Can a 12V 8Ah lithium battery replace a sealed lead-acid UPS battery?

Direct answer: Only if the UPS maker allows it and the numbers match. The small block may fit the space, but most UPS units draw a high current from the battery during an outage, and Redway’s 12V 8Ah page lists only 10A maximum continuous discharge. The UPS charger is also designed for lead-acid and may not suit LiFePO4. Check the UPS battery current, the charger behaviour and the maker’s approval before swapping.

Page figures. The 12V 8Ah (product address 12v6ah, listed capacity 8Ah) lists 12.8V, 102.4 Wh, 151 × 65 × 95 mm, 0.9 kg, 14.6V maximum charge, 10V cut-off, 10A maximum continuous charge and discharge, T2 terminals and UPS among its applications. The 12V 12Ah lists 153.6 Wh, 151 × 98 × 97 mm and 15A continuous discharge; the 12V 18Ah lists 230.4 Wh and 15A.

Discharge current decides it. Battery current during an outage is roughly the UPS output in watts divided by battery voltage and inverter efficiency. With an assumed 85% efficiency, a 12V UPS delivering 300W draws about 300 ÷ (12 × 0.85) ≈ 29A, far above 10A or 15A. The battery’s protection would cut off and the load would drop. Find the UPS’s battery current or maximum battery watts in its manual before choosing a battery.

Charger and firmware. UPS chargers usually hold lead-acid at a float voltage continuously. LiFePO4 needs a charge profile up to its limit (14.6V maximum here) and does not need float in the same way. A lead-acid float setting may leave the battery below full, and some UPS units run battery tests or report faults based on lead-acid behaviour. Ask the UPS maker whether lithium is supported and which settings to use.

Fit and terminals. Measure the battery bay and check the terminal type and polarity. Similar outer dimensions do not guarantee the same terminal position or the same cable reach.

Where it can work. Small DC loads or UPS designs that specify LiFePO4 can use these batteries within their ratings. For other jobs, see sizing a 12V LiFePO4 battery for loads and reading a LiFePO4 datasheet.

A safer test. If the UPS maker approves lithium, try one unit first: fit the battery, let the UPS charge it for the time its manual gives, then run a supervised outage at a known load and note runtime and any alarms before converting the rest.