Batteries in parallel, one charger

[Rich W]

I have a question about charging 2 batteries with a single charger.

Scenario:
You have 2 batteries, equal in age, brand, size, etc.
Batteries are wired in parallel
You have one charger;
- positive clamp to positive post on battery 1
- negative clamp to negative post on battery 2

Question:
The charger sees both batteries as a single battery and will charge both equally. If a cell goes bad/weak in battery 1, won't that prevent battery 1 from reaching full charge and continue to charge both (even if battery 2 does reach full charge), boiling and destroying both?

[rmkrum]

Yup. And that's the issue with parallel 12v batteries. If one gets bad, it hoses up both of them eventually. Many of us have gone to two 6 volt golf cart batteries in series to avoid that scenario.

Do be careful doing that change, however. There are some golf cart batteries that put out EIGHT volts that look mighty similar to the SIX volt ones. Don't ask me how I found this out at almost the last moment...before I connected the last cable. Count the fill caps. There better be only three...per battery!

[Rich W]

Originally Posted by rmkrum

Yup. And that's the issue with parallel 12v batteries. If one gets bad, it hoses up both of them eventually. Many of us have gone to two 6 volt golf cart batteries in series to avoid that scenario.

Do be careful doing that change, however. There are some golf cart batteries that put out EIGHT volts that look mighty similar to the SIX volt ones. Don't ask me how I found this out at almost the last moment...before I connected the last cable. Count the fill caps. There better be only three...per battery!

Thanks for the confirmation. I bought new Lifelines (12v) in 2015 so no upgrading those for a while. Back in my military days I worked on helicopters, which had a special type of battery. Battery cells were individual and you could remove a single cell and replace it from another battery. (we did a lot of parts scrounging.) That's when I learned about 2V per cell and each cap was a cell.

My question was related to chargers in AS, one connection for 2 batteries. My boat has a separate 15 amp bank for each battery. My son was asking why not charge them as a single unit, like the AS. My reply to him was my question here. I just wanted to make sure I was right. Thanks!

[OTRA15]

Yes, as you suggested in the OP -- one single battery, so a bad cell can affect the whole thing. Thanks for posting and the clarifications -- helpful to have clarity on this.

Have a great Fourth!

Peter

Originally Posted by richw46

. . .
The charger sees both batteries as a single battery . . .
. . .

[Rich W]

Originally Posted by rmkrum

Yup. And that's the issue with parallel 12v batteries. If one gets bad, it hoses up both of them eventually. Many of us have gone to two 6 volt golf cart batteries in series to avoid that scenario.

Sorry, another question. If you have 2-6 volt in series and you're charging them with a 12 volt charger, how does that eliminate the problem? You still connect to positive on one battery and negative on the other. If one battery has a bad cell, isn't the charger just going to keep attempting to charge both? If not, what prevents it?

[dames7]

If you want a DEEP CHARGE on a 6V get a charger that has both setting for 6V & 12V.

I have two 6V Lifeline an do a deep charge over night one battery at a time, (or until the charger deep light comes on) by doing this when I am done it will show when hookup 13.4.

Everyone has their own way to do things, I like mine, works ever time.

[uncle_bob]

Hi

Ok, so if you have two 12V batteries, they are 3 years old, one is defective and a cell shorts. Your battery stack is toast.

If you have two 6V batteries, they are also 3 years old, one is defective and a cell shorts. Your battery stack is toast.

In both cases, you can't do anything until you you get replacement parts. In both cases a normal charger will mess things up big time if it continues to run. The dual 6V stack will be damaged by over voltage quicker than the dual 12V. In both cases, you would replace both batteries in the set to keep them matched.

Is series the right way to do it? Of course it is. However in the case of a battery that dies a year or three later ... not much advantage either way.

Bob

[MelGoddard]

My question is:
Are there 6 volt batteries that will fit into an Airstream?
As I remember, the two 6 V. batteries that I had in my Hi-Lo were rather high and just fit into it's box.
Just looking at them, they seem to be too high for my FC battery box.
??????

[Rich W]

Originally Posted by uncle_bob

Hi

Is series the right way to do it? Of course it is.
Bob

How come? What's the advantage?

