Do you replace the batteries in your
Symmetra every three years or wait until they go into a faulted state?
I generally get 4 years or so before they start acting up. At that point I start pulling battery packs (there are 12 in there) as they fail. When the runtime drops to an hour or so, I order a whole new set of (raw) batteries and rebuild all the packs at once.
You might want to read my blog entries,
New Year, new UPS batteries… and
[Another] New Year, new UPS batteries…
For those not familiar, the older Symmetra battery packs were pretty dumb, with 10 12V batteries in series and a sense line connected at the center point. The Symmetra control cards (actually, the communication cards) just look at the sense voltage and decide if each pack is good. All of the battery packs are in parallel across the main 120V DC bus. So if a battery pack develops high internal resistance and the communication card doesn't notice, you now have a pack that is rapidly heating, leading to thermal runaway. Low internal resistance is even worse, as then you have the main 120V DC bus across 9 batteries instead of 10, and the next one fails so now it is 120V DC aross 8 batteries and so on. Unfortunately, APC "fixed" this by adding intelligent battery management (AKA a security chip) in the newer Symmetra models, so once a newer-model pack decides it is dead replacing all the batteries in it still won't help. It is possible to re-write the data in the security chip, but it is enough of a PITA that some users are sticking with these older units. APC still sells the exact Symmetra RM that I use, and only discontinued parts and service for the original Symmetra in the last year or 2 (it was a gradual phase-out).