Glyn wrote: ↑
Feb 11, 2018 2:25 pm
I understand the precharge resistor is needed to fill the capacitors, otherwise the current draw is too large when you connect the battery which can weld the contactor. My first question is, do I need a contactor or can I use just a bog standard switch, and, if I just go with a switch, will it still need the precharge resistor?
Since you'll have six controllers' worth of caps to charge, there'll be six times the amount of current flowing at pack connection time, so a precharge is a good idea whether you use a contactor or not.
If you do use a precharge, I recommend having it switchable, so if you turn the whole system off it also turns off the precharge. Otherwise the controller is still connected to the battery, and if you leave it sitting for a few weeks or months (dpeneding on controller idle current), it can drain and kill the battery (unless the battery has a BMS to shut it off when low to prevent that, but then the BMS itself may kill the pack (at least some cells) if left in that low state for too long).
Also, it means the contrller is still active, and if someone accidentally hits the throttle it could do something (even if there isnt' sufficien current thru the precharge resistor to do much), which could surprise someone at the least.
That said, I don't use a precharge, on either SB Cruiser or CrazyBike2, with two controllers and a 14s NMC pack (58v full). But this does cause contact damage over time; if I hot-plug my battery (ANderson SB50 connectors) I can see/hear the pop/arc, and it leaves a little damage on teh tips every time. (the Anderson cotnacts are designed iwth this in mind, so ti doesn't affect their connnection, but on a switch it can and eventually does, unless the swtich is like the Andersons, with sacrificial tips and wiping contact surfaces).
On SBC, I'm using a common battery-disconnect as my switch. (one of those with a red T-handle you insert and twist 90 degrees to turn on, twist back to turn off and you can pull the "key" out, as theft protection or safety, etc). Been working a couple years like that, I guess it's been. Maybe less, cant' remember when I installed it.
On both SBC and CB2, I also have a circuit breaker I can flip as a switch, and on CrazyBIke2 that *is* my switch. It's survived for years without issue, and still works as a breaker, but there's never a guarantee that will continue.
I also don't use a contactor, but that's mostly because I don't have one I can repurpose.