Give me feedback on my battery set up

[Brad Lustig]

I agree, the batteries will equalize, but if one desides to crap out, it's going to take the other one with it. Like someone mentioned earlier, the marine battery selector is cheap insurance. That way, you can run one, both or none.

[webbedouin]

Batteries wont kepp discharging into each other, they will equalise a bit and then stay matched to each other
the only time this wont happen is when
a one battery is faulty and wont hold its charge
b batteries arent the same model, the smaller one will discharge quicker and the larger one will equalise with it but that capacity will still be there to use just at a lower voltage

Diferences in potential will be equalized at all times. The act of equalizing the other battery will drain the first lower. Now i don't know the frequency with which this happens, but i'll wager it is within Hz (cycles per second) Touch a battery to ground and see how fast it tries to equalize! Speed o' light baby!!! Watch the heat melt stuff. There's a very good extreme example. But on a smaller scale it's happening all the time!!! No two batteries will ever contain the exact same potential because the won't be carrying the exact same amount of solutuion, even battery cell wall size varies, within specifications, within a single battery!

[denverd0n]

Yeah but of course solar systems are very different concept.

No they're not. They have a bank of deep-cycle batteries. They all drain together. They all charge together. Any differences are at most very minor, probably completely insignificant.

That's really different than charging up a pair and heading out for a weekend and trying to conserve every last bit of juice.

You don't think people who live off the grid and rely on solar power and their batteries for all of their electricity aren't concerned about conserving every last bit of juice!!??!! You need to spend some time on some of the solar power forums. Those people are completely obsessive about squeezing every last watt from their systems!

And yet, they find no need for battery isolators between every battery in their banks.

It's likely to be only be a piddling few millivolts

Exactly. The difference is so insignificant that it won't matter a watts worth. The cost of a battery isolator is simply not justified unless you have significant differences between the batteries that you're wiring together.

By way of example, my expectations for battery life is three to five years.

And again, the solar powered home users, who find no need for isolators, typically get 7-10 years of use from a bank of wet-cell batteries. Hmmm.

I really don't wish to offend you, but I'm quite sure that you're imagining a problem here where one simply doesn't exist. The cost of a battery isolator, in Dee's situation, would be a complete waste. At best, it might keep the lights on for 10 seconds longer at the end of a weekend, and it might make the batteries last for 5 years and two days instead of just 5 years.

Dee, your system is wired up just fine the way it is. Don't waste your money on a battery isolator between your two batteries.

[SteveH]

Dee,

For what it is worth, Denverdon is right IMHO.

Battery isolators are most commonly used when there are two different LOADS and one charge devise as in the RV industry when there is a battery in the tow vehicle and one in the RV, both being charged by the alternator in the tow vehicle.

[webbedouin]

Say what you believe fellows, but i'll stand pat on my 48 years of electronic training and 36 years as a professional...

[bdosborn]

Hmmm. Interesting thread. Last Wednesday I watched a factory rep test the battery strings on a 100kW UPS supplying the phone switch in a hospital. This is about as critical a load as you can get. Redundant battery strings wired in parallel. Each battery in a string wired in series (approximately 25 batteries in each string). Not an isolator in sight. I've also looked at dozens of battery piles in telecommunication buildings and I've never seen a battery isolator in a string, wether it was wired in parallel or series. There may be a small loss in pile capacity or life if you don't isolate batteries, but it doesn't appear to be significant enough to compensate for in mission critical applications. This is assuming you start off with identical type batteries in a string. All bets are off if you mix batteries of different types or age. I think the money spent on isolators would be better invested in a quality battery charger.
Bruce

[SteveH]

Say what you believe fellows, but i'll stand pat on my 48 years of electronic training and 36 years as a professional...

Webbedouin,

I certainly don't want to ruffle any feathers on this subject, but I too have spent my career (35 years) in the electronics and telecommunications industries, and running batteries in parallel is common place...done all the time to increase capacity.

You may be thinking of the problems associated with running batteries like nical cadmium (NiCad) in parallel. It will not work with these batteries because of their extreemly low internal resistance. For that reason they WILL self discharge one another when used in parallel.

