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Here’s a great green idea – especially if you want to do work around the car or boat! Recycle your cordl drill – make it co Handyman cordless drills are very cheap – most readers probably have at least one or two in their tool collection. But their big weakness is the battery pack and associated charger. What do you do when the battery pack dies? Just convert the drill to corded operation. It’s a simple conversion and ideal for working close to 12V power. B ack 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 “endof-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 20  Silicon Chip 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 siliconchip.com.au DOING THE CONVERSION: less orded! Step 1: Pull the battery pack apart. This is usually pretty easy and involves removing four Philips head screws and this gives access to the sub-C cells which are typically arranged in a circular array with at least one or two other cells on top, all with their tab connections spot welded together. By LEO SIMPSON 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. So why not run a 9.6V cordless drill from a 12V battery or power supply? Why not indeed! Now admittedly, a fully charged 9.6V nickel cadmium battery pack will not stay at 14.4V for very long. In fact, as soon as you start to do any work with the drill, the pack will quickly drop to 9.6V or thereabouts. But the point is that the drill motor itself and its internal speed controller will have (hopefully!) been designed to take supply voltages up to at least 14V. It will probably cope with more than that – say up to 16V or more although power dissipation is likely to be a problem if you start to use the drill for heavy work for more than brief periods. Before we go into the details of this straightforward conversion, let us state from the outset that it works well and the drill itself probably works better than with the original battery pack. Nor does the conversion need to be confined to drills rated at 9.6V. It can certainly be applied to drills rated at 12V, 14.4V or even higher if you have a suitable DC power supply – we’ll discuss the details later. Similarly, there is no reason why you cannot run an 8.4V drill at 12V; seven nickel cadmium cells fully charged will provide a no-load voltage of more than 12V. However, we would be hesitant to run lower voltage drills (ie, 7.6V and below) at 12V. siliconchip.com.au Step 2: Identify the internal connections. The negative end of the battery pack, one of the sub-C cells, may be spot-welded to one of the terminals which make internal contact with the drill circuitry. The other end of the battery pack will connect via a short wire to the other terminal which makes contact with the drill circuitry. So before you start ripping out the sub-C cells, your second step is to identify how the connections are made. Step 3: Remove the outside array of cells from the central one or two cells. In our case, we left one cell connected to the main connection terminals, since it was spot-welded. December 2010  21 Step 4: Drill a 12mm hole in the centre of the battery pack case. This hole takes a 6.5mm IP65 cable gland which will clamp the new power cord. Don’t rush this step. Unless you have a drill press which can provide low speed settings of no more than, say, 400 RPM, you will need to drill the hole in several stages. In fact, even though we do have such a drill press, we used a succession of drills to do this task. Alternatively, you can start with say, a 5mm drill and then expand the hole to the right size with a tapered reamer. Step 7: Strip and terminate one end of the power cable in a standard cigarette lighter plug – Jaycar Cat PP-2000 or equivalent. Make sure you terminate the red wire to the centre terminal of the cigarette lighter plug. Step 5: Install the 6.5mm cable gland. It mounts from the outside with a nut holding it in place on the inside. Step 6: Prepare a suitable length of flexible doublesheathed 2-core power cable for the drill. You need to decide how long you want this power cable to be. If you want to use it powered from the cigarette lighter socket in your car, you will want the cable to be three or four metres long. A shorter cable will mean that you are too closely “tethered” to the lighter socket. Err on the side of making the cable longer – you can always shorten it at a later date if you find it is too long. We recommend that you use Cat WB-1754 flexible 2-core cable from Jaycar. They describe it as speaker cable but the flexibility and the current rating is what we want for this job. It has red and black wires to make + and – identification easy. If you have a suitable length of 2-core double-sheathed 250VAC cable rated at 7.5A or more, that will also do the job but it probably won’t be quite as flexible as the suggested speaker cable. If you’re using mains cable, it makes sense to use the brown wire as + and the blue as –; in any case, make a note of which wire you connect to the centre terminal of the plug. Why? Because if the DC supply to your drill has the wrong polarity, it won’t work. Worse, you might burn out the internal speed control. We will come back to this point later. 