Wiring two batteries together question.

Mikeg · 02-08-2013, 11:57 PM

Ok, I know there have been postings on this elsewhere. I tried responding to the specific thread, but it was too old. Herk posted a picture showing two 12V RV Batteries wired together in parallel. The picture showed running a wire neg to neg and pos to pos. from battery 1 to 2. It then showed the 12V power running from battery one to the TT and the ground running from Battery 2 to the TT. And I cannot figure out how to copy and paste the picture. Now, the thread also referenced "The 12 Volt side of Life", which I read thoroughly. No where on the site does it say to do this, it says to run the 12V power and Ground from Battery 1 to the TT while having the two wired together as above (Neg to Neg, Pos to Pos). My online research also comes up with most people saying to do it this way. Hence, my confusion, I know Herk is very knowledgeable from reading his numerous and detailed posts. Anyway, I have two 12V batteries and will be wiring them together for this season. My questions are:

1) Which way is correct?? Or does it even matter?
2) Will the batteries charge together, either way, when either hooked to electricity through the TT being plugged in or the vehicle when towing?

I realize you don't need the batteries when hooked to electricity, but we sometimes camp without being hooked in.

Thanks for helping clear up this fellows muddled confusion.

Mike LeBlanc
09 Ram Crew Laramie 5.7L
2012 Shamrock 21SS

Twisty · 02-09-2013, 01:17 AM

"...neg to neg and pos to pos. from battery 1 to 2. It then showed the 12V power running from battery one to the TT and the ground running from Battery 2 to the TT"
That is how I hooked up my 2 batteries.
After reading quite a lot about wiring bats in parallel, I decided to just hook them up in a proven way and use them

The bats will charge either way. Ideally, they should be charged separately. Ever see a bank of bateries in a forklift or RV? The owner or user never separates the batteries and charges them independently.

Witch Doctor · 02-09-2013, 06:20 AM

I wired pos to pos and neg to neg. Increase the wire size do to a voltage drop, I think I used #4 copper. I bought the wire at NAPA and they even soldered on the ends. Your 2nd battery will charge the same as the org. Either from your converter or your TV. You can also install a selector switch with bat #1 Bat #2 or all. I did not think I needed that my disconnect switch is connected to the #1 battery and will control both. With the selector switch you would shorten charge time by doing one at a time but not really needed.

Pipeman · 02-09-2013, 07:34 AM

Mine is wired parallel and has the + side from one and the - side from the other hooked up to the electrical panel. Have been doing it that way for a few decades now. Works for me.

Triguy · 02-09-2013, 07:53 AM

Mikeg,
Not knowing precisely the setup Herk was describing in the post you mention, I can't be 100% sure, but I think his method was describing a "balanced" approach to wiring 12-volts in parallel.

The 12-Volt Side of Life offers a safe but unbalanced approach.

It is best to have the pos on one battery going to the trailer and the negative on the other battery connected to ground. This balances out the batteries.

Wiring both the positive and negative to the same battery in a parallel grouping will put more of the the load on the first battery in the string.

See this for reference to different methods of connecting parallel.

I'd attach a diagram but iPad,internet and weather are not playing together well right now.

Triguy · 02-09-2013, 08:00 AM

This is a balanced approach as I was trying to describe ...

**Herk7769** · 02-09-2013, 08:04 AM

The proper way to wire a pair (or more) 12 volt batteries is to use exactly the same length wires between batteries (to keep the internal resistance of that battery equal to all the others) and attach the positive of the closest battery to the charge/discharge source and the ground for the system to the negative terminal of the farthest battery from the charge/discharge source.

Otherwise, if both the ground and the charge/discharge wires are attached to the closest battery, and all the other batteries are piggybacked onto the battery with the load on it, each successive battery will have the resistance of the wires connecting them added to the battery's internal resistance.

This means that each battery with the higher resistance will "work" less hard than the battery in front of it and the last battery will work the least hard of them all. That means the BANK will have less total capacity than the sum of each battery because once the lead battery goes totally dead it suck power from the higher voltage batteries to try to charge itself up until they are all dead.

Here is some reading to better explain the concept. I have also included the proper way to wire 6 volt golf cart batteries.

