[A/S] Re: Boondocking Battery specs, which is most important?

["Maurice Riggins" <maurice.riggins@xxxxxxxxxxxxxxxx>]

There are pros and cons to both the 6V T-105 in series and 12V 31XHS 
in parallel.

If you look at Trojan's specs for them:

http://www.trojanbattery.com/GolfDC.asp?Product=51

http://www.trojanbattery.com/GeneralDC.asp?Product=65

You'll find 225AH at 11.25A (20 hour rate) discharge for the T105 and 
2 X 130AH = 260AH at 2 X 6.5A = 13A (20 hour rate) for the 31XHS.

So if you mainly operate at lower discharge rates, and that's what 
most folks do, even when the furnace is on, the 31XHS has at least a 
35AH edge over the T-105s.

I overlooked something in my earlier post when I said they were 
roughly equal at a 25AH discharge, and that is if each of the 
parallel 31XHS have a 25A discharge, the total current would be 50A, 
so the 31XHS would still have a significant edge since a 25A 
discharge would only be 12.5A out of each battery.  I'd have to 
calculate the Peukert exponent for each of these ratings, and use 
that to make a direct comparison, but I won't do it here since it 
deals with logarithms.  The truth is probabably that the T-105s are 
more comparable to the 27TMH batteries even at 25A discharge and that 
the difference in life, given the same usage, is a less than the 40% 
I used.

If you have to squeeze a pair of batteries out on the tongue, where 
their additional height may not be a factor, the T-105s have an 
advantage of smaller base measurements.  

But if your trailer has Group 27 sized compartments (that may take a 
Group 31 battery) that are internal (i.e. under the couch for 
example) where they will be warmer, even the less capable Group 27s 
will have much more capacity than the T-105s (or other batteries) out 
on the tongue when its cold.  They can lose as much as 30% of their 
capacity by the time the temperature gets down to freezing.

Let's look at what happens with a battery failure.  If a cell shorts 
in one of the batteries in series, the voltage drops, perhaps by as 
much as 2 volts.  Lights will continue to work less brightly, but the 
furnace fan may not work sufficiently to allow the burner to start.  
If a cell opens in one of the batteries in series, you're dead in the 
water and have lost all battery capability.

If a cell shorts in one of the batteries in parallel, it will also 
lose as much as 2 volts and a loop current will develop between the 
batteries, discharging the good one.  However, all you have to do to 
continue to operate on reduced capacity is disconnect the bad battery 
and recharge the good one.  If a cell opens in one of the batteries 
in parallel, you have to do nothing and can continue to operate at 
reduced capacity.

Some final advantages to the T-105s are that there are half as many 
cells to maintain, and they are probably cheaper than the Group 31.

Personally, I'd rather have the Group 31 in my under couch 
compartments (where T-105 are too tall) where they're warm rather 
than a pair of T-105s out on the tongue behind the LP tanks.  And I 
want the max in capacity.

Late model trailers are more resource-intensive and need all the 
battery capacity they can get.  The LP detector can take 6AH/day.  So 
can the electronics in the refrigerator, where the gas is now 
controlled by an electrical solenoid that can draw an amp.  If the 
burner's on 50% of the time, that's 12AH in addition to 6 or so for 
the circuit board.  Car radios can use a few AH/day when off, just 
remembering station settings.  The water heater may have electronic 
control and ignition, as might the furnace.  My air-conditioner/heat-
pump and furnace are controlled by an electronic wall box.

I'm also of the firm belief that there will NEVER be ANY heater that 
competes with living things for oxygen, and has the potential to 
produce carbon monoxide, in our trailer.  That means, in our case, 
there's a 9.8A furnace, which if running constantly (it would have to 
be pretty cold) would need 108AH (and about 18 lbs of propane) for 
the 11 hour period between 9PM and 8AM (quiet hours?) for example. 
That's a bit over 40% of a pair of warm Group 31's and right at the 
point to start the generator back up.  Most furnaces are closer to 
7.5A though.

Anyway, I hope this helps.  I'd opt for the Group 31 if you can get 
them in internal compartments.

--
Maurice
WBCCI 5446