How Can I Wire 6 6V Batteries to Make a 24V Battery Bank for My RV Solar Setup?

[eddie90]

Good day,

I am adding a solar setup and inverter to an RV and have a question about how to design my battery bank

I will be using 6V AGM Solar Batteries. I need to make a 24V battery bank.

Problem is I only have space for 6 batteries. And currently I only know how to get 24V out of 6V batteries by having either 4 batteries(all series) or 8 batteries(half series half parallel).

Is there a proper way to wire 6 of these batteries to make 24V? Or would this cause issues like unbalanced current draw or other things?

See attached image for an idea I had. Would this work is it very wrong lol
P.S. sorry for the poor Microsoft Paint Graphics

THANK YOU!,
Eddie

 

[anorlunda]

The batteries will not charge or discharge equally in that setup, because some are in series and others in parallel.

I may be wrong, but only multiples of four 6 v batteries (e.g. 4, 8, 12, ...) will work as you hope.

 

[gleem]

You will get 24 V with your circuit but there is a problem. You draw the same current through the circuit. Batteries are rated to supply a certain number of ampere hours before they are spent. A battery terminal voltage drops as you use it from about 12.7V fully charged to 11.8V at half charge at which time you should recharge the battery to maximize the battery life. You have essentially four batteries connected in series two with capacities twice the other. As you draw the current the single batteries will use up there capacity twice as fast as the the sets connected in parallel. This will cause the terminal voltage of the single batteries to drop twice as fast as the others. you end up with a situation where you have to charge two of the batteries before the others. When you do recharge the batteries the single batteries will charge faster leaving the parallel batteries not fully charged which is not good for the batteries. If you try to bring the parallel batteries up to full charge you will overcharge and damage the single batteries.

Thus the bottom line is the batteries connected in parallel do nothing for you but create a problem and may shorten the life of the bank.

 

[Averagesupernova]

It is definitely not an ideal situation. However, the parallel batteries will only discharge half as far as the singles so they will require less time to charge. I would say go ahead and do it. It might work out. If not, just pull 2 batteries out and you will have four in series with 2 as spares.

 

[Jeff Rosenbury]

Why 24V?

A six volt battery is only nominally 6V. It's actual voltage varies. Also charging it will require more than 6V. (exactly how much depends on how fast you charge it, and charging too fast is a problem as well.)

My point is that simply attaching four batteries in series without some control electronics isn't usually a good solution.

As long as you're using electronics anyway, perhaps put two banks of three or three banks of two and use a DC-DC converter in your controller. That way you get the the voltage you want out and the charge voltage you need in.

Designing with batteries is tricky. You might want to read about it on the Battery University.

 

[anorlunda]

Jeff made a good point about charging controllers. You probably should get a MPPT charge controller, then follow the manual for instructions on the best way to wire up the batteries. With MPPT, you may get 30-40% more energy in a day.

 

[NascentOxygen]

Instead of fitting 6 batteries of the size you have in mind (then not knowing what to do with the 2 spare ones), why not see whether you can find 4 physically larger batteries to fully utilize that space?

 

[William White]

Eddie,
what you have is this:
Anorlunda and Gleem are correct that you will have problems with charging.

Is there a physical reason why you must use 6 cells?

 

[nsaspook]

Thus the bottom line is the batteries connected in parallel do nothing for you but create a problem and may shorten the life of the bank.

Exactly, skip the extra batteries and buy more solar wattage or a better charge controller with the money.

 

[eddie90]

Thank you guys for all the great information. I had a feeling this was not the way to go but I didn't exactly know why.

Ok so the reason why I need a 24V bank is because the inverter I am installing is a 24VDC inverter.(http://magnumenergy.com/wp-content/uploads/2012/08/64-0007-Rev-D-MS-Series_Web.pdf )
It grabs 24VDC and inverts it up to 110VAC. The inverter also takes care of the battery charging, it looks like it charges at 25.6VDC

Would this work with (4) 6V batteries in series? I know Jeff mentioned just wiring (4) 6V batteries in series this is usually not a good solution.

Anyway my only reason behind using (4) 6V batteries is because these are rated as 300 AH which would be my overall battery bank capacity.
As opposed to using (2) 12V batteries(LINK) which are rated for 258AH wired also in series which would make my bank 258 AH. I figured 300>258 = good

Is this a good enough reason to go with the 6V batteries? Sounds like I might be better off just doing (2) 12V battery for less room for error.

Thank you all again

 

[nsaspook]

If both types are AGM batteries and the capacity is adequate for your power needs I would use the 12V battery to make the bank to reduce the number of connections. Make sure the inverter/charger has the correct charge profile for AGM type cells.

CHARGING INSTRUCTIONS

Initial charge or recharge: 2.37 to 2.40 volts per cell at 25 C (77 F). Float charge: 2.20 to 2.22 volts per cell at 25 C (77 F). Equalize charge: 2.40 volts per cell at 25 C (77 F). Temperature compensation = �3.75 mV. per cell per degree C [Reference to 25 C (77 F)]. This is for battery temperature (not ambient temperature) and is useful for battery temperatures from 0 C (32 F) to 40 C (104 F). Contact Concorde Battery Corporation for temperatures that exceed this range.

 

[anorlunda]

6 volt golf cart batteries are often used and wired in series because they cost substantially less per AH. I think that is simply because so many are sold for carts.

How many watts of panels are you using? How many AH of load per day are you planning to consome?

Do not forget the charge controller. Without it you might fry your batteries and lose all your investment.

 

[nsaspook]

6 volt golf cart batteries are often used and wired in series because they cost substantially less per AH. I think that is simply because so many are sold for carts.

How many watts of panels are you using? How many AH of load per day are you planning to consome?

Do not forget the charge controller. Without it you might fry your batteries and lose all your investment.

I currently use 4 GC-2 6 V batteries for my solar battery bank. It's hard to beat the bang for buck at COSTCO prices but being wet cells it can be messy watering them. That's a good reason to keep a set of polyester clothes from Goodwill.

 

[jim hardy]

As many have stated, the two extra batteries do you no good unless you have some 12v loads you could run from them. The capacity of your bank is set by the cells that aren't paralleled. Asymmetry complicates charging.

I'd go with 4 batteries (or two 12 volt) and make the extra space into storage for jumper cables or extension cords or coveralls or something.
Storage around an RV is always handy.

 

[Jeff Rosenbury]

It seems like your inverter is designed to act as a charger and an inverter, so it is likely what I meant by getting some electronics.

When figuring your amp hours, they are the amp hours of one battery (in your case). Even if 4 are in series, the same amps flow through all 4. You get 4 times the voltage, not 4 times the amp hours.

If you read about lead acids at the university link, you know to get deep discharge batteries and try not to use more than ½ the amp hours (30% is better). I didn't see it, but you lose about 20% on charging, so you need to replace ≈1.2 amp hours for each one you use.

I agree with nsaspook, more solar is likely a better investment than more batteries.