# chargeing a 12 volt battery bank with 24 volt panels

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Solar Expert Posts: 54
hi guys

well i finally got a new volt meter after i blew out my last one trying to read the amps when it was only rated to 10 a on a 50 amp array

so i dissconnect the panels and tested each one and they all read the same 25vdc even a single one that i know is twelve volt
i got these panels for free from a neighbor that upgraded there old system was 24 volts but i thought when i seperated them from two hook together to each one in parallel
\they would be 12 volt (guess i was wrong)

well i consulted the manual on my c60 charge controller and it said it allows up to 55vdc maximum while chargeing a 12volt batterie bank(guess thats how my batteries have been getting charged,i read the voltage coming out of the charge controller to the bank and its in the normal 12 volt range so im guessing this is okay

going from the combiner to charge controller its 30-36

is there any harm to running 24 volt panel to my c60?? charging a 12 volt batterie bank

if i was considering using my array as a diversion load would it be better to leave the panels configured for 24volt??

thanks guys

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Re: chargeing a 12 volt battery bank with 24 volt panels

Can you read the data plate for the panels? Battery bank voltage (and AH rating)?

The numbers you are using do not make a lot of sense.... 30-36 volts from the array... 25 volts from panels, 12 volt battery bank?

C60's are very simple charge controllers. They are either "ON" (just a short from input to output), "OFF" (if battery is fully charged), and will PWM (pulse width modulation) On/Off some percentage (90% on is almost full charging current, 50% is 1/2 charging current, 10% on is almost no charging current).

The battery voltage should move relatively slowly around the nominal voltage range (~12-15 volts) depending on battery load/charging current/temperature.

The Voltage of the Array--Should either be Voc (i.e, 18 volts, ~36 volts, depending on panels and wiring) and will other wise be near Vbatt (i.e., if battery is at 14.3 volts--The array voltage will be around 14.3 to 15.x volts). So, the actual Varray voltage depends on how much current the charge controller thinks it needs (0%=V open circuit, 100%=Vbattery, 50% somewhere in between).

Just need to know the panel Vmp/Imp, Array Wiring (number of panels in series, number of strings in parallel), and Vbattery for a start.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 963 ✭✭
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Re: chargeing a 12 volt battery bank with 24 volt panels

I don't quite get your post. Are you getting 25 volts each individual panel and 36 when two are connected together?

Is your c60 a pmw controller?

It doesn't hurt if you run 24 volts to you 12 volt battery eccept you only get the amps times the battery voltage at the time. 12 times 5 amps is not as much as 24 times 5 amps. quite a loss.

With those panels if they are 25 volts and your cc is a pmw. You don;t really have a choice unless you buy more equipment. Your job would be a bit easier to do on the fly without unhooking everything if you bought a cheap clamp meter. It you have a combiner box you can check voltage there if you have breakers or if the battery has a breaker in between the combniner and battery. I only say this cause it is easier then unhooking everything. You have already did that so this is for later. How many panels are we talking. If enough, it might make it worthwhile to buy differrent equiptment, like an mppt cc. If the c60 is mppt forget all the above and you are probly doing fine.
good luck
gww
• Solar Expert Posts: 54
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Re: chargeing a 12 volt battery bank with 24 volt panels

hi guys sorry for the confusion

the panels are sp75 they can be wired to 12 or 24 volt
i thought i had set them back to 12 volt when i seperated the strings but there are putting out 36 voc in full sun and 24-30 voc in the partial shade (so they must be wired 24volt right?)
the current at the cc to batt bank is between 12-15 volt depending on if the light says bulk or float so it seems the batts are being charged correctly
the c60 is not a mppt controller its a pwm contoller ,i consulted the manual and it states that the max open array current is 55vdc in 12,24 or 48 volt systems

is there a risk to the cc or batts doing this

if i want to use a diversion load am i better off leaving my array at 24volt or even going 48

i know dc current is more efficent the higher the voltage and it would cut down on wire size???

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Re: chargeing a 12 volt battery bank with 24 volt panels

In cool weather with any direct sun, a "12 volt panel" should be around 21 VDC open circuit. And a 24 volt panel should be around 42 VDC open circuit (just the Digital Multi Meter -- DMM -- as the load--virtually zero current).

For charging your 12 volt battery bank with a C60 controller, set the panels to "12 volts" (Voc~21 volts, Vmp~17.6 volts or so). That will give you 2x the current into your 12 volt battery bank.

Assuming the panels are all good and properly configured and array properly wired (basically all Vmp~17.6 volt panels in parallel, each panel should have a fuse/breaker on its positive lead--Series fuse should be in the data sheet--although many older panels did not give this information).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 963 ✭✭
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Re: chargeing a 12 volt battery bank with 24 volt panels

solar
Do the panels have a tag on the back. I don't know if you say wired for 24 you mean putting two together or you mean they have a button on back that lets you select. Either way unless you buy a differrent cc you can't wire them for 48 volts and put them on a twelve volt battery cause you would lose so much power trying to charge your batteries you would probly never get to divert anything. Did you understand what I was saying when I pionted out that if your solar was putting out 25 volt at 5 amps = 125 watts and charging a twelve volt battery 12 volts at 5 amps =60 watts. so with your cc you have 125 watts going with only 60 watts going to the battery. If you went to 48 volts, this would use double the panels and you still end up with 60 watts in the battery. It doesn't hurt anything equiptment wise but doesn't make sense either.

