Using two batteries in the TD

[Steve F]

My Jeep has a dual battery setup at the moment with a second battery under the bonnet with an isolator etc.

I was thinking to save on weight in the TD I would put a battery in it the same as the second under the Jeeps bonnet (its quite small but also light and will run my 50lt fridge for 2 days if not charged from a drive). When camped I could leave the TD hooked up the to Jeep with the two batteries connected but isolated from the starting battery. This would give me the equivalent of a large battery in the TD and the bonus of at least one of the batteries charging when I go out for the day and leave the trailer in camp. I figure this way the fridge would run longer as the only additional load from the TD would be some LED lights etc.

Does this sound like a good idea? or am I better leaving th two independent of each other except when they are being charged whilst towing?

Cheers
Steve

[Wos]

Batteries are not created equal. The output voltage of one, if slightly higher, will discharge into the other. The weaker one will accept the extra charge but will not be able to maintain the higher voltage. The batteries will eventually discharge back and forth and they will die. I wouldn't leave them hooked up for very long, and try to charge them every day or two at the very least.
I was thinking of using a similiar system in mine (when I start) but only hook up the trailer battery when running the engine. If the trailer battery gets weak I would swap it for the one under the hood.
My batteries will be small motorcycle batteries which will probably only be used for lighting.

[Alphacarina]

Batteries are not created equal. The output voltage of one, if slightly higher, will discharge into the other. The weaker one will accept the extra charge but will not be able to maintain the higher voltage. The batteries will eventually discharge back and forth and they will die.

Excellent (and correct) advice

Charge batteries in parallel if you like, but disconnect them and use them separately or you'll not only waste some of your energy, but your batteries won't have a long lifespan either . . . . because they are slowly killing each other

Don

[jeep_bluetj]

Yup. Unless they're 100% identical batteries. (Same purchace date, same use, same brand, etc) AND the manufacturer can recommend being tied in parallel. Optimia does, some don't. For any long term storage I'd still isolate them (unless on a trickle charger).

[Steve F]

Thanks guys, they are going to be exactly the same, same purchase date etc as the second battery in the Jeep needs replacing. I'll see what the manufacturer recommends and take it from there but it sounds like leaving them isolated when not charging is the safe way to do it.

Cheers
Steve

[Alphacarina]

Even if they are identical in every way, lead acid batteries don't age the same, so once they are a few months old they will still begin to charge/discharge each other if left in parallel

Don

[Dale M.]

Some interesting reading....

http://jgdarden.com/batteryfaq/carfaq7.htm#wiring

http://www.smartgauge.co.uk/batt_con.html

Dale

[jeep_bluetj]

Thanks guys, they are going to be exactly the same, same purchase date etc as the second battery in the Jeep needs replacing. I'll see what the manufacturer recommends and take it from there but it sounds like leaving them isolated when not charging is the safe way to do it.

Cheers
Steve

You'll be fine as long as you cycle them together. Yes, they degrade differently, but with common cycle times (I.e. same charge, discharge) you're good to go. Long-term storage they'll discharge faster if paralelled. For a vehicle batt (like you have in your jeep), one of the primary reasons for isolation is reserve - so you can actually start the thing after winching/grooving out with the radio/etc all day.

I just tie mine together. 2 optimas sitting in the TJ in parallel. 3 years going strong. Only time I'd ever pull them apart is to use em to weld with. Never needed that yet, so they've been hooked in parallel since I installed em. (And I bet my welding rods are soaked... should get new ones....)


It's VERY common in ambulances, fire trucks, motorhomes, etc to have parallell connected batteries for more capacity. (Also pretty common for serial connected, but that's better covered in the links that Dale posted.)

[Tombstonebilly]

In one of my motorhome rebuild I had three batteries for the "coach" two hooked in parellel and one for the motor all put on a three battery isolator. all new batteries, if you were to put an old battery online it would wearout the isolator quicker from the uneven charging. A Ford starter relay works as a good one battery isolator, just have a large enough wire for the connection between the car and trailer. and AWAYS put a fuse or breaker as CLOSE to the source of power ( battery or isolator power ) as possible, that way if something goes wrong the power is stopped at its source.

[Alphacarina]

I just tie mine together. 2 optimas sitting in the TJ in parallel. 3 years going strong. Only time I'd ever pull them apart is to use em to weld with. Never needed that yet, so they've been hooked in parallel since I installed em. (And I bet my welding rods are soaked... should get new ones....)


It's VERY common in ambulances, fire trucks, motorhomes, etc to have parallell connected batteries for more capacity. (Also pretty common for serial connected, but that's better covered in the links that Dale posted.)