Mikeg · 02-09-2013, 04:48 PM

Thanks for the answers.
This is the way I will be doing it.
Come on spring!!

Witch Doctor · 02-13-2013, 04:37 PM

Quote:

Originally Posted by herk7769

The proper way to wire a pair (or more) 12 volt batteries is to use exactly the same length wires between batteries (to keep the internal resistance of that battery equal to all the others) and attach the positive of the closest battery to the charge/discharge source and the ground for the system to the negative terminal of the farthest battery from the charge/discharge source.

Otherwise, if both the ground and the charge/discharge wires are attached to the closest battery, and all the other batteries are piggybacked onto the battery with the load on it, each successive battery will have the resistance of the wires connecting them added to the battery's internal resistance.

This means that each battery with the higher resistance will "work" less hard than the battery in front of it and the last battery will work the least hard of them all. That means the BANK will have less total capacity than the sum of each battery because once the lead battery goes totally dead it suck power from the higher voltage batteries to try to charge itself up until they are all dead.

Here is some reading to better explain the concept. I have also included the proper way to wire 6 volt golf cart batteries.

Thanks herk, I did not know of the balance system. I rewired my battery's to this approach. The only problem is that I can not make the plus and the minus the same length. will cause a problem? both are the same interstate battery's. How important is that?.........

**Herk7769** · 02-13-2013, 04:41 PM

Quote:

Originally Posted by gljurczyk

Thanks herk, I did not know of the balance system. I rewired my battery's to this approach. The only problem is that I can not make the plus and the minus the same length. will cause a problem? both are the same interstate battery's. How important is that?.........

The critical wires are the "inter battery" ones since they effect the "effective" internal resistance of each battery. As long as the red wire goes to the positive terminal of the closest battery and the Black wire goes to the farthest Negative terminal the batteries will charge/discharge evenly.

up2speed4 · 03-08-2013, 11:06 PM

Hello, With all this battery knowledge I thought I would post this here. My problem: I will be camping without hookups for the first time in my new 20' Flagstaff and I was told my one year old 550 amp battery will power my water pump and 12v lights and occasional heater use for one or two days. Is this true or was the dealer trying to sell me additional batteries and generator I don't need?

QuietTimes · 03-08-2013, 11:27 PM

We've been wondering too if we need to buy another battery. We have LED lights which should help. And we are also planning to buy a genny. I'm hoping that with the genny we won't have to add another battery but in the long run that might be best. Won't know till we try it out I guess. One place we go has no hookups available at all and we like to stay there five to ten days.

up2speed4 · 03-08-2013, 11:56 PM

How do you like your Ford EcoBoost when towing. Thinking of a future purchase. Thanks.

garbonz · 03-09-2013, 12:05 AM

Your battery is not 550 amps HOURS and that what you need to figure out. Look for the amp drain information on this site for the items you plan to use, estimate the amount of time you will use each one and then multiply the amps the device used times the number of hours of expected use and add them all together. This will give you the total amp hours needed per day. Then figure out the amp hours of your battery and divide it in half (that is the most you can safety use). Based on personal experience a single battery in a typical RV will only last one full day without charging unless you are REALLY careful, especially if it is not a good deep cycle.

Do the calculations and you can see for yourself.

GlacierGus · 03-09-2013, 12:10 AM

I've added the 2nd battery. For boondocking, I just want the extra capacity. Attached is a photo of my connections for 2 12 volt batteries (taken so I would remember this spring). I haven't pruchased a genny yet.

Pipeman · 03-09-2013, 06:54 AM

We boondock about 95% of our camping time. There are 2 batteries wired in parallel, with positive off of one battery and negative off the other. We also have a 4ooo watt genny. we also have a portable solar array. We don't camp for two days any more. We go out for at least a week and more than likely at least 2 weeks. Last summer we were boondocked in the Rockies for a 3 week stretch. The solar array is big enough to keep the batteries charged with the fridge, water pump, detectors and any other 12 volt use. For the 110 volt stuff the genny is used. I can also use the genny here at home to power the main parts of the house during a power outage.( which hasn't happened since the wiring was done for that). I have always used 2 batteries on any of the trailers we've owned and that's 5. If you plan on any more than a couple of days boondocking, I would go with the extra battery and then start looking for a good genny and maybe a solar array. The solar array doesn't have to be a lot of wattage but enough to keep your batteries charged. That's my take on it and I'm sticking to it.

