Cheapo battery charger auto shutoff?

[Iceman_Chris]

Hi,

I have a Digital Concepts Rapid Charger that I use for my 1800mAh NiMH batteries. This is the previous model to the one that is currently sold by D.C.. Now, my question is, earlier today, after the batteries had been charging for approximately 2 of the 7 required hours, I accidently bumped the large protrusion, causing it to fall out of the socket. I swore and put it back in but I was wondering if it is going to start the whole charging cycle again, or if it will continue charging and stop once the voltage reaches a threshold. Hopefully, it's not just a timer; it's not too cheap of a charger so I have my fingers crossed that it will stop once the batteries are charged (since I have to leave it on overnight and don't want to wake up to a burnt down house). Does anyone have any experience with these?

Thanks in advance, Chris.

PS - It is the windowless version of the charger.

 

[audioguru]

Hi Chris,
Why do they call a charger that takes nearly all night long "a rapid charger"?

It might have a timer that you started all over again. Then the batteries will be charged for 9 hours instead of 7 hours and since the charging current isn't very high then no harm is done.
It might be "smart" and detect the voltage that it is fully charged.
If you try to charge batteries that are already charged then does it stop soon or just time the entire 7 hours?

Ni-Cads and Ni-MH cells get warm when overcharged then vent gasses or a liquid to avoid an explosion or fire. It is a lithium battery that burns very hot if it is overcharged.

 

[awright]

I don't claim to know a lot about chargers - just enough to be cautious.

A rudimentary charger like many automotive and older NiCad chargers may just have a built-in current tapering function as the battery comes up to charge. These will usually overcharge batteries if left connected, but they aren't sophisticated enough to have any abnormal reaction to a "hot restart."

The next level up is just a timer like the early Makita tool chargers that you had to manually start and they just ran for an hour or so with no sensing of battery condition. These will overcharge if restarted. You also find many in salvage because the charger would burn out easily if restarted too quickly without a cooling off period.

The next level of sophistication is the voltage sensing type that will switch off or to a float charge at some threshold. These are usually safer to leave connected or the reconnect unless the float charge current is too high for continuous overcharging.

The most sophisticated chargers are the "slope-delta" chargers that keep sampling the battery voltage and cut off when they sense that the voltage has peaked and is beginning to descend. This rescent is a result of the heating of the battery and the consequent reduction in terminal voltage. These use quite sophisticated electronics, often a chip dedicated to the slope-delta sensing function. The bad thing about these is that if you do a hot start, the baseline voltage it is starting the second charge cycle with is for the hot cell and the controller may not be able to detect the slope-delta (i.e., the reversal of voltage trend) that indicates full charge, so it will not turn off. This can destroy your battery, depending upon the charging current. Since many of these are "fast-chargers," with high charging current, damage is likely. Especially, chargers that will work with any number of cells in series (i.e., "Will charge 6V to 14v battery packs," they generally don't have discrete voltage sensing cutoff in parallel with the slope-delta cutoff as a backup charge terminator. Chargers that have a separate temperature sensor (often on a magnet) can be safer for careless use because they sense fault temperatures and cut off charging.

I am sure that there are much more sophisticated chargers out there that are safe for hot start, but the bottom line is you risk damaging your battery by restarting in the middle of charging unless you KNOW what the charging algorithm is.

It is always best to ask the manufacturer how his product will handle unusual situations.

awright

 

[HiTech]

Why do they call a charger that takes nearly all night long "a rapid charger"?

Prolly from the model before that one which took 3 days to charge a 500ma battery!