I noticed the fan on my APC SMT1500RM2U was running, and after a quick check.. batteries are toasty, puffy and time for replacement! No big deal, as I've done this before on many other UPS's, but the thought occurred to me: do I REALLY need to replace all four cells, or can I safely "convert" it down to a 750VA unit by only replacing two cells? They're wired in a series/parallel (ie two cells in series, paralleled to another set of cells), and the cells on a SMT750 unit are just the one set. Obviously this would mean half the capacity, but what's got me concerned is the charging parameters, since the number of cells will be halved. I don't need the extra runtime, and even though it's notthat costly to replace all four it will obviously be cheaper to replace only two.
For background, runtime isn't really a concern here; it's running my...