Lipo to A123 conversion, two successful stories (part 1)
In this article I will show the results of two conversions from Lipos packs to A123 systems packs. A123 is the brand of one of the latest technologies in batteries; I will talk more about it in the second part.
The new cells used in both cases were:
Brand/Model: A123 System, 26650
Capacity 2300mA Hr
Rated voltage 3.3 V
Current (continuous/peak) 70A/138A
Charge Current (rec/max) 5A/10A
Minimum voltage (cut) 2V
Flotation voltage 3.6 V
How to select the proper pack
To select the A123 replacement you need to balance two factors, the weight and the energy (V x mA Hr), and at the same time maintain a practical voltage for the motor you are using. Due to the A123ís 40% overweight (compared with average lipos), keep in mind that a good trade off is to have a 20% heavier pack with 20% less energy with a similar voltage. To see some examples refer to the first tables attached.
To select the power plant from zero is easier, and it will lead probably to a motor with a different Kv than the used for lipos. If you are going to buy the cells to make the pack yourself, it is very advisable to buy cells with tabs and add the balancing plug.
First test, the Extra
My AJ Extra 330 is a 3D plane that weighs 1.7 kg and has 1,25 m wingspan. I have flown it more than 200 times, 100 of them with the A123.
I used to have a lipo pack 3S2P 11.1 V 4400mA 22C (400g), and it showed to have an excessive autonomy, then it was perfect for a 4S or 5S A123 2300mA packs replacement.
The first test was performed with a four cells pack that I had to make at home (13,2V; 2300mA; 30C). Since both packs, Lipo and A123, were similar in both size and weight, the model was easy to trim. The ESC was replaced by one with a BEC able to work with the highest voltage provided by the A123 pack.
Important note!: the A123 supports to go down until 2V per cell, then I programmed the ESCís cut off voltage in 8.1 V, that is 2.7 V per cell for 3 lipos, which gives 2V per cell in the 4S A123. When I connect the ESC it has to count 3 cells both with the lipo and with the 4S A123. If it counts 4 cells with the A123, it will cut the power before it is needed and will end the flight at the middle.
As the CG remained unchanged, I went out to fly with only one uncertainty, how well the new pack would behave. The power plant was solvent at the take-off and in the air it powered the Extra with the same agility and authority of a very good Lipo. Good vertical climb, very comfortable knife-edge. In the first three flights after 5 minutes of the mild acrobatics pack charged 1300mA each time, which makes us think that we do not have a lot of autonomy for 3D. The pack always gets land cool and the performance was sustained during the whole flight.
This was the original stuff under test:
Motor: Hyperion 3013-14, 110g, 1085rpm / V
Propeller: 12x6 APC-E
ESC: 80 A BEC
Lipo Pack original: 3S2P; 11.1 V; 4400mA; 22C; 340g
Voltage under load 9.6V
Current peak 47A
Power in 451W
A123 Pack 4S1P; 13.2 V; 2300mA, 310g
Voltage under load 10V
Current peak 55A
Power in 550W (over the motorís suggested limit)
But the best was still to come, the charge speed was awesome and it took between 20 and 30 minutes without abusing the cells charging at 5~6A. Note that the system requires a charger compatible with A123, a lipo charger will spoil the cells toping them at lethal 4.2 v per cell instead of the 3.6 v needed (but I prefer to cut at 3,55V). In addition, to be able to charge the A123 fast, it is needed a charger and a power supply capable to deliver the high current needed.
Second try on the Extra 330
To improve the autonomy, I added the fifth cell to the packs and tried with the same motor, but it pushed the motor too much and the next step was to replace the original motor by a bigger and cooler one. The cut off voltage in the ESC was fixed to 2,7V again, and the 5S A123 counting must be as 4 lipo cells, if the ESC counts 5 lipo cells, run the motor 20 seconds, and re-plug the pack, it will count 4 lipo cells this time.
Sunday morning in the field, before take-off, I made a general checkup of the model (including the verification that the ESC was counting correctly the number of A123 cells) I measured the RPMs and read the current, voltage and power consumed. Once in the head of the runaway I started to rise the throttle stick smoothly, and the new power plant moved the Extra easily and gently. The weight to power ratio was better than with four cells, giving almost unlimited climbs. Cool!
Second and final configuration
A123 Pack 5S1P: 16.5 V, 2300mA, 400g
Motor: AXI 2830-14 (150g, 860 rpm / V)
Propeller: 13x6,5 APC-E
ESC: 80 A with UBEC 3A
Model: Extra 330S (1.25 m and 1.7 kg)
Current: 47 A (Abusing the mAXImal power, but not advisable in summer)
Power In 600W
Graphics, what is happening up there?
I plugged the e-Logger to my plane, flew as I always do, and at the end of the day I got these field results.
In the graph #1 we will see the performance (current in green and voltage in blue). There are 3 parts remarked, the first is a series of two knife-edges, here we can see the current and the voltage at full power and then at cruise power. Inside the second oval we can see the performance during a vertical climb followed by a similar fall. And the last one is the landing, characterized by a progressive reduction of the power and the final the taxing with the classic throttle touches. According to the measurements the peak takes 45A at 13V.
The second graph shows how much energy was taken in this 5 minutes flight, it was about 1580mAHr, meaning an autonomy of 6~7 minutes plus a little reserve.
The third graph was made using an old pack. It is remarkable that the new packs and the old ones, with more than 50 flights, have practically the same performance. Here there are two zones marked both at full power and tested in land before and after the flight. You can see that the A123 can hold, or even rise, their voltage after a hard discharge, and it means to have the same performance from the beginning until the end, another difference with Lipos.
Ii is worth to say that in this case the pack under this last test is one year old, it was under +65A load few times, and by mistake it was discharged below 2V and overcharged at 4.2V. These are conditions that today most of the Lipo packs can not stand without consequences, and give an idea about the main characteristic of the A123 cells: robustness.
Electrifying flights and clean landings!