This post will serve as an archive for Quazar’s Battery Load Test that was previously posted on the VEX Wiki. This was done by a member of the community (Quazar) and should not be considered official. That said, we applaud Quazar for his thorough engineering work and want to ensure that the information is retained for future users.
Equipment
An accurate integrating power meter was used in conjunction with a homemade dummy-load to capture the discharge curves of these batteries under constant 4A, 8A, and 12A loads.
The batteries tested were:
- 7.2v NiMH 2000mAh Robot Battery
- 7.2v NiMH 3000mAh Robot Battery
- 7.2v NiCd Robot Battery
Since batteries are chemical systems that yield varying results even in the most controlled setting, I used a pair of each battery type in every test. This will graphically demonstrate the variance that can be expected even from identical batteries under identical conditions.
The NiMH batteries I used were brand-new, but the NiCd batteries are several years old and have seen periods of heavy use during that time. Interestingly, the captured data clearly shows that one of the NiCd batteries is in poor condition, while the other is in fair shape.
Methodology
Before each test run, the battery under test was fully charged using the new Smart Battery Charger on the “Safe” setting. The battery was then allowed to rest for at least 10 minutes to cool down to room temperature before being placed under load.
Each battery was placed under load until its output voltage depressed to below 5.1V. This cutoff voltage was chosen because that is the level at which the Power Expander’s battery LED turns red. During the period of discharge, the power meter captured Volts, Amps, Watts, mAh, and mWh four times a second.
Batteries were loaded at 4A, 8A, and 12A. These levels were chosen because they represent loads that might be generated by combinations of actual Vex hardware. The Vex v0.5 Microcontroller and Power Expander each have a 4 Amp over-current protection device, and the Cortex Microcontroller has a pair of 4A protection devices.
I then used the R analytical software to analyze and graph the results. The raw data and graphs are available as a Battery Test Data.zip.
Battery Pack Ratings and Internal Resistance
Battery packs are rated in mAh (milli-Ampere • hours), which roughly indicates how many milli-Amps the pack can provide for 1 hour. However, it says nothing about the voltage that will be delivered during that time; cell voltage is a function of cell chemistry (NiCd or NiMH) and the number of cells in the pack (6 in this case). These packs are rated at a “nominal” 7.2V (1.2V per cell x 6 cells = 7.2V).
These mAh capacity and nominal voltage ratings are usually provided assuming a 0.2C load, which means a discharge rate of 1/5 the pack capacity (400mA load for a 2000mAh pack). The test data shown below are taken at 4A, 8A, and 12A. These loads respectively represent a 2C, 4C, and 6C discharge for the 2000mAh packs, and 1.33C, 2.67C, and 4C discharge for the 3000mAh packs.
Battery packs have an additional characteristic called “internal resistance” which becomes important under heavy loads. Each cell behaves as if it has a series resistor inside of it, which will produce a voltage drop proportional to the current load: Vdrop = Amps x InternalResistance. For instance, a 0.1Ω internal resistance would cause a 0.4V voltage drop at 4A.
So, what does all this mean? The heavier the load, the lower the voltage the battery delivers. A small internal resistance will result in a smaller voltage drop, but all batteries depress to some extent. The voltage drop also results in the packs apparently running out of power earlier than their mAh rating suggests they should; they may still have energy in them, but because the voltage is depressed too low, you can’t effectively use the full capacity. Remember that lower battery voltage means lower motor RPM and increased chance of a microcontroller brown-out.
For all of these reasons, I prefer to use Watts to evaluate power being delivered, and Watt•Hours to evaluate energy capacity. Watts (for DC circuits) are simply calculated as Volts x Amps. This means that Watts tell the whole story, representing both the current load and delivered voltage in a single number. Likewise, Watt•Hours captures both the mAh delivered, as well as the voltage at which it was delivered. I’ve included these values in the statistics below for completeness.
Discharge at 4A
First, I ran all the batteries at 4A, which represents the maximum continuous load that a single v0.5 microcontroller, power expander, or bank of Cortex motor ports would be able to place on a battery. This is the resulting discharge graph:
You can see that the two 3000mAh NiMH batteries (red lines) performed almost identically. They also had the least amount of voltage drop of all the batteries, as a result of having the lowest internal resistance.
The two 2000mAh NiMH batteries also performed similarly, though one went a bit longer than the other but at a reduced voltage. One of the NiCds delivered less voltage and for a shorter time than the other. Regarding voltage drop, you can see that the NiCds did a bit better than the smaller NiMH batteries, which is a result of their low internal resistance.
All six batteries performed well at 4A, and produced textbook discharge curves. Also, you can see that the lower the internal resistance, the higher the voltage during discharge.
Here are the relevant statistics:
2000mAh NiCd Battery #1 at 4A*(light green line) 2000mAh NiCd Battery #2 at 4A*(dark green line)
Runtime: 25:13.50 Runtime: 23:25.50
Delivered: 1678.7mAh, 11.74W*hrs" Delivered: 1561.3mAh, 10.75W*hrs
Internal Resistance = 0.118 ohms Internal Resistance = 0.115 ohms
Volts: Min = 5.090 Mean = 6.994 Max. = 7.846 Volts: Min = 5.094 Mean = 6.887 Max = 7.898
Amps: Min = 3.990 Mean = 3.993 Max = 4.034 Amps: Min = 3.996 Mean = 4.000 Max = 4.224
Watts: Min = 20.32 Mean = 27.93 Max = 31.65 Watts: Min = 20.37 Mean = 27.55 Max = 31.95
2000mAh NiMH Battery #1 at 4A*(light blue line) 2000mAh NiMH Battery #2 at 4A*(dark blue line)
Runtime: 26:20.00 Runtime: 22:31.50
Delivered: 1753.4mAh, 10.98W*hrs Delivered: 1499.5mAh, 9.75W*hrs
Internal Resistance = 0.248 ohms Internal Resistance = 0.224 ohms
Volts: Min = 5.099 Mean = 6.263 Max = 7.142 Volts: Min = 5.094 Mean = 6.501 Max = 7.377
Amps: Min = 3.992 Mean = 3.995 Max = 4.041 Amps: Min = 3.992 Mean = 3.994 Max = 4.027
Watts: Min = 20.37 Mean = 25.02 Max = 28.86 Watts: Min = 20.34 Mean = 25.97 Max = 29.70
3000mAh NiMH Battery #1 at 4A*(light red line) 3000mAh NiMH Battery #2 at 4A*(dark red line)
Runtime: 40:39.00 Runtime: 41:22.75
Delivered: 2704.4mAh, 19.14W*hrs Delivered: 2757.0mAh, 19.49W*hrs
Internal Resistance = 0.080 ohms Internal Resistance = 0.078 ohms
Volts: Min = 5.093 Mean = 7.072 Max = 7.825 Volts: Min = 5.141 Mean = 7.067 Max = 7.930
Amps: Min = 3.990 Mean = 3.992 Max = 4.027 Amps: Min = 3.993 Mean = 3.998 Max = 4.043
Watts: Min = 20.34 Mean = 28.23 Max = 31.51 Watts: Min = 20.53 Mean = 28.25 Max = 32.06
(continued in next post)
