My experience with the 40 Amp fuse on my 910R - MVAgusta.net
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post #1 of 10 (permalink) Old 08-26-2017, 10:09 AM Thread Starter
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My experience with the 40 Amp fuse on my 910R

I know this will come in useful to someone one day. I could have used it. What I learned last night: 1) A low battery will cause the 40A fuse to keep blowing until you charge the battery. 2) Don't trust a voltmeter to test a battery. LOAD test it.

I went away on vacation. When I got home and started my bike, it sounded slightly slow cranking, but started right up. It was 2AM. After about 5 miles, the battery light came on. I checked the 40A fuse on the starter solenoid and it was blown. Checked all the other fuses in the fuse box and all were ok. Replaced the 40A with spare. Blew again after a few miles. I had more fuses, kept blowing every couple of miles thereafter. Used up all my fuses. Tried to limp home home, but lights died, then instrument cluster, eventually engine shut off. Rudely made a friend come rescue me. When I got home, I put a volt meter to the battery and it was 12.4V. Unplugged the starter solenoid connector and tested the diode known to fail (multimeter w/ diode test mode) and diode was fine. I wanted to find out exactly what conditions caused the fuse to blow. My ammeter is only safe to 20A. So, I used a thin piece of steel (bobby pin) to make a jumper in place of the fuse so I can feel when it gets hot. Key on, engine off, jumper felt fine. Started engine, jumper got warm, then uncomfortably hot. Revved the engine and the jumper toasted my finger. I decided to load test the battery and it failed horribly. When I let go of the load switch on the tester, it immediately jumped back up to 11.5V. Charged the battery at 5A for an hour. Load tested the battery and it held the load great. Rode for a while, checked fuse ok, rode about 30 miles. No more blown fuse.

I guess the alternator doesn't know to limit its current output. So, it's trying to charge the battery at whatever amperage the battery is willing to take. I have a Lithium Ion battery. I don't know if that makes any difference.
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post #2 of 10 (permalink) Old 08-26-2017, 09:43 PM
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Actually fuse damage is quite normal in low battery situation, much more often during a failed start attempt but this scenario is also possible.

Why during failed start attempt?
Some definitions:
In terms of electromagnetism, one watt (W) is the rate at which work is done when one ampere (A) of current flows through an electrical potential difference of one volt (V).
1W=1V x 1A

Explanation:
So if you have a battery that is empty (more often worn out) then you still have a 12V voltage (or even more on worn battery), but when you turn on the starter voltage suddenly drops to 10V or much lower (more CCA units, less will be a voltage drop).

Let's say that the starter generates 450W of power at startup, and for that it needs to constantly "pull" 37.5 A from a 12 Volt battery.
In case that you do not have 12 volts but only 10 volts, then according to the above equation starter for these 450W needs a little more Amps, more precisely 45A, which will of course cause damage to the 40A fuse.

Conclusion: Use trickle charger and always buy expensive/high quality batteries - they are actually the cheapest
esq'z me and esq'z me like this.
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post #3 of 10 (permalink) Old 08-28-2017, 12:54 AM
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The 40 amp fuse is in the charging circuit NOT the starting circuit......

Trust me, starting current draw is way more than 40 amps

In 1,600,000 mi I've never used a trickle charger......

Buy good batteries and ride your bike



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Last edited by theknurl; 08-28-2017 at 12:57 AM.
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post #4 of 10 (permalink) Old 08-28-2017, 05:19 AM
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Yes, you're right - 40A is in the charging circuit.
I wrote this up during the break in the night shift, maybe I'd be smarter that I take a nap

A trickle charger...... I agree with your advice, when I was riding 20,000+ kilometers per year I really did not need it, but now that's 5,000 km per year and he is really a best friend to my MV
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post #5 of 10 (permalink) Old 08-28-2017, 05:29 AM Thread Starter
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I was kind of thinking the same thing about the 40A fuse, it's only for charging. Don't know about bikes, but starter motors in cars have a cable directly to the battery with no fuse. The thing that goes through a fuse is the starter solenoid. Also, again don't know about bikes, but in cars the alternator's field wire is disabled during starting so the alternator's not creating drag trying produce current while cranking (and maybe for other reasons, too). Maybe they don't worry about these details in bikes. If so, I suppose the alternator could be loaded enough to pop the fuse during cranking while the battery is low, even though the fuse isn't directly on the starter motor's circuit.

