Batteries, mixing sizes in parallel

My old coach came to me with two true deep cycle (24's) house batteries. Both about two years old. Our last trip out, it was cold and I ran the furnace during the night. By early morning, the two batteries were all but dead. I'll never run the furnace during the night again.....more blankets are in order. I'm about to upgrade the 24's to 27's. In studying about battery types, I read that you should never mix batteries of different ages, types or sizes. I understand the ages and types......but why not sizes? There is room for 3 house batteries in my coach. My plans is to buy one new 27 and use another 27 that I have on hand, then add the third in parallel using one of the 24's that is currently in the coach. The 27 I have on hand is less than one year old and the 24's currently in the coach are less than two years old. This is mixing ages and sizes, but all of the same type.

What problems am I likely to have?

You won't have a problem, it just won't be as efficient as using three identical batteries.

It would probably be best to have a battery / battery charger expert weigh in for a definitive answer...
but,
batteries of different sizes have different charge and discharge rates.
batteries not the same age have a different degree of sulfation which result in different charge and discharge rates.
Most battery chargers are voltage sensing and control charging current to batteries (or interrupt the current) when reference voltage is approached. If a battery of lesser capacity is included (older or smaller) it may receive a higher voltage than desired during charging cycle causing overheating or overcharging. This overheating will further worsen charging / discharging rate difference ending in shortened life or damaged batteries. Also possible is reduced charge to the higher capacity battery. when not being charged by converter, the higher capacity battery will be tasked to maintain the charge state of lower capacity batteries.
So not a good idea to marry dissimilar batteries without isolation.

As it turns out, the battery I have on hand is a marine/RV. I purchased two identical 27's Deep Cycle (not marine) and turned in the older 24's as cores. Thanks for the replies, but it is a moot point now. The 27 Marine/RV will do service in my van. Problem solved, although more expensively than I'd hoped.

From what I've been told, you want to have batteries that are matched in size and condition, if not the stronger of the two is trying to 'charge' the weaker one which reduces the effective use of the two. In other words, you want them 'balanced' so one isn't being wasted trying to support the other.

1carguy said:

From what I've been told, you want to have batteries that are matched in size and condition, if not the stronger of the two is trying to 'charge' the weaker one which reduces the effective use of the two. In other words, you want them 'balanced' so one isn't being wasted trying to support the other.

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This is the best explanation for why you should not mix battery types I have ever read. Thanks

This is the best explanation for why you should not mix battery types I have ever read.

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I'm sure Mexray (our battery expert) will correct me if I'm wrong, but ... I believe this applies equally to batteries of the same type, but different in age. i.e. if you have a bad battery, it should not be mixed with good batteries.

Mexray has a battery tester that will weed out the bad or marginal batteries. He also sells batteries that I've found to last 10 years or so. Like a good pro, he doesn't advertise here.

There must exist a voltage level difference between batteries in parallel for current to flow between the batteries (higher voltage battery to lower voltage battery).

If you have a battery that is 12.6v rated at 125 amp-hrs hooked in parallel with a battery that measures 12.6v rated at 60 amp-hrs, the 125 amp-hr battery will not try to charge the 60 amp-hr battery.  As long as the voltages are the same there will be no current flow between batteries. 

If the 125 amp-hr battery falls below 12.6v the 60 amp-hr battery will try to charge the larger capacity battery to get the voltage back to 12.6v.

Don't confuse battery capacity (amp-hrs) with battery potential (voltage).  The reason for having batteries of the same "capacity" and "make" is so that they will decay at approximately the same rate and not become parasitic of one another.

The converter for all intents and purposes prevents parasitic charging as its voltage is higher (13.2v to 14.6v) than the batteries (this statement assumes that one or more of the batteries is not in such bad condition that it is dragging down converter by exceeding its charging current capacity).  If you are not running the converter then parasitic charging can and will occur if the batteries do not decay at the same rate thus creating a voltage difference.

Why do they say "never mix different batteries" Because it is easier than explaining the real rule.

If you are putting six volt batteries in series, then the mated pair should be twins.. Same everything.

