Sizing Off-Grid Systems And Using Gen Support

Two of your batteries in series is a string. In your case a 24 volt string. Pretend that your two batteries are just a single 24 volt battery. Then you have four 24 volt batteries that are in parallel. Your parallel wiring is way wrong. Look at the smartgauge site and pretend that each of those 12 volt batteries is one of your 24 volt batteries.

The smartgauge site is about how to take four strings of batteries and put them in parallel. It doesn't matter if the string is four 6 volt batteries in series (24 volt string) or three 12 volt batteries in series (36 volt string) or just one 12 volt battery as they illustrate on the smartgauge site.

Having said all that, Paul is absolutely correct:

--mapmaker

wiring pannels

panelwiring-solar-power.jpgpanelwiring-solar-power.jpgHi all I have just ordered some more MC4 connectors to re wire the solar panels as suggested above.
Can anyone do a quick sketch and upload it please so I am absolutely sure of the wiring, please

What or how do you suggest I wire the batteries please.

and a quick question

with the spare panel can I wire it into a spare 30amp none mppt controller and attach it to the batteries? and if I can do I wire it to the lead batteries or at the other end of the set?

thank you all for your help

regards Derek

if using the single panel is crazy let me know!!

on another note, if the charge controller is a 40 amp one, will it be ok to put the 4panels as suggested this will give me 32amps at 24 volts is that correct

You CAN NOT use the single panel, because when the 24 volts battery bank under charge the voltage usually rise higher than 26 volts, it is normal it reach to 28 volts, your single panel is only 26.5 volts so it can't charge the battery bank.

As for how to wire the panels, you wire 2 in series and other 2 in another series then parallel the two series. if you not sure how to do it, please ask some one know so you won't burn up any thing.

wiring pannels

hi thanks for the info, did you see the picture at the top of my last post showing what I think you mean, with reference to the wiring of the solar panels

regards Derek

Ok yes, the panels wiring is correct. with the batteries. it gone bad already. you already have all the batteries. What we suggest is when you buying batteries, you buy the largest ah per battery and wire them in ONE series, so you don't have to parallel the batteries. I have two bank of batteries in parallel, it give me hell to balance the charge and discharge rate of each bank. can't get them equally.

You can try to wire them like the link mapmaker give you. just treat 2 batteries as one in series and try to parallel the strings the best possible way.

I don't totally understand how people have so many problems with parallel batteries. It's not uncommon to see semi trucks, off-road equipment, and large generators with six or eight 4D or 8D batteries in parallel. We have three parallel strings on our off-grid system with battery monitors on each string and I have yet to ever see them get out of balance. Amps on each string is within 0 - 0.3A on both discharge and charging. Used to run six parallel strings with the same batteries when we had our stacked 24V inverters, and never had any problem then either.

My guess is that you get all your batteries on the same time at the same vendor and they are all the same identical batteries. Mine are from different vendor, different time and different size batteries even though they are close enough. Mine strings are about 1 to 10 amps different on each string depend on the load and the charge rate.

Yes, ours are all the same. But I have run totally mismatched parallel strings as well in the winter when I take our RV batteries out and put them on our off-grid bank and work the snot out of them all winter instead of leaving them set around on a battery maintainer. Those camper batteries are not even CLOSE to the big RE batteries, but yet they still do their part and end up at the same SOC as the big ones on either discharge or charging. And there is no cross-currents in the bank with those camper batteries on there. I put a TriMetric on those as well. When I watch during charging and the net amps to the batteries falls to zero, the amps to the camper battery fall to zero at exactly the same time. Then watch closer and the system starts to take amps from the battery bank. The camper batteries go to discharge exactly with the rest, and put out the amps they are capable of at the particular system voltage at the time.

So Big Deal if the parallel strings are mis-matched. It still works just fine. I have done this with our camper batteries for almost 10 years and have yet to destroy any batteries, or see a problem, because of it.

Chris I believe with your years of experience of using and maintaining a battery system you have developed a skill that most others may never get. Through trial and error you have found the (excuse the pun) "battery" limits of your system as well as how to not only take care of them but bring them back from what I would call an early death.

For the rest of us Newbees when it comes to really understanding how to charge, maintain, monitor and return a complex battery system to good health we are just babes in the woods.

Solar panel wiring

Hi All I finally got around to wiring the panels in as suggested, now that the rain has stopped.
Although a very overcast day as soon as I tested the result I was rewarded with 45.5volts

I notice on the back of the panel that they could achieve 33v solar-panel.jpg
so when the sun is shinning I should get 66v ? is this correct? and if that is the case the mppt controller is rated at 40amps / 100 volts so just checking all is ok please

also one more question, If I was to add another panel to the one I have left over, and wire it in in the same way, does it also have to be 205watts as these are hard to come by now

thanks for all your imput Derek

If you can't get the same 205 watts panel, buy one that is a little more like 225watts or so. make sure the VMP and IMP are close enough like with in 5% different.

Same panels

Thanks for getting back to me, I must admit as soon as I connected the panels up the light on the controller turned from red to green in about 25 minutes and the batteries came up to 25.7 volts with in about 3 hours with overcast sky!

I will have a look out for a panel with a similar specification. Then if I do and tack this onto the end and wire in the same way, will that then give me about (I think the volts on the picture I showed was 26v not the 33 doh!) output about 80 volts? 24 amps?

with all the above working correctly could I then start running some more heater lamps as follows

1 @ 75watts
1 @ 150 watts

could the system run this 24 / 7 without any need to use a generator to top up even in the winter here in southern England?

again really enjoying the education

best wishes Derek

When you put panels in series you double the voltage but keep the same current. So twice the panels, places in series, give you twice the voltage at the same current, but an MPPT controller will convert that to twice the current at the same battery voltage in either case.

great post.. then may I ask you how to convert off grid inverter to grid tie inverter? how about the system block of the controller?

The is no legal way to convert an standard off grid inverter to work as a grid tie. There are Hybrid systems the allow you to connect to the grid as well as charge a battery system but that equipment is manufactured and UL listed. Making your own equipment to connect to the grid would never be accepted by the POCO or your local code team.