Off grid solar system for maintaining half a dozen batteries (ATV's, tractors, boats, etc).

[sobee]

Hi, I have a shed in my yard that has no power to it. The shed stores half a dozen "toys" that have batteries. I currently run a cord to the shed, and move a battery maintainer between the different pieces of equipment, during the winter, but I'm not as diligent as I should be. I would like to have a maintainer on each and a source of power to the shed that doesn't involve a cord running across the lawn. Is there an easy way to do this with solar? I'm brand new to solar. I've read enough to know that it's not safe to connect a single charge controller to multiple batteries of different sizes/makes/lead acid/AGM etc. I'm wondering if it's possible to install a single 100 watt panel, and connect half a dozen charge controllers to it (one for each battery). Would these charge controllers work for small ATV batteries? I suspect they were designed for larger batteries. Is it possible to send the current from a single panel to half a dozen charge controllers? How do you make these connections? Would the charge controllers function on the little bit of current they are getting and maintain all six batteries simultaneously or is one controller going to get most of the current?

Thank you for the thoughts/suggestions. If there are references to equipment in your reply, it might help to know I live in Alberta, Canada.

 

[GXMnow]

There are several ways this could be handled. If all of the batteries are the same voltage (12 volt lead acid) They could all be paralleled and a solar panel and charge controller could hold them all at float voltage all winter. Float current is usually quite small, so even a 100 watt panel would likely be enough. Even in winter, you should hit close to half power. The cells actually get more efficient when cold. 50 watts at 12 volts is about 4 amps. So you still have more than 0.5 amp for all 6 while the sun is on the panel.

The next step up would be a tiny off grid system with it's own battery. Then use a battery maintainer on each battery. This is not as efficient, but it has the advantage of giving power full time and you could run an inverter for AC power and have LED lights in the shed. Figure 300 watts of solar, a good charge controller, and a decent sized deep cycle battery.

 

[sobee]

Thank you. All my batteries are 12 volts. I had understood from another website it wasn't a good idea to charge in parallel if the batteries weren't identical sizes/ages. Is that faulty thinking? I've definitely got quite the mix of sizes and ages of batteries. Some are AGM. Some are lead acid.

I'm not sure if this is a fair question as it will be jurisdiction dependent. However, I was curious if it's common to need an electrical permit for an off grid solar installation? I'm in Canada and our governments up here love rules......if I could keep this all low voltage (e.g. 12 volt DC), perhaps I wouldn't need permits?

 

[GXMnow]

I certainly can't speak for the rules where you live.

If you keep it all low voltage, typically under 24 volts (up to 70 in some places) and make it a "portable" system, most will usually look the other way.

If you were trying to run high currents, then the batteries need to match to share the load, but for a low current charge, there should not be any problems. To run all the batteries in parallel, they should be the same chemistry. Therefore the float voltage will be the same. There are slight differences between FLA, AGM, and GEL types, but their float levels are all pretty close since they all work on lead acid. Each battery should have it's own fuse, so if something does go wrong, it should not melt any wires. If you want a little more isolation, you could put a 2 ohm (at least 5 watt rated) resistor on the lead to each battery. At 0.5 amps, 2 ohms will drop a full volt, but as the battery tops up, the current will fall and the voltage will come up more, but if one of the batteries has some leakage current, it won't soak down the others. Separate chargers would be better, but to just maintain them over the winter, I think this would work just fine. Check the batteries before you connect them, make sure they are all charged at a good long term float voltage. Typically 2.25 volts per cell, or 13.5 volts for the 6 cell 12 volt batteries. 13.25 volts at 25C is in the float range of all lead acid type cells, FLA, AGM, GEL, VRLA, and Sealed. The charge controller should also have temperature compensation. The voltage should increase about 2mv/cell/deg C as you go below 20 degrees C. At 0C on a 6 cell pack, that comes up to 13.74 volts. If you use just a solar panel and a charge controller, the batteries will only get about 2-3 hours of real charging each day in winter. That should be fine as long as there are not static loads on them. If you do measure any load, it might be a good idea to install a cutoff switch, or just disconnect the cable from the vehicle.

I would also suggest you separate all of them before you power up any vehicle.

 

[rin67630]

You are correct. It is NOT a good idea to link them all up together.

Hmm, I would not be that stringent.

We are not speaking of very high-power batteries, and we just need a trickle float charging.

OK AGM and flood have slightly different voltages, but not really that different. Putting all them at 13,8V float will not be wrong.

If you really want to be smart with your batteries, you might additionally full charge them to absorption individually once or twice during the winter, that should be enough.

So, if all of those batteries are initially charged to 13,8V there is no reason not to connect them in parallel and float charge them altogether.
Normally in Canada, I will not expect a 100W solar panel to deliver a lot of energy on overcast weather. Expect may be 5 Watt, for a couple of hours, and maybe 25Watt on the few sunny hours you may expect.
But that is absolutely enough to float charge all batteries during the winter.

