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.

[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 !

[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.