I have 8 or 9 things I SHOULD have connected to trickle chargers, but I'm too lazy. I ride the bike often enough to where I shouldn't need to keep it charged. I was only gone for a week. I think this all happened because of the $18 alarm I hooked up last month. I should test the parasitic draw. But it doesn't matter anyway. It's not like it's killing my battery every night.

Some other stuff happened that night that really sucks. When the lights died, I rode on the shoulder so nobody would run me over. I ran over some things. One thing poked a hole in my new rear tire. Something else flung upward and cracked my carbon fiber tire hugger. And when I took the seat off, I bumped the bottom against the fuel tank and chipped the paint. The moral of the story is you may have saved a bunch of money on that motorcycle alarm, but it will leave you stranded, pop your tire, scratch your paint and make you lose a friend when you call him at 3AM to rescue you.

Hey Noel! Thanks for sending me that injector last year. I don't remember if I told you... Unfortunately, I had no luck repairing it. I ended up buying a used one from eBay in Germany for $100.
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post #6 of 10 (permalink) Old 08-28-2017, 09:58 PM
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Don't forget why circuits are protected in the first place. If the charging circuit fail, it could short the battery - poof, up in flame. If the starter fail, there is the relay, and the operator could also intervene.

Who moved my cheese?

MY04 Brutale 750S
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post #7 of 10 (permalink) Old 08-29-2017, 03:51 AM
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Quote:
Originally Posted by Paul.RP View Post
Don't forget why circuits are protected in the first place. If the charging circuit fail, it could short the battery - poof, up in flame. If the starter fail, there is the relay, and the operator could also intervene.
Sorry, Paul, you're absolutely wrong...... they're separate circuits

Connect a 100 amp fuse across the battery terminals.....
What happens?
Fuse pops, battery is fine.....
I've been using a SnapOn YA 271 for 40 years......
Push the load.test button....
125 amp short..... It's how you load test a battery.... won't hurt it at all

Welding with.batteries....


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post #8 of 10 (permalink) Old 08-29-2017, 04:02 AM
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Quote:
Originally Posted by theknurl View Post
Sorry, Paul, you're absolutely wrong...... they're separate circuits
Noel, you've misunderstood what I've wrote, or I haven't made myself clear...I'm saying the same thing, ie the reason one (the charging circuit) is protected by a fuse is ....per below

Quote:
Connect a 100 amp fuse across the battery terminals.....
What happens?
Fuse pops, battery is fine.....
whilst the other, the starter circuit has a relay (open when not in use), AND the attention of the operator (when in use).

If the charging circuit is not protect by a fuse, and it got shorted, the result could be as per your vid, bike up in flame.

Who moved my cheese?

MY04 Brutale 750S

Last edited by Paul.RP; 08-29-2017 at 04:07 AM.
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post #9 of 10 (permalink) Old 08-29-2017, 06:19 PM
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Quote:
Originally Posted by Paul.RP View Post
Don't forget why circuits are protected in the first place. If the charging circuit fail, it could short the battery - poof, up in flame. If the starter fail, there is the relay, and the operator could also intervene.
Paul;
Charging circuits fail 2 ways
Either open nothing happens
Or shorted to ground which pops the 40amp fuse

Nothing happens to the battery

If you hook the battery up backwards you pop the 40 amp fuse protecting the alternator

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Last edited by theknurl; 08-29-2017 at 06:22 PM.
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post #10 of 10 (permalink) Old 08-30-2017, 01:00 AM
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Don't know how he got a hold of some nuke subs batteries, but looks like fun


Who moved my cheese?

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