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Old 07-05-2017, 01:30 PM   #15
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Hi
With a series connection, all the charge current goes through every cell. The current is what charges the cell. (no current = no charging action). In a parallel hookup there is no guarantee the current will split equally.
Bob
Thanks, Bob. But in parallel, if you connect the plus to battery1's plus post and the minus to the battery2's minus post, it should flow the same as if they were in series. Doesn't the current flow through both that way? Maybe you were thinking I was saying to connect to the + and - of the same battery and then that battery is connected to the + and - of battery2. Maybe not, but your point is taken. Fewer cells to go bad, lower risk.


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Originally Posted by lsbrodsky View Post
This whole debate is the reason it is always recommended to replace both batteries, new, at the same time.
In my boating days, with a 3 battery bank of Lifelines, I would start to see capacity degradation after about 6 years. Those batteries were truly cycled. My Airstream does not see those kind of cycles and batteries do care about that.
Larry
Thanks, Larry. I always replace batteries in pairs. I usually have a couple of dead batteries in the garage anyway, so I keep the good one of the pair, trade in the bad one and give them one of the dead ones as the other trade.

I have a bass boat too and the trolling motor batteries get a good workout over their life. My motor has as a 12/24 switch and I thought by keeping it on 12 I was extending my run time. That switch just takes it out of one battery all the time, so now I keep it on 24 and they run down evenly. I have a 3 bank charger in the boat so they are all charged independently.

Summary:
I think I have the answer to my question. If you have only one charger, you can charge in series or parallel (using 1 connection on each battery so the charger sees it as a single battery). If one cell goes bad, it can overcharge the other battery. Six volt batteries have fewer cells, less likely to have cell failure, are built stronger and have less of a chance of failure.

I think that pretty well covers it.
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Old 07-06-2017, 07:39 AM   #16
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Originally Posted by richw46 View Post
Thanks, Bob. But in parallel, if you connect the plus to battery1's plus post and the minus to the battery2's minus post, it should flow the same as if they were in series. Doesn't the current flow through both that way? Maybe you were thinking I was saying to connect to the + and - of the same battery and then that battery is connected to the + and - of battery2. Maybe not, but your point is taken. Fewer cells to go bad, lower risk.


Hi

If the two batteries are in parallel, you have no control over where the current goes. It could all go through only one battery.

If the two + posts are hooked together you are in parallel. If the + post on one battery + goes to the other's - post you are in series.

If you hook + to - on the *same* battery you get a spray of acid steam. I have empirical data on this

Bob
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Old 07-06-2017, 09:43 AM   #17
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Like most things in life the current will flow to the "path of least resistance"
As a battery charges the internal resistance increases.
When you connect a charger to a battery the current reading is higher.
As the battery charges the current is reduced.
Smart converters and chargers handle charging in a different way than the common home shop charger.
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Old 07-06-2017, 10:08 AM   #18
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Hi

Ok, charger basics:

First phase is a current limited charge, one way or the other any practical charger is indeed current limited (if they were not, they would put out infinite power).

Second phase is a constant voltage charge. Things run in current limited mode until some magic voltage it reached. You then hold the battery at this voltage. All practical chargers do this.

Third phase would be a drop back in voltage. This keeps the battery from going a bit nuts. How much it drops back and under what conditions are a "that depends" sort of thing.

Fourth phase would be a bump up in voltage to "equalize" the cells and stir things up a bit. The when and how much are again a "that depends".

A normal / cheap / simple charger goes through the first two phases. Even though there are two of them, a lot of people call this a "single stage" charger.

A basic smart charger goes through the first three phases above. It gets called a three stage charger. A fancier smart charger goes through all three phases. It gets called a four stage charger. Some smart chargers are better than others. I've seen three stage chargers that are *really* bad to put on a battery for more than a single charge cycle.

Bob
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Old 07-07-2017, 07:28 AM   #19
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Quote:
Originally Posted by TG Twinkie View Post
Like most things in life the current will flow to the "path of least resistance"
As a battery charges the internal resistance increases.
When you connect a charger to a battery the current reading is higher.
As the battery charges the current is reduced.
Smart converters and chargers handle charging in a different way than the common home shop charger.
Hi

Ok so off to battery modeling basics:

A battery is most simply modeled in something like SPICE as an ideal voltage source and a series resistance. Yes you can get more complex. The voltage and resistance both typically vary with charge state. The voltage varies with temperature. Resistance is affected by temperature, but not to the extent voltage is. Both parameters vary with age and condition of the battery.

If the two batteries have identical voltage sources but un-equal resistances, the current will split between them. If one has twice the resistance of the other, it will get 1/3 of the current. If the batteries have both un-equal resistances *and* unequal voltages, you can work out what will happen. It is a bit messy.

Batteries are designed to deliver a *lot* of current for a short period of time. To make that happen, you minimize the resistance part of the battery. When you drop a 150A motor load on, you still want to see 12V (if possible) on the motor. If the charged battery is at 13.5 that's a 1.5V drop. The resistance involved would be 0.01 ohms. At a 1A charging current 0.01 ohms would give you a 0.01 volt drop.

Again, this is a *very* simple model of a battery. For anything important, you would use a much more complex model.

Bob
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