Building a Laptop Trickle Charger?

chimpo13 asks: "In 18 months, I'm planning on riding around the world on a 1966 Ducati 250 single. I have some problems, but there's one that can hopefully be answered on Slashdot. I think my Powerbook G3 will take the vibration of the bike, but I'm trying to find out how to trickle charge the laptop battery on my bike. It's a 6 volt bike that will be converted to a 12 volt. Has anyone built a trickle charger for a laptop?"

Comments

Re:HUH

[chimpo13]

That's dc to dc. The bike is ac. The alternator will probably put out about 100W. It also needs to run my headlight, my taillight, and my brakelight.

I don't know much about electronics, but I'm learning now.

Here's stuff I've heard from an electrical engineer who has old Ducatis:

First, if you choose to implement a charger running off the bike, the charger will have a cord that plugs into the computer presumably through the carrying case. You'll have to plan in advance how long the cord needs to be, and you'll have to route it so it doesn't conflict with any moving parts.

The AC configuration presents some special challenges. Batteries are DC creatures. A rectifier (a diode) handles the job of converting AC to DC nicely and cheaply.

We like to design something called a full-wave rectifier, because that uses both positive and negative AC (alternating current) cycles. The trouble is, you need a reference contact to the center of the AC transformer, which in your case is the bike.

The AC bike doesn't provide that. It can't be done because one side of the "transformer" is tied to the bike frame as a ground reference.

That means all you can implement is a half-wave rectifier. So exactly half the time you're getting some voltage, and the other half of the time you're getting no voltage whatsoever. You have to average the two together, so some voltage averaged with no voltage equals significantly less than some voltage.

Now we have DC voltage, but its pulsating DC. This isn't any good either. When the instantaneous DC voltage is above the voltage of the battery, the battery will charge. But when the DC pulse goes to zero volts during the unusable half-wave, of during the usable cycle when the DC voltage hasn't risen to the at least the same voltage as the battery, the battery will actually loose charge as it tries to send power to your charger!

So now another diode is needed to limit current flow to the battery and prevent current flow away from the battery.

Confused, I hope not. But there's more. We've got pulsating DC that's going from 0v to 10v DC. The 3.3 volt battery of the laptop might be destroyed by too high of a charging voltage. Now we need a special diode called a zener diode that regulates (not rectify) the voltage so we don't apply too much voltage to the battery.

So now, by using a few diodes, we're shooting a tiny charge into your battery when the voltage is above your battery level, but not above the upper limit of battery charge. So for a few milliseconds of every cycle, as the pulse of DC rises and then again as it falls we get a tiny amount of charge into the battery. We can make up some ground by adding a capacitor to store some energy which we take when the DC pulse is too high a voltage for the battery level. But still, we can never make up for the fact that we can't use exactly half of the AC cycle.

What I'm trying to get at here is that the constant 6 volts DC from a DC motorcycle can easily be regulated to the charging voltage of the batteries in order to supply a continuous recharge. The AC system would take some doing to make an effective charger for your laptop batteries.

You need to see what your AC adapter or your car adapter say their output voltage is to the laptop. They should be the same. Maybe it will say something like 3.5 VDC, and it might even give a current specification, like 100 mA or 500 mA.

You'll need a cord with the correct plug to mate with the laptop. I don't trust radio shack quality, but I'm not sure where else you can get the cord. If the car adapters are cheap enough, you can butcher one of those. They should have a higher quality cord.