Data center construction: Should UPS be equipped with lead-acid or lithium batteries?

I. Safety: Failure mode determines the risk floor

Prioritizing safety is crucial because it directly determines the consequences of a data center incident. The fundamental difference between lead-acid and lithium batteries lies in their different failure modes.

Lead-acid batteries use aqueous electrolyte and are non-flammable. Their failure is usually a “chronic” problem; for example, when the float charge voltage is too high or the ambient temperature is too high, the initial symptoms are bulging and leakage, giving maintenance personnel ample time to detect and address the issue. Of course, lead-acid batteries are not absolutely safe; long-term overcharging and the accumulation of hydrogen gas can trigger an explosion upon contact with a spark. Therefore, proper installation, venting, and regular inspections are still essential.

Lithium-ion batteries follow a different logic. Currently, data centers commonly use lithium iron phosphate batteries , whose thermal runaway trigger temperature exceeds 500°C (far higher than the 200°C of ternary lithium batteries), offering greater structural stability and a slower heat generation rate. However, this doesn’t mean they are immune to fire hazards. Once a large-capacity lithium battery experiences thermal runaway, the fire is intense and difficult to extinguish. Recent lithium-ion battery fires in a Dutch data center and several domestic server rooms serve as stark reminders. Therefore, current fire safety regulations increasingly require lithium-ion batteries to be deployed remotely, independent of the main server room, and equipped with independent fire suppression systems.

In summary, lead-acid batteries have a lower safety threshold and are suitable for most small and medium-sized computer rooms that do not have strict fire protection upgrade requirements; while lithium batteries have extremely high requirements for operation and maintenance capabilities and fire protection facilities, and must be based on a reliable BMS (Battery Management System) and strict temperature control.

II. Service life and cost: Don’t be misled by the purchase price

Since UPS batteries in data centers are in a float charge standby state for a long time, float charge life is more valuable than cycle life.

Lead-acid batteries typically have a float life of 5-8 years under ideal constant temperature conditions. However, in actual projects, they often experience significant capacity degradation after three or four years, and may need to be replaced 2 to 3 times within 10 years. In contrast, high-quality lithium batteries can have a float life of 10-15 years, and generally do not require replacement throughout their entire lifespan.

This leads to the crucial issue of “Total Cost of Ownership ( TCO )”. Looking at the purchase price alone, lithium batteries of the same specifications are indeed 2 to 3 times more expensive than lead-acid batteries.

However, if we extend the timeframe to 10 years, the multiple purchase costs of lead-acid batteries, the high labor costs for maintenance, and the waste of electricity due to the relatively low charging efficiency (approximately 80%-85%) will significantly increase actual expenditures. According to research by Schneider Electric , the 10-year total cost of ownership (TCO) of lithium-ion batteries is approximately 39% lower than that of valve-regulated lead-acid batteries.

If you have a tight budget and need to use the product for a short period, lead-acid batteries are a cost-effective option; however, if you are looking for long-term stable operation, lithium batteries are a more economical choice.

III. Space and Adaptability: Not all data centers can accommodate lithium batteries.

Many industry peers advocate for lithium batteries, with their core selling point being their “small size and minimal footprint.” At the same power output, lithium batteries are only about one-third the size and weight of lead-acid batteries. In space-constrained core data centers, the saved space can accommodate several more server racks, directly translating into revenue.

However, there are two easily overlooked major drawbacks.
First, there are the installation requirements. Lithium batteries are extremely sensitive to temperature control and fire protection. If a significant amount of money is spent on upgrading the server room’s fire protection system and precision air conditioning just to install lithium batteries, this hidden investment is substantial.
Second, there’s the compatibility issue with older UPS mainframes. Many older UPS models, which have been in operation for many years, have charger parameters set for lead-acid batteries (e.g., 13.5V-13.8V float charging).

Forcibly replacing the battery with a lithium battery that requires constant current to constant voltage and stops charging when fully charged can lead to prolonged overcharging, accelerated aging, and even safety accidents. Upgrades are smoother with newer modular UPS models that support battery type switching on the panel.

In short, there is no absolute good or bad when it comes to whether a data center UPS should be equipped with lead-acid or lithium batteries. The core factors to consider are: safety, lifespan, cost, space, and host compatibility.