GT3000R NiMH Batteries
I have already tested a couple of NiMH batteries (GMVIS, Panasonic, Saft..) but the result was not as positive as required for electric helicopters. Even if the manufacturer labels the cells with "3000" it is not garanteed that the cell has really 3000 mAh. Especially with high discharge currents the capacity is significantly lower.
Since some time I watched the rumors around the Chinese Powers GT3000 NiMH batteries. When I got the first samples I was very interested how they will run in my electric Acrobat-10 (very similar to the well known ECO-8) helicopter.
|Speed controller:||Kontroniks 3SL 40-6-18 with FlyHeli's 5A-BEC|
|Weight without battery:||1300grams (with Telemetry System, but without canopy)|
|number of battery cells:||10|
|Weight of one 10-cell battery pack GT-3000:||abt. 590 grams (including cables and connector)|
|Battery Charger:||Schulze isl 6-430d|
Label: Powers GT-3000R
Manufacturer: somebody in China ?
NiMH cells should be charged always. Empty NiMH cells have only a short life. Therefore NiMH cells are already pre-charged after delivery. Be careful during soldering of the cables, not to do a shortcut ! I discharge the cells before soldering. This has the advantage of a lower pressure inside the cells.
The Schulze Charger has a "normal" and a "sensitive" setting which is receommended for NiMH cells. I don't know why, but my cells do not get full in the "sensitive" setting. Therefore I also use the "normal" setting to charge NiMH cells. Possible the sensitive setting is only useful for smaller low-power NiMH batteries.
To prevent the battery from overheating during the first chargings, I use a digital thermometer to measure the cell temperature. A n NiMH battery is full if the temperatur reached 35 to 40 deg.C. The Schulze charger always switched off when the battery was full.
|The charging current should be abt. 3-4Amperes.
The picture shows the thermometer and a cooling fan. Since I want to change immediately after flying, I have to use this fan. A hot battery should not be charged.
The charger showed that the battery pack consumed between 3050 und 3200 mAh. This is a good value which I have not seen with any other pack until now. Should it really have 3000 mAh ? We will see after the first flights.
It was clearly visible that the battery got better and better with every discharge-charging cycle.
Slow flight with 10 cells:
The first flight was a really slow flight with some hovering and forward flight. No high power aerobatics. This flying style was to test the battery under medium discharge currents.
The flying time is not comparable with a light-weight Eco-8 due to the relatively high weight of my Acrobat-10.
We can see this in the higher hovering current consumption of 17A. A standard Eco-8 would need only abt. 14 A under the same conditions.
My Acrobat-10 needs a minimum voltage of 9 volts. The voltage must be higher than 9 volts in order to have a stable rotor head speed. This voltage was reached at 06:32. At this time the engine needed 272 watts (abt. 30A) but the head speed was still constant. During the whole test flight I had not head speed problems.
NiMH batteries can be used for dischrge currents of up to maximum 30A, therefore it was very interesting how the GT3000 will perform at high currents, which was tested next:
Power flight with 10 cells:
Dear Helicopter Pilots: NiMH batteries are usually not designed for the follwoing power flights with high current consumption. The optimum operating range is not above 20 or maximum 30 A or the battery life will be reduced drastically!
This time I did a real power flight. With a higher rotor speed of1550 rpm I flew all I was able to fly. It was a lot of fun for me, but maybe not for the acrobat. But the poor bird handled all situations very well and did not crash during this test.
The result is shown in this diagram. The current went up over 45 amperes. Even with these high currents the rotor speed was stable during almost the whole flight because the voltage was always over 9V. Only in the last minute the voltage went below 9V.
This result is very surprising since I did not expect this constant voltage from ann NiMH battery !
Flying with 12 cells:
In order to reduce the maximum current but getting the same power, it is possible to use more than 10 cells, i.e. 12 cells. Of course, the engine and the speed controller must be designed for 12 cells. Due to the higher voltage only less current is needed to get the same power. This reduces the current losses in cables and motor and tehrefore the efficiency of the engine is better. Unfortunately the weigth of the additional two cells is abt. 100 grams which makes the Acrobat-10 more heavy.
During a hovering phase at 02:03 the diagram shows a lower current than during the 10cell-tests. With 10 cells and the high rotor speed about 22A were needed while with 12 cells only about 18-19 A were consumed.
The diagram shows also some high power flying phases, i.e. a pirouette with a flip over (needs a lot of power). The engine needed up to 50 A for this maneuver but thanks to the 12 cells the voltage was good enough to keep the rotor speed stable. 12 cells have a lot reserve.
Unfortunately I was not able to do a low-power flight with 12 cells because I have flown into a tree during the last measurement.
Results of the measurements:
The following capacity values show only the capacity used for flying. Since some rest is still in the battery (abt. 10 to 20%) the real capacity of the battery pack is higher.
|Battery Pack and Flying Style||average Battery Voltage||average Power Consumption||Flying Time||Battery Capacity|
|10 cells, slow flight||11 Volts||163 Watts||11,2 Minutes||2800 mAh|
|10 cells, power flight||10,5 Volts||207 Watts||7,8 Minutes||2600 mAh|
|12 cells, power flight||12,3 Volts||231 Watts||8,8 Minutes||2800 mAh|
for comparision: values from previous measurements:
|Battery Pack and Flying Style||average Power Consumption||Battery Capacity|
|141 Watts||2328 mAh|
|GMVIS 3000 (1998) 10 cells||143 Watts||1868 mAh|
|Sanyo RC2400 , 10 cells||149 Watts||1924 mAh|
the Powers GT-3000 are the first cells that I know which are not only labeld with "3000" but really have 3000 mAh. They deliver 3000 mAh not only under laboratory conditions but also in real flight. For me these batteries are the first alternative to the widely used RC2400.
But I must repeat that these NiMH batteries are not designed for continous high discharge currents !
Usual small electric helicopter like the Eco-8 need not more that 30A. This is the ideal usage for these GT-3000 cells. It should be possible to get over 20 minutes flying time when using a 12cell pack of GT3000 and a HB20-12 motor. Who will test it ?