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Using Lithium Ion batteries is an option but would require modifying the charge circuit in the UPS. A lead acid battery will charge using a voltage of 14.4v unlike a 3S battery which would charge at a voltage of 12.6v. 

Example:

3S7P

14AhAdding

an additional car battery might be a better optionUsing Lithium Iron Phosphate (LiFePO4) may be a better idea since it is a drop in replacement for Lead Acid. Charge Voltage for a 4S pack would be 14.4v (3.6v/cell).

Conclusion

Bad idea... move onWill leave it for now. If I had some good LiFePO4 cells kicking around I may see how many I could fit. The ones I have are old and have a max capacity of about 1500 mAh.

References

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