So I would go for a 100W panel, a cheap PWM SCC and 1,5mm² wires, that will be plenty enough to maintain everything charged, so you can enjoy your toys well charged for the next spring.

 

[rin67630]

However, I was curious if it's common to need an electrical permit for an off grid solar installation?

You will probably not get any trouble from the government, but your DIY installation could probably become an excuse for your fire insurance, not to pay in case of a mishap. They LOVE to find those excuses, even if that has absolutely nothing to do with the fire cause. It is up to you to evaluate the risk...

 

[GXMnow]

Agreed.

The individual 12 volt lead batteries should all be fully charged seperately first. Then they can be connected together to float in the mid 13 volt range.

If one of the batteries is at a higher state of charge, it will lose charge into the other batteries. A weak battery, or one with leakage current can pull down the others. But if they were on separate chargers, that weak leaking battery would probably die. With the other batteries helping to hold it up and the solar during the day, it might survive the winter. It would probably be a good idea to have a voltage monitor so you can check in on it. If the voltage does start to drop off, you should be able to run your cord out and save them. Solar panels do lose some output when covered in a few inches of snow.

As for the insurance issue... Yeah, they don't like to pay any claims. If you use portable gear and have it set up in a safe manner, you should not have any real issue. But if it is cludged together and looks sketchy, that is a different story.

One old story, that may be fake goes back to the old rule, it is not illegal to be stupid.
On guy was seen pouring a liquid in his building hours before it caught fire. He ended up in jail for arson.
Another guy admitted to using gasoline to clean his gun. It spilled on his lit cigar and burned his house down, they honored his claim.

 

[45North]

How often do you use these "toys" in the winter?
Have you considered bringing the batteries into the house (basement) and keeping them charged on AC and then just taking them outside and installing them as needed?

 

[sobee]

I don't use the "toys" that often......I have taken batteries out just as you suggested. Shed is too small and not enough doors, so sometimes I have to start the toys to get other things out of the shed. It's a pain when I'm lugging batteries around. One of the toys has a slow drain (not sure why), so it needs a tender in the summer as well if I don't start it every 2 weeks. And then when the mower battery gets old, a tender can keep me going for another season before I need to replace it. Thus, I would really like to have the ability to charge in the shed.

Is there any reason (besides cost) not to wire 6 charge controllers in parallel to one or two solar panels as per George65? The costs of the controllers are quite low. It seems more "idiot proof" to me in that if I happened to connect a bad battery it wouldn't wreck anything else and I wouldn't have to be monitoring battery voltage before connecting every battery into the system every time I disconnected one.

I'm not very familiar with solar. I assume it's fairly "plug and play". Is it even possible to connect 6 charge controllers to a single panel?

 

[HRTKD]

It all depends on how much you want to spend and how much control you want over each battery.

My initial thought was to install a good size battery bank to support an inverter. Then run individual battery chargers off the inverter.

 

[rin67630]

A weak battery, or one with leakage current can pull down the others.

Weak batteries get to 99% a high resistance and will not drain the rest down. But it's a good idea to have a look from time to time.

Overcast in my observation drops panel output to 10-20%.

With 5% you are closer to reality, and 1% with 5cm of snow.
But it does not matter, if you dont get a float charge for 2 weeks...

 

[rin67630]

The costs of the controllers are quite low.

It is not a matter of cost, but every controller will have a quiescent current and that would draw down your batteries more than everything else...
the less stuff you have, the less trouble and the less risk.

 

[GXMnow]

If the cost is not a problem, I would go with a small off grid system with 300 watts or more of solar panels, a good charge controller, and a fair sized LFP (Maybe AMG will be better for a Canada winter) battery bank, and then either use DC-DC chargers, or an inverter and AC-DC chargers/tenders on each one. There are some minor risks with the parallel setup, but if cost is a big concern, It should work pretty well. A very bad battery that internally shorts should blow it's little fuse and take it out of the system. I did suggest fusing each batter, just in case. With the small currents we are talking about, each battery could be on a 1 amp fuse. Have the charge controller set to just 3 amps, for 0.5 amp max to each battery. Once they all reach the float voltage internally, they will pull much less than that. Even in the dead of winter, you should be able to hit 3 amps at 13.5 volts for a shot time each day. Make sure the panel is angled correctly to be flat to the sun at solar noon.

I used to deal with snow when I lived north of Chicago. Up to about an inch of snow and a fair amount of light would still get through. It is just water crystals. Sure, it will drop output a bit as the light is being scattered and reflected. Never actually tried measuring how much it dropped a solar cell output, but we sure did have days where there was snow on the ground and very bright sun. The glare was blinding. If the snow is fluffy, you can just brush it off. If it is the heavy wet stuff, and then the temp drops, then you have a pile of ice. No fun. We used to get "lake effect" snow when the wind would blow off of Lake Michigan. If you didn't get that cleaned off before the temp dropped, you were not going anywhere. 2-6 inches of rock hard ice encapsulating your car is a disaster. Good luck even getting a door open.

