Help chosing batteries, 2x12V parallel or 2x6V serial?

System · February 2011

Hello Everybody,
I'm about to finish my solar generator project and I can't decide about my batteries.
There are advantages and disadvantages to both of these options.
I picked Sun Extender PVX-1080T at 12V each and PVX-2240T at 6V each.

2x 1080T in parallel will give me 216Ah , or 108Ah at 50% DOD
2x 2240T in serial will give me 224Ah , or 112Ah at 50% DOD
Those are 24 hour rates
Prices are about the same on both sets, that's not an issue, weight it slightly better on 2x 6V batteries, but not by much.

Advantage in 12V is that I can just use 1 battery in case the other one goes bad or if I wanted to use it let's say for camping and want to take less weight in my car.
I also read that 6V in serial are better and will last longer, I'm not sure.

Anyone here have experience with this? I'd welcome your inputs and experiences.
Thanks.

BB. · February 2011

Re: Help chosing batteries, 2x12V parallel or 2x6V serial?

I like series connected batteries as you can have issues with proper current sharing in parallel connected banks.

However, if you wire them correctly, they can work just fine for your needs.

I would suggest you might want to look at getting a simple DC Clamp Current Meter so you can monitor your current sharing between battery "strings".

May be overkill for a a small 2 battery bank--but your batteries are not cheap and checking that they are charging/discharging correctly can catch problems (like dirty connections) before they damage your batteries.

-Bill

System2 · February 2011

Re: Help chosing batteries, 2x12V parallel or 2x6V serial?

Thank you for that article Bill, interesting. Since this is only 2 batteries, I wouldn't have much problem with this, but I'll keep that in mind in case I expand.

Since I'm here, I'll post other components of my setup for comments.
I have 2 panels each consisting of 72x 3"x6" cells, so each at about 120W.
30A MPPT charge controller : http://www.amazon.com/SolarTorrent-STCMPPT30-Solar-Regulator-MPPT/dp/B0031WV9FY/ref=sr_1_1?ie=UTF8&s=home-garden&qid=1298063115&sr=8-1

Above mentioned batteries, either 2x12V or 2x6V at either 108Ah or 112Ah (at 50% DOD)
Inverter I chose is PowerBright 1500W 12V inverter from Costco:
http://www.costco.com/Browse/Product.aspx?Prodid=11540218&whse=BC&Ne=4000000&eCat=BC|3960|21248|56378&N=4041965&Mo=3&pos=8&No=1&Nr=P_CatalogName:BC&cat=56378&Ns=P_Price|1||P_SignDesc1&lang=en-US&Sp=C&ec=BC-EC10615-Cat21248&topnav=

The reason for this setup is mainly emergency backup power, but I'll use it of course daily to unplug some of my appliances and possibly lower my PG&E bill.
Now, I understand that my solar panels won't fully recharge in 1 day, but I think for now this is close enough if I don't use too much load on the batteries.
Am I going in the right direction for what this is?

Thanks,
Tom.

ggunn · February 2011

Re: Help chosing batteries, 2x12V parallel or 2x6V serial?

Martan wrote: »

The reason for this setup is mainly emergency backup power, but I'll use it of course daily to unplug some of my appliances and possibly lower my PG&E bill.

A plain grid tied PV system will do more to offset your bill and cost a lot less. When you disconnect from the grid and run off your batteries, you are just using power that you pulled off the PV that you could have used to offset your bill directly, except that it will be at a much lower efficiency. Running off batteries when the grid is up cannot possibly save you any money. The second law of thermodynamics says that the best you can do is break even, and the third says you can never break even.

If you need the backup, then get batteries, but don't use them unless the grid goes down; your batteries are rated for a certain number of discharges and it's unwise to be using those cycles up when you don't need to. And like I said, it costs you more to use that power than it does to get it off the grid anyway, probably 2X the cost or more when you figure in all the factors.

Photowhit · February 2011

Re: Help chosing batteries, 2x12V parallel or 2x6V serial?

ggunn wrote: »

If you need the backup, then get batteries, but don't use them unless the grid goes down; your batteries are rated for a certain number of discharges and it's unwise to be using those cycles up when you don't need to. And like I said, it costs you more to use that power than it does to get it off the grid anyway, probably 2X the cost or more when you figure in all the factors.

I fully agree that a grid tied system is the best energy saver...

...but couldn't disagree more with the battery based systems. Batteries have a finite life reguardless of if they are used and some actually befefit from being used! Anyone who has used a battery based system wil tell you the a large portion of the waste comes when the batteries are fully charged and energy generated by the panels is not used. Some sort of timer (available at NAWS) to run during the afternoon, doing tasks that can be ddone at any time, charging power tool batteries, running a pump, activating a sprinkler systen,...

I would run batteries in series, less wiring, only advantage I see, to 2 in parallel would be in case of a failure.

For what it's worth, this won't back up much or for long, It would allow for some compact florescents and a radio or small TV for a few hours and a few days... As with parctically any battery based system you will be wasting energy as the system will NOT generate as much energy as it costs to produce.

n3qik · February 2011

Re: Help chosing batteries, 2x12V parallel or 2x6V serial?

2X on the batteries. They will go bad in 5-10 years just sitting there with a trickle charger.

I replace 10-15 computer UPS batteries a year at work. Power has gone out 3-4 times in 12 years.

Help chosing batteries, 2x12V parallel or 2x6V serial?

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