Quote:
Originally Posted by 3dplane
Hey guys!
I read this thread and I feel I have to chime in because of my experience using parallel connections disagree with the RV article.
Key word in that article is "CHEAPER"
Quote:
"That is why the RV coach batteries have two marine 6 volt cells in series rather than cheaper 12 volt deep discharge in parallel. You get longer battery life with the two in series."
When you have two similar batteries parallel connected and they are being discharged,the article says that the one with lower internal impedance will give it's max current and basically die sooner than the other...
Let's think about this for a second.
What happens to a battery's voltage when you load it to make it produce large current flow? .... Correct! It's voltage drops or sags! Right?
Now how far can that voltage sag RELATIVE to it's buddy pair who is parallel connected to it? Not very far since they are "clamped" together so as soon as the stronger battery is trying to give it's all and it's voltage tries to sag,the other one with now slightly higher voltage will be forced to "help out" and equalize or even out the current flow.
If we look at the other example with two batteries that are also not exactly equal in regards to internal resistance and now they are connected in SERIES.
When we put an electrical load on this circuit, the batteries "see" or share or in other words have to produce the same current! The one with higher internal resistance will work harder /tries to get hotter than the other one.
How is that supposed to be better when over time they could get out of balance if not monitored?
I don't know but in reality using quality batteries that are pretty close match and are cared for,will not cause a problem.
I've got a couple of real world personal examples:
#1: I have a golf cart that I run on (big no no) car batteries! Not new car batteries but used/failed ones. All different age and size. Ranging from 55A/h to 74A/h.
I parallel connect two to make a pair,and then I series connect more of these pairs to get my desired system voltage.
After three years of use I only replaced the weakest pair because I was trying to set new personal speed records with it.
#2: Alternator disable switch in my car... first I added two 7A/h little sealed lead acid batteries parallel to the existing battery in my car! (no they did not explode).
Monitoring my system voltage showed improvement daytime driving but at night with the headlights on, voltage sagged close to 12.0V by the time I got home.
So I added another 55A/h car battery mounted in my trunk and now I can make my 58 mile round trip day or night without voltage sagging below 12.2V (under load of fuel pump,ignition,accessories etc.)
So I have a 55A/h ,60A/h, and two 7A/h batteries ALL parallel connected in my car for the past two months with no problems.
Sometimes a few hours after the trip home from work,the voltage recovers to 12.7V and I have to trick my automatic charger by turning on the headlights so it will start charging.
The key is to know your voltage in all situations , learn the limitations of the system so the batteries don't get abused.
Just my opinion.
Barna
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Sorry for such a long Quote but lets leave it in context.
The problem with having two batteries of slightly different impedance's or plate condition is that one battery, the good battery which holds a full charge at 12.77 volts is always trying to feed the 12.62 weaker battery more voltage to even the voltage differences between the two. The weaker battery eats up the better/newer battery.