800 watt off grid PV system

[wildcat]

I have a small 12 volt system I have been tinkering with. Started with one 100 watt Renogy mono-crystalline panel and the PWM charge controller that came as part of the kit. Originally had been working off 12 volt batteries for my battery bank.

Over the last ~5 years I've slowly upgraded and expanded to my current setup: Eight 100watt Renogy panels, wired in two strings of 4 panels wired in parallel. Each string of 4 panels is hooked to its own 30amp Renogy MPPT charge controller. I have a 1500 watt inverter/charger. Currently working off one string of four 6volt batteries wired in series and parallel to make a ~400 amp hour 12volt battery bank. I'd like to expand this in the future to give me more storage capacity.

Goal for my setup is to run an electric fencer, small 10 cubic foot fridge, a few 12volt LED lights, and charge phone/PC/etc - which it has been doing just fine. I'd like to add a MOSFET battery protect switch between the battery bank and inverter/charger to be able to adjust low voltage disconnect to 12.2 volts instead of the 10.5 that is preset for inverter. I also like the feature of auto reconnect once the voltage rises back up that my current inverter does not have. I know the MOSFET doesn't take reverse current. My thoughts are that I can use two DC disconnect switches and add an extra positive circuit to bypass the MOSFET if/when I do need to top batteries off with genny. Will this work? I haven't been able to find any solid information online from retailer or manufacturer. It is a 220amp Victron battery protect unit that I am looking at using. Also, is this unit big enough?

Any suggestions for improvements? Constructive criticism on overall setup?

I'm trying to avoid having to buy a high dollar super fancy inverter if I can add components to my existing setup that will do the same thing.

[Dave Sparks]

Are you offgrid?

If so, you really can't expand a (normal) battery bank. If the batteries are mixed up in strings and different ages it is just a poor practice.

My comment is that you should just buy something decent from outback or schneider and use a 24V inverter. They have good warranties and will last a long time.

If this is not offgrid and just an exercise, be sure you wire this gear safely and follow the standards for wiring electronics. Would not want a fire right?

[MountainDon]

Dave knows his stuff.   In simple terms when one mixes newer lead-acid batteries with older lead-acid batteries the new batteries act like the older ones. Reduced capacity, etc. The internal resistance will differ and that can cause problems. It is best practice to purchase batteries from the same manufacturing batch.  Sometimes that can be difficult.

I second the comment about buying good quality equipment and basing it all on a 24 volt system. Series is better than parallel. Batteries can last longer.

You mentioned designing add-ons to change the LVD to 12.2 volts instead of the 10.5 that is builtin to the inverter you have. I believe that the reason 10.5 volts is chosen is mostly to protect the inverter from damage. With a 12 volt battery bank you may find that under heavy load the voltage may easily drop momentarily to less than 12.2 and rapidly rise when the load is removed. With an automatic reconnect this may result in the inverter cycling on-off, on-off repeatedly.  Dave will correct me if that is not true or not likely.

We have an Outback based system running on 24 volts that is 11 years old now. I love it. Super inverter. It just works with no hiccups; other than a lightning strike and that can happen to anyone.

A benefit of buying all Outback or all Schneider is the charge controller & inverter can be tied into a system where everything can be monitored readily, even remotely.

[wildcat]

I guess I should have been more specific - never know until folks go to asking questions. I've got it all in my head just doesn't always make it out. I literally just have switched over to the 6volts GC batteries a couple weeks ago. I plan on getting another 4-8 batteries in another month or two. My local supplier had a large number of these and said he doesn't sell many this time of year, and shouldn't be a problem for me to get that many out of the same batch as what I got these from. He may or may not have them set aside for me as well,  just wanted to see how the 6volts were going to do compared to the 12volt batts I was using before I invested a big chunk of change.

I had been using 12 volt system because my current inverter is 12 volt. It is modified sign wave inverter/charger with the bypass function when plugged up to generator to top off batteries. It came out of an old snap on tool truck and was free to me. I fire the genny up when I need large loads. I'm really only running 10cf fridge and electric fencer and a few super efficient 12v LED lights that use 5 watts on the high setting. I'd like to keep using this inverter for the time if possible, until I decide which route I want to go long term - 12 or 24. But I need something in the short meantime to protect my batteries- which brought me to the MOSFET. It's rated for 220 amps, formula I found said for every 100 watts drawn from inverter requires 10 amps, so that's 200 amps for a 2000watt inverter? The working load on my current inverter is 1500 watt with a 3000 watt surge, I think - it may only be 2000 watt surge. Research said the MOSFET is designed to handle 70% of what is rated at best I can decipher. So my two questions there are is it compatible with my current inverter, and secondly since they aren't designed to handle reverse current, would a couple of DC disconnects and an extra circuit from inverter to batteries AROUND the MOSFET allow me to still use the charge function of the inverter? I'm thinking yes, but I haven't found where anyone has done an application like this with an inverter/charger, only a regular inverter. Also the disconnect and reconnect voltages of these units are programmable - I was planning on 12.2 and 12.8 as the cut out and in points?

