Lead Acid Battery Life

Valve Regulated Lead Acid Battery Design Life.

Battery Life or “design life” of a battery is based on average use at room temperature (20-25°C) operation. For a modest UPS System, the design life is typically 5 years. Since, UPS applications are standby applications, the batteries are float charged, and the life is also referred to as “float life”.

The moist gel interior of VRLA batteries dries up over time, gradually reducing the effectiveness until the battery capacity is no longer viable for the application. This is why batteries will wear out regardless of how well they are maintained.

Typically, you have around 200 charge/discharge cycles in a 5 year design life battery. This is because the charge and discharge process involves a chemical reaction and this causes corrosion within the battery itself.

As this limit is approached the battery capacity starts to tail off, and can become very low very quickly. You can see that if a battery is used daily for example, the life expectancy is lower than one year.

Battery Life in relation to depth of discharge
Note how cycle life can be extended significantly by reducing the battery depth of discharge


If the battery is allowed to stand unused for a prolonged period of time, lead sulphate crystals form- blocking recharge. If this happens the UPS charger is usually incapable of recharging these batteries. It is possible to sometimes recover such batteries using high charging voltages that break down the sulphate but also having a current limited charger. Temperature monitoring is also required and as such, this is beyond the scope of most UPS built in chargers.

Sulphation occurs mainly when batteries are allowed to stand in an uncharged state. This is why it is important to have your UPS charged as soon as possible after an outage.


The float life of batteries is rapidly reduced with heat, and I mean rapidly.

HIGH TEMPERATURE will reduce battery service life often quite dramatically, and in extreme cases can cause Thermal Runaway, resulting in high oxygen/hydrogen gas production and battery swelling. Batteries are irrecoverable from this condition and should be replaced.

Yuasa Little Red Book of Batteries
Temperature (°C)20°25°30°35°40°45°50°
% Expected Float Life100%100%80%60%40%20%10%

Based on this, if the batteries are locked in a cupboard with little ventilation and temperatures allowed to build, for example to 50°C, then a 5 year float life battery would be expected to last no more than 6 months, regardless of how it has been used.

Results of over thermal runaway on batteries exposed to excessive heat
Thermal runaway results on VRLA battery

Battery Life Conclusions

  • A battery cannot be expected to last in excess of its design life so schedule a replacement before this.
  • Regular cycling of the battery will diminish its performance. If your application is for regular charge/discharge cycles then the life expectancy reduction needs to be considered.
  • Avoid heat build up. Ensure the UPS and batteries are well ventilated with adequate air flow though the air intakes. Ensure vents are free from a build up of dust and the UPS is not in direct sunlight.
  • Always recharge the batteries as soon as possible after an outage to prevent the possibility of sulphation.


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