WFrom the service manual or spec sheet (alternator or stator output). Often quoted at a specific rpm; output is lower at idle.
WIgnition, fuel injection, fuel pump, ECU, standard lights. Use a measured or published figure if you have one.
W
W
W
W
Ah
Result
Spare charging capacity55 W
Extra current drawn by accessories at 13.5 V7.0 A
Hours until the battery is half drainedNot draining
How it is calculated, with your numbers
- Accessories
acc = grips + lights + jacket + gps40 + 40 + 0 + 15 = 95 W - Spare charging capacity
spare = alt − base − acc350 − 200 − 95 = 55 W - Extra current drawn by accessories at 13.5 V
amps = acc / 13.595 / 13.5 = 7.0 A - Hours until the battery is half drained
hours = if(alt − base − acc < 0, ah × 0.5 × 12.5 / (base + acc − alt), 0)if(350 − 200 − 95 < 0, 10 × 0.5 × 12.5 / (200 + 95 − 350), 0) = 0.0
Working uses the metric units the formula is written in.
- alt Charging output at riding rpm (W)
- base Bike's own consumption (W)
- grips Heated grips (W)
- lights Auxiliary lights (W)
- jacket Heated clothing (W)
- gps GPS, phone, camera (W)
- ah Battery capacity (Ah)
- acc Accessories
- spare Spare charging capacity
- amps Extra current drawn by accessories at 13.5 V
- hours Hours until the battery is half drained
What it means
A motorcycle charging system makes a fixed amount of power at riding rpm. The bike's own systems take a share; what is left is all the accessories can use. If they want more, the battery makes up the difference and slowly goes flat, which shows up as a no-start after a cold, wet day with everything switched on.
The drain estimate uses half the battery's capacity, because draining a lead-acid battery much further harms it and may stop the engine running properly.
Assumptions and limits
- Charging output as published at riding rpm; at idle and in traffic it is lower.
- Battery at about 12.5 V during discharge.
Sources
- Ohm's law and power: P = V × I.