Got one of those. It would seem that it's a "smart" charger and it always take a while to work out charge rate. I presume part of the battery testing involves putting a small load on it. If the battery is completely flat (and probably knackered) it will just sulk.
More than one of us have had one of these chargers with a non-working mode button. You can replace it if you are OK with soldering on circuit boards.
If the battery voltage is below somewhere around 8V, the chargers that also do 6V batteries will assume you connected a 6V battery, and stop charging when it reaches around 7V. I have got around this by connecting a second 12V PSU just long enough to get the battery voltage up to 10V or so.
I'm pretty certain it needs to see above a certain voltage from the battery to switch on. To do with the reverse connection protection.
If you have another car battery - or a jump start pack - connect that across the flat battery and the charger will now power up. Leave for a couple of hours and disconnect the 'good' battery - it should now remain on charge.
Do any 'smart' chargers have a 'dumb' mode. [If not, maybe they should.]
I've got a very cheap 'Tronic' fast (smart) charger, which does AA and AAA. Occasionally, it will decide not to charge a battery (the red fast charging light flashes instead of being solid) - despite the battery voltage being around 1.2V, nor is it high-impedance. A minute on an old 'dumb' charger, and it will then fast charge as normal.
You can usually cheat it by putting a failed AAA cell onto the AA terminal for a few seconds or AA onto C, C onto D which will put enough charge in to satisfy the right size fast charge algorithm. Eventually you will get one that emits the magic smoke/steam and caustic.
Don't leave one for any length of time on the wrong higher charge rate - that is asking for trouble.
I've had that problem with the 2 hour fast chrager that was supplied with my little 3Mpxl Nikon Coolpix and certain AA NiMH cells that have seen a few recharging cycles. Even when those cells are placed on a dumb charger for half an hour, they don't always play nice with the MH-71 Coolpix NiMH battery charger, producing the rapid flash on its charge lamp (indicating charge failure due to a cell fault or overheat condition (presumably overheat of the charger itself rather than the cells which _can_ become almost too hot to hold if the charger fails to detect whatever it's supposed to detect that tells it to terminate the rapid charge phase).
For those cells, I have a nice little 4 cell Uniross Compact Fast Charger (350mA for AA and 150mA for AAA types at 2.8v for each pair) which takes 6 hours to fully charge one or two pairs of 2AH AA NiMH cells. The input power rating is marked as 5W and its compact and light weight indicate that it's based on smpsu technology.
Since there's no indication of battery type, I strongly suspect it was sold as a NiCd charger (Fast in this context usually refers to a charging time of around 2 to 4 hours rather than 6 - a typical AA NiCd would have had a capacity of around 700mAH resulting in a 2 hour charge time on this charger).
Although it's almost certainly not designed with NiMH cells in mind, it's proved quite useful as a medium rate dumb charger to kick life back into NiMH cells that otherwise fail to be charged by a NiMH 'Fast' or 'Intelligent' charger.
It's always handy to keep hold of an older 'Dumb Charger' or two to deal with rechargable cells that have taken to upsetting the intelligent charging devices. I've got two or three such chargers tucked away for exactly that purpose now.
Similarly for car batteries. Keep hold of your ancient simple mains transformer and bridge rectifier with ammeter klunker of a charger. You never know when it might rescue your latest flashy "Intelligent" battery charger from its own 'cleverness'.
Remember, a rechargable battery isn't truly dead until it either explodes, bubbles over, starts to look a 'little chubby' or keeps blowing fuses or chargers. :-)
There might be one for protection against reverse connection, but for charging, they usually rely on the impedance of the transformer and rectifiers, and sometimes use a leakage reactance transformer which has an artificially increased reactive impedance to limit the output current without causing the transformer to overheat.
Quite. But the protection which most modern chargers have against damage by reverse connection is the very reason why they won't charge a very flat battery. Of course a blown fuse does much the same. ;-)
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