I'm not against an eventual changeover to 24v as all my equipment but the inverter/charger is made for 12 or 24, was not in a hurry to change because of current shading issues where my PV array is. My understanding was wiring in parallel helped negate some of the issues if panels were shaded? I've also just gotten to my current setup of 2 strings of 4 100watt panels, been buying them in pairs as I could afford them and thus hooking up in parallel. Same manufacturer and ratings on the panels in each string.

I have Bluetooth monitoring on my charge controllers and the MOFSET switches are Bluetooth capable, anything more won't do me any good because I don't have WiFi at this location.

Thank you all in advance. I am currently working my way through the 61 page pinned solar thread trying to glean anymore useful info that I can. You never stop learning!

[MountainDon]

I cannot comment on the efficacy of what you propose with the battery low voltage protection.   

If you are planning on staying with a 12 volt system and increasing the number of batteries by adding another 4 to 8 may not be a good idea for battery longevity.

Even from the same batch, there may be, will be, small differences in internal resistance between batteries. Over time and charge/discharge cycles this IR difference can cause some cells to reach full charge sooner than others. They can end up being "cooked" while the charge controller tries to bring the cells with higher resistance up to full.  Do a search on parallel lead acid battery problems. There are references to this issue out there. Sorry I have not got links bookmarked.

Have you had a look at the forum over on solar-electric.com.... Northern Arizona Wind & Sun?

More batteries also means more cells to monitor, more cells to keep watered, more connecting cables to maintain.  Bigger batteries or bigger cells are better than more smaller batteries, IMO.

Speaking of more connections... in creating parallel strings you should try very hard to make the total length of the parallel connecting cables the same length. Let's say you have 3 parallel strings. There are 3 negative cables and three positive cables each connecting to a common neg. and a common pos. point. Maybe a buss bar. The total length of the neg and pos pair of each string should be the same length. That's to help keep the resistance in each parallel string the same, or as close to the same as possible.

[Dave Sparks]

What Don says   Seems crazy to do this with 12V. Seems a bit dangerous also unless each string is fused. It hurts my head also to think of putting time into an outcome that looks bad on paper. Sorry to be blunt.  Remember no shading offgrid! NONE !

[wildcat]

I'm not grasping why I can't add additional batteries within 4-6 weeks of the ones I just installed, especially when I am reading many of your responses to others stating that a year is the cutoff point for which you would want to add any new batteries to an additional bank. I understand purchasing and installing all at once is ideal, just not feasible at the moment. Are you really saying that I can't add batteries to the ones I just installed 2 weeks ago?!?

Forgive me for being obtuse, but other than batteries not liking being paralleled and being able to use smaller gauge wire, I'm really not grasping what the benefit of 24v would be for my particular situation. I don't have any long wire runs, the system is at point of use. I don't have any large loads. The 10cf fridge is the largest load that will run off this. Kill a watt meter says it's ~500 watt surge and ~60 watts running. I'm working on rectifying the shading situation, but it's a one woman show around here and I can only cut down and clear so many trees by myself in a year.

I've figured on approximately 1kw of use a day, and that is with overestimating and rounding up. Battery temperature compensated to 20F, 2.5 hours of sun per day, and ideally I'd like to have about 5 days worth of capacity. Calculator I used said between 1.5-2kw of storage to accomplish that. It also said 600 watts of panels, I have 800 watts.

I do keep all my wiring identical lengths. OCD pays off sometimes. Everything has inline fuses in appropriate locations, although I am considering combiner boxes just to clean up the rats nest of wiring and branch connectors.

I want this to be safe and the most efficient as possible, in terms of producing the most power from what I have. If 24v is really the way to go then I guess I will start shopping for an inverter and a charger.

Thanks again for your responses and again, sorry for being so dense. I've been reading and researching from every resource I can find, but so far it still hasn't helped me understand.

[MountainDon]

I was not saying you cannot add batteries after even a short period of time.  If they are from the same batch you can probably not have much of an issue adding some. Adding batteries from a different batch can lead to mismatch problems. However, I was saying that more than two or three parallel strings of batteries is a poor idea. (three at the most).  You mentioned possibly adding 4 to 8 more batteries to what you have.  If you already have 2 parallel strings at 12 volts, 4 more batteries gives you a total of 4 parallel strings, 8 more batteries gives 6.  That is just too many parallel strings for the best battery life.

24 volts series strings cuts the number of parallel strings in half. That is one of the advantages of 24 volts. A 24 volt battery bank should outlast a 12 volt bank of the same capacity.

60 watts running seems low for a refrigerator, but great if correct. Have you compared what the kill-a-watt reads on something more readily verifiable, like an xxx watt electric resistance heater?    I believe a kill-a-watt meter does not have a fast enough sampling rate to accurately measure surge.

I had meant to comment before that a modified sine wave inverter can cause electric motors, like those in a refrigerator, to run hotter and make more noise than a pure sine wave inverter. A hotter motor means a shorter service life. It also means wasted power.  Some devices like microwaves hardly work at all on modified sine wave. Just FYI.

The more neat and tidy the cables are, the safer the installation when doing future maintenance.

[wildcat]

I guess I'll start shopping for a 24v inverter/charger. Better to wire the PV panels in 4 sets of 2 .... or 2 strings of 4?

Thanks again.