**Herk7769** · 03-09-2013, 08:04 AM

Quote:

Originally Posted by up2speed4

Hello, With all this battery knowledge I thought I would post this here. My problem: I will be camping without hookups for the first time in my new 20' Flagstaff and I was told my one year old 550 amp battery will power my water pump and 12v lights and occasional heater use for one or two days. Is this true or was the dealer trying to sell me additional batteries and generator I don't need?

First off, 550 CCA is not amp hours, it is "amp seconds."
30 of them to be exact. The CCA is then number of amps that battery, fully charged, can deliver in the cold for 30 seconds until dead.

There should be a "RC" (Reserve Capacity) number on that battery as well.
Post the model number. RC is used for how long the battery will last if your fan belt breaks before IT is dead.

These numbers being "on the battery" means it is most likely a dual purpose "Marine" type battery, used to start a boat's engine and run some boat items with the engine off. Not the best type for travel trailer house batteries that will never start an engine.

Amp hour rating is used to determine a deep discharge storage battery's capacity to deliver a reasonable house load over a long period of time "hours" not seconds.

The typical DP (Dual Purpose or Marine type) has between 60 and 75 amp hours capacity. 60 - 75 amps over 20 hours to discharge from "full" to 20% of initial charge remaining.

To determine "how long it will last" you will need to look at the discharge curve for your model battery. They are slightly different based on each manufacturer's construction.

Also due to something called the Peukert Effect, trying to pull more amps from battery than it is rated at (to drive an inverter or start a car) dramatically reduces the "amp hours" because the battery's construction has a hard time "serving up" electrons at that rate. Dividing that demand over several batteries means they ALL will last longer.

acadianbob · 03-09-2013, 08:23 AM

Quote:

Originally Posted by QuietTimes

We've been wondering too if we need to buy another battery. We have LED lights which should help. And we are also planning to buy a genny. I'm hoping that with the genny we won't have to add another battery but in the long run that might be best. Won't know till we try it out I guess. One place we go has no hookups available at all and we like to stay there five to ten days.

We have one 225SCS Trojan 12 volt battery (130 amp hr) that does just fine for extended boondocking. We use a Honda generator to top it off; usually about 45 min at breakfast (DW can also use her hair drier and the MW) and about the same amount at dinner time. We've boondocked for 2 weeks at a time in the mountains with no issues. I honestly do not see the advantage of 2 batteries for our purposes. It would get you an extra day or two initially without running the generator. But after that, you are running the generator anyway. So, for us, the extra expense (and weight) of another battery doesn't make sense. At 5 to 10 days, you are going to need a generator anyway.

**Herk7769** · 03-09-2013, 09:15 AM

Quote:

Originally Posted by acadianbob

We have one 225SCS Trojan 12 volt battery (130 amp hr) that does just fine for extended boondocking. We use a Honda generator to top it off; usually about 45 min at breakfast (DW can also use her hair drier and the MW) and about the same amount at dinner time. We've boondocked for 2 weeks at a time in the mountains with no issues. I honestly do not see the advantage of 2 batteries for our purposes. It would get you an extra day or two initially without running the generator. But after that, you are running the generator anyway. So, for us, the extra expense (and weight) of another battery doesn't make sense. At 5 to 10 days, you are going to need a generator anyway.

Your single 225SCS is the equivelent of TWO OEM DC-24 batteries.

Rolling buckeye · 03-09-2013, 10:18 AM

As I read about the Trojan batt,I am curious as to how a yellow top optima compares? As adding at least one more batt.is second on my list(just after buying a spare tire,I can't write my thoughts of why a 28-44,000 $ tt would not have come with a spare.) this seems deceitful by both my dealer and forrest river,but too late now,it would seem a spare tire is considered an option by FR Really!!! Anyway,is the extra $ of the optima worth it as compared to the Trojan,and is it even as good?

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