How to wire two Battery's??

[Rustic Eagle]

Just thinking out loud here........,

When adding a second 12V battery to an existing 12V battery, I would think it would be important to have the same specified "amp/hr capacity" with each 12V battery. The existing battery (if dated) should be checked to insure it can still take a full charge to maximize the battery bank balance with the discharge/charge cycles.

On the subject of which of the two 12V parallel wiring configurations is optimal..., IMO as long as the two batteries are positioned next to each other and connected with short-length jumper wires sufficiently-sized, I believe the difference would be ever so slight under it's intended use.

I'm a mechanical guy, so I tend to learn more than contribute in these electrical discussions

Bob

[Seann45]

Originally Posted by Rustic Eagle

Just thinking out loud here........,

When adding a second 12V battery to an existing 12V battery, I would think it would be important to have the same specified "amp/hr capacity" with each 12V battery. The existing battery (if dated) should be checked to insure it can still take a full charge to maximize the battery bank balance with the discharge/charge cycles.
Bob

Always use the same size of battery and the same age of battery..

Here is the prefered wiring diagram from the "RV DR"

[Terry]

Speaking of loss from the battery to converter - I have a pretty good loss right now (about 1.5 volt or more). I have narrowed it down to the fuse holder. It has a little coorosion in it and is causing a problem. Will be replacing with better quality soon. Amazing how many failure points along the way....

[White Knight]

The biggest thing one must be concerned with is having clean connections. With clean connections, either of the above methods will work equally well( eg. pos on one battery and negative connection on other battery or neg and pos connections on one battery with batteries jumpered together). To keep connections from corroding, dielectric grease is best, but if unavailable you can use vaseline. The idea is to keep the moisture and oxygen away from the connections. Use this on all connections possible, fuses, batteries, etc.

[SmokerBill]

I finally went outside and opened up the battery boxes and checked for voltage drop. First I checked the wing nuts, and one of them was just finger tight, so I loosened, then tightened up all four.

After that, there is only 0.1 volts drop from the battery terminals to the power converter inside, no load. That's not too bad, IMHO

Dang, I forgot to check for drop with a heavy load on the battery. Well, that'll give me something to do tomorrow.

[Bob Landry]

In a 2 battery system(12V), positives are connected together and negatives are connected together. The positve lead needs to come off of one battery while negative uses the other battery. The reason is that during both charge and discharge, current will take the path of least resistance. If you are using the same battery for + & -, you will get more current flow through the battery that has the leads connected to it and they will not charge or discharge equally. And will age differently as noted.

[Lady Fitzgerald]

Originally Posted by Bob Landry

In a 2 battery system(12V), positives are connected together and negatives are connected together. The positve lead needs to come off of one battery while negative uses the other battery. The reason is that during both charge and discharge, current will take the path of least resistance. If you are using the same battery for + & -, you will get more current flow through the battery that has the leads connected to it and they will not charge or discharge equally. And will age differently as noted.

Bob, in theory that would be true, but only if there was a large enough distance between the batteries to introduce significant resistance in the wiring between the batteries and the batteries are electrically identical (which will never happen in actual practice). When the batteries are adjacent to each other, the internal resistance of each battery will be considerably higher than the resistance in the jumper between the batteries (a huge difference; I seriously doubt anyone here would even be able to measure the resistance in the jumper, let alone the voltage drop), effectively wiping out any difference in current draw due to resistance in the jumper.

Electrically, it doesn't matter which arrangement is used but connecting the ground and the feed to the same battery has the advantage that it might be a bit easier to take one battery out of service while the other one remains connected (depending on how the jumpers, ground and feed are connected).

[Terry]

What I am gathering from all this is that in theory one way may be better; in practical terms - another way may be better --- but, in the end - either way will work.

Sounds like then it is just personal preference as to which way you feel more comfortable.

[SmokerBill]

I agree. As long as you have clean, tight connections and properly sized wire, either way will work. There may be very small differences in charging or output efficiencies, but those are probably insignificant for RV purposes.

Now in a laboratory it might be an important consideration, but for us, probably doesn't make a hill of beans which positive and negative posts we connect to the trailer.

[ldlogue]

Another point to consider is both batteries need to be the same age. If not the weaker battery will pull down the good one. I had one of my 12 v batt to go bad and so when I replaced it I replaced both batt at same time.

Some may argue this point but I have worked in heavy trucks for 30 yrs and with a bank of 4 12v batt with 2 batt on each side of truck set up parallel for 12v or in series for 24v when 1 went bad we have replaced just that batt and in short order the others would fail. It became obvious that when 1 batt went bad it was better to replace them all.

I'm not saying this is a hard and fast rule but the better way of doing it. That way all batteries are of the same age and condition.

[RoyBraddy]

http://www.amazon.com/Blue-Sea-Syste.../dp/B000K2MCR2
I solved the problems of doubling up on BATTERY CABLES on the terminals by using this Blue Sea Systems 9001e e-Series Battery Switch Selector. This switch uses three standard size lugs on the back so I placed it in the POSITIVE cable path whick allows me to only run one battery cable to each POSITIVE battery terminal using same length cables. Each NEGATIVE Terminal goes directly to frame ground using same length cables.

This switch has four positions OFF-BAT1-BOTH-BAT2 which allows me to connect to either BATTERY or BOTH batteries or none (OFF). This also serves as my BATTERY DISCONNECT switch. If the need arises I can charge one battery from the on-board converter and the other battery from an external charger at same time... This is also great for troubleshooting as you have complete control of which battery is connected or off without removing any battery cables.

This is perfect for installing two +12VDC batteries in parallel... ($35 Amazon.com)