15A Home Depot light switch on ebike

[fitek]

I've been using a 15A light switch to connect my batteries up to the controller. It worked for a couple of months but then one day the bike wouldn't go. I tested the switch with a DMM and it's toast.

Current limit on the bike is 28a and I don't ride around at full throttle so I figured 15a might be OK.

I had it hooked up straight to the andersons, no cap and resistor or anything.

Would adding a cap/resistor help the switch survive? Any other suggestions?

 

[Drunkskunk]

do you mean a 15A AC lightswitch?

If thats the case, then ultimatly, no. might make it last longer, though.

AC doesn't burn the electrodes the way DC does, so DC switches are built with heavier contact pads to avoid being burned by a DC arc miliseconds before the gap closes. AC switches don't have the heavy pads, so even though they can withstand 15 amps constant current, they can't handle switching even haslf that much current.

 

[fechter]

If you put a resistor across the switch it will eliminate the arcing.
I'd use about a 1k, 5w or so (not critical). The resistor could allow for some leakage current depending on your controller, so if you plan to park the thing for months at a time, disconnect the battery.

You can also get a 20 amp 'heavy duty' switch from Home Depot.

 

[billvon]

Would adding a cap/resistor help the switch survive? Any other suggestions?

There are two times you will see large/damaging currents in your switch. You'll see a lot of current when you first turn the switch on; this can be on the order of 50-100 amps as the controller caps charge up. It's very short duration so it's not a problem for wiring, but can be hard on switch contacts (smaller conductive area.) You'll also see some arcing if you disconnect the battery under load, since DC does not stop twice a cycle like AC does. Turning off under low load conditions helps here.

Best way around this is a real DC switch. Cheap DC rated switches have larger separations in the open position, faster operating times to get the arc quenched as fast as possible, and heavier contacts. You can get a 20 amp DPDT DC toggle switch for around $5, and you can parallel the contacts for greater current carrying capability. I got a 30 amp 30 volt toggle switch that I've used successfully for ebikes but it's pricier ($40 or so.)

 

[philf]

Yup, DC isn't AC.

I'd built a box that pre-charged the controller caps through a resistor before engaging a relay (intended for AC switching, and rated at 30A/277VAC), thus eliminating the arc on "turn on". It worked flawlessly, until I had a failure which caused the relay to fall out while the controller was drawing a significant current. The contacts in the relay pulled an arc when they opened (or, more like it, "tried" to open) which was sustained until I let off the throttle, rendering the relay contacts instant welded garbage. It all worked perfectly to that point.

Using a proper DC contactor seems overkill, so I'm continuing to play with automotive relays and better circuitry - but I'm treading lightly. AC is easier on relays because it falls through zero volts as the current "alternates". The same reason people survive much higher voltage AC shocks than anyone who has been unfortunate enough to touch a high potential DC source. One feels like a buzz that throws you. The other feels like cooking and apparently locks your muscles, even at similar potential.

I have used small rocker switches (DPST with both poles wired in parallel) on my controllers, which work well, but I know that every time I switch 'em that I've taken a big chunk of their life away. Whichever contact makes first "gets it", until the opposite one starts making first. The switches I got (at a bargain price) are actually cheaper than the Anderson connectors that would otherwise be taking the hit. But I recognized that the switches were being treated as an expendable part in this scheme - hence the pursuit of a pre-charge/relay scheme.

There are simpler solutions, if you don't care about the switch. It's a matter of "feel" for me. I *like* having control of the power at my finger tips, rather than via connectors that are only accessible once I dismount.