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Do you have cordless drills or other battery-powered tools lying unloved and unused because their batteries have failed? Don’t do the usual and buy another one because they’re so cheap: think of the planet and resurrect the ones you have! And then build our new power tool charger to really care for your batteries! Bringing a dead cordless drill back to life by Ross Tester A while ago, a builder mate of mine put together a steel-frame house. These are somewhat like a huge Meccano set, with all of the mostly-prefabricated steel frames bolted together. To screw the bolts onto the nuts, he used a cordless drill. Well, that’s not strictly true. He used lots of cordless drills. When I called in to see him he had about half 24  Silicon Chip a dozen cordless drills lying on the scrap heap. When I asked him what they were doing there, he said “One has stripped gears, the rest have dead batteries.” “It’s a lot quicker, easier and cheaper for me just to buy another drill,” he said. That made me think about my own collection of cordless drills. At last count, I had in my “junk box” (and in my case it’s just that) about eight of the things. Or at least they would be cordless drills, if they still had batteries to power them. As it is, they’re just about worthless – but I haven’t the heart to throw them out! My junk collection, for want of a better word, dates back the best part of 20 years – ever since the first electric siliconchip.com.au drills, free of the shackles of a power cord, came onto the market. Every year or two since, I have added another one. And before you think that has been a bit (a lot?) frivolous, I have to say that only a couple of those purchases have been by choice – perhaps to get a more powerful model or one with more features. In one case it was an absolute bargain during a stocktake at a hardware store that I couldn’t walk past – like about 90% off normal price! The rest have been by necessity. Their battery packs have failed – either failed to charge properly (a sure sign is when you only get a few holes drilled out of a charge!) or have been damaged by overcharging. Perhaps the former were caused by the latter. I’ll tell you how I know they were overcharged shortly. I would have preferred to buy a just new battery instead of a new drill. The problem was, and remains, that if you can find a suitable battery pack (not at all easy!) invariably they are significantly dearer than buying a new drill, complete with new battery and probably a case and goodly selection of drill and driver bits into the bargain. Look , we are not talking expensive drills here. All my drills are imports, mostly from China. The dearest one would have cost me about $100 – and that was a hammer drill as well. The cheapest (and most recent) was the princely sum of $18.88 – with carry case, charger, six drill bits, six sockets and socket converter. Of course, you can pay a lot more for a cordless drill. Some of the models intended for “industrial” or tradesmen use can easily set you back several hundred dollars. A sparkie mate of mine has one with so much grunt it almost turns the building if you hang on tight! But he paid more than five hundred dollars for it. The big advantage of these “industrial” models is that they are not only great performers, their battery packs are usually very high quality and most importantly, they usually include a smart (and often very fast) charger which will not allow the battery to overcharge. But we’re not talking about those exotic models here: we’re talking about the everyday models that the average handyman – and some not so handy – would buy from their local hardware chain or department store. The prices would range from sub-$20 to, perhaps, $100 or so if you bought a kit with all the works. As a general rule, the higher the voltage, the more expensive the drill. These days, you can find cordless drills anywhere from 4.8V (toys!) through to 24V (and more). Most common, though, are those in the 12-18V range. Replacing dead batteries When I looked at all those dead drills, it seemed to me that it was a terrible waste of money to keep buying complete new ones just to get new batteries. Surely there was a better way to go? Sub-C (4/5) Nicad cells, the size predominantly found in cordless drills, are easily obtainable, so some time ago I looked at the idea of repacking the batteries with new cells. (You may recall an article in SILICON CHIP about ten years ago where I described doing just that for a mobile phone). I disassembled a couple of drill batteries and found that while they were packed in like sardines, replacing them in like manner was certainly not beyond the scope of the average person (eg, me!). Then I priced the replacement cells. Uh-oh! In a 12V battery there are 10 such cells (10 x 1.2V) – and even in bulk (10+) they were going to cost about eight dollars each. That’s eighty bucks just for batteries. Back then, a new cordless drill sold for about a third to half of that – they’re even cheaper now! Chalk and cheese So I gave that idea away and kept buying drills – that is, until I had the opportunity to talk to Jaycar’s Gary Johnston. He told me that he had been looking at the same idea. What he found was that the Nicad cells used in the cheap imported drills were just that – cheap. And nasty. Even if treated in the best possible way, with charge monitoring and so on, it was highly likely they would fail quite quickly. He maintained that with better cells, even if more expensive, you would not only get your drill back but you would Here’s a collection of sub-C NiCad cells from Jaycar – the same ones we used to repack our cordless drill batteries. These are a higher quality cell than you will find in most cordless drills these days and should last much longer – especially if looked after properly and charged with our new cordless drill battery charger, described elsewhere in this issue. siliconchip.com.au