Transcript
Technical Aspects of the P2Quadcharger
Lithium Battery Technology
The type of battery used in the DJI Phantom 2 UAV “Intelligent Flight Battery” is a version of a rechargeable lithiumion battery called a lithiumion polymer battery, commonly referred to as “LiPo”. It is one of numerous battery chemistries utilizing electrolytes of lithium compounds. The polymer reference is to the lightweight plastic packaging; the chemistry is the same liquid form as regular lithiumion batteries used in many products such as toys, tools, computers and mobile phones.
Rechargeable lithium batteries work on the principal of intercalation and deintercalation of lithium ions from a positive electrode material and a negative electrode material. The liquid electrolyte provides a conductive medium. The electrodes are separated from each other physically and electrically by a separator material that is porous only to the exchange of ions.
Voltage present at the external terminals depends on the specific chemistry of the cells. A normal LiPo battery measures about 3.0 volts when discharged and about 4.2 volts when fully charged. The DJI battery has three internal cells so the terminal voltage is indicated as a nominal 11.1 VDC which falls between the discharged and charged values. A fully charged Phantom 2 battery will measure about 12.6 VDC which is 3 x 4.2VDC.
Lithium Battery Charging
Charging LiPo batteries is not a trivial task. They need to be charged with a very specific charge profile to allow for maximum life and to prevent damage and even dangerous consequences. They must also be protected from overcharging and overdischarging.
The DJI batteries, as promised by the ‘Intelligent’ part of their name, will protect
themselves from being overdischarged or overcharged but they don’t protect themselves from being charged at an excessive rate. These batteries are rated at 5200mAH (5.2AH) and a maximum 35C discharge rate. This equates to a maximum discharge rate of 182A, but the charge rate is a different story. The DJI charger charges at less than 1C (4.2A) and many LiPo manufacturers recommend no more than 1C.
The P2 Quad Charger (P2QC) will charge 4 batteries at least as quickly as the DJI charger will charge 1 battery. Our data suggests that it will charge faster than the DJI charger because of the sophistication of our design. We follow the industry recommended charging profile for LiPo batteries which includes three phases: ● Preconditioning ● Constant Current ● Constant Voltage
The Preconditioning phase is mostly to detect and properly handle a LiPo battery that has been overdischarged. If the voltage is below 3V it starts the charge at 1/10 C until the voltage reaches a preset value at which point the charge ramps up to the maximum charge rate. This is probably not going to occur with a DJI Phantom 2 battery but adds another level of safety.
The constant current phase assures that the battery is charged at the maximum safe rate. Some users charge LiPo batteries at a higher (2C to 4C) rate but this should only be done very cautiously with constant observation. LiPo batteries have been known to bulge or explode and catch on fire if charged at an excessive rate. The P2QC charges at a rapid but safe rate, but lithium batteries should always be monitored during the charge cycle. The intrinsic temperature monitoring of the P2QC adds even more safety to the charging process. No lithium battery charger can claim to be completely safe under all conditions and damaged batteries should be safely discarded..
Once the terminal voltage reaches a specific value the P2QC switches to the constant voltage phase and the charge current tappers off to safely top off the battery for maximum charge.
The DJI battery will normally shut off when the charge reaches 98% or 99%. The P2QC will terminate the charge if the battery fails to selfterminate. The P2QC also provides an internal safety timer that will stop the charge when a maximum time has elapsed (about 4 times the normal charge time) which would indicate that there is a problem with the battery.
Figure 1. P2QC Charging Profile
The P2QC also has individual temperature sensing elements in each port that will modify the charge rate if overheating is detected. In addition, many battery manufacturers recommend not charging Lithium batteries if the ambient temperature is below a certain temperature. The number varies with each manufacturer but anything below 32°F (0°C) is probably a good average. The P2QC will disable charging below about 40°F (4.5°C).
Figure 1 shows the intelligent charging profile of the P2QC. Measured data taken on the DJI charger suggest that it uses a much simpler charge profile. This appears to be a constant voltage charge only. Future data will display the DJI Charger charge profile from empirical measurements. The primary difference is the charge rate. We deliver more power faster which is evidenced by the demo video results. For all of the math folks out there, the integrated area under the power curve is greater.
P2 Quadcharger Design
Our design utilizes stateoftheart surfacemount components and design methods. There are four independent, intelligent LiPo battery charging and management ICs in the base unit. These circuits monitor and analyze numerous conditions thousands of time per second. With computer precision they watch the supply voltage and current, the battery voltage and current and the temperature of each battery slot. We decided upon an external DC voltage source for maximum flexibility. A user can choose their own power supply (as long as it meets our recommendations) or purchase an approved power supply from us at a discounted price. The use of an inadequate power supply will yield some less than satisfactory results. Even if the supply says that it is 12V at 17A or more, 12.0V is not enough (see Vehicle Charging below). The P2QC requires at least 12.8VDC and really works best between 13.6VDC and 20VDC. The wires also need to be at least 14AWG (gauge) copper.
Mobile Charging
The P2QC comes with a vehicle power cord in the box. There are some serious considerations when charging in a car or truck. Prior to 2000 many vehicles had real cigarette lighters in them. Consequently most of them had fairly robust wiring and fuses to support the highcurrent required for the heating element in the lighter. Many had fuses rated at more than 15A. New cars and trucks typically have "Accessory Plugs" or some other fancy terminology that really means "Cell Phone Charger". The power required for this task is much less and many vehicles have 5A or lower rated fuses. The worst problem is the size of the wires going to the accessory plug; often 16 or 18 gauge wire that goes through multiple connectors and a relay before it gets to the receptacle. In addition to this, the venerable cigarette lighter receptacle and plug are horrible connectors for delivering reliable power to a device like the P2QC which requires in excess of 17A to charge four depleted batteries. We have purchased numerous cigarette lighter plugs that the
manufacturer rates as 15A or more only to find that the spring inside heats up considerably with any more than a 5A load. Heating the wire or plug robs power from the charger.
The P2QC needs at least 13.6VDC and 17A to operate properly. We could have put a voltage booster in the base or as an addon accessory, but this would only exacerbate the problem by drawing a lot more current. The DJI car charger is an example of this. The output is rated at 4A and the input is 8A, evidenced by how hot it gets during a charge. The P2QC will charge batteries in a vehicle if it meets the minimum requirements that we have listed in a table we will provide with the base unit and on our website. The charger will behave differently in a car or truck than with an AC line power supply. As the batteries charge the intelligent controller throttles the charge current and voltage to protect the vehicle supply and the batteries. The vehicle should also be running while chargingyou don't want to be left sitting with a dead battery! For someone requiring the best in vehicle charging I will publish an article on how to (or have a qualified car electrician) connect the proper size wire directly to the vehicle battery and an adequate highcurrent connector. We will have the wire and connectors available on our website soon.
Next I will talk about balancing cells, active discharging and what's next for us.
