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

Invert

[Ledge]

Why not just hook an inverter up if you are going to convert it to 12 volts?

Re:Invert

[cymen]

In my experience inverters are very wasteful in terms of energy requirements and motorcycles are very fruagal in their providence of said energy. In other words, even modern bikes 700cc often have a hard time powering heating riding gear let alone inverters. This old bike probably won't have enough juice to power little more than the headlight.

Will it work?

[TheSHAD0W]

I don't know if your notebook can be easily connected to trickle charge. Most laptops' DC in are designed for a full-current connection, and the charging circuitry won't function properly with a less-than-optimal supply.

There are two possibilities I can see; one is, to remove your battery (or get a spare) and trickle-charge it outside of the laptop; or to trickle-charge an external battery and use that as a voltage input. I recommend the latter; I don't know what the Powerbook's battery terminals are like, but I suspect it'd be inconvenient to hook up to.

Re:Will it work?

[GigsVT]

I don't know what the Powerbook's battery terminals are like, but I suspect it'd be inconvenient to hook up to.

If it's Lithium Ion, better be careful charging it. They can and will explode if not used with a proper charger setup. Other types of battery are much more tolerant of dumb chargers.

HUH

[mugnyte]

this guy built one and it seems rather trivial.

I hit it on the first link of a yahoo search. So, for that, you get the big middle-finger-in-the-face award.

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.

Few options...

[stienman]

There are probably 6v to 120vAC power inverters which should do the job just fine, and will be the easiest and fastest way to go. Just plug your power adaptor into it and then into the laptop and you're done.

I suspect they are rather expensive though.

The other alternative is to make an adaptor that goes directly from the battery to the powerbook. Since you didn't give any specs on the power requirements of the laptop you're talking about (and yes, the different powerbooks and ibooks have different power adaptors) then I can only give some general suggestions.

Unless you are an EE (or aspiring to be one) then don't get involved with switching regulator design and go with the option above. If you must, then go to National Semiconductor, Linear Technology or Maxim and look at their parametric guides to the power regulators. What you need is a switching boost regulator. If you are exceptionally lucky, you'll find just the part you need with the exact application note for input voltage, output voltage, and supply current. I doubt it though. Then you'll need to buy the parts and assemble it. Getting all those tiny surface mount parts on a bread board is only half the fun, though you can still get many parts in through hole.

Of course, neither of the above two methods result in a 'trickle' charge. It'll still suck power from your bike while charging, which leaves the last option:

Charging the battery directly. Just don't. If you don't know enough that you must ask slashdot (of all the places...? Why not sci.electronics.design or something? Egad...) Sorry, uh... Yeah, like I said, if you think the best place to find out is slashdot, then you don't know nearly enough to properly charge your battery directly without damaging it. Plus you still have the problems of building a step up switching regulator on top of the charger, since the battery will need a higher voltage than your 6v motorcycle cell.

However, I will give you a general overview: Battery charging is essentially a current (not voltage) operation. You force a certian amount of current in for a period of time, and the battery releases some as heat and stores the rest. Lithium Ion batteries are very finicky when it comes to charging. You will reduce your batteries capacity by 10% or more each time you charge it incorrectly. If a battery costs you $100, then it's worth doing it right, and in your case I think the only 'right' way is to use an inverter and the laptop's internal charger. Furthermore, Lithium Ion batteries do not like being trickle charged. When done properly it doesn't hurt them, but it doesn't give them a good full charge, either. Lastly, LiIon batteries are well known for their inability to take many charges. Early cells couldn't handle more than 500 charge cycles without losing most of their useful capacity. State of the art cells now don't go over 800. Trickle charging excacerbates this issue.

If you can get NiMH batteries for your laptop then I'd say you have a good chance, since you can go to your local hobby shop can get a charger from them, but they still take 12v so we're back to square one. If your LiIon is only 10.8v then you can get LiIon chargers from the model airplace community, but those are also usually 12v or 120vAC.

So, in closing, I'm still strongly advocating usage of a regular inverter with the laptop's own power brick.

Good luck on your trip!

-Adam

Yet another reason I loathe Apple (not the products, the company): they don't have power specifications on their website about the laptops. The have the input specs to the power brick, but not its output or the laptops input and current requirements, nevermind the battery V and I. I know they have to dumb things down for the average user, but at least put some real technical specs on the technical specs page instead of the fluff they currently have there.

Ask Greg Frazier

[gmhowell]

Ask Greg Frazier. He writes many stories for Rider magazine on circumnavigations via motorcycle. I believe he was wired with laptop for his latest jaunt.

As far as charging, I'd look into some of the solar rechargers. Don't think a vintage Ducati's charging system could keep up.

I found this address with a quick google search: gregfrazier@yahoo.com