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New LIFEPO4 to replace bad in older series

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I have questions about keeping an older battery pack viable. My Miata was converted to EV in 2010 and batteries are somewhat creaky. (

The car seems to have been parked outside and unused for some years but still runs, although with a range of approx 50 miles versus original claims of 80-90 miles.

The original batteries (36) are CALB LIFEPO4 200Ah (afaict), so I decided to try replacing the weakest batteries in the system to keep it going a little longer ahead of a full battery refresh.

In any case, I replaced one battery (the one most with low voltage when under load, and prime culprit for low charge ahead of rest of pack). A couple of weeks later, I've gone back to check individual voltages per battery after full charge, and the new battery I added to the pack has a charge of 0 and resistance of 0. This decline was not reported by the battery monitoring system.

I suspect that the issue is that it doesn't match the draw characteristics of the rest of the pack, so has gone into a safety state, but wondering if it might just be a bad battery.

Any insights or suggestions would be much appreciated.