battery temp compensation

[Vic]

Hi fiddler ..,

36 Volt systems are not common for off-grid power.  It will be difficult to find a quality Inverter for a battery bank of that voltage.

As td mentioned,  generally,  when adding PVs in series,  the maximum difference in Imp of two different PVs should be 10%.   A 5% or lower variation in Imp would be better.

And when adding different PVs in parallel,  the Vmp should be within 10% maximum,  with a 5% variation or less being better.

The above are basic rules,  but it you try to be at 5% or less variation in either case,  you should,  generally,  be fine.

FWIW,   Have Fun,     Vic

[fiddlerkelly]

I have mostly just lights at the  cottage 2 12 volt water pumps a 42 inch flat screen with basic receiver but plan on adding a 7 cut fridge with draws 1.5 amps and at times a laptop .Average draw on system will be approximately 4 amps /hour at most 8 hours a days little less in summer . Water pumps only run when needing water.

I heard so much about wiring batteries and got from it the less parallel is better.

[tecnodave]

Fiddlerkelly,

Please clarify one thing for me. Are all the panels that you plan to use from the same manufacturer?
The ones shown in the chart you uploaded?

If this is true then you have 144 cells in each string?   String voltage will be about 68.8  not the 160 volts that you would see with higher cell count panels.


If you do what you suggest, making a series string of different current panels of three 150 watt panels in series with a set of two paralled 50 watt panels , the two 50's will add  to only 100 and will throttle down the 150 watt panels to 100 watts so total string watts would be 400 watts , not 550 watts as you might calculate.

Now then if you were to try 3 of the 50 watt panels in parallel in a string of 150 watt panels it would balance.

The more info that you post the better that others will understand what you are trying to accomplish.

td

[fiddlerkelly]

Here is a formula I found for adjusting ST according to temp. Look right?

[fiddlerkelly]

Yes from same company and guy told be they can be combined. The difference in VOC is minimal.

[tecnodave]

Fiddlerkelly,

These panels are all "12" volt 36 cell panels but the current is what matters in a series string. The 50 watt panels are 2.91 amps and the 150 watt panels are 8.72 amps so if you were to put two 50 watt panels in parallel in a string of 150 watt panels you will throttle down the 150 watt panels to 2.91 times two or 5.82 but if you were to use 3 of those 50 watt panels the current would be 8.73 amps and would not limit the 150 watt panels.

Three 150's in series with 2 paralled 50's will produce 400 watts........they all act like 100's
Three 150's in series with 3 paralled 50's will produce 600 watts........they all act like 150's

This is not the way to design a new system but as all the panels are same make, all will be using same cells so this will work......add 1 or 2 more 50 watt panels and balance the strings.

Now that you have described your use a bit more I think definitely I would recommend 24 volt battery set,  a Classic 150 with MNBTS and Whizbang jr. and shunt. And use 68.8 volt strings.....144 cell...

The 48 volt comes into play when you have a washer and dryer and a bunch of kids to keep it full,
Small house, getaway cabin, etc.  24 volt will do all you need.  4 strings of batteries is not the ideal but your big ticket items , charge controller , inverter/charger , and your use of system should dictate battery size and voltage.

td

[vtmaps]

The guy I am buying off suggested putting two groups on roof comprised of 3 150 watt panels and a 100 watt panel made of two of the 50 watts that  I already have.

Technodave explained why that is a poor idea... the 50's will limit the 150's.

He also explained that you could put 3 of the 50's in parallel, and then put those in series with the 150's.

The problem with both of those schemes is fusing.... I really don't think it will be cost effective to do either of those schemes. 

I agree with Technodave that you should be at a system voltage of 24.

Probably the best approach is to buy another controller for the new panels and use your present controller with the old panels.  There will be no problem having two arrays, each with its own controller, connected to one battery bank.

--vtMaps

[tecnodave]

Fiddlerkelly,

Vtmaps said it best! For simplicity, keep it simple, all panels in string should be same for best results.
I am only using legacy 36 cell panels because I get them surplus for .10-.15/watt. If I had it to do over I would use the big standard, the 60 cell type commonly available, but I have 2 kW of "12 volt" panels and I have been doing Lektrix way tooo looong.