22  Silicon Chip Step 8: Strip and solder tin the two wires at the other end of the cable. Pass this end through the cable gland and leave about 15cm slack. Step 9: You need to work out how to make the wire connections to the 2-way connector of the battery pack. In our case we left one of the sub-C cells connected to the connector – since it was spot welded to the can. Now there is an important point here. Since the siliconchip.com.au can of the sub-C cell was connected to one of the battery pack connectors, this was the negative side of the battery pack supply. Hence the negative wire of the 2-core power flex needs to connect to the same point. Fortunately, with a well-tinned and good, hot iron we found that we could solder directly to the can of the sub-C cell. If you cannot do this, you will have to devise some other method of making a reliable connection. It’s also important that you don’t connect both of your wires to the “dummy” cell – even dead, it will attempt to charge (very rapidly!) when connected to 12VDC and the heat could cause damage; perhaps even a fire. Break one of the connections between the cell and battery connector. Step 10: Check your connections. Make sure that the central terminal of the cigarette lighter plug connects to the correct terminal in the battery connector. drill speed. In order to do any real work with the drill, your power supply will need a rating of at least 10A. Power supply options Step 11: That done, reassemble the battery pack with its Phillips head screws. In our case, we found that we need to ensure that internal connections were actually held into the drill’s connector when the battery pack was clipped into place. The best way to achieve this was to leave the dead cells in place. If you don’t that, you would need to make up a suitable packing piece. So as noted at the start, we removed the central cell from the array of cells around it. This allows space for the inside portion of the IP65 cable gland and the cable itself. The photos tell the story. You have a number of options for powering your modified no-longer cordless drill. First, you can run it from the cigarette lighter socket in a car or boat. Note that some cars these days do not have a cigarette lighter socket, or if they do, it is termed an “accessory socket” and it may only have a light-duty fuse, say 3A rather than 30A. The modified drill will blow the fuse in an accessory socket and we would caution against increasing the fuse rating unless you know the harness wiring to the socket can take high currents. Note that the peak current of the drill, when first switched on or when it is stalled (say when driving screws) can easily exceed 20A. Another point to note is that most of these cordless drills short out the internal speed control when you pull the trigger all the way in. So if you want to avoid the initial switch-on surge, squeeze the trigger smoothly rather than jerk it fully in. Your second option is to connect the drill directly across a 12V battery, either a car unit or an SLA battery rated at 9Ah (amp-hours) or more. Either way, you have to be very careful of polarity, otherwise you will definitely blow the speed controller in the drill. In fact, if you are going to use this method of connection (rather than a cigarette lighter plug), then you would be well advised to connect a 20A Schottky diode in series with the positive lead. This can be installed inside the battery pack. Another point to consider if you are using a 12V SLA battery is that you must not discharge it below 11V. If you do, the battery will not be able to be recharged. Third, if you have 12V lead-acid battery charger rated at 10A or more, that could also be pressed into service. Finally, you could also modify a discarded PC power supply to run your drill. We will show how to adapt such a PC power supply in a future issue. Summary: suitable power supplies Step 12: Use an adjustable DC power supply to check operation of the drill. Start out with a low voltage setting, say 6V. If you drill has variable speed control (most have), use the drill’s trigger control to check that it will vary the siliconchip.com.au • • • • • • Car cigarette light socket (fused to 30A) Boat cigarette lighter socket (fused to 30A) 12V SLA battery rated at 9Ah or more Adjustable power supply able to supply at least 10A 12V lead-acid battery charger rated at 10A or more Ex-PC power supply – able to to supply at least 10A at 12V (“XT” supplies usually easier to use than “AT”). SC December 2010  23