Some Notes about Sparkfun 6000 mAh LI-POLYMER BATTERY PACKS
This is some notes about the Sparkfun 6000mAh LiIon battery pack. My complete lab notes can be found in a PDF file here or alternatively look at the reference section. Sparkfun have these batteries on their web site as: "Polymer Lithium Ion Battery - 2000mAh, sku: PRT-08483".
I did buy three of these batteries from Sparkfun. My intention is to use them with a Park Flyer with an Outrunner motor. The outrunner will draw about 2-4 amperes in level flight and up to 10-12 amperes at full speed. I will use 7,4 volt as nominal motor voltage. These battery packages seem to be a good choice and will give a prolonged flight time with the 6000 mAh capacity.
A picture of the battery can be seen in figure to the right. The 6000 mAh batteries actually consist of three 2000 mAh cells in parallel. Each cell has a nominal voltage of 3.7 volts. The cells are specially matched for internal impedance and can be fully charged and discharged in parallel. It looks like figure below. The first thing I did was to weight the batteries. I got the numbers as below. According to the specification it says 37 grams but that number is referring to just one cell.
Weight of one battery A = 111 grams (117 Grams after modifying)
Weight of one battery B = 114 grams (120 Grams after modifying)
Weight of one battery C = 112 grams
The next thing I did was to do a little load test. Using a car halogen lamp I loaded the battery with about 1,5 amperes. Just by looking at the connection cables for the battery you conclude that they are under dimensioned for a load of up to 12 amperes. I realized that I had to modify this. A simple calculation (and some tests) gave the result that at 12 amperes load you will have a voltage drop of about 0,57 volts. This would give an efficiency of about 84%, totally unacceptable!
The next step was to increase the current; I decided to connect two of the lamps (12V/55W) in parallel. Nothing happened! The voltage over the battery dropped to zero (0) and no current at all could be measured. It was time to dissect the battery and take a closer look. I had a clue by reading on the Sparkfun web site (user comments) that there are a built in protection circuit into the battery pack.
Partially dismounted battery can be seen in figure below to the left. At the battery connectors you can see a small circuit board with the protection electronics. A schematic over the battery protection circuit can be seen in figure below to the right.
Partially dismounted battery
A sensor IC, DW01, is connected to the battery and senses the voltage and current. It controls the N-channel Power MOSFET's M1, M2. The circuit trigger on a current slightly above 2 amperes, the nominal current for one cell. Obviously it doesn't take consideration to that there are three cells in parallel giving a nominal capacity of 6 amperes and 12 amperes if you load it with 2C.
According to the datasheet the max discharge current is 2C5A (0 - 60 degrees). According to the datasheet for the MOSFET FS8205A (M1, M2) the maximum current is 6 amperes at 25 C Celsius. I wanted to go to 2C so modifying the trip point for the circuit was not an option. Therefore I totally removed the safety circuit board. When loading the battery with 12 amperes (2C) the battery wires and power connector are far too under dimensioned. Therefore we replaced the wires with larger area.
The new cables can be seen in figurebelow. The area is increased more than tenfold to about 0,9 square millimeters. At maximum load of 12 amperes there will be a big difference of the voltage drop in the power cables compared to the old thin ones.The voltage drop decreased from about 0,58 volts to 0,050 volts. The new cables were soldered directly on to the tin plates of the battery. You have to watch up with the applied heat to the battery whenn soldering the cables. The mounted cables were then connected to a high capacity power connector. The reworked battery package can be seen in figure below after insulating the package with tape.
The next step was to fully charge the battery with a special LiIo charger. Let the battery rest for an hour to cool down. Then connect a ceramic resistor setup of 0,61 ohm, start the clock, and frequently measure the voltage of the battery as well as the current. As an additional check I also measured the battery temperature. The total time for the load test was about one hour and 19 minutes. The temperature went from 26,4 degrees Celsius to 36,3 at the end of the test.
The test was terminated when the battery voltage dropped to about 2,75 volts which is the minimum recommended discharge voltage. Furthermore, for each point, as the discharge continues, we calculate the power and the integral of this over time. We can use the trapezoidal rule to do the numerical integration and get an approximation of the value. The trapezoidal numerical integration is defined by:
By applying the this on the measured values we get column C(t) [mAh]. For details, see the full PDF report. We can see how this figure evolves with the time. Just before the battery voltage drops to the minimum recommended discharge value of 2,75 volts we can see that we have produced 6 475 mAh. This is in accordance with the specifications of the battery. The nominal figure is 6 000 mAh.
To conclude, the Sparkfun 3*2000 mAh battery delivers the promised figures that you would expect after some minor modifications of the battery. The modification exist of removing the built-in safety circuit as well as replace the under dimensioned cables with suitable ones. These modifications are quickly and easy to do without any bigger effort.
SM6FIE, Bo Gärdmark
You can also download the total lab report in PDF format, see just click here.
Lab report, Some Notes about Sparkfun 6000 mAh LI
Sparkfun 6000 mAh Battery product page
LI-Polymer Battery Packs Specification
DW01-G, One Cell Lithium-ion/Polymer Battery Protection IC
FS8205A, Dual Channel Enhancement Mode Power MOSFET