December 2006  25 Here’s why you can easily cook cordless drill batteries . . . Here’s a typical battery and charger stand from a typical “cheapie” cordless drill, in this case a 12V “XU1” model from Bunnings Hardware. It was purchased for the princely sum of $18.88 (ever seen a battery for anything like that?). have one which would perform better (ie, give more power) and would last a lot longer on each charge. And so that started me thinking about the subject again. But we are getting a little ahead of ourselves. Cheap and nasty! Elsewhere in this feature we have shown some photographs of a typical The positive and negative connections to the battery are clearly visible in recesses at the top of the battery – they’re even marked with polarity. But you can also clearly see some form of “sense” connector/terminal (the little metal tab facing the camera). Opening up this battery revealed (presumably) a thermistor connected to this terminal. Note that the insulation has been removed from this cell to allow intimate contact (ignore the fact that in this case the thermistor has been assembled not touching the cell!). However . . . low-cost cordless drill. We’re not singling out this particular drill for any reason – it was one of several similar models we could have shown. As you can see from the photographs and the captions, the battery itself contains a thermistor to (theoretically!) limit charging when the batteries get too hot. But the charger itself contains no terminal to connect to this sensor nor any circuitry to affect charging. Not only that, but the thermistor doesn’t even make intimate contact with the cell it is supposed to. Duh! Worse is the fact that the charging circuitry consists of just one significant component: a resistor. Even the LEDs which show the charging state (ie, charging and charged) are not at all smart: the manufacturers rely on the fact that red and green LEDs light at different voltages and will therefore (hopefully?) come on at somewhere around the right battery charge point. Charge rate and time 12V drill packs from two different drills. The one on the right can be repacked but the one on the left is severely distorted by heat (from overcharging) and no longer fits into either the drill or the charger. Note how the vertical section leans to the left and its edges are wavy, not straight. 26  Silicon Chip While the single resistor will limit the charging current to a “safe” level (and we’ll look at charging in more detail shortly), it does absolutely nothing to prevent overcharging due to time. Most cheap cordless drills have batteries designed to take the standard C10 charge rate – that is, the charging current is 1/10th the rated battery current – so a typical 1.8Ah battery should suffer no damage if it is charged at about 180mA. Mainly because the current decreases as the battery charges and its voltage rises, the normal charging time is not 10 hours as you might expect but is siliconchip.com.au . . . looking inside the charger, you can clearly see the positive and negative charging terminal connectors, identified on the top of the well with “+” and “–” symbols. But there is no other connector for the sense circuitry – even though there is clearly a provision for it in the moulding (top of pic, bottom of well). increased to about 12-14 hours. And, as they say in the classics, there’s the rub: more batteries (I’ll admit it, mine included) are ruined by being left on charge for far too long than wear out through use. It’s easy to see how: you use the drill until it starts to lose its power, then pull the battery and place it in the charger. You mean to take it out next morning before you go to work – but forget. In fact, it might be a couple of Here’s the evidence! The charger’s PC board essentially contains just one significant component – a current-limiting resistor! The red (charging) and green (charged) LEDs sort-of monitor the voltage and the diode makes sure a reverse-polarity plugpack won’t cause damage. No wonder with chargers as simple as this that so many batteries are cooked! days before you remember – and all this time current has been forced into a now very-much-overcharged battery. Batteries overheat when overcharged (they also overheat when charged too fast). Heat is the biggest enemy of rechargeable batteries. Apart from the fact that overheating can – and does – kill the cells themselves in various ways, it can also cause deformation of the plastic battery case. One of my cordless drill battery cases is actually deformed so badly due to heat that it won’t even fit into the drill any more! So you may end up with a double whammy: dead cells AND a battery which won’t fit the drill even if you replace the cells with new ones. It’s therefore important not to overcharge these batteries – and that brings us to the nub of this article: a cordless drill (or any other low-cost tool) battery charger. Before we get there, though, we are going to look at repacking your dead cordless drill battery with new cells. After all, you want something worth charging! Dissassembly Fortunately, most battery packs are assembled with Phillips screws, so you shouldn’t have too much difficulty there. You might come across some with tamper-proof screws, in which case you’ll need a suitable tamper-proof screwdriver to tamper with them! Jaycar have a number of sets of these screwdrivers and/or bits for cordless drills. Very occasionally, you might come across a battery which is welded rather than screwed together. If you refer to the article I mentioned earlier on repacking the mobile phone battery (SILICON CHIP, April 1996), you’ll see how to get around this wee problem. Basically, it involves gently squeezing the longest edge of the case in a vyse and tapping it until the weld cracks, then prising the joint open. Once you have removed the screws Yet another dead cordless drill battery, shown with the case opened at left and the cell pack removed (above). Note the way