Back in the December 2006 issue we had a feature article on repacking the battery pack in cordless drills whose batteries had died (or were at least very sick!).
The idea was to buy a ten-pack of sub-C 1.8Ah nickel cadmium cells and simply pack ’em into the battery pack.
It is not rocket science – just follow the steps in the article. But there was a catch – there generally always is.
In most cases, if your battery pack had a short life it was probably because the charger was a rudimentary design, perhaps not much more than a small transformer, diode rectifier and a charge limiting resistor. These chargers – and they are extremely common with cheap cordless drills – spell early death for the battery pack. They have no proper “end-of-charge” circuit cut-off or timer to prevent over-charging.
So, as well as the article on repacking the battery pack, we presented a proper charger – the Power Tool Charging Controller – also in the December 2006 issue.
This charge controller was designed to go between the existing rudimentary charger and the battery pack. It was based on a PIC microcontroller and incorporated adjustable timer, temperature cut-out and dT/dt sensing for correct end-of-charge cut-off.
Fast-forward to December 2010 and everything we wrote four years ago is still valid and even the cost of a ten-pack of the same sub-C nickel cadmium cells is still the same.
A new set of NiCads is likely to be around $70 or more and you need to add the cost of building the Power Tool Charge Controller which might be around $30 or more. Total cost is going to be at least $100 and that is the outstanding reason why so many cordless drills end up in the garbage bin or just gather dust on a shelf in the workshop. It’s a shame though, since the drills are probably capable of many more years of work.
What about replacing the battery?
Just go to eBay and you will find that there are now a lot of battery packs available. They’re not particularly cheap though, especially when you add in the cost of postage. And good luck in finding one that suits your particular drill.
Even if you do find one, the cost of a new battery pack may well be just as much as (or even greater than!) the cost of a brand new, more modern drill kit, often including two batteries, from your local hardware store.
One thing hasn’t changed though. If your charger is one of those rudimentary designs, you still need to build our Power Tool Charging Controller. And while the kit is no longer available from Jaycar, the design is still valid and you can get all the bits.
An easier fix!
But why bother with all of the above? Why not just convert the drill to “corded” operation? This way, you avoid the cost of repacking or replacing the battery pack.
Just recently one of our staff members was faced with the same conundrum – his cordless drill battery pack had died – so what to do with it? It was a 9.6V drill and superficially, that means there is no easy battery/cheap replacement.
But let’s think about that. A 9.6V battery pack means that it contains eight nickel cadmium or nickel metal hydride sub-C cells rated at 1.2V each. Hmm.
When those sub-C cells are fully charged, they will have a terminal voltage up around 1.8V or thereabouts. That means that a fully charged 8-pack of sub-C cells has a terminal voltage around 14.4V. That just happens to be the same voltage as a fully charged 12V lead-acid battery.