I agree with that - Optimas are special batteries (I have 4 in different vehicles, including one in my tear) and paralleling them won't 'cost' you nearly as much as with cheaper lead acid batteries

Paralleled batteries in vehicles which are driven daily, or in vehicles which only use them for starting work very well too - Many diesel trucks use a pair of batteries for starting, but they don't deep cycle discharge them in parallel . . . . that wouldn't work well. Paralleled starting batteries is not the same thing as deep cycle batteries which are used for an extended period before recharging them, like in a camper or a sailboat - I have many years experience with large battery banks in sailboats which often power many loads for a week or more between charges and you seldom see lead acid batteries paralleled in such service . . . . the ones you do see don't last more than a couple years or so

If you must parallel a pair to use in something like a teardrop which is used so infrequently, then either separate them when not in use or keep them on a float charger 24/7 and they will last you much longer

Don

[alanm]

Don:

Like you, I have 27 years of boating experience with separate starting and house batteries. The starting battery (in the tow vehicle in this case) is a high CCA (cold cranking amp) unit and is isolated from the house batteries.

The house batteries are deep cycle high amp hour units and here you have the choice of lead acid, gel or AGM battery construction. I have always used lead acid of a common brand (Interstate in my boat right now) for ease of replacement in an emergency. I have preferred to use 6 volt golf cart units so I make pairs of them in series for 12 volts and then join the pairs in parallel for the amphours. Running lead acid, or any other battery construction in parallel is not a problem and is found in virtually every cruising boat. As has been previously mentioned, they should be matched in age, size, etc. for best results, and kept on a multistage shore charger when not in use. My boat batteries have lasted 7 years or more this way.

The biggest consideration in powering any DC device like a boat or RV is knowing your loads in amps and how long those loads will be used between chargings. This gives the total amp hours used and should be no more than 50% of the amp hours available. In my boat, I have a total of 690 AH available, so I can use 345 AH before recharging is recommended. This gives the best possible life for the batteries.

Finally, charging is the key ingredient for best battery life and use. The normal towing vehicle's charging system is aimed at keeping the starter battery up to snuff. It is not suitable for deep cycle batteries because the built-in regulator is not the right type. On my boats, I have used an aftermarket regulator that allows the aftermarket high output alternator to charge on a three stage basis.

The shore charger, if you put one in, should be a three stage unit like the True Charge 40 from Xantrex (just an example - there are many good units available).

I know this has been too long a post, but there are many sources of electrical info available from the boating world that are applicable to the trailer world.

[brian_bp]

...you have the choice of lead acid, gel or AGM battery construction.

Just to reduce confusion a little when comparing this great information to other sources, I'll note that all of these are "lead-acid" batteries. The first item in the list should presumably be "flooded'; all of the construction types include lead plates with electrolyte between them, and the difference is that the electrolyte can be a pool of liquid in which the plates are immersed (flooded), gelled (gel), or liquid absorbed into a glass mat (AGM).

[kartvines]

I understand that overall most of you think the Optima is a good battery for the price, I thought would use the blue top, after review many post it seems as if most of you have gone with the yellow top is that correct?

I do want one that will not have fumes being like most I am storing mine with all of my other electronics. I have installed a computer fan at each of my exhaust vents to insure that I keep everything well vented, I also have a slit in the bottom rear of my box to insure there will not ever be a fume build up.

Please excuse me if this end being a hijacked post

[brian_bp]

I understand that overall most of you think the Optima is a good battery for the price, I thought would use the blue top, after review many post it seems as if most of you have gone with the yellow top is that correct?

In the Optima line, blue-tops are "marine" (most are dual-purpose starting and deep cycle) and yellow-tops are also dual-purpose. While it's relatively clear that the red-tops (starting) are not the right choice, the difference between yellow and blue is not so obvious.

Looking at the specs, I think it's mostly a matter of terminals: the blue have threaded posts (good for attaching cables in a trailer) and some sizes have automotive-style posts as well, while the yellow generally have automotive posts and some sizes also have GM-style terminals. Only the biggest (D31) size of yellow has threaded posts. Otherwise, the specs are identical.

It appears to me that we would want Optima models starting with "D" for "Deep cycle" (e.g. D34M is a group 34 blue-top), not those without the "D" which are starting batteries (e.g. 34M, for starting your boat). The "M" on the end indicates a marine - or blue-top - model; again, that's right for a trailer, if only because of the threaded studs (unless you are getting a big D31, then I think a yellow D31T would be the same as a blue D31M).

I suspect that people are buying the yellow-tops because that's what stores have in stock, or at a reasonable price. I agree that the better choice is the blue-top (for the threaded post, even though the insides are the same), and the Optima RV page describes the blue-tops, not yellow.

In the yellow, the largest sizes (BCI Group 31) was listed under Commercial, not Dual-Purpose. I saved my copy of the Optima specs a couple of years ago, so some of the details of model identification and description may have changed.

All of the Optima products are AGM type, and thus have the same venting characteristics.

[Trailer411]

Hello,

We have run into a compatibility issue before on some of the larger trailers that we build. You can see some pic's at http://www.ultracomptrailers.com.

We deal directly with interstate batteries and i have talked to the engineers about this problem on one of our applications where we "combine" 2 banks of batteries as the unit travels down the highway.

Interstate recommends always use the same "group" number on battery banks. With our units we use to use a deep cycle for the "house" and when we combined them with the truck the deep cycles would be shot in about 6 month's. The deep cycle battery against the large truck battery wasn't so much the issue as our house bat's where a different group #. Common groups are group24, 27, ect.