Load Bank Design Joshua Bardwell http://www.fpvknowitall.com
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What s a Load Bank? Definition: Develops an electrical load (most commonly resistive) Applies the load to a power source Converts or dissipates the power output (typically to heat) Uses: Generator test and run-in Braking of electrical motors Discharging Batteries!
Load Bank Considerations Discharge Rate How much current will flow through the load bank? High-end load banks use computers and electrical components to control current precisely Excess current may damage the power supply (battery) Watts Dissipated How hot will each part of the load bank get? The load bank must get rid of the energy somehow If it gets too hot, it will burn itself up West Mountain Radio Computerized Battery Analyzer ($3000+ for 2 kw)
Two Key Formulas I = V R P = V2 R The current (amps) flowing through a resistor is equal to the potential (volts) applied across the resistor, divided by the resistance (ohms) of the resistor. The power (watts) dissipated by a resistor is equal to the potential (volts) across the resistor, squared, divided by the resistance (ohms) of the resistor.
Ohm s Law Has Many Forms I = V R R = V I V = I R
Examples How much current will flow through a 0.5 ohm resistor when it is wired across the terminals of a 4S battery? I = 16.8 volts / 0.5 ohm I = 33.6 amps How much resistance is needed to get 3C of current out of a 1500 mah battery? 3C @ 1500 mah = 1500 ma * 3 = 4.5 amps R = 16.8 volts / 4.5 amps R = 3.7 ohms How much resistance is needed to get 60 amps of current out of a 4S LiPo for test purposes? R = 16.8 volts / 60 amps R = 0.28 ohms How many watts are dissipated by a 0.25 ohm resistor applied across the terminals of a 4S battery? P = 16.8 volts * 16.8 volts / 0.25 ohms P = 1,128 watts!
Series Resistors Total resistance = sum of individual resistance values Explore this circuit at http://tinyurl.com/yaecgmch As resistors are added, total resistance goes up As resistors are added, total current goes down (more resistance = less current per Ohm s law) As resistors are added, total power dissipated goes down (because total current goes down) As resistors are added, power dissipated by each resistor goes down (because total current goes down)
Parallel Resistors Explore this circuit at http://tinyurl.com/yc67dhgx Total resistance: 1 / Rt = 1/ R1 + 1 / R2 + 1/ R3 ) As resistors are added, total resistance goes down As resistors are added, total current goes up (less resistance = more current per Ohm s law) As resistors are added, total power dissipated goes up (because total current goes up) As resistors are added, power dissipated by each resistor stays the same (because total current goes up, but there are more resistors too)
General Guidelines If you want to dissipate MORE POWER, add resistors IN PARALLEL or use lower-value resistors If you are dissipating TOO MUCH POWER, add resistors IN SERIES or use higher-value resistors
Choosing Resistor Values 1. Decide how much current you want to flow 2. Based on the voltage you are working with, calculate the necessary resistance value 3. Calculate power dissipated by that resistor 1. Want to test 4S battery at 60 amps. 2. R = 16.8 volts / 60 amps = 0.28 ohms 3. P = 16.8 volts * 16.8 volts / 0.28 ohms = 1,008 watts
Excess Power? 1. Calculate resistance for the rated power dissipation of your resistor 100 watts = 16.8 volts * 16.8 volts / R 100 watts = 282 volts^2 / R R = 282 volts^2 / 100 watts R = 2.82 ohms 2. Calculate amps for that resistor value I = 16.8 volts / 2.82 ohms I = 6.0 amps At 16.8 volts, a 2.82 ohm resistor will produce 6.0 amps of current, dissipating 100 watts 3. Add resistors in parallel to bring total current flow up to the desired level If one resistor produces 6 amps of current, then we need 10 resistors in parallel to get 60 amps of current Resistors can easily be added or subtracted to produce more or less current Power through each individual resistor remains constant, so no additional calculation is needed Current simply scales up and down as resistors are added or subtracted
Choosing Resistors This type of resistor is typically rated from 100 to 200 watts. It needs to be mounted to a heat sink and have good air flow to achieve its full power rating. It usually costs around $10. But you also have to buy the heat sink. This type of resistor is also rated from 100 to 200 watts. It is designed to work without a heat sink. It still benefits from good air flow. It usually costs closer to $40.
Alternative Load: Halogen Bulbs!
Halogen Bulb Considerations Bulbs produce a LOT of light consider buying aluminum flashing from the hardware store as a shield 12v bulbs typically work fine on 4S voltage, but blow out immediately on 4S HV packs or higher Buy 24v bulbs or wire two 12v bulbs in series if higher voltage is a concern Bulbs are not necessarily costeffective compared to power resistors Approximate amps are difficult to calculate because the resistance of the bulb is not constant At the rated voltage, you know you will dissipate approximately the rated wattage At other voltages must experiment to find out I get about 20 amps @ 4S through 3x 24v bulbs in parallel, which equates to about 6 amps per 24v bulb or per two 12v bulbs in series
Wiring Recommendations 12 gauge wire is okay up to about 60 amps 8 gauge is okay up to about 120 amps Use heavier wire if at all possible! Fine-strand silicone wire is best. Get a big, chunky, knife switch for making/breaking connection If you try plugging in an XT60 or bullet with a 30+ amp load attached, you will probably arc the connector and burn it or even weld it together Solid-core copper is okay too if you aren t going to move anything very much. XT60 is good up to 60 amps or probably a little higher even Above 60 amps consider using bullet connectors and/or ring terminals instead of XT60 6mm bullets are okay up to 120 amps