Two 6v in series or two 12v in parallel?

[davenrino]

Currently I have a Start bAttery and two 90AH 12v house batteries, all Grp27. I am thinking that I would be better off with two 230AH 6v Trojan T105s in series for the house bank. Am I correct in thinking this would give me 12v @ 230AH vs the 180 I have now? Any disadvantages?

[thomm225]

Probably.

I used two 12 volt 90 ah batteries in parallel for years and have already notice how much stronger my two 6 volt 220 ah Golf Cart batteries in series are.

My batteries are charged via solar only and I run a fan all night usually. Plus computers after dark with my inverter.

One difference from your idea is that my 6 volt batteries were not Trojan but another brand that I got for $135 each.

They are like 3" taller than the 12 volt batteries were which sort of give you an idea of their lasting power as they are true deep cycle batteries.

[davenrino]

Thanks Thomm. Are your 6v the bigger Golf cart batteries like Grp 902? I can only fit the GC2 or T105.

[Uncle Bob]

Originally Posted by davenrino

Currently I have a Start bAttery and two 90AH 12v house batteries, all Grp27. I am thinking that I would be better off with two 230AH 6v Trojan T105s in series for the house bank. Am I correct in thinking this would give me 12v @ 230AH vs the 180 I have now? Any disadvantages?

You will be very happy with the performance of the 6 volt deep cycle batteries over the 12 volt general purpose grp 27. Or at least I have been.

[davenrino]

Originally Posted by Uncle Bob

You will be very happy with the performance of the 6 volt deep cycle batteries over the 12 volt general purpose grp 27. Or at least I have been.

Thanks Rob, I have also read that you can take them down to 20% SOC, but I am guessing it is more like 50%.

[Uncle Bob]

Originally Posted by davenrino

Thanks Rob, I have also read that you can take them down to 20% SOC, but I am guessing it is more like 50%.

There is no way I would intentionally go below 50%.

[valhalla360]

Originally Posted by davenrino

Thanks Rob, I have also read that you can take them down to 20% SOC, but I am guessing it is more like 50%.

You can but don't expect them to last long taking them to 20%.

Lead acid batteries are happies at 100% charge...the further below, the worse it is for them. Doesn't matter if they are staring or or deep cycle. Usually the lower limit is considered to be 50% and taking them at least back to 80% daily (though again 100% is better but often not practical).

Yes, the 6v should do better for house loads. They have fewer but thicker plates which hold more energy...hence the high amp hours

Keep in mind outside of rare and expensive specialty batteries, most 12v batteries you find in big box stores are starting batteries (even those listed as combo or deep cycle), these have many thin plates which creates more surface area. Surface area allows for more amps but the thin plates don't hold as much power. Great for pumping out hundreds of amps to start a motor for a few seconds but not great for low amperage long duration house loads.

Assuming a relatively small motor that needs a modest amount of amps to start, there is no harm using deep cycle batteries to start the motor.

[thomm225]

Originally Posted by davenrino

Thanks Thomm. Are your 6v the bigger Golf cart batteries like Grp 902? I can only fit the GC2 or T105.

They were Season All GC2 220 ah 6 volt batteries. Heavy also so I bought the little carry strap.

[mako]

Also over the long run, FLA in series are safer than in parallel. When they get old, a dead cell (low voltage) in one battery, with its huge internal resistance, will draw huge amps and can cause a fire. Even without the extreme situation of fire, charging of the good battery will be hindered.

Not trying to be an alarmist, but nothing beats a couple of Costco golf cart batteries in series, especially for value.

For very high kW banks, the best FLA would be 2v industrial (OpZS) which can be sourced in nearly any amperage capacity and can draw down to 80% regularly, and even to 100% without damage.

[Adelie]

If you can only fit 2 batteries I would go 12v, that’s what I did.

As the batteries age eventually 1 cell will die or short out.

With 12v batteries you are left with diminished capacity until you make it home and can replace both.

With 6v batteries you might be getting 10v or you might only be getting 6v. 10v might run some items, 6v won’t run anything.
Check your nav lights and gps/chart plotter for minimum voltage.

I would modify the space for 2 T-1275 batteries, which would give you about 300Ahr.

[guyrj33]

Two 6v volt GC2s.
Least $ per AmpHour

[shadow]

love 6v batteries.. easy to place and manage and better life so far IMO..

[Tugwit]

I have two banks of batteries, each consisting of two Dyno D145 6V, 245AH batteries. Dyno Battery - Premium Quality Battery Manufacturing - Made in the USA

I had to modify the the battery boxes to accommodate the extra height of the batteries , but I love 'em.

I replaced both banks last year after 15 years of service. They were still working fine, e.g. to start the engine, but didn't seem to hold a charge as long as they once did.

Basically, I gave up on a dedicated start battery in favor of the 6V deep-cycle batteries in series. On odd days, I switch the ship loads to Battery bank #1; on even days to bank #2. I switch the batteries to "both" to start the engine and charge both banks when motoring.
When these give out--in 15 years or so--maybe I'll consider Lithium when the price comes down....