Did you get the questions we ask? how many panels, what do they say on the tag on the back. how far is your wire run and how big is your wire.

Then it would be easier to answer further.
Good luck
gww
• Solar Expert Posts: 54
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Re: chargeing a 12 volt battery bank with 24 volt panels

yeah the series fuse is 15 amps

the specs on the panel are 21.7 voc thats funny cause im getting 30-36vdc at the controller and all the panels are parallel

any ideas whats going on ???
• Solar Expert Posts: 963 ✭✭
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Re: chargeing a 12 volt battery bank with 24 volt panels
the specs on the panel are 21.7 voc thats funny cause im getting 30-36vdc at the controller and all the panels are parallel
Is it really cold out? If your batteries are full or if you are measuring the wires going to you cc but unhooked from the batteries, cold wheather and a cheep meter could make up the differance. I have three muti meters and there is a two volt differance in them. Cold would raise the voltage and if there is no load the voltage would be open (voc). That is a lot.
gww
• Solar Expert Posts: 963 ✭✭
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Re: chargeing a 12 volt battery bank with 24 volt panels

How many panels and how many/what batteries?
gww
• Solar Expert Posts: 54
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Re: chargeing a 12 volt battery bank with 24 volt panels

wierd i was just reading the sp75 manual and it says can be configured for 12 or 6volt

how am i getting this reading ??? some panel ive doubled up in the combiner could that be it
• Solar Expert Posts: 54
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Re: chargeing a 12 volt battery bank with 24 volt panels

not that cold out 70 degrees my meter is kinda cheap, analog not digital , might borrow a better one tommorow
• Solar Expert Posts: 54
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Re: chargeing a 12 volt battery bank with 24 volt panels

the batteries just got equalized before the reading

9 shell sp75
2 48 watt panels
2 70watt panels
• Solar Expert Posts: 963 ✭✭
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Re: chargeing a 12 volt battery bank with 24 volt panels

Did you check the panel polarity before putting them in you combiner box. I remember asking on your other post asking how many panels?

I remember you said you were using a twelve spot combiner. Is it a secret how many panels you have?
gww
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Re: chargeing a 12 volt battery bank with 24 volt panels

Would not be the first time a DMM with a dying 9 volt battery (or other meter problem) has lead somebody down the wrong path.

Measure you car battery--Should be around 12.5 to 13.5 with everything off.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 963 ✭✭
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Re: chargeing a 12 volt battery bank with 24 volt panels

Ok, now we have the panels, You are getting out of my relm of knowlage. I would unhook the 48 volt panels and the 70 watt panels unless the voltage listed on the panels was the same as the voltage on the sp panels. You shoud consider borrowing or buying a dc clamp meter. Then you could see what is getting to your batteries with and without those panels to see if there is an issue. I honestly don't know if all the voltages of the panels are above battery voltage, if it would transfer all the amps or not. My inclination is it would be no problim. How many and what are your batteries?
gww
• Solar Expert Posts: 963 ✭✭
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Re: chargeing a 12 volt battery bank with 24 volt panels

I think most divertion load are taken from the battery its self. I know it could be set up to come from the panels but mine always came from either the batteries or the inverter.
gww
• Solar Expert Posts: 54
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Re: chargeing a 12 volt battery bank with 24 volt panels

thats the strange part cause the reading i was getting from the cc to batts was normal on the meter 12 -14.5 volts
but when i touch the combiner it 36vdc

i belive the polarity is correct on the panels would a reverse wire kick the volts up like this???

these are mono-crytsal panels ive heard those tend to run a little more efficent could that be whats happening

no secret about these panels my nieghbor gave them to me when they upgraded and i found a guy that took em off a building and was selling them for 30 cents a watt so i got another 8 ive yet to hook up
• Solar Expert Posts: 5,183 ✭✭✭✭
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Re: chargeing a 12 volt battery bank with 24 volt panels

OK you said Voc is 21.7 V and when testing 2 panels in series you got as high as 36V....this is exactly what you should get when you are checking the output and the panels are connected to the CC, you're measuring VMP at that point NOT VOC...

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• Solar Expert Posts: 54
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Re: chargeing a 12 volt battery bank with 24 volt panels

its not two panels in series

its one panel in parallel thats the strange part
and the manual does say these are 24 volt it says 6volt or 12 volt
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Re: chargeing a 12 volt battery bank with 24 volt panels

About the only things that explain the high voltage at the panels... One is that the meter is not working right. Another would be inductive kick from the wiring as the PWM controller cycles.

