Fuse Size

[RobbyRob]

OK so here is a drawing of my current power set up.

I am looking at wanting to place a fuse on both charging leads and one in between the batteries and the controller. Probably between the controller and first Anderson Connection. I am a newb when it comes to electrical any idea the size of fuses I will need for these?

[RobbyRob]

I am also having a hard time finding a simple 48v on/off key switch if anyone knows of any please do tell. Gary's have a lock on them which I do not need

[Morgen 3Eman]

If I were doing this, I would put a fuse on the output of each battery, and one on the input to the controller.  But remember, I'm a very cautious old fool.    As for amp rating, your controller should determine that.  My MP3 internal controller only needed a 30 Amp fuse, while my external controller needs a 50 amp fuse. 

TTFN,
Dennis

[RobbyRob]

After much searching I think I have found a key switch that will  do the trick. It is from a 48v Yamaha Golf Cart part number JN8-82510-02-00. I will be contacting my local Yamaha dealer to get a price for the part on Monday. I will keep you posted. If anyone sees a problem with this switch not working please let me know.

[RobbyRob]

Ok I have noticed there have been several similar posts for an on/off key switch for the Batteries to the Magic Controller. Sorry about that. One thing I do not understand is why GM hasnt incorporated a simple on/off switch for power shut off to the controller. We do not all have the aluminum case batteries =) Anyhow the switch I previously showed works wonderfully wired in line on pos+

I do however still need help sorting out the fuse situation with my battery outputs and my controller inputs. Just need the correct Amp Fuse. I think 60Amp will work but I am unsure still. Any help.

[truly_bent]

The keyswitch you have there is suitable for switching the controller's motherboard supply - breaking the line to terminal S1. It is not suitable for breaking the connection between battery and controller. For that you'd need something that could carry a substantially greater load, like the one offered on Gary's site.

I would use a switch, like the one you've shown, to break power to the controller board (S1 again) and a separate fuse for each of the battery's positive (+) leads, before the wye junction into the controller. You want to protect each battery from the possibility of a dead short.

I disagree with Dennis on putting a third fuse on the input to the controller though. I don't see how that would serve any useful purpose.

If you wanted to protect the controller, you'd fuse the output side, between the controller and the motor. This would protect the controller's output stage (FETs) in the event of a dead short at the motor. Unfortunately, this would require a (fast) fuse for each of the three windings.

I only have one battery with a high-amp automotive fuse into the controller (rated to my battery's max output spec). I haven't fused the output side of the controller, nor have I noticed anyone else mentioning doing so either, but that's how we protect motor drives in industry.

[RobbyRob]

Thank you very much. The switch listed there is rated for 48v off a golf cart. I would think it would be suitable would it not? Anyhow I better just be safe and use Gary's. Looks like 60 Amp fuses will work.

[truly_bent]

The point is that the switch you've shown is suitable for breaking the "control power" to the controller's circuit board (terminal S1) only, not the "battery power" line going into the controller.

The leads from your Yamaha switch appear to be about 20AWG, while the leads coming off your battery are probably 10AWG. Big difference in amp carrying capacity. The GM switch is intended to break the power line, and as such, has substantially beefier contact points.

If your controller is dead (as in turned Off), then it isn't drawing current and and breaking the battery line is no problem. If however, your controller is live (as in turned On), it may be drawing some current from the battery and breaking the line then will create an arc, which pits the switch contacts. Do that often enough and it'll eventually destroy the switch contacts.

[RobbyRob]

Thanks completely understand now.  You guys are awesome!!

[Morgen 3Eman]

Why three fuses?

It depends on what you want to protect from over current.  For the sake of argument, let's assume the controller draws 30 Amps at full power, and we use a 30 amp normal blow fuse.

The fuse to the controller is to protect the entire system from a controller caused overcurrent event.  If the motor winding gets dead shorted, the controller will try top draw 100?150? or so Amps, and the fuse blows in less than a second.  The FETs can probably stand that mistreatment for a very short time.   If the short is less well made, maybe the controller tries to draw 50-60 amps, and the fuse blows in 10-20-30 seconds or so.  The battery fuses don't get blown.  So why have them?

The fuses on the batteries is to protect the batteries from a battery fault.  You and I would never do this, but let's imagine that "Alan" meant to charge both batteries, but failed to turn on one of the chargers.   When he connects the batteries, the first one to be connected will charge the caps in the controller to some level depending on which battery he connects first.  No big deal.   But when he connects the second battery, we now have a set of cells  at 57 Volts supplying charging current to a battery that is at 40 some volts.  Guess how big the current pulse will be! We effectively have a short across a 17 Volt battery.  We can also guess that the 50? 100?150? Amp charging current will not doing anything good to either battery, but large reverse currents are generally bad for most cells. 

