21700 Li-Ion Battery Pack (35A, 48V, 1680 Watts)

This is my original design creation of a battery pack that I personally created out of 21700 5Ah or 5,000mAh LG cells. The array is (7) parallel cells equaling 35 Amps @ 3.7V (nominal) and is in series with (13) other parallel cells yielding a nominal 48 Volts. The 21700 cells have up to 40% more capacity than the common (hidden) cells ! This pack can be either created as (7 x 13) for 35A @ 48V or as (13 x 7) for 65A @ 25.9V, but all horizontal bus bars will need to be resized and repositioned appropriately. All bus bars are made from standard 10mm wide copper stock and can be sized for either 1.0mm, 1.5mm, or 2.0mm thickness. A 1.0mm thick x 10mm wide bus bar is equal to (12.73) 18 AWG solid copper wires. An 18 AWG solid copper wire has a resistance of 6.4 milliohms, so 6.4 / (hidden).(hidden) = 0.(hidden) milliohms. This is critical since P = I^2 * R and P = (35 amps)^2 * 0.0005 ohms = 0.6158 Watts dissipated in each foot of bus bar. Since there are (13) top to bottom bars and (2) end bars that are each about 9 inches, the total bus bar length is 15 x 9" = (hidden)" / (hidden)" = (hidden).(hidden) ft * 0.6158 Watts = 6.93 Watts (and this is with no contact to contact resistance - the reason usually for welding). However, if only 7 amps are drawn then the power dissipated in the bus bars becomes more manageable at P = (7 amps)^2 * 0.0005 ohms = 0.0245 Watts * 11.25 ft = 0.2756 Watts !! Bear in mind that there are 0.28 watts lost during a 7 amp draw and must be accounted for in your power budget as a parasitic power loss. At 7 amps * 48V = 336 watts, 0.28 watts from the 1mm x 10mm bus bar so you will be losing 0.08% power not so much of a problem. :-b All bus bars use PEM clinch fasteners with part number SL-M3-2ZI for metric M3 screws that are attached to 65mm long standoffs that can be in either round aluminum or round plastic. All batteries are connected thru upper positive (+) PCBs that monitor current and will limit over-current and/or over-temperature conditions for each battery as well as providing an *individual* cell charger. All negative side (-) PCBs also limit over-current and/or over-temperature conditions for each battery as an added measure of protection and to ensure electrode balancing. Circuit design is left for the creativity of the pack designer. The 'sweet spot' for the LG 5Ah 21700 battery is with a current draw of 1 amp, where in the capacity of the battery is really 5,100 Ah (see my LG model for energy dissipation curves). So since there are (7) batteries in parallel and if each draws about 1 amp, a very good usage model is 7 x 1 amp = 7 amps @ 48 volts. This will not cause any excessive internal heating and also prolong the life of the battery. It is also recommended that charging be at the same rate of 1 amp per hour to recharge the battery in about 5 hours. Herein again, fast charging will cause excessive internal heating and reduce battery life. Both discharging and charging at about 1 amp will also reduce any risk of thermal runaway, reducing a fire risk. This equates to a power delivery of 7 amps x 50 volts = 350 watts. Obviously more can be drawn, but will cause other risks. At 10 amps (each delivering 1.43 amps) x 50 volts = 500 watts which may be necessary for some applications, but again a preferably 1 amp recharge rate is better. Overall dimensions are 330mm long x 180mm wide x 83mm tall (13.0" L x 7.1" W x 3.3" H) Also this pack can be stacked with two other packs (total of three) to yield 105 Amps with clearance on the sides for vertical bus bars. Each additional pack is rotated 180 degrees to minimize vertical bus bar length.

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  • Steven Minichiello
  • xoo
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