Using 12 volt lead acid batteries in parallel 2

[knowlittle]

A battery failed to provide enough power to drive a motor (gate to be exact) on successive operation (open and close in repetition). I replaced it with a new one. The old battery shows full charge voltage. As far as I see, the old battery is just reduced in capacity and can be connected to the new one in parallel. My goal is to extend the life of the new battery. Longer trouble free years.

I am a materials engineer myself and know the battery chemistry. I just don't see why the old battery would damage the new one, but internet is full of No's. Anything I am missing? Thank you.

By the way, the batteries are sealed AGM type.

 

[TugboatEng]

The old battery will short internally and become a drain in the system. It's best to replace it. Why do you need more capacity? If one can open the gate, 2 only doubles the chance of failure. Lead acid batteries have an expected life of 5-7 years. Internally shorted batteries can explode. Deteriorated batteries leak and corrode your cables. Save yourself money and hassle, replace your batteries regularly.

To extend the life of the new battery don't deep cycle it. If your application requires deep cycling, use NiCad batteries.

 

[3DDave]

As TuboatEng suggests, the short answer is - no. It's not a chemistry problem; it's a failure of the mechanical elements as the lead is unevenly replated with each charge cycle of the battery. Adding a second, already damaged battery that is reacting to charge and discharge differently is unlikely to make a meaningful change in exchange for a lot more bad options to exist.

 

[knowlittle]

Thank you. I didn't think about shorting. I don't know if deep cycles or not. According to a youtube, current draw is just over 1 amp. If verified (actual measurement), I will just replace battery + trickle charger with a DC power supply.

 

[IRstuff]

In redundancy reliability, having two systems in parallel improves reliability, BUT that assumes that one can disconnect the failed unit and use only the working unit, and that one can tell the difference soon enough to not cause harm downstream.

Batteries do not fall into that category; although one can, presumably, construct some sort of current switching to ensure that the failed battery does not take down the entire assembly, but then, you have to worry about the reliability of the current switch.

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[knowlittle]

IRstuff,
Your post clearly explains what is missing in my analysis. Thank you for the teaching.