Lipo to "SLA" adapter (for UPS battery replacement)

[chaseadam]

I am getting ready to replace the battery pack in my 48VDC APC UPS. Before I go out and purchase new batteries, I am exploring if I can adapt some very large LiCo02 Lithium Cells from a decommissioned Chevy Volt battery pack I have laying around.

The UPS supports external battery packs in addition to an internal pack consisting of 2x 4s 12V SLA batteries. I do not (believe) I have direct control over the charging circuitry.

I can imagine several situations where we may want to safely drop in a LiPo battery as a replacement for SLA or other battery technologies.

Are there existing "adapters" (preferably circuit) which would allow a LiPo battery to interact with a system which is expecting to be charging an SLA battery?

Any obvious issues with a "simple" solution such as prevent "charging" from the UPS (i.e. a diode in front of the battery assuming the UPS charger is "satisfied" when it reaches float voltage) and use a separate dedicated charger?

Thanks!
Adam

Reference Material:
https://www.eevblog.com/forum/projects/converting-2200va-ups-from-sla-to-li-based-batteries/
https://www.powerelectronics.com/markets/mobile/article/21858074/replace-sla-batteries-with-liion-technology

[Pineapple Dan]

Like your reference material suggests, LiFePo4 lends itself better to this. It would be possible to convert I suppose but it will be a complicated affair involving a cacophony of TI bqXXXX chips, power mosfets and pesky sensors to prevent the much-dreaded exploding battery event from happening. You can probably shoehorn a bunch of off the shelf BMS into it but then you're still stuck with the different charging voltages. Ideally you would boot out the existing charging circuit and replace with your own

[chaseadam]

a cacophony of TI bqXXXX chips, power mosfets and pesky sensors to prevent the much-dreaded exploding battery event from happening

Yep, had those datasheets up already for this and another project.  I will try the large diode and dedicated charger and see if the UPS stays happy.

[ajb]

Assuming you can incorporate a sufficient BMS to prevent catastrophe, the discharge curve of the Lithium battery is going to be different from the SLA, so any state-of-charge or capacity/runtime estimates the device does may be thrown off.  Whether or not that's a problem I guess depends on the application and how finicky the particular device is.

I just came across this site, which is interesting, it discusses using lithium batteries in parallel with lead acid, which maybe could be a good option to extend the runtime of a UPS that supports external batteries: https://www.zwerfcat.nl/en/open-hybrid-bms.html