Question about adding a battery

[loneshark64]

Question for the battery wizards.
I have a house bank with one 12v flooded lead acid that I bought last year. It was used for 10 or 12 days. It was rated by the manufacturer at 105 ah (I have the other specs). I want to add one or two more batteries and in parallel and create a larger bank. The manufacturer doesn’t make that battery any more. I just bought two new flooded batteries rated at 100 ah from a different manufacturer. The question is, can I connect all three in a single bank or should I just go with the two new ones or buy an identical third and ditch last years battery.

Most of the advice is, do not put different batteries in a bank because one will be trying to charge the other, and you won’t get the full capacity of the larger battery. And there are potential concerns about heat. But in this case I wonder if this is really worth worrying about.

First, these batteries have differ specs but they are not far off, and I wonder how precisely accurate manufacturer specs are anyway.
Second, my (limited) experience with 12v batteries is that often 2 identical batteries give very different results due to manufacturing quality issues, slight differences, who knows. God only knows why one battery is “good” and another is “a dud”. Who knows if the identical batteries are actually identical?
Third, I have had situations where one battery in a bank starts to go downhill, but everything still ran ok for a while and the boat didn’t blow up even though the batteries had degraded differently and were not the same.
Fourth, am not really concerned about getting the max capacity out of the original battery. I would rather get 80% of it than have to buy another battery or just go with two.
Fifth, I know people replace batteries in existing banks. This may be dumb, but….

Bottom line, it just doesn’t seem like a real problem. I may still just not take the risk, but I am interested in double checking my thinking on this. As an aside, I am not super excited about the idea which has been suggested to equalize these because they are brand new batteries.
Thanks

[ranger58sb]

Originally Posted by loneshark64

Question for the battery wizards.
I have a house bank with one 12v flooded lead acid that I bought last year. It was used for 10 or 12 days. It was rated by the manufacturer at 105 ah (I have the other specs). I want to add one or two more batteries and in parallel and create a larger bank. The manufacturer doesn’t make that battery any more. I just bought two new flooded batteries rated at 100 ah from a different manufacturer. The question is, can I connect all three in a single bank or should I just go with the two new ones or buy an identical third and ditch last years battery.

Most of the advice is, do not put different batteries in a bank because one will be trying to charge the other, and you won’t get the full capacity of the larger battery. And there are potential concerns about heat. But in this case I wonder if this is really worth worrying about.

Not a wizard, but... if specs are close enough, and the "old" battery was only used for a few days... I think I'd possibly go ahead and tie the three together.

I'd check and compare manufacturer recommendations for charging and float voltages first, though, to be sure they're all compatible with a common charger setting.

-Chris

[YANDINA]

The warnings about matching battery characteristics, size, age, etc., only applies to using batteries in series, it does not apply to batteries in parallel. The only requirement is matching chemistry.

[Statistical]

105 vs 100 is fine. Ideally the batteries would be identical but what you describe is close enough.

What you don't want is widely difference capacities in parallel because the load will be shared evenly for batteries that aren't even.

Example 105 ah and 50 ah. When you pull a 200A load it would be 100A per battery which pushes the 50 Ah one to a higher C rate which likely means it will wear out quicker. The SoC in the smaller battery will drop quicker meaning you are doing deeper cycles further accelerating wear. As the SoC diverges the larger battery will charge the smaller battery meaning efficiency losses.

[citlab]

Originally Posted by ranger42c

Not a wizard, but... if specs are close enough, and the "old" battery was only used for a few days... I think I'd possibly go ahead and tie the three together.

I'd check and compare manufacturer recommendations for charging and float voltages first, though, to be sure they're all compatible with a common charger setting.

-Chris

I tend to agree with this approach. We currently run 3 AGMs 110ah each for the 11thh year with no clear degradation. Our 1st set for the housebank was a 2x100ah FLA batteries installed by the manufacturer - they were of a different brand ! They lasted for 2 seasons, The next set was a 2x110ah FLA identical batteries, which also lasted for 2 seasons.
This limited limited experiment tells me that having 3 batteries has had more benefits than the fact that one of them would be slightly different. We do rotate the batteries each season so that they, on an average, experience the same electrical environment.

