6 volt batteries vs 12 volt ???

I read that having two 6 voly batteries in tandem is better than two 12 volt batteries as they tend to last longer powerwise .... I also read that some folks had ordered their TM's with 6 volts and was wondering if someone could explain this setup .... and its benefits ... my simple way of thinking is that two 12 volts woould be better as you could switch from one to the other as I do on my boat ....the RV world is a little different I am finding out .... also sorry if this question has been asked already...I looked but didnt find anything ..thanks!

[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.

However, I am also under the impression that when you come right down to buying batteries, there are 6-volt deep discharge batteries made for golf carts, etc, but very few 12-volt deep discharge batteries.

Either way, make sure you buy true Deep Discharge batteries, not a so-called marine battery such as the Interstate group 24 battery that most dealers install in RVs including TMs. The Trojan T-105 deep discharge 6-volt battery is well-regarded. If you are more comfortable with 12-volt batteries, I am told that the Everstart Deep Discharge battery (Wal Mart!) is one of the best on the market. This nugget comes from the gurus in the solar power industry - solar system batteries are subject to frequent deep discharge cycles, which play havoc with ordinary car batteries.

Finally, the reason you have two 12-volt batteries in your boat, and switch between them, is that on a boat, one battery must be reserved for engine starting. This is a heavy load, and calls for a fully-charged battery. In other words, you must not deplete that battery very far or you'll be stuck at sea with no engine. Meanwhile the second battery takes care of the electronics and lights, etc, a much lighter but longer term load. You can deplete that battery without major consequences. In a trailer, there is no engine, of course. The battery, in whatever form, serves only the electronics, lights, and appliances. So it is best to always use both batteries. Hook 'em up and leave 'em. By doing so, you assure that both are always charged equally, which is a good thing.

Hope this helps.

Bill

Thanks Bill, so if and when I order my TM is the 6 volt route the best route ?
This is my thought process ... I have a Diesel and it has two 12 volt car batteries ... in a pinch I could swap batteries out in either the Truck or the TM if I stayed with 12 Volt or is the batteries for the RV's a totally different animal ie... the terminal post etc and size ? I know this may sound strange but where I intend to go there aint no hookups or Walmarts...... thanks!

[Bill]

Well, that is something I hadn't considered, and it is something to think about. If you are talking about needing an occasional swap-out to keep your TM going for another day or two, then you might do well to have 12-volt batteries all around. I wouldn't plan on doing this on a regular basis, though.

The batteries in your truck are motor-starting batteries. They can put out a tremendous burst of power for a short time, and that's what is needed for starting a diesel in cold weather. But they are not really good for long-term small power requirements. Of course they will power your TM in an emergency. But if you install them in a trailer and drain them slowly but deeply, as trailers do, they may not recover.

In contrast to the huge but short-duration power demand of motor starting, a trailer presents a modest but long-duration power drain, and you need a different kind of battery for that. As I mentioned, a deep discharge battery is really needed.

Finally, you probably can't start a big cold diesel with deep discharge batteries, though I haven't tried it. They simply aren't built to provide that mammoth burst of power.

Have you considered buying a small generator such as the Honda EU-1000i? Kinda pricey, but you can fire up the battery charger in the TM's converter, and not have to worry about dead batteries.

Bill

Be careful wiring in two batteries of different types and/or ages. The weaker one will tend to discharge (and also ruin) the better one.

Originally Posted by 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.

Bill 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 not sure of is if you connect two golf cart batteries in series (to up the voltage to 12 volts) is your capacity 360-440 amp hours or does it remain at 180-220 for the pair? The dealer who sold me on the upgrade to 6 volt batteries in my 5er told me they were good for 400 amp hours (AH) compared to 100 amp hours in the original type 27 battery that came with the unit. I have a great deal of respect for your experience and knowledge and do not think that double the capacity would qualify as "Slightly" in your book. That suggests that the pair of 6volts are only good for 180-220 AH(which IS slightly better than the 170-210 AH that you would get from a pair of 12volts in parrallel).

Is there an electrical engineer in the house?
Bill

[Bill]

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 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 at the 20 hour rate. 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. So 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".

By the way, I am an electrical engineer.

Bill

Holy smoke ..... I'm getting a generator

[Bill]

I'm doing this 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 they are not 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, just as they did to you, Bill. They know they can snow you.

To return to the example, a 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 times 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 x 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. The electricity- producing part of a battery is lead and acid, nothing more. 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.

As noted, a battery is basically a very simple device. Dunk some lead in some acid, and electricity comes out. 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. However, the variations result in differences in how fast the battery can produce power, and how well it stands up to deep discharge. As a result, some batteries can be characterized as motor-starting batteries (huge current for a short time, but you can't discharge them very deeply or they break), and some are characterized as deep-cycle batteries (small current for a long time, and you can discharge them very deeply without damage).

So, did anyone read all the way through this? Or are we firmly in "eyes glazed over" territory?

Bill

[commodor47]

Bill,

Makes perfect sense to me. I never tire of learning something new . . . especially when it relates to a TM.