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 01-21-2017, 04:18 PM #3015 (permalink) MPaulHolmes PaulH     Join Date: Feb 2008 Location: Maricopa, AZ (sort of. Actually outside of town) Posts: 3,832 Michael's Electric Beetle - '71 Volkswagen Superbeetle 500000 Thanks: 1,368 Thanked 1,202 Times in 765 Posts oh yes! precharge resistor/capacitance/time. You need to figure out the RC time constant for your setup. Like this: Let's say you are using 1000uF of bus capacitance, and you are using a precharge resistor that is 500 Ohms (just random values). Also, let's say you are using 400v for your battery bank. Things to consider: 1. The precharge relay will have to be able to make 400v/500Ohm = 0.8 amps. 2. The time you need to wait is 5*R*C seconds. So, 5*500Ohm * 0.001Farad = 2.5 seconds. (0.001 Farad = 1000 microfarad) 3. The energy that your precharge resistor must be rated for is: Energy = 0.5 * C * V * V = 0.5 * 0.001Farad * 400v * 400v = 80 Joules That's a lot of energy for a resistor. You need a pulse handling resistor like the king of surge resistors, the Ohmite pulsEater (I love that name!): http://www.mouser.com/ProductDetail/...0Td1E32eWcg%3d That can handle way more than 80 Joules, so it would be very safe. If a resistor doesn't list its pulse rating in Joules, I would say you should probably not use it. __________________ kits and boards
 The Following 2 Users Say Thank You to MPaulHolmes For This Useful Post: kalidasbala (01-22-2017), motorulf (01-21-2017)