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TM Batteries
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  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 6volt batteries in series has slightly more capacity than a pair of 12volt 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 7085 Amp hours Group 27 batteries are rated at 85105 Amp hours Golf cart batteries are rated at 180220 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 360440 amp hours, or does it remain at 180220 for the pair? Bill: When you put two 6volt batteries in series, the amphour capacity remains the same as a single battery, because all of the amps flow through each battery. When you put two 12volt batteries in parallel, the amphour 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 amphour capacity of a T105 6volt battery is 225 amphours. So a pair of them, hooked in series to provide 12 volts, has a capacity of 225 amphours. According to the Interstate Battery web site, their SRM24 Marine/deep cycle 12volt battery (Group 24) has a capacity of about 82 amphours. So a pair of them would have a capacity of 164 amphours. This is the battery that most dealers install in your TM. So a pair of T105 batteries has a capacity of 225 amphours. And a pair of SRM24s has a capacity of 164 amphours. The T105s provide 37% more capacity. But the T105s are much bigger batteries. The pair of T105s weighs 124 pounds. The pair of SRM24s weighs 92 pounds. So with the T105s, 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 T105s provide 1.81 amphours per pound. The SRM24's provide 1.78 amphours 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 amphours (how many amps will the battery deliver for how many hours). The other is watthours (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 watthour capacity, not amphour capacity. Dealers love to switch back and forth between watthours and amphours. They know they can snow you. To return to the example, a Trojan T105 has an amphour capacity of 225 amphours. Since power (watts) is voltage times current, the watthour capacity of this battery is 225 amphours x 6 volts = 1350 watthours. If you put in two batteries, this doubles to 2700 watthours. An SRM24 has an amphour capacity of 82 amphours. Again, since power (watts) is the product of voltage times current, the watthour capacity of this battery is 82 amphours x 12 volts = 984 watthours. A pair of them yields 1968 watthours. The T105s produce 21.77 watthours per pound. The SRM24s produce 21.39 watthours 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 shortcircuits 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 motorstarting batteries and deepcycle batteries is in the construction of the lead plates. Motorstarting 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 motorstarting. 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 deepcycle 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 motorstarting 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 motorstarting, 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 510 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. 
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