[YANDINA]

Originally Posted by Statistical

<SNIP>
105 vs 100 is fine. Ideally the batteries would be identical but what you describe is close enough.

What you don't want is widely difference capacities in parallel because the load will be shared evenly for batteries that aren't even.

Example 105 ah and 50 ah. When you pull a 200A load it would be 100A per battery which pushes the 50 Ah one to a higher C rate which likely means it will wear out quicker. The SoC in the smaller battery will drop quicker meaning you are doing deeper cycles further accelerating wear. As the SoC diverges the larger battery will charge the smaller battery meaning efficiency losses.

Sorry, this is a common misconception regarding batteries in parallel.

There is NO PHYSICAL LAW that says two batteries will share the load "evenly" or 4 batteries will each supply 25%. Each battery supplies a share of the current in proportion to its ability.
If you parallel a 12 volt flashlight battery with the 100 ah battery and draw 100 amps out, the flashlight battery doesn't suddenly explode trying to supply 50 amps.

[svfinlandia]

Originally Posted by loneshark64

Question for the battery wizards.
I have a house bank with one 12v flooded lead acid that I bought last year. It was used for 10 or 12 days. It was rated by the manufacturer at 105 ah (I have the other specs). I want to add one or two more batteries and in parallel and create a larger bank. The manufacturer doesn’t make that battery any more. I just bought two new flooded batteries rated at 100 ah from a different manufacturer. The question is, can I connect all three in a single bank or should I just go with the two new ones or buy an identical third and ditch last years battery.

Most of the advice is, do not put different batteries in a bank because one will be trying to charge the other, and you won’t get the full capacity of the larger battery. And there are potential concerns about heat. But in this case I wonder if this is really worth worrying about.

First, these batteries have differ specs but they are not far off, and I wonder how precisely accurate manufacturer specs are anyway.
Second, my (limited) experience with 12v batteries is that often 2 identical batteries give very different results due to manufacturing quality issues, slight differences, who knows. God only knows why one battery is “good” and another is “a dud”. Who knows if the identical batteries are actually identical?
Third, I have had situations where one battery in a bank starts to go downhill, but everything still ran ok for a while and the boat didn’t blow up even though the batteries had degraded differently and were not the same.
Fourth, am not really concerned about getting the max capacity out of the original battery. I would rather get 80% of it than have to buy another battery or just go with two.
Fifth, I know people replace batteries in existing banks. This may be dumb, but….

Bottom line, it just doesn’t seem like a real problem. I may still just not take the risk, but I am interested in double checking my thinking on this. As an aside, I am not super excited about the idea which has been suggested to equalize these because they are brand new batteries.
Thanks

It sounds to me like you answered your own question.
Personally, I would buy the third new battery, but From what you are saying I don’t think it makes a lot of difference in your case.

Al, S/V Finlandia

[donradcliffe]

Originally Posted by YANDINA

The warnings about matching battery characteristics, size, age, etc., only applies to using batteries in series, it does not apply to batteries in parallel. The only requirement is matching chemistry.

Spot on. The old wives tales about one battery stealing capacity from the others only apply in the case of a shorted cell, which is an extremely rare event.

[captstu]

One concern with any parallel setup. If one battery fails with an internal sort on one cell, the remaining batteries try to charge the failing battery - usually at a very high current.

I had this happen with nearly new AGM batteries in a 1200 Ah bank, one cell shorted and the rest want ape@@$# trying to charge the damaged battery. The result was lots of heat, smoke, boiling stuff and fear.

The 1200 A bank was factory original - but when I replaced the failed battery, I added 100 A inline fuses to each one.

Be safe.

[Cadence]

Go for it. Don't worry about it, you will be fine.

