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VRLA vs lithium: which one, and when
VRLA is the lower cost choice for standby duty where discharges are rare, floor space is available, and the room is temperature controlled. Lithium iron phosphate wins where the battery cycles regularly, space or weight is constrained, or the site runs warm, because it delivers thousands of cycles at high depth of discharge and a lower cost per usable kilowatt hour over life.
The comparison is usually framed as old versus new. It is really standby versus cycling, and capital cost versus cost per cycle.
Side by side
Figures below are typical for good quality industrial product, not best case marketing values.
| Criterion | VRLA (AGM/GEL) | Lithium LFP |
|---|---|---|
| Design life | 5 to 15 years | 10 to 15 years |
| Cycles at 80 percent DoD | 200 to 600 | 3000 to 6000 |
| Usable depth of discharge | 50 percent typical | 80 to 95 percent |
| Footprint and weight | High | About one third |
| Tolerance to heat | Poor | Good with BMS |
| Upfront cost | Lower | Higher |
| Cost per usable kWh over life | Higher when cycling | Lower when cycling |
Where VRLA is still the right answer
Substation DC systems, UPS strings in conditioned rooms, and telecom sites with rare outages and generous space. Discharges are infrequent, the duty is float, and the lower capital cost is not offset by cycle wear. Replacement is also simple: a known form factor, widely available, no software.
Where lithium pays for itself
Peak shaving, solar self consumption, load shifting, frequent outage areas, and any site where the battery room is expensive or hot. In these cases lead acid is replaced two or three times inside the life of one lithium system, and the usable energy per installed kilowatt hour is nearly double.
Frequently asked
- Is lithium safe for indoor installation?
- LFP is the most thermally stable common lithium chemistry and is installed indoors routinely, with a battery management system, compartmentalisation, detection, and clearances defined at design stage.
- Can lithium be retrofitted into an existing UPS?
- Often yes, but the UPS charger profile, communication, and low voltage disconnect must be compatible. Confirm with the UPS manufacturer before ordering; an incompatible charger will shorten life or trip protection.
- Which is cheaper overall?
- For pure standby with rare discharges, VRLA. For anything that cycles weekly or daily, lithium is usually cheaper across ten years once replacements, floor space, and cooling are counted.