In series you add the voltages but the AH stays the same, right? In parallel the voltage stays the same but you add the amp hours. If a 6 volt has a bad cell, you have no power in the AS. With 12 volt, if you lose a battery, you still have power.

[alano]

Originally Posted by uncle_bob

Ok, so if you have two 12V batteries, they are 3 years old, one is defective and a cell shorts. Your battery stack is toast.
Bob

If one of your batteries fails, this doesn't toast the "good" one. It just means the "good" battery will likely not see a full charge.

One of the advantages of 6 V batteries is that there are fewer cells, so the anode/cathode construction is more robust mechanically and environmentally - one of the reasons they are popular in golf carts. They typically cost more but you end up with a slightly higher capacity too.

[rmkrum]

Yup. Exactly. Parallel cells/batteries are much harder to match and charge correctly. A series string is also a lot easier to troubleshoot. The connection between the two series batteries should be at exactly half the total voltage. Otherwise, you have easily found which battery is failing.

In any case, you replace them in sets. Trying to replace just one battery in a set is an exercise in futility. They won't match up in capacity...hard enough with new ones.

The other disadvantage of parallel batteries is that if one goes south, the 'better' one will dump current into the bad one. That can lead to overheating the 'bad' battery at a minimum. Bad enough, and we are talking melted cables... A series set just beats up the battery charger, and is less likely to cause high current issues between batteries like a parallel set can.

[uncle_bob]

Hi

Ok so:

Why is two in series better than two in parallel?

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. One of many issues is "thermal runaway". As the battery gets hotter, it's preferred voltage gets lower. That means it will hog more of the current. It then gets hotter and hogs even more .... There are indeed a long list of odd issues like this.

How does one kill the other in parallel?

A cell either shorts or it does not (at least in this context). A "12V" battery has six cells in it. When the stack has 13.6V on it, you have about 2.3V on each cell. Loose one cell (it's shorted) and 13.6 goes to 11.4V. The "good" battery will struggle to get to 12V. In the typical case, it will pull down to 11.x volts pretty quickly. That's below 50% charge on the good battery. You are now in the region where you are doing damage to the battery with repeated discharges.

In series or in parallel, when one cell's voltages or resistances get "off" from the other due to age, they begin to charge unequally. One is over charged, the other is undercharged. The net combo does not produce the amp hours you expect. The overcharged battery likely is being damaged by the process. The under charged battery isn't getting any better due to the treatment.

None of this will be taken care of by a normal charger. Various charger approaches will make one or the other case a bit less of a problem while making a different case more of a problem. An ideal charger would monitor the cell voltages on each cell in the stack, good luck finding one. It also would monitor the current in/out of each battery in a parallel battery combo ... (that could be done, but rarely is).

By far the biggest question is "how do I know my batteries are healthy?". A specific gravity check on flooded cells is a good start. A charge up and measure amp hours to 12.0V at a 10A load is the only really trustworthy way to do it. On a 200AH stack run down to 12V, that will take about 10 hours to test ....

Bob

[lsbrodsky]

In the end, there is nothing you can do about it either way except take care of your batteries. Use and charge them according to the battery type and manufacturer recommendations. 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

[uncle_bob]

Hi

Size of lead acid battery vs typical use matters a lot. A battery that typically runs between 100% and 80% will last way longer (greater number of charge / discharge cycles) than one that cycles between 100% and 60%. If you get into the 100% to 40% range ... yikes.

Number of cycles is a big deal. If you do a full cycle once a day (whatever that cycle is) and that happens 200 days a year, your batteries know about that. If somebody else is "only" out 40 days a year, their batteries are a lot happier. If the batteries are good for 400 cycles, the first guy will get 2 years on a set. The other guy probably will loose the batteries based on something other than the number of cycles.

You need to know a *lot* about the setup and how it is used to judge if the information on one setup applies to another setup.

Bob

[Rich W]

Originally Posted by uncle_bob

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.

Originally Posted by lsbrodsky

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.

[uncle_bob]

Originally Posted by richw46

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

[TG Twinkie]

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.

[uncle_bob]

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

[uncle_bob]

Originally Posted by TG Twinkie

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