If we just had fuses for each battery, and none for the controller, in the case of a motor fault, we would have to blow both of the battery fuses before power was removed to the controller.     In  dead short, they would blow pretty quick, maybe not as fast as a single, but still pretty quick.  Probably  the 200?300? amp pulse will damage the controller FETs.   But if we got a not so good short, drawing 50-60 Amps, neither fuse would be drawing enough current to blow.  Is a fire going to improve the short enough to blow the fuses?  I don't know.

What I do know is that I paid $2.90 for 5 fuses yesterday, and it is cheap protection for a  kilobucks worth of battery , motor, controller and work.

Regarding fusing only the controller outputs , I would point out that if the controller itself fails, you have no protection.  My personal experience is with two MP3  internal controllers.  One of them developed an internal short, and the other had the motor windings short.  The fuse on the input side blew both times. 

Please feel free to ignore any and all of the above, and do what you think is right for you.


TTFN,
Dennis

[RobbyRob]

Wow information overload the last couple days.

I need to see what my controller draws for amps at full power. Does anyone know what the Magic Controllers draw for Amps? I cant find the specs anywhere.

Based on what I am hear overkill would be to do both sides of the controller and both positive lines on the battery?

I will probably just do the battery fuses at 60 Amps and the controller input (after I find out how much it draws) based on the last post. I dunno I am so new to this, I would rather over do than under

[Bikemad]

I disagree with Dennis on putting a third fuse on the input to the controller though. I don't see how that would serve any useful purpose.

With a single battery it probably wouldn't be necessary, but as Rob is using two batteries wired in parallel, a short within the controller or motor would only blow the fuses if the current draw was approximately three times greater than the actual fuse rating.

any idea the size of fuses I will need for these?

The GM BAC-0282P controller is stated as being 48V/50A, but in reality it is not likely to draw more than 30Amps, so I would recommend using 3 x 30 Amp fuses as per Dennis's suggestion.

A 30Amp fuse on each of the batteries would be small enough to prevent any damage to the batteries/BMS in the event of a short circuit on the battery power lead and would easily deliver a combined constant current of 60Amps until the batteries were nearly completely exhausted with no chance of blowing the fuses, whereas a 30Amp fuse on the controller feed should blow if the current exceeded 45Amps.

A 30Amp fuse on the controller's battery supply lead should protect the circuit board from being roasted if two of the FETs failed in a shorted state:

I am also having a hard time finding a simple 48v on/off key switch if anyone knows of any please do tell.

If you want a key operated switch to carry the full controller current I would suggest fitting a battery isolator switch:



These are usually available from car accessory shops and are typically rated 100Amps continuous and 300Amp peak.

Alternatively, as you have a 12V battery for your lighting etc. you could use this to power the coils of a 12V 40A relay using a low current key switch, and then connect the switch contacts to your controller battery supply lead.

Alan
 

[RobbyRob]

As usual Allan you came through for me

So a 30Amp Fuse on each of the battery Pos+ terminal leads, and 3 30Amp fuses on the output to the motor? (5 Total)

Or 1 between the supply to the controller and 1 on each battery? (3 Total)

[truly_bent]

A third fuse on the input, eh? I have to ponder some on this.

I've always worked on the premise that fuses protect components "upstream". In the case of one battery not having been charged when they're put back in parallel, that potential difference between battery voltage levels would (should) blow the fuse protecting the fully charged battery. This agrees with the premise.

Remembering that I am a proponent of fusing the motor windings (although I haven't done it on my own setup); that would protect the controller in case of a motor winding short.

That only leaves the case of the controller's FET output stage shorting. Nothing else on the controller board is likely to short and draw as much current (the board's control section should be optically isolated from the power stage anyway). The board's power supply wire would probably vaporize before the battery fuses even noticed.

If the FET output stage shorted, it would be a race as to what burned first - the fuses protecting the batteries or the board's copper traces and/or FETs. If each battery were protected to 2C discharge, and using fast (rectifier) fuses, I would expect them to blow first. However, not before feeding 120A (2 x Max Discharge Current: 60A) into the FET traces.

If you want to protect your FET output stage from that 120A surge (although something in the output stage must have already caused a short), then I can see the validity of using a third fuse on the input. Not that you'd be protecting anything that wasn't already protected, but you would let less smoke outta the wires, because the 30A fuse would blow first.

I'm still undecided, but, if you have the space, it's not a bad idea.

[Bikemad]

So a 30Amp Fuse on each of the battery Pos+ terminal leads, and 3 30Amp fuses on the output to the motor? (5 Total)

Or 1 between the supply to the controller and 1 on each battery? (3 Total)

I would fit a 30Amp fuse on each of the battery positive leads as close to the battery as possible, and another 30Amp fuse on the positive feed wire connected to the controller's battery input.

I would have thought that a single fuse on the input side of the controller would be sufficient to protect the controller, but if you want to put three more fuses on the motor phase wires (as Jeff suggests) they should ideally be placed at the controller end of the wires, just in case the phase wires chaff through and short together between the controller and the motor.

Alan