# Volts to charge

chuck
Registered Users Posts:

**7**✭
Here is a good one:

A guy has a bank of batteries wired and at 24 Plus or minus volts. Also a rack of 12, 12 volt panels, and a controller.

Knowing that amps charge a battery, how many 'VOLTS' would the panels, wired series/parallel (that are putting out amps)

would it take to charge the 24 volt battery bank ?

May sound like a dumb question, BUT needs an answer.

Almost like:

How much wood can a woodchuck chuck if a woodchuck could chuck wood. Hard wood or soft ?

A guy has a bank of batteries wired and at 24 Plus or minus volts. Also a rack of 12, 12 volt panels, and a controller.

Knowing that amps charge a battery, how many 'VOLTS' would the panels, wired series/parallel (that are putting out amps)

would it take to charge the 24 volt battery bank ?

May sound like a dumb question, BUT needs an answer.

Almost like:

How much wood can a woodchuck chuck if a woodchuck could chuck wood. Hard wood or soft ?

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## Comments

17,615✭✭To charge

anybattery you must have more Voltage than the battery's rating.A "12 Volt" deep cycle battery of the FLA type is typically charge at 14.2-14.4 Volts. A 24 Volt system is usually 2X that.

This is one of the confusing things about "nominal" Voltage ratings: a "12 Volt" battery isn't 12 Volts, a "12 Volt" inverter actually operates over a range of about 10.5 to 15, and "12 Volt" solar panels typically have a Voc of over 20 Volts and a Vmp of around 17.

So for a 24 Volt system the twelve 12 Volt panels would be wired in six parallel banks of two in series.

Clearer? Or more confusing?

7✭For a 24 Volt system the twelve 12 Volt panels would be wired in six parallel banks of two in series.Clearer, but we need a picture (drawing) of

six parallel banks of two in series. Is there a place on the net to find this ?Almost like:

How much wood can a woodchuck chuck if a woodchuck could chuck wood. Hard wood or soft ?

9,363✭✭✭✭✭here's my setup has 3 banks of 5 panels.

Note that it's a 48V system with 5, 12V panels to have enough juice (85v) to charge on hot days.

|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||

|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar

gen: http://tinyurl.com/LMR-Lister ,

17,615✭✭Right. The ol' series & parallel explanation. I'll try one of my famous (infamous?) "text drawings".

Two panels in series:

(-) Panel 'A' (+)---(-) Panel 'B' (+)

For parallel, each 'series bank' of panels gets wired (-) to (-) and (+) to (+), with no connections between banks on the (+)---(-) connections.

Helpful? or are you now wondering about the difference between softwood grown in the South and softwood grown in the North?

And yes; we really could do with some basic diagrams/schematics/actual images.

32,027adminBasically, it would look like:

- Vmp-array = Vbatt-charging + 2 volts for wiring/controller drop + a bit because solar panels drop Vmp a volt or so as they get hot

So, ~17.5 volts or so is optimum for a 12 volt system.For a 24 volt system, Vmp = ~35 volts is optimum (this Vmp assume a panel running at ~77F -- really quite cool for a warm summer day and under full sunlight).

You can use a calculator (Xantrex has one for the XW charge controller) or you can put the formula in spread sheet and do your own assumptions/calculations:

The forumula I have used is:

- Vmp = Vmp-STC + (temp coeff * (ambient+assumed temp rise-STC temp)
- Example:
- -0.157 V/C * 1/1.8 C per F = -0.0872 V/F (temperature coefficient converted to F)
- Vmp-hot = 34.9 Vmp + (-0.0.0872 V/F * (100F+63F-77F)) = 27.4 volts Vmp Hot
- 26.8 Volts * 2 panels in series = 54.8 Vmp Hot for 2 series panels
- Voc-cold = 43.2 Voc + (-0.0.0872 V/F * (14F -77F)) = 48.7 volts Voc Cold

Double check my work (this is the Internet after all :roll:).-Bill

7✭Getting closer to understanding, but would this still only give 24 volts to the controller ?7✭Say what ?17,615✭✭Not exactly. It will give approximately 2X the Vmp of the panels to the charge controller. If the Vmp (Voltage at Maximum Power) is 17.5 as Bill suggests (typical) then the input side of the charge controller will get 35 Volts. This is enough to charge a "24 Volt" system. Do not worry about the difference; the charge controller will do just what the name implies; regulate the current to the batteries so that they do not get too much power. There's a couple of different types of controller (PWM & MPPT) but the net result is the same; giving the battery the right amount of current to reach and hold its proper Voltage level.

