Comparison between one Battle Born LiFePO4 battery and two 6V GC2 batteries in series
A common battery configuration in RVs and golf carts involves the use of multiple 6V GC2-style batteries connected in series to form 12V or 24V systems. In this post, we examine the differences in discharge and charging profiles between a single Battle Born 12V 100Ah (BB10012) pack and two GC2 batteries connected in series (see photograph below). The BB10012 weighs 29 lbs and the two GC2s weigh a combined 150 lbs.
Figure 1: Photograph of comparison test setup
Two brand new Duracell Ultra high capacity 6V batteries (Part number SLIGC115) were purchased from Batteries And Bulbs in Reno. These batteries have a capacity rating of 230 Ah. The batteries were fully charged by applying a 14.4V charging voltage until the charging current fell below 1 Amp. After settling for 10 minutes, the batteries were discharged at a constant current until the battery voltages fell below 11.8V. The discharge current among the tests varied between 10A and 50A. The results of the tests for 3 discharge currents are shown below.
Figure 2: Voltage discharge curve comparisons for three discharge currents.
What is clear from the curves in Figure 2 is that the relative performance is highly sensitive to the discharge rate of the batteries. At a 50A discharge rate, the BB10012 can sustain above the 11.8V cutoff for almost 2 hours, whereas the GC2 batteries cutout in less than an hour. At the lowest discharge rate, the BB10012 can deliver 10A for around 9.5 hours, whereas the GC2 batteries last for over almost 12 hours.
Figure 3, below, shows the cumulative delivered charge and energy over the discharge curves for all 3 discharge current tests. What is immediately clear from the figure is the known Peukert effect in lead acid batteries. The useable capacity drops from 120 Ah at 10A to 40Ah at 50A for the GC2 batteries – despite their capacity rating of 230Ah. By contrast, the BB10012 pack remains within several percent of 100Ah, independent of the discharge rate. The slight drop in capacity with current is entirely due to the ohmic voltage drop during discharge.
Figure 3: Discharge curves: Voltage vs Capacity
Finally, Figure 4 below shows the head-to-head charging profiles. In this test, a charging current of 40A was used with a bulk and absorption voltage of 14.4V. Absorption was allowed to continue until the charging current fell below 3 Amps. What is evident from the figure is that the BB10012 pack charges completely in less than half the time as the GC2s (2.5 hours, compared to 5 hours), even with the identical charging parameters. The reason is that internal resistance of Li-ion batteries is much lower, and so the high-current bulk charging phase can continue for longer. Therefore, less time is needed for absorption.
Figure 4: Charging profile comparison