Thanks for all the great information you provide BHM readers. I’m planning to install a small PV system using the instructions in your article in issue #116. This will be a very small system and used primarily for back-up during a black out. I have a Uni-Solar US-32 panel and Sunsaver 10 amp controller. I’m installing the panel on a carport roof, (I live in Massachusetts), running the wires to connect to one or two 12-volt deep-cycle marine batteries in my attached garage. I’ll be using the 2-pole fused disconnect and correct wire size for the panel and ground wires you recommend.
I have three questions.
1. What can I use as a bracket to mount the panel to the roof? I’ve seen small RV racks for sale and wondered if there is an alternative mounting bracket that can be purchased at a hardware store.
2. The wiring diagram in your article shows the load wires going to two 12 volt lights, hooked-up in parallel. I plan on wiring one light for the garage and continuing the wires about 30 feet to be run to the inside of the house. Those wires will be hooked up to a wall plate with a 12-volt cigarette plug receptacle mounted on the wall. I want to use the plug for a 12-volt TV or any other 12-volt appliance/charger that works using the auto style plug. What do you think of this arrangement and do you have any suggestions or recommendations.
3. Do I need to vent the batteries outside if they are in an un-insulated garage? I plan on putting the batteries in an insulated box for the winter and wondered if I can drill a hole into the top of the box and let the batteries vent into the garage.
Thanks for the help.
Ernie Smith
Ernie,
I can give you some general answers, but since we would not know everything about a specific installation, we cannot be too specific.
Although most solar dealers offer an “approved” angle “foot” to attach any solar module to a roof, I have taken 1-1/2 X 1-1/2 aluminum angle and installed the leg standing up to the side of the solar module and the flat leg bolted into the roof framing. Keep in mind that if you do not use the pre-drilled mounting holes on the back of the module you will void the warranty, so you might try a combination of two short lengths of angle back to back to make a “Z” shape. This will give you a way to utilize the bolt holes on the back of the module.
As long as your loads are 12 volt DC and wired in parallel, they do not all need to be at the same location. However, I would increase the wire size to the more distant load as there is 10 times the voltage drop at 12 volts DC that there is at 120 volts AC.
Finally, a few RV or golf cart batteries out in a garage will not give off enough gas to cause a problem due to the large space, but don’t locate them next to a gas fired hot water heater. Hydrogen gas is only explosive when highly concentrated like in a small sealed up closet or battery box. If you do place the batteries in an insulated box, you will need a vent at the top. For larger battery systems we use a 1-1/2″ PVC pipe vented outside, with screening to prevent insects from entering. The pipe needs to slope uphill since gas rises, but you will need something to keep out the rain.
I wanted to thank you ahead of time for you assistance.
We are going to have an off grid solar panel system. Is it possible to wire two 6- volt batteries together both parallel and series to get double the voltage and double the amperage? If not, what are your recommendations?
Thank you so much,
Jenelle Sherstad
Jenelle,
You can wire two 6-volt batteries in series to make 12-volts. You can also wire two 6-volt batteries in parallel to double the amp-hours. But if you wire the same two 6-volt batteries both ways, you will get a lovely cloud of smoke from burning plastic insulation, lots of fire, and chunks of burning copper wire flying through the air. This is because you will be making a dead short from the positive to negative terminals on both batteries.
Either use four batteries, with each pair wired in series, and then the 2 pairs wired in parallel, or buy a really good fire extinguisher.
Would these batteries work as a battery bank — Caterpillar 175-4370-pho Cat lists these as heavy duty, deep cycle, deep discharge they are 12 vdc. I can get a good price on these from the cat dealer and they weigh 62#
On using a array of PV panels, for aesthetic reasons, would like to mount in back of house. This would mean a cable run of approximately 50 feet.
For a 12vdc system what size cable should I expect to use? Same question but for 24vdc? I have access to some free cable if right size. Also is it normally multi strand or single solid conductor.
Cable from battery to inverter, approximately 5 feet run. Expected cable size?
In choosing a inverter, if I use a 12vdc supply source (pv) would the inverter have to be 12vdc input or is there one that is selectable input voltage? I expect to increase the incoming voltage as I can afford 2 or 3 more panels.
This unit will not be grid tied. It will be used for my shop, Expected maximum power usage at any one time, 1000 watts, then would drop off. I can only operate one tool at a time.
Biggest load to be expected for short bursts (3-5 minutes) 115 vac wire feed (currently grid connected) other loads is just small hand tools, circular saw, drill, saber saw , one flourescent light etc.
Thanks
Choo
Choo,
The 12 volt Caterpiller battery you referenced weighs 62 pounds. A 6-volt golf cart battery weighs 63 pounds. Since you would need two 6-volt batteries to provide 12 volts, this means a 12 volt battery made from golf cart batteries would weigh 126 pounds, or double the weight of the 12 volt Caterpiller battery you referenced. The heavy weight indicates more lead plates which provides more amp-hour charge capacity. If your system will not cycle every day from full to low charge, then this battery might give you good life. However, most starter batteries are designed to provide one heavy discharge during starting, then several hours of slow recharge. Solar systems do not operate this way.
