Tips (1) - Batteries


Handling, dismantling, charging or discharging a battery, are hazardous operations (risk of fire, explosion, etc...) when carried out by unauthorised personnel. Use described here is out of the scope of normal use of this battery and you may apply it to your own needs only if you solely take the responsibility. The author is not responsible in case of accident.

Note : The tests presented hereafter were carried out in 1996 and 1997. Since then, Lithium-Ion batteries have made a lot of progress and some of them are now (in 2003) able to deliver current up to 5 to 6C continuously (Lithium-Ion Polymer batteries). However, safety rules regarding charging and balancing are still valid and more than ever indispensable. I am still using the Nokia charger to charge and balance my Kokam Li-Po 2S1P 640 mAh packs.

Lithium-Ion battery:

At first glance, using a Lithium-Ion battery on an Indoor plane could seem a little bit luxurious. However, a cost analysis in actual situation leads to the opposite as shown hereafter (prices in US$ are approximate):

Ni-Cd battery

Lithium-Ion battery

7 x 50 mAh cells:

Price: 28 $

Nokia BLJ-2 battery:

Price: 98 $


28 g


32 g

Delta-peak Ultra duo plus II charger:

Price: 300 $

Nokia DTK-9 desktop kit charger:

Price:118 $


328 $


216 $

Moreover, you need at least three 7 x 50 mAh packs (total price: 84 $) to achieve 3 flights of 3 minutes each during a one-hour session in your favourite club. As a matter of fact, it is hazardous charging 50 mAh cells, standard charge type, at more than twice their nominal capacity, i.e. 100 mA (and delta-peak chargers operating at such current are not very common). While only one 400 mAh Lithium-Ion allows for 4 flights of 6 minutes each without difficulty!

1/ Exceeding 6 minute flights could overloaded the motor...
2/ Without more information, you should not try to draw more than a 0,5 A mean current (0,4 à 0,9 A) from this battery. Only use it on the XXS Hi-tech fitted with the WES DC 5-2.4 motor.

Technical data:

The nominal voltage of a Lithium-Ion cell is 3,6 V. Therefore, we only need two cells to get a 7,2 V battery. During charging, voltage on each cell must not exceed 4,2 V. While discharging, you must not go below 2,75 V. Hence, the charging equipment must monitor each cell individually and continuously balance charging between the two cells.
In a Nokia BLJ-2 battery, you have in fact two 400 mAh cells connected in series (with middle point output) and an electronic control circuit for charging and decharging each cell.
For Indoor use, the two cells are separated from the electronic circuit and wired with a 3-pins connector. The BEC speed-controller will limit the discharge to 5,5 V (ideally, discharge of each cell should be monitored). For charging, the electronic circuit is put on the desktop charger and connected to the cells by a 3-wire cable (positive terminal, negative terminal and middle point).

Original battery on the desktop charger.

Original battery before and after modification:
On the right: the 2 cells under a shrinking sleeve.
On the left, the electronic circuit of the battery.

Lithium-Ion technology is quite new and there is large disparity between samples. Theoretically, life time is about 1500 operating cycles (500 for Ni-Cd), self discharging rate 5% per year (1% per day for Ni-Cd). In a word, a very promising technology, when settled...
Various capacities are available: 400 mAh, 900 mAh, 1300 mAh, 2300 mAh. An higher capacity should allow for an higher output current but that needs to be confirmed as these batteries are currently designed for mobile phones the current of which is about 0,4 A.
These data are only based on my own experience. If you have further information, please share them within the Indoor community.

Nickel-Metal-Hydrure battery:

Lithium-Ion are still expensive, fragile and not very reliable and NiMH batteries are a good alternative choice. They can be found in standard sizes at low price but only some of them are suitable for Indoor use. Only the first model presented below is able to supply our motors with the right current. Its capacity to mass ratio is good : 34 g for 7 150mAh cells (and 12 $ !!) to be compared to 28 g for 7 NiCd 50 mAh cells or to 44 g for 7 NiCd 120 mAh cells.

Brand: GP
Name: 15F8H
Type: NiMH
Size: 6F22
Metal external case
Voltage: 9 V
Capacity: 150mAh
Cells: 7
Mass: 42 g

When dismantled, this battery weighs only 34 g. Excellent capacity/mass (and price) ratio.A standard charge seems to be needed about every 5 fast charges to re-balance the cells.

Brand: VARTA
Name: AccuPlus Ultra
Type: NiMH
Size: 6F22
Plastic external case
Voltage: 9 V
Capacity: 120 mAh
Cells: 7
Mass: 43 g

When dismantled, this battery weighs only 40 g. Unfortunately, due to a too high internal resistance, it cannot be used for our indoor motors.

Brand: Panasonic
Name: Rechargeable
Type: NiMH
Size: 6F22
Plastic external box
Voltage: 9 V
Capacity: 160 mAh
Cells: 2 (equivalent to 4+3)
Mass: 42 g

When dismantled, this battery weighs only 35 g. Unfortunately, due to a too high internal resistance, it cannot be used for our indoor motors.

More info...

If you want to get more information on batteries and learn more about some strange effects (the "memory effect" is not at all what people think!), you should visit the two following sites:

For safety precaution rules when using NiMH batteries, also visit this page.


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