Did you intend to type:
and the manual does say these are NOT 24 volt, it says configurable for 6volt or 12 volt

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 54
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Re: chargeing a 12 volt battery bank with 24 volt panels

ok so im sure they are 24 volts

my c60 seems to be charging the batterys fine

the people i bought my c60 from tryed to sell me a mppt cc when i ask if this was ok
the manual state max array voltage at 55vdc for all systems

is this hard on the controller
am i wasting energy doing this
• Solar Expert Posts: 963 ✭✭
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Re: chargeing a 12 volt battery bank with 24 volt panels

Does it say on the panel 21.7 voc? If it does you have a good setup depending on the wire size and it is apparently ok if the amps are getting to the batteries. If you add seven more panels you may be over the cc amp limit of 60 amps. What do the panels say the working amps are? I am assuming a bit under 4 amps per panel. except for your fusing of two panels on one breaker/fuse in my opinion it is probly as good as it gets if 21.7 voc is what it says on the panels. what size and how many batteries are you charging? This could make a differance on what you send to them. If you send to many amps to fast, you could hurt them. If bill counters anything I said here, listen to him as he is much smarter then I am but the above is my opinion based of my guess of what you are trying to tell us you have.

I hope this helps.
gww
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Re: chargeing a 12 volt battery bank with 24 volt panels

Really need Vmp/Imp, and Voc/Isc would be handy.

The problem is that "24 volt panels" means almost nothing these days...

For us in the off grid battery system play ground, "24 volt panels" are intended to charge 24 volt battery banks. That means a 72 cell panel with Vmp~35-38 volts and Voc~42 volts (at 25C).

Many "24 volt panels" these days are GT Panels (for grid tied solar systems) with 60 cell panels and Vmp~30 volts. This is not high enough voltage to reliably/efficiently recharge a 24 volt lead acid battery bank (especially in hotter climates).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 54
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Re: chargeing a 12 volt battery bank with 24 volt panels

okay thanks for hanging with me

well the specs on these panels say 12 or six volt

all these panels are in single parallel to the combiner

the spec on the back max watt 75 min watts 70

imp 4.4

vmp 17.0

isc 4.4

voc 21.7

series fuse 15A

does this sound about right for what im experiencen
• Solar Expert Posts: 54
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Re: chargeing a 12 volt battery bank with 24 volt panels

my cc is running at float even when the sun barely hitting the panels
• Solar Expert Posts: 963 ✭✭
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Re: chargeing a 12 volt battery bank with 24 volt panels

It is hard to say what you are experiancing. what size is your battery bank. The panels and cc work well together although you only have room for 4 more or less cause of your mismatched panels. Otherwise that part is fine. The battery we don't know about and we also don't know if you are discharging it, as you have not said. I really would like to help but have to ask several time for info to be able to help.

What are you seeing that concerns you? How big is your battery bank? What are you doing with the batteries after they are charged?

You have seven more panels and your cc is pretty good now with not a lot of room. If you set your cc up differently you may be able to use all the panels, of course that would depend on how big your battery bank is. Also this is the first time I have ever seen the short circuit amps and open amps listed as the same value. What do you want to know?
gww
• Solar Expert Posts: 54
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Re: chargeing a 12 volt battery bank with 24 volt panels

woop isc is 4.8

i have a 12 volt batterie bank with 4 6 volt surrite s550 420 amp hours at 20hr so total bank of 820 at 50%dod

i probley discharge these guys to 10% per cycle

im not at the cabin full time but will be more soon

would i be better off giving these batteries a full charge and turning off the cc if im gone a month or more

or better to leave it hooked up with these shallow cycles???
• Solar Expert Posts: 963 ✭✭
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Re: chargeing a 12 volt battery bank with 24 volt panels

Ok, I would leave them as they are but most of the advise I have seen here is to charge well and unhook loads and charging and every thing would be fine. Doing that get rid of the chance of equipment failure causing problims. I have never been in that situation so my request would be to wait for someone with more experience to advise you of the plus and minus of unhooking and letting set. I don't know the batteries, can you check their sg to see if they get full the way you are charging? Some say once a month you should take your batteries down to 20-30% instead of constantly only going down 10%. I kind of believe it myself. You of course couldn't do this while letting them sit while you are away but while you are there, you might think about it or do a little reserch and decide for yourself. I think you are doing fine Though if I where you I would get rid of the odd panels and put 12 of the same panels through your combiner box. with derating of the panel output that would give you a tiny bit less then a 5% charge rate for your batteries and as long as you keep the discharge at 10% you might be ok. you could buy another pmw cc and combiner box and put all you panels on your batteries and you would be about perfect in my mind.
I hope this helps
gww

PS it would be a 820 ah 12 volt bank and 410 usable amps at 50% dod you had it typed backwards. basically a little under 5000 usable watts daily if you have enough to recharge after using. So if you don't want to stress anything and you add the panels a safe 2500 watts usable daily and maby even if you didn't add the 7 more panels.