As noted in many past answers to email questions, we cannot provide specific design answers like what wire size to use as there are many variables we would not know about your specific application, and we do not have the time for this level of help. Please check the most recent article I have in Backwoods Home Magazine that describes how to make an off-grid solar system for a remote cabin which will answer many of your other questions.
I have 4 or 5 12V batteries similar to the Vision CP1270 Battery (that style). I want to put 2 of them in my truck to run any emergency items or simply to charge various items. I am going to have them charge from the regular system, but I need assistance on how I should run the wires. I am going to connect one DC to AC converter to them as well, to connect a laptop or other item. Can you provide information to or explain how I will want to wire them?
I am also investigating what it would take to use the other 3 to have as “backup” power to a small amount of items in my home and I am curious if you can give some advise as to how best utilize them for that if it is possible.
Thank you!
Jon Webb
Jon,
The information you have provided is not detailed enough for me to provide a specific answer, and our web site is not intended to provide free detailed system designs. However, I have published many articles in Backwoods Home Magazine that cover systems like you are planning and I suggest you check these articles which you can read on line:
I need to know how to wire 8 Trojan T-125 6 volt batteries to get 24 volts.
Thank you.
Orlando
Orlando:
First take four (4) batteries and connect the positive of the first to negative of second, and so on down the line. This requires 3 cables, so you should end up with 4 batteries in a line with an empty positive terminal on one end and an empty negative terminal on the other end.
Now do this all over separately with the other four batteries.
Place these two separate “strings” of batteries side by side.
Now connect the positive terminal of each separate “string” to each other — thats positive to positive.
Now connect the negative terminal at the other end of each “string” to each other — negative to negative.
Finally, when you connect this battery set to an inverter or other load, be sure to connect at the opposite corners from each other. This means, if you connect the positive load cable to the positive post of the first row of batteries, you connect the negative to the negative post of the second row of batteries. This balances the electrical flow so each “string” of batteries will have the same length of path for the electricity to travel, which forces all batteries to receive the same charge and discharge.
Remember to NEVER connect the positive to the negative of the same battery as this is a dead short and will let all of the smoke and fire escape from inside the battery!
I have two solar panels 80 watt each on my boat wired in series to make 24 volts 160 watt. I have two sets of two batteries also wired in series 24 volt. Start and house batteries. I also have a Morning star 24 volt 10 amp controller.
When I leave the boat I would like to charge both sets of batteries. When using the boat I would like to charge the house batteries only as they will be running a fridge etc and the start batteries will be charged via engine Alternator unless it fails at sea. So I will need a switch for off, battery 1, battery 2, or both.
Is there anyone that can help me with a circuit diagram required for this project?
Regards,
Simon Brock
Simon:
As noted on the web site, we cannot provide specific wiring diagrams and other detailed design assistance since we do not have all of the information about your specific application, and this is a free site so time is very limited for each reply. However, I can give you some basic guidelines. You have to be careful because if one battery gets really discharged, and the other is fully charged and you close a switch between them, there can be a huge in-rush of current from one to the other that could easily melt any smaller wires between them. There are 2 ways to charge these batteries separately without tying them together and have a potential melt down.
You can buy a battery isolator at the RV store which has a connection for a charger or alternator input, and two separate outputs to connect the 2 batteries. This device has diodes that will allow charging current to flow from the single solar charger or alternator out to 2 separate batteries, but will not allow one battery to flow back to the other. Although intended to connect one alternator to 2 batteries, you can use the solar charge controller as the charging source. Make sure the isolator has a common negative and one in and 2 outs. Just let both batteries stay connected to the solar charger. If one battery starts to get charged separately from the engine, when it raises the voltage of that one battery, the higher voltage will stop the flow in from the solar charger so all of the solar charging will divert to the battery not being charged by the engine because its voltage will be lower. You would not need any switches to do this, but your connection should be on the battery posts so the existing 1-2-both engine switch will not isolate the charger from the batteries.
The alternative is to use two identical solar charge controllers with one to each battery, and both connected to the same solar modules. The battery with the lowest voltage will receive most of the charge until they are close, then the charge controllers will be charging equally and sharing the same solar modules.
Be sure to install a DC rated fuse in each charger output connection to each battery.
I have some solar questions and I cannot seem to find the answer to them.
I am trying to connect my inverter to a sub panel but I am not having much success. First let me explain my set up to you:
I have a small solar panel charging 2 6-volt and 2 12-volt batteries (the 6 volts are in a series then to parallel with the 12’s).
