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[2020_SRS_Commuter]

I added lights to my 2020SRS, they are claimed 20w ea. I remember measuring them at about 1.5A each so that makes sense. When they are on, therefore, close to 3A load on accessory.

I then replaced the stock horn with two other horns. FIAMM 72112 FREEWAY BLASTER LOW, and FIAMM 72102 FREEWAY BLASTER HIGH.
I did not do a current measurement for these but they are specified to be 6amp max draw each. So, worst case, 12A.

Summing yields 15A.

There is construction on the highway I use at night. As nuts as this sounds, twice now, construction vehicles have pulled into the highway at night right into traffic to go wherever, entering the roadway perpendicularly directly crossing into the path of traffic going 80mph, and I've had to slam on the brakes and wound up within a few feet of them at a dead stop. I have the high beams on AND the extra lighting. Its very bright. No excuse for their carelessness.

So anyway, last night it happened for the second time and I lay on the horns, then they stop blowing and the extra lights go out. At home I unpack, get everyone settled and head out to do a bit of recon to see what happened, and now they work again.

So today I did an experiment. With a voltmeter across the 12v battery I read 13.3v with low and high beams on. Turning on the extra lights drops its to around 13.25v. Laying on the horns draws it down further. Its impossible to say with precision if its a specific voltage because of the reaction time of the meter, but it appears that at 13.0v the lights and horn go out, and accessory power is somehow deactivated. From that point on, its out. If I key the bike off/on again. Its back.

Any ideas? My guess is either my 12v battery is weak, or its insufficient for what Im asking, or there is some other malfunction. But my instinct is I've just overloaded the thing. I wonder if I can replace the stock battery with a significantly larger one in the luggage or something and run some cables to where the old battery is kept.

Thing is, I really really want to keep that lighting AND the horns. If my extra lights arent on, the horns dont cause the power to drop out. And if they are on, as long as I am brief with the horns its ok too. But holding the horns down for a full second with the lights on kills it.

Edit: I guess one option is to have a separate battery just for the horns, and connect that battery only to their relay, and charge it manually from time to time.

[fencingdude101]

Funny, I've been dealing with a similar thing recently on my '20 SR/S, but with heated gear.
If I max out the heated grips, heated seat, gloves, and jacket liner, power to all of these cuts off until I restart the bike. No dash warning. It was right on the edge, as I didn't have the issue when slightly lowering the jacket power.
It has been getting worse over the past few cold days (lower power levels were tripping things), and it turns out my heated jacket has a short in it that I'm doing a warranty claim on. Hopefully the new jacket pulls less amps without the short and I can stay warm this winter!

Anyway.

There is a 10A fuse under the seat for the 12v Accessory circuit, so if more than 10A was pulled, I initially expected the fuse to blow, and to not get 12v Accessory power until a new fuse is put in. Since a bike restart resets power delivery, there's something else going on.

There is a comment in the unofficial zero manual that may explain things: https://zeromanual.com/wiki/12V_Fuse_Block

Gen3 models have a fuse block but the protection has redundant upstream components in the MBB, so the fuse block just helps with isolation.

It sounds like the FST platform has more than just fuses protecting the 12v wiring.

Either way, I think you're pulling more than the allowed 10A, and the bike is being nice and not forcing you to replace a fuse after doing so.
I've seen forum posts of people putting bigger fuses in for the Accessory circuit, but I'm not sure if that will work for us, since the MBB is monitoring things as well.
Other options I've seen include adding another DC-DC converter and adding another 12v battery.

[Fred]

Not too much of a mystery. It sounds like Zero are using a resettable fuse.

[karlh]

Look at page 6.33 of the 2020 SR/S Owner's Manual for a description of the Power Distribution Unit (PDU).  The unit acts as the fuses for all the circuits listed.  If you exceed any of the current limits, the PDU cuts power to that circuit until the key is recycled.

My 2007 BMW R1200ST has a similar unit, called the ZFE (a German acronym).  The bike has no fuses, and the ZFE monitors the current to several circuits just like Zero's PDU.  It can be very frustrating if you're not familiar with its operation.

[fencingdude101]

Thanks karlh for the reference to 6.33, good info. So it's not the MBB, rather the PDU that controls all the 12v circuits other than ABS and the Dash. There're also no redundant traditional fuses for each individual PDU-controlled 12v circuits, only 3 10A fuses going into the PDU.
Helpful to know.

[2020_SRS_Commuter]

Look at page 6.33 of the 2020 SR/S Owner's Manual for a description of the Power Distribution Unit (PDU).  The unit acts as the fuses for all the circuits listed.  If you exceed any of the current limits, the PDU cuts power to that circuit until the key is recycled.

Reading is fundamental 
Thank you.

Looks like I'll be adding some kind of additional power source for the horns. A battery or something.

[atomicdog]

The battery can't handle that much current draw. https://antigravitybatteries.com/products/starter-batteries/small-case/ag-401/

the MBB is monitoring current draw of the 12v battery and  since the rest of the system is getting starved for current the MBB tries to correct for that.