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Apr 17, 2017

# How to Calculate Battery Runtime (1)

Know about hidden battery losses when estimating the energy reserve.

If the battery was a perfect power source and behaved linearly, charge and discharge times could be calculated according to in-and-out flowing currents, also known as
coulombic efficiency. What is put in should be available as output in the same amount; a 1-hour charge at 5A should deliver a 1-hour discharge at 5A, or a 5-hour discharge at 1A. This is not possible because of intrinsic losses and the coulombic efficiency is always less than 100 percent. The losses escalate with increasing load, as high discharge currents make the battery less efficient.

Peukert Law

The Peukert Law expresses the efficiency factor of a battery on discharge. W. Peukert, a German scientist (1855–1932), was aware that the available capacity of a battery decreases with increasing discharge rate and he devised a formula to calculate the losses in numbers. The law is applied mostly to lead acid and help estimate the runtime under different discharge loads.

The Peukert Law takes into account the internal resistance and recovery rate of a battery. A value close to one (1) indicates a well-performing battery with good efficiency and minimal loss; a higher number reflects a less efficient battery. Peukert’s law is exponential; the readings for lead acid are between 1.3 and 1.5 and increase with age. Temperature also affects the readings. Figure 1 illustrates the available capacity as a function of amperes drawn with different Peukert ratings.

As an example, a 100Ah lead acid battery discharged at 15A should theoretically last 6.6 hours (100Ah divided by 15A) but the actual time is less. With a Peukert number of 1.3, the discharge hours are about 4.8 hours.

Figure 1: Available capacity of a lead acid battery at Peukert numbers of 1.08–1.50.

A value close to 1 has the smallest losses; higher numbers deliver lower capacities. Peukert values change with

battery type age and temperature:

AGM:      1.05–1.15
Gel:         1.10–1.25
Flooded: 1.20–1.60

The lead acid battery prefers intermittent loads to a continuous heavy discharge. The rest periods allow the battery to recompose the chemical reaction and prevent exhaustion. This is why lead acid performs well in a starter application with brief 300A cranking loads and plenty of time to recharge in between. All batteries require recovery, and most other systems have a faster electrochemical reaction than lead acid.

The instruments for testing the battery

We have a series of instruments which can be used to tets the battery, such as SAT-AC battery conductance tester, SAT-AR Battery resistance tester,SAT-DC200 Full-Automatic Battery Charger, SAT-DC500 Battery Charger-Discharger Monitor etc. For more information, please send mail to sales@criane.com