To make best use of what you have already ordered use strings of three panels. If you do a 24 volt battery bank this will be enough voltage at 52.2 volts, then if you want to use your 5 50 watt panels add one and make two strings of 3 each. This will work much better. Circuit breakers on each string, 1.25 times the max amps for that string.

td

[fiddlerkelly]

Yeah  I am considering selling the 5-50 watt panels and getting another 2 -150 same type as rest that way everything is identical and I gain an extra 50 watts .

[fiddlerkelly]

resthome

•Correction factor = (0.595 x T/ºF - 12.5) / 100 or 0.03 pts per 5 ºC

Jut done a quick calculation as spot check on one battery and was getting 1.3 on all 3 cells of 6 volts and ended up with result of 1.264 in each cell. open circuit voltage was 6.3 volts on one battery.
Sound like in good condition ? temp of cell was 65.2  F.

[tecnodave]

Fiddler,

That is a pretty good idea, have all strings identical, way easier to diagnosis a fail mode and repair.
My system is way too complicated, charge controllers from 3 different companies, panels from 5 manufacturers, 3 panel types.......that said......been an electrician for 46 years, would never do anything like this for a customer! Way too difficult to diagnosis a failure!


td

[tecnodave]

Fiddler,

I was writing while you posted so I didn't see your post.  If that is your specific gravity then you are ok on that battery. Full charge when new should be 1.265-1.275 , but it's important that cells do not vary more than .020 from each other.  Check and record readings for all batteries , then pick and choose those that are real close in readings for each string.   How are you charging now?  Having a charger with a battery temp sensor makes job much easier.

td

[fiddlerkelly]

These were 4 batteries that I  had taken out of the system with plans on charging them with two 6 volts done in series each. When I had got home from cabin with them I just done a spot check on their voltage prior to charging and they were reading 13.3 volts. I suppose it was the shaking up they got on the way out the woods road. So I decided to let them sit for a couple of days to level of their voltages. So when I went out today to get their SP the batteries had leveled off to about 12.65 volts. I bought a digital temp probe used for cooking to measure temp of battery acid.

Hopefully I will be able to get 2 more panels from same company for the price I got on the others. This will give me a few more watts for my system also which is a plus.

So if I go with all same panels is there any reason why I just can't connect these in series for a total of 136 VOC approximately as this would help reduce amps on wires going to charge controller.

[Vic]

Hi fiddler ..,

Am not sure where what you are using for a CC.  Know that you were considering a Classic 200,  previously.

Normally, if  one wanted to use an MPPT CC,  that person would use one with a 150 V input capability.   And only use a Cl 200 or 250 when the cable run from the PVs to the CC is long,  dictating a higher string Vmp to reduce the loss and or copper cost.

Running high string Vmps reduces the efficiency of the CC,  causing it to run hotter than it would with lover Vin.

We do not know where you are located,  but running a string Voc of 130 V or more usually means that the Voc in Winter can easily exceed 150 V,  and will also reduce the efficiency of the CC.   Most MPPT CCs will be damaged and/or have any warranty voided when 150 V input is exceeded.   Classic and KID CCs have Hyper VOC,  which extends this problem point by at least 12 V input.

AND,  if you are still considering/using a 12 V battery with this proposed system with 135-ish Voc,  then it is  even less good than stated above.

Here is the Classic String Sizer - it is very easy to use and look at various scenarios:
http://www.midnitesolar.com/sizingTool/displaySizing.php

Should you need it,  here is the String Sizer for the KID:
http://www.midnitesolar.com/sizingTool_kid/index.php

If you are able to come up with two identical or balanced strings,  this is often a good way to go,  and you will still not need a Combiner with Fuses or Breakers in it.   When setting up strings,  String Sizing Tools will help avoid strings with too low a voltage for the CC,  or ones with excessive string voltages.

Opinions,   YMMV,     Have fun,    Vic

[fiddlerkelly]

What do you mean by CC. Not familiar with that term.