I need to slow-charge a bank of capacitors that power a load which draws up to 150A at 12V, because I don't want to draw a massive amount of current when the battery is connected to the ~20000uF cap bank.
I thought I could do it by having a relay switch a high value resistor in series with the cap bank for a few seconds, then bypass this resistor for normal operation. However, such a relay would have to pass 150A in the normal operation mode. Many relays can't do this, because they are rated based on their interrupt current and not operational current. My application will only require 150A when a battery is plugged in, and when disconnected the relay will continue to be energised by a support circuit until the 150A load has shut down and draws nominal current. This will occur several seconds after the battery disconnect, so the actual interrupt current will be small, in the milliamp range. The 150A load is electronically controlled, and can be shut down very quickly, almost instantaneously. So, no arcing should occur on the relay contacts, which is what I believe gives them their normal current rating?
Are there any relays specifically optimised for this job?
Best Answer
You want a contactor, which is a heavy duty relay specially constructed for high currents.
Digikey lists more than a dozen of them capable of >150A (they're all either 500A or 600A devices, so the give you a lot of headroom).