2 AGM 100ah batteries, different ages and brands. Safe to parallel?

[Bob Van Go]

I have 2 100Ah AGM batteries, one is about 3 years old, the other new, and different brands, so likely different internal structures and charge/discharge rates. My understanding is that if I parallel them, one would likely be overcharged, and one undercharged, shortening the life of both batteries. I can live with that if there is no other danger. The overcharging possibility has me reluctant to hook them in parallel. I'd like to be safe, so is hooking the 2 in parallel ok, or should I just use a switch to switch between the two to keep it safe? Is there an automatic switch that would switch between the two when one is getting low? Or anything that can be done to make it safe in parallel? Thanks.

 

[sunshine_eggo]

Yes. Ensure you hook them up with best practices (link #6 in my signature). The new battery will almost certainly wear faster as it will take a greater share of the work BUT it will wear slower than if it was installed by itself.

If possible, validate that high current charges/discharges are appropriately shared between them and within specification with a clamp DC ammeter.

 

[Bob Van Go]

Yes. Ensure you hook them up with best practices (link #6 in my signature). The new battery will almost certainly wear faster as it will take a greater share of the work BUT it will wear slower than if it was installed by itself.

If possible, validate that high current charges/discharges are appropriately shared between them and within specification with a clamp DC ammeter.

Thanks. I'll check out the link.

 

[sunshine_eggo]

Thanks. I'll check out the link.

It's a wonderful education. It's victron specific in many aspects of it, but there is a lot of good best practices info.

 

[Bob Van Go]

Just to clarify, I can parallel the old and new without any real danger? Just shortening the life? I'll ensure voltages are equal when connected. Someone said put a light bulb in line 1st to act as a resistor while they equalize. Good idea?

 

[sunshine_eggo]

Just to clarify, I can parallel the old and new without any real danger?

Yes.

Just shortening the life?

Of the newer one, but it will last longer than if it was installed by itself.

I'll ensure voltages are equal when connected.

Meh.

Someone said put a light bulb in line 1st to act as a resistor while they equalize. Good idea?

Someone is hyper paranoid. If one was dead empty sitting at around 10V, sure. If they are both in the operating range and of remotely similar states of charge, no way I'd bother.

 

[Texas-Mark]

FWIW, I was running parallel FLAs for over 20 years. I would always just replace the one that died for whatever reason. I had up to a 5 year spread between oldest and newest. None ever lasted less than 6 years. I have seen people post that you should never mix old with new because it will degrade the newer. I never really saw that in actual practice.

 

[sunshine_eggo]

FWIW, I was running parallel FLAs for over 20 years. I would always just replace the one that died for whatever reason. I had up to a 5 year spread between oldest and newest. None ever lasted less than 6 years. I have seen people post that you should never mix old with new because it will degrade the newer. I never really saw that in actual practice.

That's the best practices answer from manufacturers and professional installers, who need to minimize their support time and maximize their income. It's absolutely the best answer, but with DIY, good/bad isn't binary... it's a spectrum...

 

[Mattb4]

FWIW, I was running parallel FLAs for over 20 years. I would always just replace the one that died for whatever reason. I had up to a 5 year spread between oldest and newest. None ever lasted less than 6 years. I have seen people post that you should never mix old with new because it will degrade the newer. I never really saw that in actual practice.

It really comes into play if you hook in serial to get a higher voltage. In parallel the only issue is voltage droop as the weaker/older battery is not adding as much to carry a large load.