isn't going to overheat the batteries with the 300 AH wired in.  I'm not sure that you'll have enough amperage to desulphate, either. Remember that on hot days, when your panel really heats up, your power output drops pretty quickly. Good Luck!

[Radar67]

FDH, that is an awesome site. It gave me so many ideas I haven't even considered...now I got ta go back to the planning board.  :D Are you working on the diesel bike he shows?

Greg, the desulphator feature on this setup is designed to keep the battery desulphated. You first have to desulphate the battery using a 120v desulphator.

Scsmith, when I start setting this up, I plan to take plenty of pictures. I had the same idea for my tractor...it may set 2 or 3 weeks between uses and this setup should keep the battery fresh. So far, just looking the panel over, I like what I see. It seems to have a good quality glass on front and back and the aluminum frame is by no means lightweight.

Stew

[beav]

  Radar 67, the biggest concern is that the batt. types are the same. A flooded lead acid (fla) type batt. can withstand a considerable overcharge, and is an actual maintenance procedure known as an equalization charge. If the batt. is a gel cell (maintenance free,vrla,etc.) the voltage required to fully charge it is less. If both of these batt.s are in the same string either the fla will never fully charge, or the gel cell will boil out and die.
I am guessing the tractor batt. is a fla, but the 7a?
Generally gelcels need to be limited to 14.1 volts, and fla's can go over14.4(temp depending)
I would use the charge controller on the small batt. One amp will fill the lil pup in a day or two.The tractor batt will eat up 1A and ask for more.The tare current on these small controllers is small,and worth the peace of mind. If it is the harbor freight controller,throw it away.

[Radar67]

Beav, the controller is set for 14.2 volts max, if I remember correctly (I'll have to check out the spec sheet when I get home). The 7a battery is a gel cell. The whole purpose of connecting to the tractor battery was not to charge the battery, but to keep the battery fresh between uses. (sometimes the tractor is not used for 2 or 3 weeks). That is good information to know for my house design though.

Stew