How are we doing so far?

7✭Think I have got it now. (I think) Sure would be nice to see a drawing by someone in the know.8)32,027adminGo ahead and ignore that...

All I was trying to say is that the Vmp of the panels needs to be at 2-3 volts or so higher than your 12 volt battery charging voltage (around 14.4 to 15.0 volts). Or roughly 2x higher for a 24 volt battery bank.

Let's try a different way.

What charge controller, what solar panels, how much power, how long a wire run (from panels to charge controller) are you looking to do? And battery bank voltage/AH rating.

We can run the numbers with a specific answer.

-Bill

7✭12) Panels = Solarex MSX-60

Controller = Xantrex C60 DC Controller

11) Batteries = Each at 1600ah, float 2.17 volts, are wired for 24 volts.

25' from panels to controller

17,615✭✭Twelve 60 Watt panels = 720 Watts @ 80% efficiency = 576 Watts on a 24 Volt system is approximately 24 Amps current, which the 60 Amp charge controller will handle. The panel specs say 3.5 Amps max so * 6 =21. No problem.

Okay, there's problem #1: 11 "2 Volt" batteries is 22 Volts, not 24. Disregard the numbers after the decimal point. And who sold you on so much battery capacity? That's 38,400 Watts! Enormous for an off-grid system.

Remember: a "24 Volt" system will run a

realVoltage of around 25-27. Low Voltage s typically 21 on this, and maximum of about 30. But you have to get the nominal in the right place or else all the others are out of whack. You're short 2 Volts.Problem #2 is that you've got about 20 Amps of charge current at most, which will only handle about 400 Amp/hrs of battery at most. The panels are no place near sufficient to charge 1600 Amp/hrs of battery.

Vmp on these panels is 17.1, so two in series would be 34.2 Maximum of 24 Amps of current.

And now I have to let Bill run the calc because it doesn't work on my Linux netbook.

7✭OK,

I need one more battery and 12 more solar panels ?

32,027adminRegarding the size of the solar array and battery bank... Typically, we size the battery bank to the loads, and size the array to the size of the bank (aim for 5-13% rate of charge based on 20 Hour rating) and how many Watt*Hours (or Amp*Hours @ 24 volts) per day you will be requiring from your system.

- 1,600 AH * 0.05 = 80 amps charging (minimum recommended rule of thumb)
- 80 amps * 29 volts charging * 1/0.77 system efficiency = 3,000 watts of solar panels minimum (Ideally)

You can use less solar panels--but you will probably have to run your genset more often during the whole year to keep get the batteries > 90% charged once or twice a week, plus any equalization needed (approximately every 1-4 months for flooded cell batteries).Also, approximately where the system will be installed (so we can figure out how much sun you get during the year).

What seems to work out well, is to design the system for 3 days of no sun and maximum of 50% battery discharge (combination of good life and not too much money).

Also, we look at how much sun you get for ~9 months of the year, and size the solar array to supply ~100% of the power. For the other 3 months of the year, it usually requires you to run your genset during periods of bad weather.

Again, the above are just suggested starting points.

-Bill

17,615✭✭Yep. You either need 4X as many panels or you need to run the gen set or you need to change the entire battery bank to something smaller. None of those are very attractive options, especially $ wise.

I'll differ with Bill on the "3 day rule". I find the cost of having 3 days batteries and panels is prohibitive. My personal rule-of-thumb is to shoot for 25% DOD for one day, use the "remaining" 25% is you need it for the second - and always have a back-up generator so the matter of dead batteries never comes up.

Personal note; mine got thoroughly toasted this year. The kids were at the cabin on their own for a week and a half. Refrigerator went off every night, they said. Turned out they were running the water pump 4-5 times a day, mostly with the batteries

outof Float. Drawing batteries down to the low Voltage shut-down point every day for a week isnotrecommended.9,363✭✭✭✭✭But they were clean kids !

|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||

|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar

gen: http://tinyurl.com/LMR-Lister ,

17,615✭✭Must be: that's like 300 US gallons per day!