August 18, 2022

The problems with using a car SLI battery for energy storage or trolling motors.

Car batteries are cheap and ubiquitous, why can't they be used for deep cycle energy storage? The answer is that they are designed for low cost, light weight, high current and shallow discharge. But it is rare to see actual data explaining what the problem is.

Below is a graph showing the voltage during deep cycle discharging of a SLI (Starting, Lights and Ignition) battery. It is an Exide 60AH absorbed glass mat, valve regulated lead acid car battery. This data was taken by Christopher Suozzo for his Master's Thesis at Ohio State University.

The battery was charged to 100% and left to rest for 4 hours before discharging. The initial cycle "Zero" was done after a 24 hour rest, which allowed some of the plate surface charge to dissipate. This explains its lower starting voltage. The discharge rate is low, 2.5 Amps, which is initially a C/24 rate. So each of these curves required at least 24 hours of Christopher's time, and the 301 discharges required over a year of calendar time.

discharge curves for deep cycling a car battery

As can be seen, after 7 cycles the capacity is already down to about 50AH, 83% of the initial capacity. After 48 cycles the capacity has dropped to 60%, and after 141 cycles only half of the initial capacity is remaining. So, yes, these batteries can be deep-cycled, but are not at all economical. This is battery that can start a car for 3-4 years or longer without a problem.

Another interesting use of this curve is to see what the discharge curves for a degrading battery look like. For the first 16% of the discharge the curves are very similar. Since a motor starting battery only discharges 1-3AH during cranking even the lowest capacity curve can start the car. Actually the test was stopped when the battery would no longer start an engine. Otherwise, the curves look very similar in shape.

I hesitate to say that other types of SLA, such as flooded batteries, will have the exactly the same discharge curves degradation, but they would still degrade as quickly as this one does.

Reference: Lead-Acid Battery Aging and State of Health Diagnosis Christopher Suozzo, Ohio State University, 2008.

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