I also have a charge controller hooked up to these. I then have my wires which go to my inverter. I then have a two prong pigtail which goes to a subpanel.
This subpanel supplies power to 2 outlets and 2 small fluorescent lights. This part of the system was not set up by me but worked with our previous cheap 250 watt inverter but this was stolen so we purchased a Cobra 1000 watt modified sine wave inverter and it won’t work. (By the the way this system is entirely off-grid. There is not even electricity in the neighborhood.)
I am confused as to why the cheap inverter worked fine yet this somewhat nicer one doesn’t. I have included some pics of the subpanel wiring. Any help that you can give will be appreciated.
Thanks,
Rod
Rod:
Although I have seen many small off-grid systems wired like this, before I make any suggestions, for the record I will tell you this would never pass an electrical inspection. There are many code violations and although most are not major, this really should be re-wired to code. Since this web site is for general questions and cannot get into specific design or code issues, I cannot be more specific than these general comments below.
First, since all of your circuit breakers are rated higher than the output of your inverter, they will never trip. Any over-load condition will shut down the inverter by its internal overload or temperature safety limits. Although this inverter is most likely too small for what you are trying to do, it still has a 2000 watt surge rating for a brief period. This equals about 18 amps which is greater than the temperature rating for your “plug” assembly, and this is un-safe. Your inverter problem is most likely due to the mis-matched battery bank being too small for this larger inverter. The battery bank is the first place I would check. Now that you have an inverter that is 5 times larger in capacity than your old inverter, it is drawing 5 times more amps from the battery bank. For example, at the full rated load of 1000 watts @ 115 VAC, this equals almost 100 amps draw at 12 volts DC assuming a 90% efficiency. If you added in the surge load, this inverter could draw up to 200 amps for a very brief period. If you are using car batteries or lightweight RV batteries, or if they are deep cycle batteries but are over 3 years old, you will most likely need to replace them all, and do not mix and match. Use the same size and voltage batteries if you have more than one.
For the past couple of years I’ve been scouring the local Craigslist for used PV panels in preparation for my ultimate plan to move off-grid. I have been very fortunate in that I have amassed approx. 1.3 (rated)Kw of 12v nominal panels for an average of just about $3/watt. I realize that when dealing with this much rated capability it is normally recommended to up-size to a 24 or 48v system, but to keep things as simple as possible (KISS) I’m planning on sticking with 12v.
The drawback to this is that they are a mish-mash of manufacturers, sizes, and specifications (which also makes it difficult to wire in series for higher voltages). My primary concern lies in the fact that the open circuit voltages range from around 17v up to over 21v. It is my understanding that (in layman’s terms) voltage is roughly equated to ‘line pressure’.
With this in mind I’m wondering if the panels with a higher voltage specification would generate a ‘back pressure’, as it were, preventing the panels with a lower voltage specification from adding to the overall current (kind of like a garden hose trying to augment the flow of a fire hose). I asked an engineer friend of mine who’s been living off grid for almost 20 years now, and even he was stumped.
Chuck Farley
Chuck:
You did not say how many modules you have, but with a total of 1300 watts and all are nominal 12 volt, I assume we are talking about 22 to 26 modules. With your desire to keep everything at a nominal 12 volts, this means you will have all 22 to 26 modules wired in parallel, and that is not good. There are many reasons why this is not practical, but the big one is the UL label on the back that says the size of a series fuse required. For example, a typical module in this size range will probably be listed as requiring a 15 amp “series” fuse. If the modules have an average output of 5 amps, this means if you have more than 3 modules in parallel, it is possible for one module to have an internal “short” and all the current from the other 3 modules will be routed through this failed module and could cause a fire.
To reduce the number of parallel modules, I would set this up for 24 or 48 volts nominal. This means the modules will be wired in “groups” of 2 or 4 in series. Even with identical modules with the same model number you can have as much as a 10% difference from one module to the next, so you want to “match” the strings. Lay them all out where they receive the same amount of sun at the same time, then go down the line on a clear day and mark on the back what voltage each is producing. Next, using a meter that can read DC amps, short the positive and negative leads through the amp meter and also note this reading. You need to be fast as any change in sky conditions will throw off your readings.
Now try and “match” high and low modules in the same string so each separate string has the same voltage and amperage. The problem is not in having slightly different voltage modules in series, the problem is avoiding having multiple strings in parallel that have far different voltages. This is the same problem as hooking up different batteries in series. You will always have one or two individual cells wired in series that are lower or higher than the others, and the battery will settle on a voltage that levels these highs and lows. But if you have batteries with different voltages wired in parallel, the higher voltage battery will try to “charge” the lower voltage battery.
You should use a good quality MPPT type charge controller which will try to sort out any remaining mis-match. If your modules are really mis-matched and it is hard to do the above, you may want to use multiple charge controllers so you will have fewer modules wired in parallel. These can still charge the same battery bank.
