General Alternator Capacity Query

[Chris09]

Just as you reach the all critical ignition point, [with a thumper] the crank slows, the field drops and available voltage [with out a battery] is at it's lowest.

 

[oude geit]

240 w is a small generator. Put a decent voltmeter on the bike. It will tell you when you've exceeded supply voltage. ie...12.5 volts or lower not good.

 

[thekaz]

This isn't specifically ST related, but I'd like to know anyway.

My brother has purchased a very nice looking Deauville NT650V2. Nice looking little brother to the ST and I must admit to having looked at them with a keen eye in the past.

He wants to fit some daytime driving lamps. So I start delving into the circuit diagram. There are some very interesting figures in there.

The total value of the 6 fuses in the fuse box is 46A (4x10A and 2x3A). The main fuse on the starter relay unit which supplies the power to all of these fuses is 30A
So it is clear that if all of the fuses were subject to their maximum power rating, the 30A fuse would blow without affecting the individual fuse. That's OK - its normal to put in the next higher rated fuse to protect a circuit. But it means that I can't rely on the fuse value for finding any spare capacity.

The lamps would need to be connected direct to the battery anyway - with its own fuse and with a relay. That's not a problem either.

Ive edited my original post - I referred to an 'alternator', but in view of @mjc506's post, I need to change this to 'stator' or 'generator'. The fiches say 'generator'.

The battery has an 8 Amp/Hour rating, and the generator is .24kw = 240W (which is about 20A to keep the maths simple).

There's a distinct possibility that the 2x60W driving lamps (say 10A to keep the maths simple) that he want to install will exceed the capacity of the generator, and the extra power demand will then be taken fromt he battery, which will eventually go flat while he is riding. So if there is no spare capacity on the generator the full 10A will have to come from the battery, which gives him about 48minutes.

All of this I can understand and work out for myself. For example, brake lights, both headlight filaments, indicators, horn are not drawing power all of the time, and the battery can provide the power and then recharge for the periods when they are not being used. So in effect, I can dismiss some of that power demand.

But my knoweldge hits a buffer at that point. So the question is (and you knew I'd get there eventually).....

What happens to the generator if the load of switched on equipment exceeds the capacity of the alternator - apart from the fact that it cannot charge the battery ?

Does it risk damage ? Does power start to flow from the battery and try to operate like a motor? Does it overheat ?

I haven't got a clue.

I'm sure less inquisitive people would just stick the lights on the bike and not worry about it and say it seems to work. I don't know how many of them then get left at the side of the road with a flat battery !

I would stick with "generator" and really the only important part is the 240W power rating
Most 12v electrical guys will try for a max of 80% capacity at a specific rpm because most "energy generation units" have graphs that show heat increases and energy creation efficiency slows ALOT after that.
This is the reality of MANY vehicles. I want all this stuff but I can only create X amount of power.

When you switch heavy loads on the system will try to keep up but if you go into what some call a negative charge or discharge the system will eventually just round out of power . It definitely does risk damage at some point. If it happens randomly probably not but still a risk.

If it has a stator its has a rectifier which well they love to commit suicide when over heated. There are three common upgrades though. A better rewound stator which maybe purchasable or maybe rewind the original. And the other is a Mosfet rectifier if it does not already have one. The third is to remove any connector between the stator ac phases and the rectifier. These can make a huge increase in wattage BUT math is still king here and you just cannot over come serious negative wattage.

 

[Willsmotorcycle]

There are clearly very smart riders here with electrics. The 120 watt advertised number is probably not accurate… However changing all the bulbs to LEDs and not using the the load resistors, but changing the relay for turn signals will give you a surplus that would not have me concerned about draw. I would run a volt meter to keep me from wondering.