Li-Po Batteries & LEGO Mindstorm NXT

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Both the LEGO 7.4 Volt Rechargeable Batteries for Mindstorms NXT (Part #x1678 & #bb495), are of Lithium-ion Polymer construction. Lithium-ion polymer batteries, polymer lithium ion, or more commonly lithium polymer batteries (abbreviated Li-poly, Li-Pol, LiPo, LIP, PLI or LiP) are rechargeable batteries (secondary cell batteries). Normally batteries are composed of several identical secondary cells in parallel addition to increase the discharge current capability.

LEGO NXT Li-Po Batteries

Lithium-ion polymer technologically evolved from lithium-ion batteries. The primary difference is that the lithium-salt electrolyte is not held in an organic solvent but in a solid polymer composite such as polyethylene oxide or polyacrylonitrile. The advantages of Li-ion polymer over the lithium-ion design include potentially lower cost of manufacture, adaptability to a wide variety of packaging shapes, reliability, and ruggedness. Lithium-ion polymer batteries started appearing in consumer electronics around 1995.

 

Li-Po-Battery
Lithium-ion Polymer Battery

Cells sold today as polymer batteries are pouch cells. Unlike lithium-ion cylindrical cells, which have a rigid metal case, pouch cells have a flexible, foil-type (polymer laminate) case. In cylindrical cells, the rigid case presses the electrodes and the separator onto each other; whereas in polymer cells this external pressure is not required (or often used) because the electrode sheets and the separator sheets are laminated onto each other.

As you expand your LEGO Mindstorms NXT Kit with add-on Sensors, Multiplexers and Controllers, you often need an external power-supply for them because the NXT on it’s own is not capable of supplying the power requirements. Often the main reason for an external power-supply ‘s due to the current limitations of the NXT Sensor Ports. Listed below is a selection of currently available NXT add-ons that require an external Power Supply:

LEGO Mindstorms NXT Add-ons Requiring External Batteries

Mindsensor Multiplexer for NXT Motors (NXTMMX) 7.2 – 9.0 Volts
Mindsensor Relay Driver for NXT (NXTRelay) 3.0 – 18.0 Volts
Mindsensor 8 Channel Servo Controller for NXT (NXTServo-) 4.5 – 6.0 Volts
Mindsensor RCX Motor Multiplexer for NXT (MTRMX-Nx) 7.2 – 9.0 Volts
HiTechnic Sensor Multiplexer (NSX2020) 7.2 – 9.0 Volts
Dexter Industries NXT Wi-Fi Sensor (dWi-Fi) 6.0 – 9.0 Volts

 

With LEGO Mindstorms NXT Robots, 90% of the time weight is an issue. Conventional batteries like Alkaline, NMHa, & NiCd are very heavy in comparison with Lithium-ion Polymer batteries. Li-Po’s have the advantages of both lower weight and greatly increased run times in comparison and alone are sufficient justification for the purchase price.

dWi-Fi Sensor Battery
My Lithium-ion Polymer Battery modification to suite NXT Wi-Fi Sensor.

Li-poly batteries are in favour with the world of radio-controlled aircraft, as well as radio-controlled cars. With similar requirements to LEGO Mindstorms NXT Robots, this is a great source for campatible batteries. ‘Off-the-shelf‘ Li-Po’s will require the addition of suitable plugs to match your NXT add-ons.

Lipo Battery Connector Cables
Lithium-ion Polymer Battery Extension Cables.

 

I suggest you purchase a few sets of  Lithium-ion Polymer Battery Extension Cables for manufacturing adapter cables to suite your LEGO Mindstorms NXT add-ons. For my Dexter Industries dWi-Fi Sensor, I am using 7.2v 900mAh Li-Po’s with a 2 Pin 0.1 Header with Crimp Pins – 2.54 pitch to suit the on-board D.C. Connector attached. The Li-Po gives me 3 – 4 times the running time compared with a standard Alkiline or NmHi battery, for the same weight.

To mount the Li-Po to my LEGO NXT Robot, I used a Technic #2905 Triangle glued to the battery casing. My choice of Plastic Cement that works very well with LEGO® Parts is made by Revell: Contacta Professional Product Code: REV-39604. For pinpoint accurate gluing you can’t beat it! The extra fine nozzle of this glue product allows for micro-fine gluing of tiny edges of transparent parts. Excellent Plastic welding effect, liquid consistency, quick-drying, extra-fine nozzle, subsequent application also possible.

The 11.1v 2300mAh batteries have a 2.5mm D.C. Plug attached, which is the same size as used to charge the LEGO Mindstorms NXT Rechargeable Batteries. This allows me to plug the battery into the Charging Socket of the NXT to give me extra running time.

Li-Po Batteries
A selection of the Lithium-ion Polymer Batteries I use.
7.2v 900mAh, 11.1v 1500mAh, 11.1v 2300mAh

I use the 11.1v 1500mAh batteries with a 2.5mm D.C. Plug attached also. The difference with this arrangement is the fact I have 3x 3Amp 50Volt rated Diodes in series with the positive lead from the battery, too drop the output voltage at the plug from 11.1 to 9.3 Volts. This makes an ideal power supply for the Mindsensors LEGO Mindstorms NXT: NXT Motor Multiplexer, RCX Motor Multiplexer and Relay Driver.

Charging Lithium-ion Polymer Batteries

LiPoly batteries must be charged carefully. The basic process is to charge at constant current until each cell reaches 4.2 V; the charger must then gradually reduce the charge current while holding the cell voltage at 4.2 V until the charge current has dropped to a small percentage of the initial charge rate, at which point the battery is considered 100% charged. Some manufacturers specify 2%, others 3%, but other values are also possible. The difference in achieved capacity is minute.

Balance charging simply means that the charger monitors the voltage of each cell in a pack and varies the charge on a per-cell basis so that all cells are brought to the same voltage.

Trickle charging is not recommended for lithium batteries. Most manufacturers claim a maximum and minimum voltage of 4.23 and 3.0 volts per cell. Taking any cell outside these limits can reduce the cell’s capacity and ability to deliver full rated current.

Most dedicated lithium polymer chargers use a charge timer for safety; this cuts the charge after a predefined time (typically 90 minutes).

 

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