Blowen 100 AMP Fuse

[Seajays]

Yesterday, I blew a 100 amp fuse in my BM2 chair with Dynamic Dx2 controls. While entering my Van the carpet mat got hooked up with the chair tie down bracket as I was pulling into the drivers position. I must have held the forward control to long and the DX2 called for 120 amps,just as it locked in to the Tie Down. Only after getting to my destination , I discovered that me chair was dead. Back to the home shop for repairs.

[Burgerman]

Interestingly it doesent quite work like that.

If you abused the chair properly (!) it can want 120 amps PER MOTOR. But it doesent do this at stall speed. At stall speed the motor may want say 120 amps and theres 2 of them at the battery. Thats 240 amps on the motor cables quite possibly. But the motors may draw the max 120 each the controller allows at say 12v or less. Lets say 6 volts for the sake of clarity. More volts means more amps, the controller will not allow this...

So the motors take 120 amps peak, at say 6 volts each. But that is 1/4 the batteries voltage. So the BATTERY amps are about 4 times lower. Why? Because the total watts is the same. Same POWER used. The controller is behaving like a transformer. So 30 ampsx 2 at the battery. Total battery amps are then at stall speed in this case 60 amps.

As speed increases, the controller uses more volts to keep the motor amps at full 120 amps power. At say 50 percent full speed while under max acceleration, the motors may be drawing 120 amps each at full 24v. So then it draws 240 amps at the battery cables... As you get faster still the volts can no longer increase, so amps drop away. At full speed it may be taking only 6 or 10 amps per motor.

So the fuse you need is a 150 to 200 amp one. Why? Because they dont typically blow until exeeded by a large margin. But 100 is too small. Stock F55s haVe 100 amp fuses. Because they use 80 amp controllers. The max intermittent battery amps is on these is 160 then. So the fuse never blows.

If that makes your brain hurt read it very slowly about a dozen times!

[Seajays]

Bm, The fuse that I blew was in the negative feed that went to the 2 anderson connectors that is joined together with the link connector. To confirm, you say a 150-200 amp fuse?

[Burgerman]

Yes. Your DX2 can pull 240 amps for a few seconds. But usually less. Especially in a van. You need to be at about 2 mph at full power on a ramp or hill to pull 240 amps. A 200 amp fuse will easily cope. Not sure those motor couplings will though. You need to use hard steel. 2.3mm thick. Or stock couplings with the metal sleve added as per some photos I posted here somewhere!

[JoeC]

Digikey number F3325-ND looks good. It will do a 200% overload for a minimum of one second; good luck getting the DX2 to pull 350 amps!

Properly sizing a fuse is fussy, though- size it too large, and you may as well skip the fuse. With heavy wiring, good batteries, good connections, a genuine short out to blow this fuse before insulation melts. That's my best estimate.

[Seajays]

How about one of these in each cable.
http://www.ebay.com/itm/200506469151?ss ... 959wt_1165

[Burgerman]

http://www.ebay.com/itm/10x-200AMP-ANL- ... 2565d2bdad

Use these with some heatshrink. And nut/bolt and crimp terminals. If you test it at full acceleration up a steep ramp and cant blow it its the correct size. Your 240 amp max happens only at full acceleration and about 2 mph.

[Seajays]

BM, the one that I have now is like this. Would be easier to go with the one that I posted , as I do not have a crimper.
http://www.ebay.com/itm/NEW-CAR-STEREO- ... 582wt_1165

[Burgerman]

That grub screw isnt adequate. And the fuse contacts are not adequate. Resistance is bad. + If it gets a tug, and pulls out from the boy racer grub screw, then theres whole batteries capable of thousands of amps connected to loose cables... It just doesent look secure.

The hydraulic crimpers that I and others here have are cheap, and work great. But crimping alone, with andersons or the ring terminals isnt secure enough either as the materials are too soft/thin to be tight enough. I always ad 3 strips of cored solder, and lightly flux the cable bared end before crimping. Then just heat up the crimped end until I see the solder melt/run, and dip in a glass of water. Then add heat shrink with a cigarette lighter. Nothing else is really edequate.

Both for physical strength or resistance reasons. Trust me its a good investment.