Banking AGM of different Ah

Plecoptera

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I can't find my specific question asked of AGMs, tho it is an active question on LFP.

I have two 12V AGMs of mature age (years old) who live outside in or under a travel trailer. They both have always been BatteryTendered, and still show a resting charge of +12.7V. One is 12V100Ah, the other is 12V55Ah.

I am considering hooking both up to a common parallel 300A busbar, and running the trickle to the busbar. Both batteries would continue to live outside.

It seems like the simple physics of this should deal with the balancing issues, but I am neither an EE or a Physicist, so could be wrong.

Opinions sought.
 
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Substrate

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The smaller battery will be worked harder than the large one, and age more quickly. With such a gross imbalance of capacity, it will also send your larger one to the grave earlier too.

It is akin to putting a tricycle wheel on one side of your car. The tricycle wheel will wear out faster, and put a lot of strain on the mis-matched gearing so to speak for a poor analogy.

Voltage alone is no indicator of overall lead-acid battery capacity, or health such as rising internal resistance and sulfation.

If you do this, it would simply be a trivial application of sending batteries to their grave quicker.

There is a reason why one should always match multiple batteries with the same capacity, age, and model. Otherwise it is just an experiment to see how fast they trash each other.
 

Texas-Mark

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It depends on the load, otherwise there is nothing inherently wrong with paralleling dissimilar capacities.

There is a reason why one should always match multiple batteries with the same capacity, age, and model. Otherwise it is just an experiment to see how fast they trash each other.
As I mentioned in a different thread, I have FLA batteries in a parallel bank with up to a 4 year difference in age, and have not had any "bank death" issues over the past decade. I also have been running a mix of FLAs and AGMs in parallel for over 4 years. IMO proper maintenance is way more important than matching things. Note: I do not drain my battery bank more than 20% of it's capacity.
 

Substrate

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Kind of like what I was saying - a trivial application where of course one can slap stuff together. You have the advantage of maintaining that random pile. Most who slap batteries together won't! :)

Just have to be careful about the impression so that someone who might actually buy NEW batteries, doesn't go down this path and waste money, not knowing that you are a total exception to something that has been proven since the 1900's.
 

Texas-Mark

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Just have to be careful about the impression so that someone who might actually buy NEW batteries, doesn't go down this path and waste money, not knowing that you are a total exception to something that has been proven since the 1900's.

People are free to believe whatever they want. I know what has been working for me. This is just like those who say not to mix FLA and LiFePO4 batteries for the same reasons, but yet people have been doing it successfully for years. <shrug>

In case you think I was BSing about the 4 year spread. The rest have a spread of around 2 years between these. I have been doing this for the last decade (different ages) and none have lasted less than 5 years. And these are just "cheap" off the shelf marine batteries (other than my 4 AGM batteries). That 2016 battery is still going strong.

20211001_194120.jpg

A I said before, I properly maintain my batteries including monitoring the temp of them (every battery has it's own temp sensor). No boiling going on here. Note: I don't care about the humidity, the temperature meters with that feature were cheap.

20211001_175852-1.jpg
 
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