Battery to heavy?

[markdoane]

Originally Posted by lewster

Mark,

When you use 2-6Volts in series you should get about the full amp draw rating.

Thats correct. I just want to point out, in the context of this thread, that you need to double the weight of batteries in order to achieve that.

[DreamStreamr]

Lew,
My apologies. Also my point about AGM only half matters, in a way -- unless someone is installing inside the cabin the non-venting requirement of the AGMs just doesn't matter. The true maintenance-free aspect of AGMs only matters for dummies like me who didn't keep their plates covered with water. I like the double 6v solution, have seen it on several members' outfits. Certainly more cost effective on front cost and probably lifecycle cost too.

Sorry about that -- I knew I was getting out of my league on this thing anyway and sure enough can real quick end up talking about more than I know. I realized after I sent I was being a little (too) enthusiastic about my new battery. I appreciate your kind correction.

[RoadKingMoe]

One advantage to two 6V golf cart batteries in series, compared to two 12V batteries in parallel, is in discharge cycle life. You can either discharge them deeper for the same number of discharges, or discharge them more times for the same depth of discharge.

Another advantage is that while they may not have significantly higher capacity (220AH vs 200AH) at the low discharge rates typically used when boondocking (i.e. 10A or less), they ARE significantly better at high discharge rates, such as the 25A rate used to establish "Reserve Capacity," or even higher, such as you'd find with heavy inverter usage. Keep in mind, though, that you have to compare their 25A rating to a 12.5A rate through each of the 12V batteries in parallel. Here's the calculations of Peukert Exponent (n) and Peukert Capacity (Cp) for the Group 27 and Group 4C Lifelines. Note that there isn't much difference in 2 x the Cp of the 12 volt batteries compared to the Cp of the 6 volt batteries, which indicates there will be little difference in their capacities at low current draws. But watch what that huge difference in exponent does as the current draw goes up. I've calculated the time for a 10A total draw (the 20 hour rate of the two 12V batteries), a 25A total draw (the standard for establishing "Reserve Capacity", and a 100A and 200A total draws, powering a 1000W and 2000W inverter, for example:

Group 27
(logT2 - logT1) / (logI1 - logI2) = n
(log186 - log1200) / log5 - log25) = n
n = 1.1583734592428565078682881631963

Cp = (5A^1.1583734592428565078682881631963) x 20 hours
Cp = 129.03225806451612903225806451613

Cp/I^n = Time so for 10A load, 10A/2 = 5A per battery:
129.03225806451612903225806451613/5^1.1583734592428565078682881631963
= 20 hours

Cp/I^n = Time so for 25A load 25A/2 = 12.5A per battery:
129.03225806451612903225806451613/12.5^1.1583734592428565078682881631963
= 6.9 hours (415 minutes)

Cp/I^n = Time so for 100A load, 100A/2 = 50A per battery:
129.03225806451612903225806451613/50^1.1583734592428565078682881631963
= 1.4 hours (83 minutes)

Cp/I^n = Time so for 200A load, 200A/2 = 100A per battery:
129.03225806451612903225806451613/100^1.1583734592428565078682881631963
= 0.62 hours (37 minutes)


Group 4C
(logT2 - logT1) / (logI1 - logI2) = n
(log492 - log1200) / (log11 - log25) = n
n = 1.0860161267351761034826747166678

Cp = (11^1.0860161267351761034826747166678) x 20 hours
Cp = 270.39536696992743815972924455649

Cp/I^n = Time so for 10A load:
270.39536696992743815972924455649/10^1.0860161267351761034826747166678
= 22.2 hours (11% better)

Cp/I^n = Time, so for 25A load:
270.39536696992743815972924455649/25^1.0860161267351761034826747166678
= 8.2 hours (492 minutes) (19% better)

Cp/I^n = Time so for 100A load:
270.39536696992743815972924455649/100^1.0860161267351761034826747166678
= 1.8 hours (109 minutes) (31% better)

Cp/I^n = Time so for 200A load:
270.39536696992743815972924455649/200^1.0860161267351761034826747166678
= 0.86 hours (51.4 minutes) (38% better)

Finally, there's a major difference in the failure modes of two 12V batteries in parallel vs two (only) 6V batteries in series.

If a cell opens in the 12V setup, you just continue to operate at 50% capacity. If a cell shorts in the 12V setup, the shorted battery discharges the good battery, but you can disconnect the bad battery, recharge the good one, and continue to operate at 50% capacity.

If a cell opens in the 6V setup, the whole system is dead. Period. If a cell shorts in the 6V setup, voltage drops to that of a dead 12V battery, even if all the other cells are fully charged. So essentially, the system is dead.

It may not make much difference boondocking in an RV, where you can hook up and head home, but if it happens a few days offshore and you don't have fuel to motor (and power your electronics) all the way home, it's more serious. That's why most boaters that go the 6V battery route have more than one pair of 6V batteries. The same is true for the big motorcoaches. There's been a bit of a trend away from the big 4D & 8D batteries in both, mainly because of the difficulty swapping them out, if required. It's about like swapping out a small diesel engine.

Finally, where space is concerned, there are differences. The 6 volt batteries have a smaller footprint, but are taller. Depending on your space available, one or the other setup may be better for that.

[markdoane]

Maurice,

I am impressed. I agree with your conclusions. I do need to critique the numbers, however.

If you 'backtest' the numbers in the Lifetime chart, there are some glaring consistencies in the data.

I calculated the Peukert coefficient for all the batteries in the list, but I used the 8/15/25 amp discharge minutes, instead of the 25a and 20 hr rates.

The resulting numbers were about what I expected. The Peukert coefficient has some 'drift' as you go from low to high discharge rates.

However, the Peukert coefficient for the 6V (4C) battery was exactly the same for all three calculations. Too much the same. So much the same that I believe the numbers were calculated rather than based on test data.

So I guess I'm saying I agree with you, but don't have any faith in the numbers published by Lifeline for the 4C battery.

"I reject your reality, and substitute my own!"

[RoadKingMoe]

Mark, I've always used the 20-hour amp-hour and reserve capacity ratings to calculate the Peukert exponent and capacity, but I did do these in a real hurry this morning, copying and pasting from the Windows calculator, so I could've made a mistake.

I wasn't going to say anything, but the Lifeline numbers for the 4C looked a little suspicious to me also.

[markdoane]

Originally Posted by RoadKingMoe

Mark, I've always used the amp-hour and reserve capacity ratings to calculate the Peukert exponent and capacity, but I did do these in a real hurry this morning, copying and pasting from the Windows calculator, so I could've made a mistake.

I wasn't going to say anything, but the Lifeline numbers for the 4C looked a little suspicious to me also.

Maurice,

You did the calculations right. I was using a totally different method as a way to check for data integrity. When I found the data was 'too good' for the 4C battery, I had to call it out.

As you know, the Peukert coefficient is a part of an equation that represents the non-linear behaviour of a battery under different load conditions. If you calculate the Peukert coefficient at different discharge rates, the coefficient should be different. If it is different for all the batteries except one, and that one is remarkably constant, then I can only conclude that the numbers were 'worked backwards' by the person who wrote the chart.

[RoadKingMoe]

What hoisted the BS flag on it for me is that AGMs usually have just slightly less capacity than the best similarly sized Trojan flooded cells, and that's true for the other Lifeline batteries I've looked at (and use). However, Lifeline's claims for the 4C are much greater than the specs of the Trojan T-105 6V flooded cell. Something's fishy here. Nevertheless, I used Lifeline's specs for it in my calculations above. Maybe it does perform as claimed, but I'm suspicious.