Homemade Li-Poly 3S5P Battery Pack

This web page documents how I made a 3S5P (S=Serial, P=Parallel) Lithium Polymer (Li-Poly) battery pack for my ECO 8 helicopters. I purchased 15 1500 mah (milliamp hours) from ALL_XTREMEHOBBIS, the cost was $12.00 USD each. I read about the offer on the Ikarus ECO 8/16 forum. 

The first thing I did was to charge each cell using my Great Planes Triton battery charger. I then discharged each cell to see how will they perform. Each cell performed about the same with an average discharge rate of 1521 mah. I them recharged each cell to make sure the recharge rate was about the same as the discharge rate. All of the cells passed this test. Originally I was planning to build a 3S4P pack and to keep three cells as spares. Since all of the cells performed so well I decided to use all of the cells in the pack. Here's a schematic showing how the cells are wired and the location of the new wires used to monitor/charge the individual paralleled cell packs. 

To assemble each paralleled cell pack I used some thin Scotch permanent double sided tape to hold each cell together. I then bent each of the output tabs upward and then soldered them together, See photo below. Note: You can click on any image below to see a higher resolution image.

To separate the cell packs I used fiberglass breadboard material. I used double sided foam tape between each pack to hold the separators and battery packs together.

The following two images shows the top two cell packs. The orange and blue wires are used to monitor and charge the individual cell packs.

The image below shows the complete battery pack after applying the heat shrink wrap tubing. I used the 3.0" yellow tubing available on this page: http://www.dynamoelectrics.com/SearchResult.aspx?CategoryID=23

The battery pack ended up being rather heavy and long. The weight is around 615 grams! That's pretty heavy for a 7600mah Li-Ploy battery pack. I have a Thunder Power TP-8000 pack that weighs 473 grams. Both have about the same discharge capacity. That's a difference of 145 grams. The pack is 2 inches (50.8 mm) wide and just fits within the ECO 8 battery holders. The pack is about 1.1 inches (30 mm) deep and 10 inches (254 mm) long. At 10 inches the pack occupies the entire undercarriage of the ECO 8. See photo below.

After completing the pack I used my Schutze 6-330d (version 8.06) battery charger to top-off the pack. This charger charges Li-Poly cells to a higher voltage level then the Triton charger. I'm not sure if that's good or bad. The Triton charger charges three cell Li-Poly packs to 12.605 VDC.  The Schutze charger charges them to over 12.720 VDC. When I topped off the new pack it was able to add 325 mah. The terminal voltage after the charge was 12.680 VDC. I waited a few hours before making this measurement to allow that pack to stabilize. Since I have extra wires to monitor each individual cell pack I also measured their voltage. They were within 9 millivolts from each other. 

I then used the Triton charger to discharge of the pack. With three Li-Poly series cells, the maximum discharge current of the Triton was between 1.6 and 1.8 amps. The minimum discharge voltage was set to 9 VDC. The battery pack produced 8200 mah at the end of the test. I then used the Triton charger to recharge the pack. It was able to put in 7802 mah. Had I used the Schutze charger it would have put in another 300 to 400 mah. 

The next day I did a test flight with my ECO #2 helicopter. This ECO 8 is set up to run on 3 cell Li-Poly packs. It has a Ikarus H8 brushless motor, a Castle Creation's Phoenix 45 ESC (Electronic Speed Controller) controller and a 18 tooth pinion. I was able to hover in the backyard for around 29 minutes. At 16 minutes I landed and checked the battery voltage and temperature and the temperature of the motor. Both where only a little warm and the individual voltage of each cell pack was only a few millivolts from each other. I then hovered for another 13 minutes. At that time my battery voltage monitor LEDs turned on indicating that the battery voltage had reached 9.9 VDC. After the flight the battery and motor temperatures where only slightly warm to the touch. When I recharged the battery I got 7777 mah back into the pack. After the charge each cell had the following voltage levels: 4.219, 4.213 and 4.225.

During the test flight and do to the extra weight, the helicopter flew a little sluggishly. Other then that it flew just fine. The extra weight should help stabilize the helicopter when flying on windy days do to the extra mass. I'll update this web page after I get some more flights on the new battery pack.