But when you hook batteries in PARALLEL. then it's ok to mix sizes.. JUST do not mix voltage or chemistry. 

OK to mix.. Group X and Group Y (24, 27, 32, even 8d  Heck even a G-27 and a pair of six volt)

NOT OK:  Flooded wet cells, Maintenance Free, AGM, GEL and in some very few cases different brands. (This is very rare and far as I know not an issue in the US)

Each type (Flooded wet cell, like the golf car batteries), Maintenence Free and such) has slightly different charge voltages.

Now: Question: How many motor home owners do you think Mix different size batteries? (Percentagbe please)

Answer: Approx 100%, yes 100%

Why,, Cause when the engine is running the starting battery (A group 78 or 73 (not sure) in my case) is parallel with the mated pair of six volt U-2200's that light up the house.

NOTE 2: There are no six volt batteries in RV's.. That mated pair of U-2200's (Which are six volts when divorced) form a single 12 volt battery.. And that is how you treat it.. As a single 12 volt battery.

John From Detroit said:

Why do they say "never mix different batteries" Because it is easier than explaining the real rule.

If you are putting six volt batteries in series, then the mated pair should be twins.. Same everything.

But when you hook batteries in PARALLEL. then it's ok to mix sizes.. JUST do not mix voltage or chemistry. 

OK to mix.. Group X and Group Y (24, 27, 32, even 8d  Heck even a G-27 and a pair of six volt)

NOT OK:  Flooded wet cells, Maintenance Free, AGM, GEL and in some very few cases different brands. (This is very rare and far as I know not an issue in the US)

Each type (Flooded wet cell, like the golf car batteries), Maintenence Free and such) has slightly different charge voltages.

Now: Question: How many motor home owners do you think Mix different size batteries? (Percentagbe please)

Answer: Approx 100%, yes 100%

Why,, Cause when the engine is running the starting battery (A group 78 or 73 (not sure) in my case) is parallel with the mated pair of six volt U-2200's that light up the house.

NOTE 2: There are no six volt batteries in RV's.. That mated pair of U-2200's (Which are six volts when divorced) form a single 12 volt battery.. And that is how you treat it.. As a single 12 volt battery.

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Exactly what I was thinking. Assuming all else is equal, then mixing different sizes shouldn't change anything except storage capacity.

John From Detroit said:

If you are putting six volt batteries in series, then the mated pair should be twins.. Same everything.
But when you hook batteries in PARALLEL. then it's ok to mix sizes.. JUST do not mix voltage or chemistry. 

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John, can you explain exactly how putting a 6v battery in series with another 6V battery (of slight or moderately different characteristics) would harm either battery, but would NOT be of consequence if the two were in parallel? 

Please do not waste my time, or yours, with a batteries 101 lecture, just answer the question.  Thanks in advance, lou

aka Porky said:

John, can you explain exactly how putting a 6v battery in series with another 6V battery (of slight or moderately different characteristics) would harm either battery, but would NOT be of consequence if the two were in parallel? 

Please do not waste my time, or yours, with a batteries 101 lecture, just answer the question.  Thanks in advance, lou

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He did not say anything would be harmed, it just isn't the most efficient way to do things.

aka Porky said:

John, can you explain exactly how putting a 6v battery in series with another 6V battery (of slight or moderately different characteristics) would harm either battery, but would NOT be of consequence if the two were in parallel? 

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There are three stages of lead acid battery charging as can be seen at the graph at http://www.batteryuniversity.com/partone-13.htm with respect to voltage and amps.  The problem is that if the batteries are significantly different characteristics the "smart" charger won't necessarily be reading the voltages correctly.  Thus an older battery may be overcharged while a newer battery is undercharged.  Leading the first battery to boil over.  I forget what happens to the undercharged battery.

seilerbird said:

  He did not say anything would be harmed, it just isn't the most efficient way to do things.

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Okay,  John... er.. Tom,
JFD has made the statements, in the following quote, several times.  Each time saying it's okay to mix un-matched batteries in a parallel configuration, but in a series configuration they must be exact twins.  (same everything)  Here the quote....

