Proof of concept. Not reviewed by an electrical engineer, and the Amazon links use a placeholder tag.

The exact cells inside your UPS, and whether LiFePO4 will trip

Your UPS is beeping and the OEM cartridge lists at up to $392. Here is what's inside it, what actually fits, and the math behind each answer.

Enter a UPS model, cartridge SKU or serial prefix

The 4 expensive UPS battery traps

$110The plastic sled markupAPCRBC163 lists at $180. Two CSB 9 Ah blocks are $69.88 from a non-Amazon store, and you reuse the slide-on wire.51 mmThe BE600M1 width and terminal trapA standard 65 mm battery jams in the slot, and the stock cell has asymmetric +F2/-F1 tabs.236 WThe 1500 VA LiFePO4 blackout crashA 12 A BMS trips above 236 W on a 24 V unit, while the UPS can demand 50.8 A on a CP1500PFCLCD.+28.6%The CP1350PFCLCD serial splitCXY and CX5 serials take 9 Ah cells in the same case. CQB and CR8 use 90 mm cubes, and a 151 mm brick won't fit.

All 12 UPS models

Solver output for each model at its default hardware revision
UPS	VA / W	Waveform	Cartridge	Cells	Max DC	Safe at 12 A BMS	Serial split
BR1500MS2	1500 / 900	Pure Sine Wave	APCRBC163	2 x 9.4 Ah	45.7 A	236 W	No
BX1500M	1500 / 900	Simulated / Stepped	APCRBC124	2 x 9 Ah	45.7 A	236 W	No
BR1500G	1500 / 865	Simulated / Stepped	APCRBC124	2 x 9 Ah	44.0 A	236 W	No
BN1500M2	1500 / 900	Simulated / Stepped	APCRBC161	2 x 9 Ah	45.7 A	236 W	No
BE600M1	600 / 330	Simulated / Stepped	APCRBC154	1 x 6.5 Ah	33.5 A	118 W	No
BE850G2	850 / 450	Simulated / Stepped	RBC17	1 x 9 Ah	45.7 A	118 W	No
SMT1500C	1440 / 1000	Pure Sine Wave	RBC7	2 x 17 Ah	50.8 A	236 W	No
CP1500PFCLCD	1500 / 1000	Pure Sine Wave	RB1290X2	2 x 9 Ah	50.8 A	236 W	Yes
CP1500AVRLCD3	1500 / 900	Simulated / Stepped	RB1290X2E	2 x 9 Ah	45.7 A	236 W	No
CP1350PFCLCD	1350 / 880	Pure Sine Wave	RB1270X2C	2 x 7 Ah	44.7 A	236 W	Yes
CP1000PFCLCD	1000 / 600	Pure Sine Wave	RB1290	1 x 9 Ah	61.0 A	118 W	No
EC850LCD	850 / 510	Simulated / Stepped	RB1270B	1 x 7.2 Ah	51.8 A	118 W	No

All 14 OEM cartridges

List price against a non-Amazon single-unit bare-cell reference
Cartridge	Inside	Bus	Terminal	List price	Saving
APCRBC163	2 x 9.4 Ah	24 V	F2	$180	~61%
APCRBC124	2 x 9 Ah	24 V	F2	$181	~61%
APCRBC161	2 x 9 Ah	24 V	F2	$164	~57%
APCRBC154	1 x 6.5 Ah	12 V	+F2/-F1	$101	~70%
RBC17	1 x 9 Ah	12 V	F2	$103	~66%
RBC7	2 x 17 Ah	24 V	NB	$392	~69%
RB1290X2	2 x 9 Ah	24 V	F2	$117	~40%
RB1290X2E	2 x 9 Ah	24 V	F2	$129.99	~46%
RB1280X2B	2 x 9 Ah	24 V	F2	$127	~45%
RB1270X2C	2 x 7 Ah	24 V	F2	$121	~42%
RB1270X2A	2 x 7 Ah	24 V	F2	not published	unknown
RB1260X2A	2 x 6 Ah	24 V	F2	$116.95	~48%
RB1290	1 x 9 Ah	12 V	F2	$55	~36%
RB1270B	1 x 7.2 Ah	12 V	F2	$50	~30%