[MJH]

Originally Posted by loneshark64

Question for the battery wizards.
I have a house bank with one 12v flooded lead acid that I bought last year. It was used for 10 or 12 days. It was rated by the manufacturer at 105 ah (I have the other specs). I want to add one or two more batteries and in parallel and create a larger bank. The manufacturer doesn’t make that battery any more. I just bought two new flooded batteries rated at 100 ah from a different manufacturer. The question is, can I connect all three in a single bank or should I just go with the two new ones or buy an identical third and ditch last years battery.

Most of the advice is, do not put different batteries in a bank because one will be trying to charge the other, and you won’t get the full capacity of the larger battery. And there are potential concerns about heat. But in this case I wonder if this is really worth worrying about.

First, these batteries have differ specs but they are not far off, and I wonder how precisely accurate manufacturer specs are anyway.
Second, my (limited) experience with 12v batteries is that often 2 identical batteries give very different results due to manufacturing quality issues, slight differences, who knows. God only knows why one battery is “good” and another is “a dud”. Who knows if the identical batteries are actually identical?
Third, I have had situations where one battery in a bank starts to go downhill, but everything still ran ok for a while and the boat didn’t blow up even though the batteries had degraded differently and were not the same.
Fourth, am not really concerned about getting the max capacity out of the original battery. I would rather get 80% of it than have to buy another battery or just go with two.
Fifth, I know people replace batteries in existing banks. This may be dumb, but….

Bottom line, it just doesn’t seem like a real problem. I may still just not take the risk, but I am interested in double checking my thinking on this. As an aside, I am not super excited about the idea which has been suggested to equalize these because they are brand new batteries.
Thanks

The short answer is that as long as the batteries are of the same chemistry you can tie them together. The different makes, ages, and specifications of the batteries will most likely reduce the overall life compared to three batteries of the same make, age, and specification.

Moving on, you have not stated what charging sources you have; I assume just an engine alternator.

The minimum engine alternator size is generally about 25% of the total amperage of all batteries in the system. Without considering the amperage of the start battery you will have changed from a 105x25%=26Amps requirement to a 105+100+100=305x25%=76Amps requirement...a three-fold increase. I suspect you will be short if you are just using a standard alternator which will be working hard to keep them charged; if you can't they will die sooner.

Good Luck.

~ ~ _/) ~ ~ MJH

[David B]

Originally Posted by citlab

I tend to agree with this approach. We currently run 3 AGMs 110ah each for the 11thh year with no clear degradation. Our 1st set for the housebank was a 2x100ah FLA batteries installed by the manufacturer - they were of a different brand ! They lasted for 2 seasons, The next set was a 2x110ah FLA identical batteries, which also lasted for 2 seasons.
This limited limited experiment tells me that having 3 batteries has had more benefits than the fact that one of them would be slightly different. We do rotate the batteries each season so that they, on an average, experience the same electrical environment.

Just a note on this, two seasons from FLA's is very, very short unless you are really cycling them.

We run 10 x FLA's all up, and stay with FLA's as they give us the best value per usable Ah.
One set of four is now 10 years old (bow-thruster), the engine and generator start batteries gave us 8 years, and the service batteries about the same.

Perhaps yours were being deep discharged too often, or over-charged?

[Emouchet]

I would not worry. The difference in between them is not significant.
You can go that route.

Flooded batteries are actually very tolerant to this kind of usage, much more than AGM for instance.
Next time you replace your batteries, simply make sure this time to replace all them at the same time.

[Cadence]

Originally Posted by David B

Just a note on this, two seasons from FLA's is very, very short unless you are really cycling them.

We run 10 x FLA's all up, and stay with FLA's as they give us the best value per usable Ah.
One set of four is now 10 years old (bow-thruster), the engine and generator start batteries gave us 8 years, and the service batteries about the same.

Perhaps yours were being deep discharged too often, or over-charged?

I was wondering about that and this is the first I had seen it mentioned. Best value per usable Ah. It seems to make sense.

[David B]

Originally Posted by Cadence

I was wondering about that and this is the first I had seen it mentioned. Best value per usable Ah. It seems to make sense.

Not fussy either - takes a bit to kill them.