28amp alternators…what CAN they run?

GrumpiestGoblin

Joined
May 21, 2023
Messages
59
Age
44
Location
Rossendale, UK
Hi everyone,

I’ve read a lot of threads about upgrading alternators to run heated clothes and grips, aux lights and other stuff. I’d be interested in doing that in the future, but just can’t afford it during my current project.

It got me thinking - assuming my 33 year old 28amp alternator is fine, just what CAN I run from it?

At most I’d want a satnav (my old TomTom usually runs straight from the battery) and maybe a 12v socket to run a usb charger for my phone…that’s pretty much it!

if needed I can compromise and use my phone as a satnav (I’d just need a mounting kit, easily sorted!)

cheers
Mike
 
GrumpiestGoblin said:
Hi everyone,

I’ve read a lot of threads about upgrading alternators to run heated clothes and grips, aux lights and other stuff. I’d be interested in doing that in the future, but just can’t afford it during my current project.

It got me thinking - assuming my 33 year old 28amp alternator is fine, just what CAN I run from it?

At most I’d want a satnav (my old TomTom usually runs straight from the battery) and maybe a 12v socket to run a usb charger for my phone…that’s pretty much it!

if needed I can compromise and use my phone as a satnav (I’d just need a mounting kit, easily sorted!)

cheers
Mike
Click to expand...
Ay up Grumpiest Goblin, although are you really?
What you're planning to run there is a fraction of bugger all.
If it's OK, by that I mean dry and charging, just ride. Yes a 40 amp upgrade would be the way to go, but in summer? Meh.
Do they still have the bike show there, if so when is it this year.......hang on I know.......last weekend?
Enjoy.
Upt.
 
Upt' North said:
Ay up Grumpiest Goblin, although are you really?
What you're planning to run there is a fraction of bugger all.
If it's OK, by that I mean dry and charging, just ride. Yes a 40 amp upgrade would be the way to go, but in summer? Meh.
Do they still have the bike show there, if so when is it this year.......hang on I know.......last weekend?
Enjoy.
Upt.
Click to expand...

Ey’ up!

Grumpy? Absolutely! Well…my kids think so. I’m also a teacher, and being tracked down online and having your personal doings shared is an occupational hazard, so I tend to go incognito…. You’ll find me quiet personable in real life!

The Rossendale Bike Show! That’s going back a bit I think! I’ve been here 7 years now and have only heard of it in myth!
 
I'll echo the opinion that what you plan to run is a very small load and not a problem. What you can do, and it is easy to do, is replace all the bulbs on the bike with LED. Dont bother with the dash lights, not much draw there. But the headlights, turn, brake, running lights add up to a considerable draw on the system. LED can reduce that by 50+%. That can only help.
 
rwthomas1 said:
What you can do, and it is easy to do, is replace all the bulbs on the bike with LED. Dont bother with the dash lights, not much draw there. But the headlights, turn, brake, running lights add up to a considerable draw on the system.
Click to expand...
All this. Even if you don't add any more load to the bike converting even the headlights alone (but I'd do all the other stuff) gives you a very comfortable margin and as a bonus you can see for miles and miles and miles oh yeah.

I don't know the LED State of the Art for tail lights though. My early tail light LEDs don't "fill the lenses like the factory bulbs but that's a trait of LEDs. To my mind the ST1300 (don't know about the 1100) has weak tail lights but decent brake lights with factory bulbs. My LEDs in the tail are similar though not "filling the reflector".

You're doing a resto so there's no need to think about this now other than whether or not to upgrade your alternator. Your electrical needs being modest the only thing to consider your current alternator's oil-tight integrity.
 
At some point in time, an 1100's electrical systems were measured or calculated to be very close to 28 amps if all were on at the same time.
 
Thanks everyone!

ST Gui said:
Your electrical needs being modest the only thing to consider your current alternator's oil-tight integrity.
Click to expand...

Mine may have a slight weep - i’ll investigate more down the line when I get the swing arm out….

Ron said:
At some point in time, an 1100's electrical systems were measured or calculated to be very close to 28 amps if all were on at the same time.
Click to expand...

Very interesting to know this Ron! I‘ll keep this in mind!
 
Ron said:
At some point in time, an 1100's electrical systems were measured or calculated to be very close to 28 amps if all were on at the same time.
Click to expand...
Debates on the old mailing liST...
IIRC a max surplus of ~10A, which covers like a set of heated handles...
 
GrumpiestGoblin said:
At most I’d want a satnav (my old TomTom usually runs straight from the battery) and maybe a 12v socket to run a usb charger for my phone…that’s pretty much it!
Click to expand...
Those two items are very low amperage and could reasonably be considered to be "negligible" additional loads on a 28 amp alternator.

Generally speaking, if it produces heat (grips, incandescent bulbs, heated clothing) it is a high amperage load, and if it powers electronics, it is a low amperage load.

Michael
 
Ok, lets toss around some real numbers. For a Garmin XT, the cable draws less than 5 milliamps @ 12 volts when the unit is in standby mode, and less than 1 micro amp when the unit (unit = XT) is off. So. .005 x12 = .06 watts and a parasitic load when your garmin is off of 12 millionths of a watt. The gps itself does not have a published spec for current on the Garmin website, but it will happily run with a 1 amp fuse. Worst case this draw would be 12 watts, but since the battery inside the device is rated from 3.5 to 4.1 volts, I would guess its actual current draw is maybe half of that 12 watts. Your phone probably draws about the same, USB chargers are typically 1 amp (some are 2 amp) but they drop the current down to around 5 volts. Together, your GPS and phone are drawing around 1 amp or 12 watts.

Now, 12 millionths of a watt will not get a flea to exit your hospitality even with a tasty dog trotting by, and while I've done no experiments, 6 hundredths of a watt might be the energy used by said flea to leap off you onto that dog. (Hey guys, how about crunching some numbers and coming up with what kind of power Fred flea generates while jumping). Twelve watts is not negligible, but it is a very small fraction of the approx 390 watts output of a 28 amp alternator in tip top shape.
 
Mike Martin’s “ST1100 Maintenance Tips,” -
Electrical Loads for a '91 ST

*** Mike Martin's ST1100 Maintenance Tips - Electrical Loads***

koczarski.com

Edit: in stop-n-go traffic with the fan running, higher wattage headlight bulbs, and brake lights on, you are probably discharging the battery a bit.
Edit2: rated at 420 watts (15V x 28A @5K rpm), but 13.6V is more realistic.
 
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I would replace the headlamp bulbs with LED to free up some Wattage and just go ride.
You can replace all of the other bulbs with LED's as well if you like, but the biggest change that you can make that will give you the biggest bang for your buck is changing the headlamp bulbs.
 
Andrew Shadow said:
I would replace the headlamp bulbs with LED to free up some Wattage and just go ride.
You can replace all of the other bulbs with LED's as well if you like, but the biggest change that you can make that will give you the biggest bang for your buck is changing the headlamp bulbs.
Click to expand...

definitely something I’ll consider Andrew!
 
My first police ST1100 had a 28 amp alternator and it coped fine with all the extra load required of it. However it did have an uprated regulator/rectifier - a beefed up version with a chunky heatsink.

regrec.jpg

LED headlights will help a lot, though.
 
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