Battery power

[Bill]

According to
http://www.ibsa.com/www_2001/content...uct_marine.asp

Three SRM-24 (rated at 16.4 hrs @ 5 amp load each) = 16.4 hrs @ 15 amp load (total).

The three-battery bank has a capacity of about 16.4 x 15 = 246 amp-hours

Weight = 3 x 46 lbs = 138 lbs.

Two SRM-27B (rated at 20.4 hrs @ 5 amp load each) = 20.4 hrs @ 10 amp load (total).

The two-battery bank has a capacity of about 20.4 x 10 = 204 amp-hours.

Weight = 2 x 55 lbs = 110 lbs.

All else being equal, the set of 3 SRM-24s is better.

Incidentally, the capacity of a lead-acid battery is closely related to its weight - more lead and acid mean more capacity - so this result is not surprising. Most "which-is-better" questions can be answered by looking at the total weight.

As Ray has pointed out many times, these are not true deep-cycle batteries, but they are a bit better than standard car batteries (aka cranking batteries).

HTH

Bill

Originally Posted by Bill

Three SRM-24 (rated at 16.4 hrs @ 5 amp load each) = 16.4 hrs @ 15 amp load (total).

The three-battery bank has a capacity of about 16.4 x 15 = 246 amp-hours

Weight = 3 x 46 lbs = 138 lbs.

Two SRM-27B (rated at 20.4 hrs @ 5 amp load each) = 20.4 hrs @ 10 amp load (total).

The two-battery bank has a capacity of about 20.4 x 10 = 204 amp-hours.

Weight = 2 x 55 lbs = 110 lbs.

All else being equal, the set of 3 SRM-24s is better.

Incidentally, the capacity of a lead-acid battery is closely related to its weight - more lead and acid mean more capacity - so this result is not surprising. Most "which-is-better" questions can be answered by looking at the total weight.

HTH

Bill

That helped a lot!

So the SRM-24s are 1.783 amp-hours per pound while the SR-27Bs are 1.85 amp-hours per pound. Therefore, the SRM-27Bs are ever so slightly better in terms of capacity per pound, but the SRM-24s will give me what is probably the maximum amp-hours that will fit into the allotted space.

It looks like I'll go out this weekend and buy two more batteries to match the one I already have and either a double or two single battery boxes.

I used to be better at this stuff. I studied electronic engineering at CalPoly many moons ago. I just don't have any reason to use my old skills.

Maybe in a few years when the first set of batteries bite the dust I'll switch over to T105s. If I drop the battery tray down about 2 inches then the T105's would fit, but only two of them.

I intend to connect two SRM-24 batteries in parallel. This immediately brings to mind two questions:

1. What size wire.
2. Should there be a fuse.

The present pair of wires from the single SRM-24 battery to the TM power center appears to be around 14-gauge (I'm guessing here) and is protected by an in-line 30 amp fuse. To a casual observer it would appear that the same size wire would be adequate for connecting the batteries together to create a battery bank. There probably is no need for a fuse in this connection.

But then I started thinking of worst-case scenarios.

What happens when you connect one fully charged battery with one fully discharged battery? Is there any reason to believe that the current would be less than 30 amps? What size wire should be used?

What happens if one of the batteries fails and becomes a short circuit?

Can a lead-acid battery actually short out?

Am I just being paranoid?

My next set of questions is more long term. When the TM will be stored for an extended period of time should the batteries be disconnected? It appears that common wisdom indicates most TM owners at least disconnect the battery bank from the TM, mainly because the radio antennae draws current at all times, which would eventually kill the batteries.

But what about storing the two batteries connected together with no load? As the batteries drain for reasons other than current load, the connection would keep them both at the same charge level. That way when you come back later to charge them up the next time they are at equal potential and there is no current flow between the batteries.

If you disconnect the batteries for long-term storage and then connect them back together to hook up the trailer and charge them, I don't see any reason to believe that they have drained at the same rate. We are back to my earlier question. When connecting two batteries together in parallel, what happens if one battery is essentially fully charged and the other is essentially fully discharged? Would it be best to charge them as single batteries before connecting them together in parallel to create a battery bank?

