28/04/09

Molding and changing

There is a strange behavior in this solar GPS.

The specifications state that it could work up to 30 hors continuous, with the help of the solar collector. So, I think that it could be charged just laying on the sun. Nope. No way.

In fact it’s necessary to charge it via the connector. It’s a USB look-a-like B connector, although it hasn’t any USB abilities, it only uses the same kind of port. I could think of an interface to update the software inside but could not obtain any information in order to develop more in that direction, so it’s a dead end, for now. But it could be charged by a USB powered connector, namely some largely available lighter chargers with this kind of connector. But only if it is on. No charging abilities on the off side of life…

I understand it’s a best design law, but never the less it was odd. If you need to feed more power then what is available to charge the device, you will end up, some day, with it discharged and in an “ohnonotnow” situation, i.e. in voyage with your lighter taken by any other device, dead batteries on the solar GPS and a few more road turns to reach your destination in a unknown place.

So let try to open it, and see what is going on, (in a matter of fact more “what is not going on…”).

It was easy to find the 4 screws that release the 2 halves of the housing before cracking it, they just are beneath the soft pads underside it.

Inside, there is a well executed, good looking circuit and a SINONAR Solar cell. It’s a SC-6041, with 4.5Vcc,   5.3Voc and up to 17.0mA. More that enough to feed a 3,7Vcc Li-ion battery pack.

The terminals of it are just connecting the electronic circuit in a place ruled by the power button, and the tricky idea was to connect it directly to the contact pads where the battery lays.

Unsoldering the wires was easy. Adding a 1N5818, (or 1N5819), Schottky diode to protect the solar cells and soldering to the battery contact pads was also a piece of cake. Just in case, some protection via a plastic small heat shrink tubing to prevent undesired contacts was the last luxury touch. See here the added circuitry:

Difficult hãmmm?

Before reassembling some measurements here made, just to check things out and obtain the “quality control stamp”. Some 4,4V where obtained in the pads in low solar light conditions what is more than enough. A few days at full solar power and the device will attend working conditions.

Those figures are obtained inside with fluorescent light, not bad.

Of course these changes could lead to excessive charging conditions witch may overheat the batteries shortening its life span. Considering that the device is normally at full influence of the solar heat inside a shielded wind screened car, this trick shouldn’t short it much more than “normal”, if you know what I… The other side is that you have, almost always, a full charged working GPS.

(A broken glass cell has made this project a "stand by" one, until a new cell come from scratch.), ( A new cell has born, tests are being conducted...)