The gel cels that Dee is using, however, are known to have very high internal resistance making them excellent for use in parallel installations.

[Cutterpup]

Engineers like to use lots $$$$ to fix .02 problems. It justifies the degree hanging on the wall.

Adding a $20 to $100 battery isolators to save a $50 battery for an extra year?

Most engineers like complicated solutions to simple problems, HOWEVER not all engineers are like this some take the time to look at the big picture.

just my two cents. and many years of changing multi battery hookups on bouys.

Dan

[Dee Bee]

Wow Thanks for the passionate responses...

Basically I want to know that what I have installed is safe, ie won't blow up. I will watch the discharge rate of idle bateries. Thanks for the heads up on this. So far so good.

Here is something to scratch your heads over:
I tested the rig by running a 12 v DC computer fan for 6 hours. The amperage of the fan is .05. Resting voltage of the two cells after a full charge and a day of equalizing started at 13.3 v DC As I ran the fan for 6 hrs the battery voltage WENT UP!!!

After six hrs it was reading 13.8 v DC. I can't figure this out at all.

When I was finished I but the batteries back on the charger that is programed for these gel cells. They recovered and returned to their resting volatage.

Is this typical?

DEE

[Larwyn]

As I see it, isolating parallel batteries would also require isolated loads on the two batteries. Not exactly practical on the simple electrical systems we use on these tiny trailers. I would run it like you wired it, see how it works out, then try to solve any problems that might arise. Throwing dollars at a percieved problem is a trap I try to avoid. Even if there is a shortage of amp/hours with your present set up, I would be inclined to spend my money on higher capacity batteries, rather than isolation of the existing ones. There are thousands of RV's out there runniung paralell and series/paralell battery banks and I've never seen isolaton go beyond seperating the starting battery from the house battery bank.

In my opinion what you have is safe, will not blow up, and will most likely work well for you.

[madjack]

...my .02 worth...as a 20+yr trucker, big rigs run 4 batteries(typically)...no isolators in sight and trucking co. are absolutely obsessed with saving every penny they can. 30+yrs playing with troll motors with 2 bat. wired either parralell or series...no isolators. 40+yrs 4wheel deives w/winches using 2 or more bateries...no isolators. All of this without any discernible diff in battery life.
Until Web posted about this problem/solution I had been unaware of it. I understand what he is saying, I just don't see it as a problem worth the solution proposed. However I would recommend a master switch on all of our batteries for safety and to disconnect the batteries when in long term storage
madjack

[webbedouin]

Here's the deal, Dee's batteries are going to be sitting idle about 90% of the time. All they are going to have to do is equalize each other most of the time. They won't be subjected to constant loads, constantly changing loads and/or constant or varying charging conditions. Idle batteries are the devil's playground. Busy batteries have other things to do.

[madjack]

...webbe, I agree totally about "idle" batteries which is why I suggested the master disconnect however if a "maintainer" type float charger is used and it is left plugged in while tear is stored, well.......they aren't really "idle" are they
madjack

[bdosborn]

Here is something to scratch your heads over:
I tested the rig by running a 12 v DC computer fan for 6 hours. The amperage of the fan is .05. Resting voltage of the two cells after a full charge and a day of equalizing started at 13.3 v DC As I ran the fan for 6 hrs the battery voltage WENT UP!!!
DEE

Here's my guess:
Battery voltage is directly proportional to temperature. Batteries have an internal resistance. As you discharge them, they heat up and the voltage rises.
Bruce

[Dee Bee]

Here's the deal, Dee's batteries are going to be sitting idle about 90% of the time. All they are going to have to do is equalize each other most of the time. They won't be subjected to constant loads, constantly changing loads and/or constant or varying charging conditions. Idle batteries are the devil's playground. Busy batteries have other things to do.

Ok So is it better to keep the batteries constantly on the charger? The "Battery Tender" is programed for these gel batteries. It has three levels of charging, the last in the program cycle is "float."

Or is better to let the batteries alone and just charge them prior to a weekend trip?

DEE