Once upon a time I studied to be an Electronic Engineer, but my interest was limited to transistors and integrated circuits. I don’t recall any instructor ever discussing what happens when you start messing around with large batteries. I’m more comfortable with milliamps than amps.

[Bill]

Originally Posted by PopBeavers

I intend to connect two SRM-24 batteries in parallel. This immediately brings to mind two questions:

1. What size wire.

Number 10 should be sufficient, especially for the very short run between batteries. If you want to be a purist, use a pair of #10s, or a #8. Incidentally, to maintain balance between the batteries, it is a good idea to connect the TM's + (black) wire to the + terminal of one battery, and the TM's - (white) wire to the - terminal of the other battery.

2. Should there be a fuse.

There is no need for a fuse between the batteries. Maintain (but do not increase) the 30-amp fuse in the black line from the TM.

The present pair of wires from the single SRM-24 battery to the TM power center appears to be around 14-gauge (I'm guessing here) and is protected by an in-line 30 amp fuse.

I think you'll find that both the black wire and the white wire are #10. With a 30-amp fuse, #12 or #14 would be too light.

What happens when you connect one fully charged battery with one fully discharged battery?

Easy answer - don't do this. If you suspect that your batteries have a wildly different state of charge, then charge them separately before connecting them. Though I don't like the use of terminal voltage as a state-of-charge indicator, it is adequate for this purpose. But before measuring the voltage, make sure the batteries have rested for an hour or more since they were last charged or discharged. Voltage readings taken too soon after any battery activity will be inaccurate - often wildy so.

What happens if one of the batteries fails and becomes a short circuit? Can a lead-acid battery actually short out?

Lead-acid batteries can develop a "shorted cell" if they are mistreated. "Mistreatment" usually means discharging the battery all the way to ZERO state of charge - most lead-acid batteries don't like being discharged below about 50% SOC. If this happens, the battery with the bad cell will hog all the charging current, and will drag the other one down, which will probably damage the other one. The appearance is that the battery bank "won't hold a charge". By the way, the short is internal to the battery - the plates, which are covered with sulfide by then, warp a bit and touch each other. The "short" is actually a fairly high resistance connection, not a metallic dead short.

Am I just being paranoid?

A little bit. At one time or another, I'm sure you have started a dead car with jumper cables. That is an extreme example of what you are worried about - one dead battery and one full one, suddenly connected together. You get a bit of a spark when you connect the last cable, but nothing dramatic happens.
I am active in a solar power forum, and the serious solar power enthusiasts have rooms full of lead acid batteries, all of them interconnected with #2 wire or solid copper buss bar. The array is fused, but not the individual batteries.

My next set of questions is more long term. When the TM will be stored for an extended period of time should the batteries be disconnected? It appears that common wisdom indicates most TM owners at least disconnect the battery bank from the TM, mainly because the radio antenna draws current at all times, which would eventually kill the batteries.

Yes. Make sure the batteries are fully charged, leave the two batteries connected together, and then either disconnect the pair from the TM, or put a good 3-stage charger on the pair. Incidentally, phantom loads include not only the radio antenna, but the radio itself and the gas leak detector. I haven't convinced myself one way or another about the water heater controller.

But what about storing the two batteries connected together with no load? As the batteries drain for reasons other than current load, the connection would keep them both at the same charge level. That way when you come back later to charge them up the next time they are at equal potential and there is no current flow between the batteries.

Correct. Both batteries will self-discharge at a rate that is temperature-dependent - very slowly in very cold weather, faster in warm weather. As you note, they will self-discharge at rates that are similar, but not abosolutely identical. Keeping them connected will keep them at an equal state of charge (or discharge). Before hooking up the charger (or every month or so if you leave a 3-stage charger on them and the weather is warm), check the water level in all the cells.

By the way, all of this is predicated on the assumption that the batteries are the same age and the same type (cranking, marine, deep cycle, etc). It is not wise, I am told, to mix batteries which have more than about 6 months age difference. NEVER mix a sealed (gel-cell) battery and a flooded (wet) battery.

