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Heat and UPS Battery Life: What Battery Makers Say

Oct 21
5 min read

Updated: Oct 22

Heat shortens the life of a UPS or standby battery (EnerSys DDmP125-33 LP manual, Table 5.1, p. 11). This guide sticks to what two valve-regulated lead-acid (VRLA) battery makers, C&D Technologies and EnerSys, publish about it. Where their documents give numbers, those numbers apply only to the products each document covers.

Heat shortens life; cold reduces performance

  • EnerSys: "Elevated temperatures reduce operating life. Lower temperatures reduce battery performance." This is from the location table for its PowerSafe DDmP125-33 LP cells (EnerSys DDmP125-33 LP manual, Table 5.1, p. 11).

  • EnerSys, PowerSafe SBS XL: optimum performance and service life come at 20/25 °C (68/77 °F). Capacity rises as operating temperature rises and falls as it drops (EnerSys SBS XL manual, §5).

  • C&D: a VRLA battery's components deteriorate over years of normal use, and this deterioration "is greatly accelerated by abuse charging conditions and elevated operating temperatures" (C&D 41-7264, p. 1).

The temperature range to aim for

Use the range for your own battery, because each maker states its own:

Document and product

Stated temperature

EnerSys DDmP125-33 LP manual

Ambient between 72 °F and 78 °F (23 °C to 26 °C)

EnerSys SBS XL manual

Optimum performance and service life at 20/25 °C (68/77 °F)

C&D Pure Lead Plus datasheet

Nominal operating temperature range +74 °F to +80 °F (+23 °C to +27 °C)

Where the battery sits matters

  • Keep it away from heat sources and air streams. EnerSys says to minimize temperature variations between cells. To do that, it says not to locate DDmP125-33 LP batteries near HVAC ducts or exhausts, heat sources such as equipment that generates heat, or direct sunlight (EnerSys DDmP125-33 LP manual, Table 5.1, p. 11).

  • Leave space between units. C&D says to mount individual batteries at least 0.5 inch apart "to allow for convection cooling" (C&D 41-6965, p. 2). Its gassing bulletin 41-6739, which covers only C&D Dynasty 12 V products, likewise calls for free flow of air around the individual batteries, with 0.5 inch between units (C&D 41-6739, p. 3).

  • Watch for uneven temperatures. For DDmP125-33 LP systems, EnerSys gives a maximum cell-to-cell temperature difference of 5 °F (3 °C). If cell temperature is the problem, it says to review the battery location instructions (EnerSys DDmP125-33 LP manual, p. 30).

Charging, heat and temperature compensation

  • Charge correctly. C&D's rule for optimum life is simple: "do not overcharge and do not undercharge" (C&D 41-2128, p. 7).

  • Warmer batteries draw more float current. C&D says ambient temperature has a significant impact on float current at a constant float voltage. The result is increased heating, gassing and premature dry-out. In warmer surroundings the battery is also less able to shed heat, so the risk of thermal runaway rises (C&D 41-2128, p. 22).

  • Lower the float voltage for warm rooms. C&D says a battery that will run at an elevated but constant temperature can use a lower float voltage to reduce that effect. Its C&D-specific compensation figure is −0.005 V per cell per °C (C&D 41-2128, p. 22). Its gassing bulletin (41-6739, C&D Dynasty 12 V products only) says to adjust the float voltage when the ambient is expected to differ from 77 °F (25 °C) by more than 10 °F (5.6 °C). If wide swings are frequent, it says to use temperature-compensated charging (C&D 41-6739, p. 3).

  • Compensation figures differ by product. EnerSys gives ±3 mV per cell per °C around 20 °C for its SBS XL range (EnerSys SBS XL manual, §5.1). It recommends temperature compensation for DDmP125-33 LP batteries when ambient is outside 68 °F to 80 °F (20 °C to 27 °C) (EnerSys DDmP125-33 LP manual, p. 28). Use the figure in your own battery's documents.

  • Outdoor cabinets. For batteries and chargers outdoors and exposed to temperature extremes, C&D says automatic temperature compensation of the charging voltage should be considered, partly to minimize the risk of thermal runaway in hot periods (C&D 41-2128, p. 23).

  • Charging temperature limits. C&D says the temperature rise of a VRLA battery during charging should be limited to 10 °C (18 °F), and the battery should not be charged above 50 °C (122 °F) (C&D 41-2128, pp. 18, 20). For DDmP125-33 LP cells, EnerSys says to stop the charge immediately if cell or battery temperature exceeds 105 °F (40 °C) (EnerSys DDmP125-33 LP manual, p. 31).

  • Charger ripple adds heat. C&D says excessive AC ripple from the charger causes additional battery heating "and a resulting decrease in the expected life" (C&D 41-2128, p. 23).

Thermal runaway

C&D defines thermal runaway as heat being generated in the battery faster than it can be dissipated. If this goes on, the battery dries out and its temperature climbs until the plastic container could deform and even melt (C&D 41-2128, p. 24). C&D lists these conditions (C&D 41-2128, p. 25):

  • Increase heat: high charging voltage, an unlimited or too-high charging current limit, excessively high float current, and high battery operating temperature.

  • Reduce heat dissipation: a high-temperature environment and a lack of adequate ventilation around the batteries.

  • Charger features that help: float temperature compensation, the lowest practical initial current limit, a float current limit, and a charging-current disconnect if the battery reaches 122 °F (50 °C) or runs 18 °F (10 °C) above ambient.

C&D adds that a string left to operate with shorted cells can see float current rise dramatically, which could lead to thermal runaway in the remaining good cells (C&D 41-2128, p. 25). That's one reason to keep up regular checks. See UPS and Standby Battery Testing and Maintenance.

Check the temperature as part of maintenance

  • C&D's maintenance table for its VRLA UPS batteries includes pilot unit temperature, measured at the negative terminal on 1 unit per 10 in the string (C&D 41-6965, p. 4).

  • EnerSys says to check the ambient temperature in the immediate environment as part of SBS XL maintenance (EnerSys SBS XL manual, §6).

Replacing batteries that have run hot?

We supply replacement UPS and standby batteries and provide service across the Southeast USA. Browse SWU UPS batteries or send us the model of the batteries you have.

Request current price and availability: use the request form or email office@luxena.net.

Sources (checked 2026-10-21)

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