Power supply/charger needed

Umm, that's just a MicroUSB wallwart, right...?

Yes, but the last two I bought have not charged the device when completely flat!

Mike

They do have individual current ratings and AIUI some are not strong enough for tablets and the like. My Satmap navigation system is non-standard even though it has a mini USB port.

That's not a problem with the charger. It's a problem with running the batteries completely flat.

In devices where the charging is controlled by the microprocessor (such as phones, PDAs, etc), if the battery is completely flat, the microprocessor can't start the charging. On the face of it, this is a catch 22 situation, but what they do to get around this is provide a bypass which will allow the battery to charge extremely slowly until it gets up to enough voltage for the microprocessor to boot, at which point it will take over control.

You may have to wait an hour or two before the battery gets enough charge and the microprocessor boots.

The reason this has to be done extremely slowly is that charging lithium ion batteries without any control is extremely dangerous; you are almost literally lighting the fuse on a firework. The bypass charging current has to be so low that if the microprocessor never started up for any reason and the charging continued uncontrolled, it would take a very long time to over-charge the battery to the point of exploding into flames.

If you are going to routinely run the batteries flat, charge them in an external purpose-designed charger.

They are mostly rubbish if you got them from discount places. Maplin perhaps?

Brian