If you are really interested in lead-acid batteries, here are three good links. There are also good tutorials at the websites for battery manufacturers such as Exide, Trojan, and others.

http://www.batterystuff.com/tutorial_battery.html
http://www.batteryfaq.org/
http://www.uuhome.de/william.darden/carfaq4.htm

Bill

Bill,

Thanks for your insight. I only know enough to be dangerous.

It seems to me that for anyone using two batteries that it would be useful to have a small battery control center mounted in a box near the batteries that would contain:

1. A 4 position switch: Battery 1 only, Battery 2 only, Both batteries, No batteries. This switch has already been discussed elsewhere.

2. An ammeter to display charge/discharge current to the battery bank, which would display current to either one or both batteries, depending on the switch position.

3. A volt meter with another switch so that either battery's voltage could be measured (as long as the "both batteries" position is not selected) so that you could easily verify that both batteries are approximately equally charged before switching them into the "both batteries" position.

[Bill]

Hey, you're like me, a gadget guy! I installed two charge/discharge ammeters, one visible from outside and one visible from inside, in the 12-volt to my TM battery. Small ammeters (meaning 20-0-20 amps or 30-0-30 amps) are more easily available from tractor supply places like Trusty Tractor than from auto supply places. About $8 each.

A four-position switch such as you describe is readily available from marine stores such as West Marine. It seems you can pay anywhere from $30 to $200 for them.

Voltmeters are easy.

Somewhere I ran into a link for the kind of system you are thinking about - was it on this board? Anyway, these folks did a real nice installation for a very small price. Go to

http://www.rickandbarb.com/

and burrow your way down through MotorHome Projects > Technical Projects > Digital Meter Project.

Bill

Originally Posted by Bill

Hey, you're like me, a gadget guy! I installed two charge/discharge ammeters, one visible from outside and one visible from inside, in the 12-volt to my TM battery.

Bill,

Mounting meters in the TM is easy. But what did you do to mount them outside? Since my batteries are on the tongue instead of the rear compartment I will need to put some sort of weatherproof box together. I might as well mount it all in a single box: meters, switches, etc.

If I could find a long enough battery box I could install two group 27 batteries and the switches and meters inside the single box. I think I have enough room on battery tray, if I can find the right box. Otherwise I will need a box for the switches and meters.

There is a West Marine store near my house. Guess where I will be on Saturday morning.

[Bill]

In my 2720SL, the battery is in the rear compartment as you noted. I bored a hole down into the bumper compartment and ran several wires down there (including an AC cord for a 3-stage battery charger permanently mounted in the battery compartment). That was part of a project to get power into the TM when it is closed - a problem you and your 2005 model don't have to deal with.

I mounted the switches and one of the meters in - guess what? - a Tupperware box that fits in the bumper compartment. Accessible with the TM closed. The box is rigid enough to mount the switches on, soft enough to provide some shock protection, waterproof, and since it is quite translucent, I can read the meter without opening the box. Looks kinda strange, but works like a champ. Might not be so good in the exposed area at the A-frame of your TM, though.

Be sure to let us see photos of your final result.

Bill

[B_and_D]

Have you seen these battery boxes? They should fit two T-105's...or two group 24 or 27's.

http://www.alliedbattery.com/boxes.htm

A new battery box for us is on our projects list this summer. We saw a metal tool box at Home Depot that might also work.

We want to install a cutoff switch on the outside of the box, whatever it is that we end up with. For now we simply take the fuse out of the line when the TM is not in use. Just leaving the TM sitting will drain our batteries at a surprising rate. If we charge them up and take out the fuse, they still seem pretty well charged even after a month.

I got a really heavy duty cable to run between the two T-105's at our local Interstate Battery store. It's as heavy as the cables that attach your TV battery to the alternator. Didn't cost much, and had all the proper insulation and ends already attached.

I used a box from the same company and it worked great for the two t105 batteries. You will need to add a drain and vent. I have a 2005 2720SL and the cut out the nipple on the original battery box for use on the side of the new one. I did not need to move the vent through the side , it worked fine with the original flex hose. Will not work outside the TM. I belive it will not be watertight.