Question about the small battery:
Sometimes I get caught in some shitty traffic....not rush hour in LA, but pretty shitty start stop BS, plus lights at intersections, so sometimes I get caught where Im just walking in first a few feet at a time. I do have a voltage monitor and can tell when shes charging,......BUT....... how long will this little battery last in traffic like that.???
Really good question. Been pondering this for years, might as well get into it.
*Warning* - A little math is involved, and I'm still tryin' to wake up.
Going to use some rough numbers here (for convenience), and some general purpose assumptions, based largely on my starterless XS1B, which should closely match the 70-79 systems, dissentions encouraged.
Viable, working/usable energy of a battery will be voltage times Amp-hour rating, yielding Watt-hours.
A 12v 1.3 Ah battery would have about 15 Watt-hours of usable energy.
A 12v 4.5 Ah battery would have about 55 Watt-hours of usable energy.
A 12v 5.5 Ah battery would have about 65 Watt-hours of usable energy.
A 12v 12 Ah battery would have about 145 Watt-hours of usable energy.
Think of it as an electron 'gas tank'.
Now, the loads, for a stock charging system, points ignition, NOT running.
The 5.5 ohm rotor will be constantly 'on', pulling 2.2 amps, a 26 watt demand.
The mechanical regulator has an internal Vref circuit (25 ohm resistor and 12 ohm solenoid) of 37 ohms, pulling 1/3 amp, a 4 Watt demand.
The ignition, with dual 5 ohm coils, each coil can pull 2.4 amps, a 30 watt demand for each.
However, the coils are energized separately, and for 90° dwell (1/4 revolution of points cam, 1/2 crank revolution)
If one of the points is closed (a 50/50 chance), one of the coils will be drawing, a 30 Watt demand.
If both points are open, there will be no demand.
The neutral lamp will have a 3 Watt demand
If the headlight is on, and at 'low' beam, its demand is 40 Watts, plus taillight is 8 Watts, plus 6 watts for instrument bulbs, total of 54 watts.
So,
26 Watts rotor
4 Watts regulator
30 Watts coil (50/50 chance)
3 Watts neutral lamp
Gives:
33 Watts total (open point)
63 Watts total (closed point)
With headlight on:
40 Watts low beam
8 Watts taillight
6 Watts instruments
Gives:
87 Watts total (open point)
117 Watts total (closed point)
So, just turning on the key, you can have initial demands of 33, 63, 87, 117 Watts, depending on situation.
The battery will supply that power for a limited time, based on capacity divided by demand, yielding hours.
A 15 Watt-hour (Wh) battery will supply 33 watts for 15/33 hours, about 27 minutes. Worst case 117 watts for 15/117 hours, about 8 minutes.
So, calculate switched-on 'engine not running' battery exhaustion time (in minutes) for 33, 63, 87, 117 demand watts:
12v, 1.3 Ah, 15 Wh battery: 27, 14, 10, 8 minutes
12v, 4.5 Ah, 55 Wh battery: 100, 52, 37, 28 minutes
12v, 5.5 Ah, 65 Wh battery: 118, 62, 45, 33 minutes
12v, 12 Ah, 145 Wh battery: 264, 138, 100, 74 minutes
Now, with the engine running, the ignition system will be running at 50% duty-cycle (point closed for 180° crank rotation), so its draw will be about 15 Watts.
While running and/or riding, you can expect demands of 45-48 watts (headlight off), 99-102 watts (headlight on).
If the charging system has failed due to bad rectifier, but rotor is still drawing power, you can roughly expect these run times (headlight off, headlight on):
12v, 1.3 Ah, 15 Wh battery: 20, 10 minutes
12v, 4.5 Ah, 55 Wh battery: 70, 33 minutes
12v, 5.5 Ah, 65 Wh battery: 80, 38 minutes
12v, 12 Ah, 145 Wh battery: 180, 85 minutes
If the charging system has failed due to bad (open) rotor, you can remove 26 watts from the loading, and expect longer battery run times.
In fact, if you have a charging system failure, you can disconnect the regulator to eliminate the 30 watt demand of that system, and extend your crippled 'get home' time.
Then, with the regulator unplugged, you can roughly expect these 'crippled' run times (headlight off, headlight on):
12v, 1.3 Ah, 15 Wh battery: 60, 13 minutes
12v, 4.5 Ah, 55 Wh battery: 220, 47 minutes
12v, 5.5 Ah, 65 Wh battery: 260, 56 minutes
12v, 12 Ah, 145 Wh battery: 580, 127 minutes
Now, the issue of idling and plodding thru stalled/congested traffic.
I've found that if the battery voltage is near 12.6v, very little, if any, current is traveling to/from the battery. It's essentially at a break-even point, the charging system is handling the power draws. On my XS1B, headlight off, neutral light on, idling at 1200 rpm, the battery voltage is at 12.6v, and the system is barely in equlibrium, the charging system is supplying the 45-48 watts power.
However, in congested traffic, at idle speed, using the brakes with the 27 watt brakelight, the turn signals, and worse, the headlight, the system is in depletion mode.
So, in a typical scenario of headlight 'on' and holding the brake, expect extra draws of:
Headlight 40 watts, taillight 8 watts, instruments 6 watts, brakelight 27 watts. Total of 81 watts.
How much battery life in this scenario?
12v, 1.3 Ah, 15 Wh battery: 11 minutes
12v, 4.5 Ah, 55 Wh battery: 41 minutes
12v, 5.5 Ah, 65 Wh battery: 48 minutes
12v, 12 Ah, 145 Wh battery: 107 minutes
I hope that this exercise gives you a working knowledge of electrical capacities, demands, and depletion times. All this is different with modified systems, like Permanent Magnet Alternators, electronic ignitions, LED illumination, smartphone charger, CB radio, electric shaver, and such.
Ever notice that the simplest questions often have complicated answers?