Charging Batteries While In The Cradle

This mod requires a quick bit of work in the cradle (USB only I’m afraid. If you can do it with the serial cradle, let me know!) Then there is some more intense work inside the Visor itself. You can do this on the Visor or the Visor Deluxe. I’m unfamilliar with the Visor Platinum, and the Visor Prism already works this way. If you are careful it requires no cutting or modifying the plastics.

About The Batteries

After reading some research on the ‘net, I elected to use so-called rechargeable Alkaline batteries. The reason in the long-run is that when NiCd’s and NiMH’s start to wear down, they go from useful to stone-dead in such a hurry that you may not get any warnings at all. ‘Renewals’ are basically just normal Alkalines, that the manufacturer says can be recharged. Their ‘drawback’ is that if you let them get too low in power, then they won’t take a new charge. So the trade-off is that if you can leave your Visor in the cradle for an hour each day, you’re laughing, but if you do run the batteries down low, you may have to get new ones. On the other hand, they will behave with the same usage curve as normal Alkalines, so depending on your useage, you could go a few days to a week before you actually needed to top them up. Though I imagine most people hot-sync at least once a day. I make heavy use of my back-light, (I play games on the Palm in the dark before falling asleep, or read e-books on it) and with my ‘Tale-Light’ mods, use the serial port a fair bit too. I made the recharging modification on 05 December 2000, and in the evenings I leave my Visor in the cradle for about an hour or two, while I’m working at my desk. 
As of February 7th 2001, my batteries are starting to wear a bit. I’d run them down to about 2 volts, and getting them back up to the 3 + volts is taking some more time. If I leave the Visor in the cradle overnight, that does the trick… Still, it remains fully useable, and I’ve only actually been recharging it ever 3 or 4 days.
The Cradle: If you use the stock USB cradle with your Visor, and you have that cradle plugged into a powered USB hub, then your cradle has access to 5 volts at up to 500 milli-amps. What does this mean? Well, by soldering one jumper into the cradle, you have changed the stock cradle into a ‘Charging Cradle’. If you look at the 8-pin connector in there, with the cradle facing you, the 2nd button from the left is pin 7, officially called VDOCK in Handspring literature. It is not connected to anything ‘out of the box’. But Handspring’s documents say it is designed to be connected to a source of 4 to 6 volts, with a maximum of 500 milli-amps. Hmmm. 
Though I started with a simple wire jumper, I have replaced that with a 10-ohm resistor. This does not affect the voltage noticably, but it does help limit any current surges when first placing the Visor in the cradle. Solder the resistor between Test Point 1 and Test Point 4, as in the following image. It doesn’t matter which direction the resistor goes, they aren’t polarized. 

Please Be Careful!! The little plastic hot-sync button is not attached and YOU CAN LOOSE IT when you take the cradle apart. Then, when you finaly break down and buy a new cradle just to have that little button, you have to buy a serial cradle so you can claim you’re really buying it to see how it works, to try to feel a little less foolish. Until a week later, when you find the original plastic button, where the cat left it.
If you are using a serial cradle, click here. Also, some instructions for testing.
The Visor: This is where things get complicated and tricky. You will be soldering very small things onto other very small things, in proximity to other small things.
Let’s get started. Take your Visor apart (see below for details). You only need to go as far as getting the button-board free. In a nutshell, what we want to do is connect the +5 volts from the VDOCK pin at the hot-sync port, to the positive battery terminal. Along the way, though, we need to get the voltage down to about 3.2 volts. Those of you who know what you are doing and know electronics might roll your eyes, and point out that there is just gobs of space in the cradle for a full-blown voltage and current charging regulator system. If you want to do this, fine, just hook a small wire from VDOCK to the positive battery terminal on the button-board and you are done. 
However, the Handspring specification says for 4-6 Volts at 500 ma to be on that pin, and they did this for a reason. VDOCK is a direct connection to a pair of VDOCK pins in the springboard port. For those springboards with their own rechargeable batteries. So to adhere to Handspring’s specs, I’ve elected to do the voltage regulation within the Visor, without affecting the voltage that goes to the springport.
What I have done is pretty cheap, pretty simple, and cuts a few corners. I have taken two 1N914 diodes, soldered them in series, and put them in between VDOCK and the positive terminal. I would have liked to add a 10 ohm resistor in this series too, but was unable to locate a chip-sized one, so I put the resistor in the cradle (above). I have positioned the diodes and the rest of the wire, so that they do not interfere with any of the Visor’s plastics.
Diodes are polarized and it makes a big difference which way you put them in. Most diodes have a line or bar indicating which lead is the ‘negative’ lead. When diodes are installed ‘forward-biased’, the ‘negative’ lead is at a lower potential than the ‘positive’. Anyhow, you want to solder your diodes with the ‘positive’ lead of the first diode connected to the VDOCK pin of the hotsync connector, then the ‘negative’ lead of that diode to the ‘positive’ lead of the second one. The ‘negative’ lead of the second diode is connected, via a small piece of hookup wire, to the positive Battery connector.
Click here for a Very Simple Schematic.

The voltage-drop across the two diodes brings the 5 volts down closer to our desired 3.2 volts. Important: Never put your Visor into the charging cradle without any batteries in it! The diodes alone will not bring the voltage down to safe levels – the batteries are part of this circuit, and without them, the cradle-voltage may kill your Visor. If you do these modifications, then try to use the Visor in the cradle without batteries, you could kill it!
Now, it’s one thing to just say your Visor Deluxe recharges in the cradle, for my money, it’s not complete without a light to tell you it’s getting power. So I have added another wire to that VDOCK pin. Over at the backup-capacitor, I have fixed an LED. The short-lead of the LED goes to ground of the capacitor, and the long-lead of the LED is connected to the VDOCK line. To limit it’s current a little, I added a 120 ohm chip resistor. Obviously the LED’s leads have been trimmed and shaped to make it fit nicely. My Visor is ‘Ice’ colour, so I went with a white-light LED. Whatever colour you use, be sure to get a 5-volt LED for this application. If you want to use a 3-volt one, you must increase the value of your resistor. I believe, 1000 ohms is the value for a 3-volt LED.

I should point out that if you have a Visor in an opaque case, you will probably want to make a very small hole in the case, so that you can see the charging light. It is inconceivable that you would want to simply not have a light. “Real electronics should glow in the dark.”
Now, if you have been careful about positioning these new components, you should be able to fit the button board back in place without trouble. First though carefully check your work to ensure there are no short-circuits or solder-bridges. Another hint here, test the button-board’s fit while doing the work, add a part, test for fit, etc. so you can make adjustments as you go.
To test, reassemble your Visor, put in the rechargeable Alkalines, pop it in the cradle, and monitor the voltage. There are lots of 3rd party programs which give you very accurate readings of voltage. Check it in the cradle. Check it out of the cradle. I get a steady 3.27 volts while in the cradle. This is a good level, to ensure the batteries will get a full charge, without being too high for the rest of the Visor’s circuitry.