I have been given an old cordless drill (Black and Decker BD602) without a charger.
It is only 7.2V so no problems if it must go in the bin, but a spare is always useful, if only for mixing paint.
I can make a charger if someone can let me know the voltage and polarity of theirs. I am guessing that with it being a B&D there are going to be loads of them about. Equally, if someone has one that they no longer need, please name your price.
Thanks, Rob
I've had one of those for, well, must be about 15 years I should think, and it's still going strong (on its original battery too!). I can't claim that it has been used every week of its life, but more like heavy use - often as a powered screwdriver - for a few days now and again as each DIY project comes and goes. It would be a crime to use it as a paint stirrer !!!
The charger bears the legend "8.7v 480 mA" with the little symbol of a continuous line above a dashed line which I presume to mean raw unsmoothed DC. The centre pin is positive and the output measures 9.6v off load (on an old but accurate moving coil voltmeter). If 'twere me, as a best guess without knowing what cells are in the battery pack, I think I'd take note of the "480 mA" rating and build a constant current charger of about 400mA. Actually, no, on second thoughts, I don't think I would - I'd probably want to open up the battery pack to see what size the cells are and make a more informed guess at their rating and build a constant current charger to suit!
HTH.
The accepted safe charge rate from an unintelligent charger is 1/10th the capacity for 14 hours. Sub C cells are usually 1500-2200 mAhr, so I'd make a constant current charger 150mA for starters.
Your one has probably just a simple series resistance so will vary the charge current rather unpredictably. ;-)
Thus reinforcing my point about wanting to see the cells first ... thanks Dave. JFYI, back in the dark ages (!) I stumbled across an ingenious circuit that Hewlett Packard used to use for their calculator battery chargers employing just two transistors and two resistors in which the forward voltage drop across a conducting transistor junction acted as a controlling reference for the current through the load across a wide range of voltages. Therefore, 30v input to the circuit would deliver (say) 50 mA through 1, 5 or 20 cells connected as a load. It was like a 'minimalist' current limit protection circuit on a laboratory power supply. In a 'universal' version of the circuit - of which I built many over the years - changing the value of one resistor would control the current delivered to suit the capacity of the cells being charged. Simple but very effective.
Thanks to all for your help. I have 'roughed' it a bit by plugging in a 9V
150mA PSU. I realise that the current is low but it didn't get particularly hot so perhpas there is some current limiting in the drill.How long would you charge it for?
Cheers Rob
The voltage across the base and emitter of a transistor is pretty much stable once the junction has started conducting - it's only a diode in that sense, and once the current flow has gone past the "knee" point then the amps to volts gain is almost infinite.
I spent 4 years working at Mullards Semiconductors in Southampton....oh the memories.... ;)
PoP
The CC circuit I use for drills etc is at its simplest one transistor, two diodes and two resistors. I add a couple of LEDs for status. It will work ok at virtually any voltage (more than the voltage of the cells) but of course needs a larger heatsink if the difference is great. I can't remember where I found it, though, to give them a credit.
I've got a charging area in the workshop with a few 13 amp outlets all fed off a 14 hour one shot timer.
Any chance you could knock up an ascii circuit diagram?
Hi friends, I have also got this model and it's missing the trigger. Much appreciated if you can suggest me the trigger model for this drill. Thanks in advance.
No longer available by the looks of it:
The product is roughly 30 years old.
Just buy a "for parts" unit off Ebay. Not money well spent but...
And Black and Decker never learned the proper way to make a charger, so you also have that working against you. Without care, the 6-cell pack could be dead after a couple of years, because the charger... ruined it. Nickel Cadmium cells are a great way to corrode all sorts of stuff sitting next to them inside the drill. Before wasting money on parts, the product must be opened and inspected for corrosion damage.
As a 7.2V pack, you stop drilling when the pack voltage hits 6V, to avoid cell damage. The device is unlikely to have an indicator for this, or even an automatic protection feature (low voltage cutoff).
A proper charger would have dv/dt detection of the endpoint, and that only works well with fast charge cells and decent levels of charging current.
The Black and Decker charger is more likely to rely on the user being smart enough to unplug it, which is hardly a scientific method. B&D would use the "C/10 charge it forever we don't care if you ruin the cells" method. That's part of the reason it lasts for two years.
Modern goods have more protections for the Lithium cells by comparison. Even a company like B&D would use the methods everyone else uses.
Paul
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