TM Batteries

[Bill]

In the Electrical forum, a series of questions was asked about the batteries in our TMs. A good discussion followed, and it was requested that the highlights of the discussion be put here, in the library.

Bill
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Mac1sss:
I read that having two 6 volt batteries in tandem is better than two 12 volt batteries as they tend to last longer powerwise. I was wondering if someone could explain this setup, and its benefits.

Bill:
All else being equal, a pair of 6-volt batteries in series has slightly more capacity than a pair of 12-volt batteries in parallel. I think this is due to the fact that inside the battery there is a little less structure (cell separators) so there is room for a little more lead and acid.

Bill & Lisa:
According to the following source http://www.windsun.com/Batteries/Battery_FAQ.htm
----Group 24 batteries are normally rated at 70-85 Amp hours
----Group 27 batteries are rated at 85-105 Amp hours
----Golf cart batteries are rated at 180-220 Amp hours.
What I am unsure of is, if you connect two golf cart batteries in series (to up the voltage to 12 volts) does the capacity double to 360-440 amp hours, or does it remain at 180-220 for the pair?

Bill:
When you put two 6-volt batteries in series, the amp-hour capacity remains the same as a single battery, because all of the amps flow through each battery.
When you put two 12-volt batteries in parallel, the amp-hour capacity is double the capacity of a single battery, because each battery sees only half of the amps.
So, to address Mac's question, let's review the numbers.
According to the Trojan web site, the amp-hour capacity of a T-105 6-volt battery is 225 amp-hours. So a pair of them, hooked in series to provide 12 volts, has a capacity of 225 amp-hours.
According to the Interstate Battery web site, their SRM-24 Marine/deep cycle 12-volt battery (Group 24) has a capacity of about 82 amp-hours. So a pair of them would have a capacity of 164 amp-hours. This is the battery that most dealers install in your TM.
So a pair of T-105 batteries has a capacity of 225 amp-hours. And a pair of SRM-24s has a capacity of 164 amp-hours. The T-105s provide 37% more capacity.
But the T-105s are much bigger batteries. The pair of T-105s weighs 124 pounds. The pair of SRM-24s weighs 92 pounds. So with the T-105s, you are buying 35% more battery - 35% more acid, and 35% more lead. To compare the two side by side and expect them to be equal is not quite fair.
To make a fair comparison, you see from the above numbers that the T-105s provide 1.81 amp-hours per pound. The SRM-24's provide 1.78 amp-hours per pound. This is virtually identical - and this was the meaning of my phrase "all else being equal".

Bill:
I'm continuing as a separate post, so as not to obscure the conclusions in the one above.
Battery terminology is tricky, and manufacturers like it that way. In particular, the word "capacity" is tricky. There are two different but related kinds of capacity. One is measured in amp-hours (how many amps will the battery deliver for how many hours). The other is watt-hours (how many watts of power can a battery deliver for how many hours). So you have to know which one you are talking about. To make things even trickier, different manufacturers have different methods of measuring the "capacity" of their battery, so their listed specs are not quite directly comparable.
It is easy to get confused about capacity. For example, why doesn't the capacity double if I put in two batteries? If I have two batteries, shouldn't I get double the power? And the answer is yes, the power doubles. But this is watt-hour capacity, not amp-hour capacity. Dealers love to switch back and forth between watt-hours and amp-hours. They know they can snow you.
To return to the example, a Trojan T-105 has an amp-hour capacity of 225 amp-hours. Since power (watts) is voltage times current, the watt-hour capacity of this battery is 225 amp-hours x 6 volts = 1350 watt-hours. If you put in two batteries, this doubles to 2700 watt-hours.
An SRM-24 has an amp-hour capacity of 82 amp-hours. Again, since power (watts) is the product of voltage times current, the watt-hour capacity of this battery is 82 amp-hours x 12 volts = 984 watt-hours. A pair of them yields 1968 watt-hours.
The T-105s produce 21.77 watt-hours per pound. The SRM-24s produce 21.39 watt-hours per pound. Again, the numbers are virtually identical. And when you stop to think about it, you would expect them to be identical. A battery is basically a very simple device. Dunk some lead in some acid, and electricity comes out. If you have more lead and more acid, you would expect to get more capacity, in direct proportion. And you do - there is nothing magic here. All claims that one battery is fantastically better than another battery of the same size are bogus. If you buy more battery, you'll get more capacity. If you don't, you won't.
There are minor variations in the way batteries can be constructed (thick lead plates, thin lead plates, etc), but these differences have no real effect on capacity.

TucsonCarol:
So, does this explain why my originally installed battery will "take a charge, but not hold a charge"? I'm guessing we discharged it too deeply and it was permanently damaged.

Bill:
As you guessed, a battery that will take a charge but not hold a charge is damaged internally - and cannot be fixed. What often happens is that, when a battery is deeply discharged, the lead plates bend and touch each other. This short-circuits the battery internally.
Another common failure mechanism is that the lead plates shed flakes under deep discharge. The flakes settle to the bottom of the battery, and build up to enough depth to short the plates together. Same effect.
I mentioned earlier that batteries generate electricity when a piece of lead is dunked in acid. The electricity generation actually happens at the surface of the lead, where the lead contacts the acid. The more surface area there is, the more current can be generated.
The lead is formed into a series of plates, which are immersed in acid. One of the differences between motor-starting batteries and deep-cycle batteries is in the construction of the lead plates. Motor-starting batteries have many thin plates. The large surface area of the multiple plates contacts a lot of acid, and therefore can generate more current - which is needed for the huge current demands of motor-starting. But the thin plates are fragile. A big mechanical shock can bend them. And more commonly, they bend for chemical reasons as the battery is deeply discharged.
By contrast, a deep-cycle battery has fewer plates, but they are thick and strong and rigid. There is less surface area, so they can't generate the huge currents of a motor-starting battery - but when you discharge them deeply, the plates are physically stronger and don't bend. So you can recharge them, and all is well.
Just as a final note, I have referred to the "huge current demands" of motor-starting, versus the "small current" required to run the TM (or the house batteries on a boat). To illustrate this statement, consider the following. It is common for the starter in your car to call for 400 amps when starting the engine. Starting a diesel is harder, so it calls for more current. And starting a cold diesel is terrible - the demands may approach 1000 amps! But starting is accomplished in 5-10 seconds, and then the battery can relax and get recharged by the alternator.
By contrast, the battery in your TM doesn't have to supply nearly that much current. How much current does it suply? Figure four or five lights at 1.5 amps each, a television at 2 or 3 amps, the blower motor for the furnace at 10 amps, and an occasional spurt for the toilet. It seldom exceeds 20 amps of current. But that level of current demand can last for hours, not seconds! The battery may be deeply drained, and when recharge is finally available, we want it to recharge gracefully.
Quite different requirements, which leads to designation of "motor starting batteries" and "deep cycle batteries". Your diesel pickup needs motor starting batteries. Your TM needs deep cycle batteries.