Seeking advice: replace Lead-Acid Gel-Cell batteries with LiFePO4 in UPS

Question

I have an older Tripp-Lite 3000 VA UPS. This has two strings of Power Sonic 12 V 7 Ah batteries in parallel. Total is 48 Vdc 14 Ah.

Some kind soul (heavy sarcasm here) left the UPS turned ON but disconnected the incoming AC power. Then allowed the unit to freeze over last winter.

The batteries came back a little bit but do not have the kind of run time they used to have. So: I'm looking at replacing all 8 of the batteries.

I happen to have a bunch of brand-new 3.8 Ah LiFePO4 cells purchased from Battery Hookup some time ago. I've made several 12 V packs with these with great results. But I have full control of how and when they are charged.

This time, I'm thinking of using several dozen of these cells to replace the old Power Sonic batteries. But I am concerned about charge voltage.

The UPS seems to float the existing batteries (4 x 12 V nominal in series) at 54.9 Vdc. That works out to 3.425 Vdc for each LiFePO4 cell.

Question: is this reasonable?

Answer 1

LiFePO4 cells are not as dangerous as other lithium batteries (they don't catch fire and just burn), but there is still a fire risk if you charge too fast or too high.

I wouldn't do this in the home without doing the tests below, but having said that, I wouldn't object to doing it in the factory: our factory has several other fire risks and managing fire risk is part of why we have a factory.

The float voltage is reasonable: it won't overcharge the cells, and will keep them reasonably full. It doesn't matter that the cells remain on float because you aren't taking them up close to fully-charged.

So direct replacement is reasonable as long as you don't need to use the UPS (which, to be fair, is the normal case for many UPS -- it may go for years before you need it).

What you haven't told us is the charge rate of the UPS, or the float range, or the boost voltage, or if it is a 4 or 5 state charger (unlikely).

If, after the cells go partly flat, because they've been supporting the load for 20-200 minutes, the system tries to recharge too fast, or (less likely) tries to boost too high, then you could overheat and damage your cells.

You can probably find an estimate of how fast the UPS tries to charge by finding how long it takes to recharge after use, which is probably documented. And your LiFePO4 cells will have documentation for the charge rate.

You can also just measure the charge current with your existing cells, if you want to open up the UPS and use a DC clamp or in-line ammeter. And you can check the float range, and check to see if there is a boost voltage with a voltmeter.

If you don't want to do that ... well, LiFePO4 cells normally have a higher charge rate than Lead Acid batteries, so you could take a chance -- at least read the documentation and make an estimate.