battery charging question

Ray IN

Member
I am trying to understand if/how two batteries in parallel are kept evenly charged by a tractor alternator. Is it reasonable to say that if one battery gets lower, then more charging current will flow to it? What about two 6 volts in series--would that make it easier or harder to keep them equally charged? I'm talking about 12 V neg. systems with an alternator. Thanks for any insights.
 

when everything is working right all of the cells are drawn down equally. where things go wrong is when a battery gets old or damaged, one cell will not take a full charge, so it prevents all the others from getting fully charged. With less than a full charge in the battery there is less to be drawn from any of the cells or from either battery when they are hooked in parallel.
 
Hello Ray Tractor,

The batteries receive the same charge regardless of the configuration. The weak one will make the other work harder. No two batteries are created equal!

Guido.
 
One problem with parallel charging is unless the batteries are identical in size and type and age and design and especially resistance etc etc ONE CAN DRAW MORE CHARGING CURRENT THEN THE OTHER in such a current divider arrangement. The strong battery may cannibalize the weaker one

I'm NOT saying both cant get charged, I'm ONLY saying they may not be charged equally unless they are well matched. Sure voltage across each is the same but current is E/R and unless R of each is the same no guarantee each gets equal charging current in that current divider network

NOWWWWWWWW if two are in SERIES charging "current" will be the same

For an in depth article about parallel charging and how even small resistance differences, check lout the URL link by Smart Gauge

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

John T
Parallel Charging
 
I run two 12 volt batteries in parallel on my D19 Allis diesel. I don't think I've ever had two batteries that matched on it. If one goes bad, I replace it with whatever one is handy. Sometimes I take the battery out of my pickup or another vehicle and put a new one in that vehicle.

It always starts, so I've never given it a thought.
 
Even if there is a weak cell , it will still charge up the rest. A hydrometer will show full charge on the good ones & low on a weak cell. A battery is like a chain. It will only pull as much as its weakest link. A battery will only put out as much as the weakest cell.
 
Back when I was in business , I had a 78 Ford 150 & I had another battery on the left fender. I used any and all kinds for extra boosting power to start customers cars in cold weather. It worked fine &it seemed like it broke the sulfating on a poor battery & made it better. The good battery would last 10 to 12 years too.
 
Thanks for the link. It was very enlightening. When ever I see an electrical question I look to see if you respond. I have a great deal of respect when you respond because IMHO you are the most correct. Thanks again.
 
You are overhinking this.

When you install a matched pair of new batteries connected in parallel on a tractor, all is well and the system works as interned.

When one of the batteries begins to fail and has issues it will drag down the other battery.

Then it's time for new batteries.

No amount of analyzing or trying to figure out how all this works will change the $$$$ fact that it's time for new batteries at that point.

(IF you want a tractor that starts reliably.)
 
Exactly! Just what I was going to say. A battery is made up of 3 or 6 small batteries hooked in series.
 
If one battery has more resistance than the other it will receive or accept a lower charge in a parallel hook up. Electricity will flow the path of least resistance..
 
Not always true. I had a Dodge diesel pickup and it started to become hard to start. It had two batteries that were only two years old. I took both batteries to the parts store, where I had purchased them, and had them tested. One was good and the other was junk and was replaced. That solved the starting problem.
 
I gave my battery load tester away after I purchased a battery conductivity tester, saves me time and a trip to town. It's good to see you took your batteries to town and had them tested. Some on YT would say they are only a few years, shouldn't be bad.
 
Parallel set ups both batteries act as one. IE if one battery has a cell short out it will pull the other down and the whole set up will have 10 volts.

I much prefer a pair of 6 volts in series and have been changing my equipment as batteries need replacement. MUCH easier to diagnose problems as you can treat each battery individually without unhooking anything.

jm2cw

jt
 
Hello John T,

The strong one may cannibalize the weak one, but the weak one will make the stronger one work harder every time!

Guido.
 
2, OK, you got lucky and saved some $$$ upfront. The "good" battery", at 2 years old is certainly on it's way to being used up, and will eventually affect the other battery being charged equally.

I still stand by my statement that if having a dependable vehicle is important to a person it's best to replace batteries connected together at the same time, with identical batteries.

I have quite a few vehicles and tractors, and what I would have done is replaced both of the batteries in the Dodge, and finished using up the "good" battery in another machine that only takes one battery.
 

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