I have now lived in So Cal for 18 years. Only had a dusting of snow 3 times. I don't miss it at all.

 

[sobee]

Thank you for the education! I was thinking of mounting the panel on the south wall of my shed above a man door perhaps as an "awning". I'm at 51 degrees N. Is there a handy online calculator that will tell me correct angle? With the winds we get around here, I'm not sure I would have a lot of snow sitting on the panel.

One term I would like a bit more explanation on is quiescent current. Are you saying the charge controllers will deplete the battery they are connected to when there isn't enough sun to power them? A "low cost" charge controller on amazon suggests it's self consumption is <10 mA. A small ATV battery has 10 amp hours.....how long could I go without sun before that battery didn't have enough voltage to start an ATV? Is that easy to calculate?

 

[GXMnow]

Here is one site that give an estimate of how much power you can get at your location with different angles etc.

Solar Irradiance Calculator

www.solarelectricityhandbook.com

Anything connected to the batter has the potential of drawing off some current. quiescent current is just a fancy name for wasted power when it is doing nothing. A well designed charge controller should be minimal. 10 ma or 0.01 amps is not too bad at all. On your ATV, let's figure you want at least 6 of those 10 amp hours left to ensure it will start. So we could burn off 4 amp hours and still be good to go.

4 AH / 0.01 Amps = 400 Hours. 400/24 = over 16 days without sun.

 

[rin67630]

Perhaps the terminology was wrong. What I was thinking of might be called sulfated batteries where there is internal shorting across the plates from deposits in the bottom of the cell. That causes internal resistance and will pull the battery down and any other attached to it.

OK the very purpose of the installation was to void just that...

 

[sobee]

@GXMnow. Thank you for that website/calculator. That is quite handy. 39 degrees is optimum year round for my area, with 1.85 kWh/m^2/day in December. 24 degrees is optimum for winter and would get me 1.93 kWh/m^2/day in December. Both angles are sharp enough I wouldn't expect them to hold snow.

Now that I know the "solar irradiance", is there any way to predict how a panel might perform? For instance, I don't imagine a 100 Watt panel outputs 100 Watt, or it will only do it for a very short period of the day. Can I take that solar irradiance number and multiply it by the square meters of the panel and some efficiency number to come up with an estimate of real world output?

I think panels are 15 to 20% efficient, but I'm not sure if that is real world in Canada? How efficient are charge controllers? How efficient are batteries at converting power sent from the charge controller into electricity? Sorry, I'm an engineer by training (but no solar or electrical experience) so I have lot's of questions.

 

[GXMnow]

Solar panel ratings are based on an ideal sun exposure of 1,000 watts per square meter. The irradiance figure from that web site then directly tells you how much power to expect.

A 100 watt panel at 25 degrees C will put out 100 watts with 1,000 watts per meter of sun hitting it.
At your 24 degree angle, you get 1.93 KWH/meter of sunlight. So that would be 1.93 times the panel rating, or 193 watt hours out of the 100 watt panel in the entire day. You may actually do a bit better as it will be cold. Low temperatures increase the panels output voltage and will make more power. At 0 C, a typical panel could be up more than 7%. Making that a 107 watt panel, and making 205 watt hours. If you use a 300 watt panel, just multiply this all by 3.

The panel efficiency is already factored into the panel power rating. My panels are 1x1.5 meters, so if they were 100% efficient, they would make 1,500 watts in perfect conditions, but they are only rated at 300 watts. That is 20%, which is pretty good for current panels.

 

[rin67630]

Thank you for the education! I was thinking of mounting the panel on the south wall of my shed above a man door perhaps as an "awning". I'm at 51 degrees N. Is there a handy online calculator that will tell me correct angle? With the winds we get around here, I'm not sure I would have a lot of snow sitting on the panel.
...
Is that easy to calculate?

I would recommend to use PVGIS.
You place approx your home on the map, enter your panel power, and you get everything calculated in a breeze...
You have also calculators for off-grid planning including the probability to get an empty battery.

 

[rin67630]

If the battery was not isolated and was internally shorting ,it could drag the others down which would accelerate their demise.

Ok if you have a defective battery, that may happen.
You ought to notice that at the very beginning of the winter season after having left your battery fleet unconnected for a week, if one is going low on itself, you just can dump it and buy another one.

If you put all batteries on individual SCC's, without sorting them, the chances are big that:
a) you will not notice the defective one.
b) this SCC will have a higher consumption and draw most of the scarce solar energy onto that battery at the expense of the others.