Why do they say "never mix different batteries" Because it is easier than explaining the real rule.
If you are putting six volt batteries in series, then the mated pair should be twins.. Same everything.
But when you hook batteries in PARALLEL. then it's ok to mix sizes.. JUST do not mix voltage or chemistry.

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Why is one obvious mismatch okay and the other not?  We can all see the "inefficiencies" in mixing un-like batteries in the parallel configuration, which he JFD deems okay, but why can't they be tolerated in the series config.  What's the big difference?

What is this "real rule"?

Remember, I stipulated that we are talking the same small differences in each battery type.

Tony_Alberta said:

There are three stages of lead acid battery charging as can be seen at the graph at http://www.batteryuniversity.com/partone-13.htm with respect to voltage and amps.  The problem is that if the batteries are significantly different characteristics the "smart" charger won't necessarily be reading the voltages correctly.  Thus an older battery may be overcharged while a newer battery is undercharged.  Leading the first battery to boil over.  I forget what happens to the undercharged battery.

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I understand batteries and battery charging, and given the fact that I stipulated that there were no significant differences in the battery characteristics, I don't think one needs to worry about confusing the smart charger or boiling either battery.  JMHO

I'll take a stab. When batteries are in series, all the current has to pass through both batteries.  The least capable battery is the bottleneck for  whatever attribute you want to consider important. For example, a single weak cell in either battery becomes the limiting factor for the whole set-up. In parallel, there is always a path that bypasses the weak point, so it is of less impact overall. It's still a poor contributor to the total set-up, but not a choke point.

The imponderable question is how much difference in voltage or current capacity is significant? Once you stipulate that the batteries in question have "no significant differences", then the whole question becomes moot.

[quote author=Gary]
I'll take a stab. When batteries are in series, all the current has to pass through both batteries.  The least capable battery is the bottleneck for  whatever attribute you want to consider important. For example, a single weak cell in either battery becomes the limiting factor for the whole set-up. In parallel, there is always a path that bypasses the weak point, so it is of less impact overall. It's still a poor contributor to the total set-up, but not a choke point.[/quote]

Considering what we expect from either configuration the result is the same.  Less voltage and/or less current capacity overall.

[quote author=Gary]
The imponderable question is how much difference in voltage or current capacity is significant? Once you stipulate that the batteries in question have "no significant differences", then the whole question becomes moot.
[/quote]

Thanks for making my point.  I guess I'll never know JFDs un-tutored reasoning since everyone here is so eager to answer for him.

Maybe he's right, there are NO 6v batteries in RVs, just weak mismatched 12s.

Not to whip a 'Dead Horse" but what characteristics are you talking about??  While Amp-Hrs, Voltage and CCA are important, they are determined by internal Plate size, construction materials, Electrolyte quality and concentration.  These are the characteristics that determine the advertised capabilities.

As Batteries are used they discharge (decay) and voltage lowers.  If the batteries decay at different rates there will develop a voltage difference which produces parasitic charging between batteries in parallel.

Since we as consumers do not generally know the internal build characteristics and decay rates of the battery, it is prudent to use batteries of the same make, model and age so that the decay rates will be essentially the same.

I know that many of the Branded Batteries are made by the same manufacturer and therefore will have the same decay characteristics, I am not interested in doing that research.

As for 6volt Batteries connected in Series the capacity of the two will be limited to the smaller capacity battery therefore you are paying for capacity you will not be able to use.

All good points, Walt. That's why the rule of thumb is "don't mix".  Most people don't know - or care to learn - the intricacies, so its better to avoid the situation altogether. An expert can do things an amateur should best leave alone.

Okay guys, I surrender.

I specifically said that I did not need a tutorial on batteries.  I was building batteries for Gould National Battery in Chicago Heights at the age of seventeen.  In two years time I performed every task in the assembly process from inserting plates and separators to final test.  I've used and abused batteries for most of my 72 years.  I fully understand the points each of you have made, thank you.

I simply wanted to know from JFD why he so emphatically states that differences in battery capacity, physical size, etc.. were acceptable in a parallel configuration, but totally intolerable in a series configuration.

Now that everyone has taken his turn to "pee in the pool", I withdraw the question.