Alternative method for lots of amp hours in a battery bank?

[MOA_Chaser]

So, I was having trouble yesterday with my DieHard 31 group AGM battery (ended up getting it replaced for free 35 months into my 36 month replacement warranty). Before I took it back to Sears, I took it to my local battery guy to test. I use him for all my other battery uses... cars, trucks, tractor, bikes, etc. He tested it and confirmed it was bad.

He said that if I was looking for a lot of amp hours, I could consider using 6V golf cart batteries. You can wire two of them up to get 12V out of them... but these specialized 6V batteries have something like 465 amp hours! He stated that a lot of the RV guys, etc use these in the battery banks when they need extended power because of their high capacity.

Of course they're not AGM, so you'd need to make sure the water levels were maintained, and ventilate them properly, etc etc. But if you have a BIG stereo system or want to have a stereo bank that can go a long time without charging, these might be the ticket. With more 6V cells you could also split the weight evenly between port and starboard.

I'm not sure how useful these would really be, yes they're expensive. But it was an option that I didn't think of before. You know, if I win the lottery and want to outfit my super-boat.

 

[Bruce]

I use 8 volt golf cart batteries in groups of three for 24 volt power supplies.

They do provide a lot of amp hours for the cost but they are heavy.

You can not simply add the amp hours. For example this Energizer 6 volt provides 208 amp hours at 6 volts. When you wire two in series you still get 208 amp hours but at 12 volts.

They weigh in at 60.5 pounds. So two of them would provide a better amps to weight ratio than the group 31 AGMs in my boat. And a way better cost to amps ratio.

 

[maboat]

More specifically, you add volts (but not amps) when you connect them in series and you add amps (but not volts) when you connect them in parallel. Using the Energizer golf cart batteries example (208aH @ 6v):

• You can connect two in series to get 208 amp hours @ 12 volts
• You can connect two in parallel to get 416 amp hours @ 6 volts

You would need four of these batteries (two pairs) in a mixed series+parallel configuration to get 416 amp hours @ 12 volts

 

[David Analog]

Correct. Series batteries do not combine amp/hours. They sum voltage. The opposite with parallel.
Series 6 volt golf cart batteries are a great, proven option.
With 400 amp/hours of capacity you better have at minimum a big, smart 40 amp AC shore charger.
They are still consumer grade so you cannot deplete them more than 50% (no lower than 11.8 volts).
If not, then you are back in the same boat even though you have increased the capacity.

 

[txav8r]

And with golf cart batteries...you MUST maintain them on a charger. They won't like being left at states of charge less than full and will start to decline rapidly. Buy 6 or 8 of these occasionally and you won't repeat that $800-1000 mistake. Also, everything is a compromise and trade off...the weight of 4 of these puppies will add more than twice the load in your boat of 12V batteries...and take up twice the room. But they do provide more amps than the typical 12V battery for battery...but you have to have two of them to one 12v to do that. Is it really worth the room and weight...and cost? I think so on bigger boats with bigger stereo demands, not so sure on our boats.

 

[Bruce]

the weight of 4 of these puppies will add more than twice the load in your boat of 12V batteries

My initial thought was that golf cart batteries were heavy. But at 60.5 pounds each (121 total) two of the Energizer 6 volts will provide 208 amp hours at 12 volts while the two DieHard Platinum Group 31 AGM batteries in my boat weigh 75 pounds each (150 total) and provide 200 amp hours of capacity.

The amp hours per pound work out as

Energizer 6 volt golf cart battery - 1.72
DieHard Platinum Group 31 - 1.33
Optima Bluetop D31M - 1.25
Interstate SRM-27B - 1.75

 

[maboat]

My initial thought was that golf cart batteries were heavy. But at 60.5 pounds each (121 total) two of the Energizer 6 volts will provide 208 amp hours at 12 volts while the two DieHard Platinum Group 31 AGM batteries in my boat weigh 75 pounds each (150 total) and provide 100 amp hours of capacity.

The two DieHard Platinum Group 31 AGM batteries (in parallel) will provide 200 amp hours (100 x 2) of capacity

 

[Bruce]

The two DieHard Platinum Group 31 AGM batteries (in parallel) will provide 200 amp hours (100 x 2) of capacity

Thanks for catching that.