the cells are stacked to allow them to fit into the case and the use of wide sticky tape to hold them together. To repack the battery, you need to copy the old arrangement exactly. siliconchip.com.au December 2006  27 These cells have long solder tags. There are no polarity markings on the cells and polarity is important! It’s safest to identify the polarity with a multimeter. As it happens, the cell on the left has its “+” to the top, the cell on the right its “–” to the top. There is also an indent around the top of the cell at the “+” end. (and stored them where you can find them again later), pull apart the battery pack carefully, noting where any loose bits (eg, catches, springs, etc) fly out from. Now gently remove the battery pack in one piece, taking careful note of how the cells are assembled. You are going to have to copy this arrangement exactly with the new cells, otherwise it will probably not fit back together again. Also make a note of the number of cells. It is not unknown for a “12V” cordless drill to contain nine or even eight cells. Nevertheless, the rule is that what comes out must be replaced exactly. Unless you are very unlucky, the manufacturers would have used “subC” cells (which, incidentally are 4/5th the size of a C cell). And that’s what you need to buy – the same number as were used originally. For those who are mathematically challenged, a 7.2V drill should have six cells. A “9V” drill will probably have seven (though it may have eight and possibly be labelled 9.6V). A 12V drill will have ten cells, a 14.4V will have twelve and an 18V drill should have fifteen. The cells in the original pack will almost certainly have tags that are welded together. Most hobbyists don’t have spot welding equipment so you are going to have to solder – should have some residual charge. Remember the old adage: “measure twice, cut once”. In our case, measure polarity twice, solder once! Assembly and soldering the new ones and this creates a small dilemma. The solder tags are normally long enough to reach the edge of the battery (indeed, the Jaycar cells we used were even longer), giving you enough space to solder the tags together. However, this then gives you exposed metal which you must be very careful not to short circuit. These Nicad and NiMH cells, when charged, are capable of delivering enormous currents into a short circuit – perhaps 100A or more – for a short time, which can easily cause a fire. So be careful to insulate any exposed metalwork. Also when soldering be careful that you don’t melt through any plastic insulation and allow a “+” tag to touch the “–” case. It’s a common reason for cell failure. Cell polarity On many cells, there is no polarity marking as such. The very last thing you want to do is solder in a cell back-to-front. Even though you can often tell polarity by looking at the tag connections (negative connects to the cell body, positive to an insulated pad), by far the safest way to definitely determine polarity is with a digital multimeter. New Nicad and NiMH cells, even those supplied flat from the factory, + When you have worked out how the new cells are to go together (using the old pack as a template) make up the new pack, soldering the tags as you go. Trim the tags back to the minimum required for a good solder joint (there’s less danger of a short that way). Most packs have cells arranged in both the horizontal and vertical planes; again, you need to arrange your new pack the same way. It’s probable that you will need some tape to hold the cells together. Many factory packs used very wide sticky tape because it is wide and is also very thin (much thinner than insulation tape or gaffer tape). If we were making up a battery pack from scratch, we would normally think about using heatshrink tubing. But even that may be too thick to allow the pack to go back into the holder. The thermistor With one proviso, if there is a thermistor in the original battery pack, it should be included in the new one. The proviso is that if you intend to build the Power Tool Charger Controller described later in this issue, you need to install the thermistor designed to go with that charger. First, we’ll look at simply replacing the existing thermistor. Have a good look at how it is mounted and connected. Most batteries have the insulated covering on one cell removed so that the thermistor can make intimate contact – see the photos of the battery pack earlier in this article. Duplicate the original and remove the insulation from the cell in the same position. Be careful that you don’t al- – + + Once you have identified the “+” and “–”, mark the cells so there can be no mistakes. The cells which make up sticks are simply butted together (as tightly as you can) and held securely while you solder the tags. 28  Silicon Chip You may choose to leave the tags intact and solder them where they touch (use a well-tinned iron, clean the tags and make the solder joint quickly – you don’t want to heat the cells or melt the insulation) . . . . . . but to minimise the risk of short circuits, our preference is to trim the tags so that as little metal is exposed as possible – just enough to ensure a reliable solder connection. You still may need to insulate the metal. siliconchip.com.au The battery conectors on the original pack, which must be transferred to your new pack (see photo below). Once again, there are no polarity markings on the battery connectors, so check and check again. A reversed connection to the drill won’t hurt but a reverse connection to the charger most certainly will! low anything to short to it! When the new cell pack is completed, check that it still fits inside the case the original pack came from. If necessary, adjust the cell positions so that it does. The new charger thermistor One of the features of our new Power Tool Charger is that it closely monitors battery temperature. It does this via a thermistor fitted in similar manner to the original thermistor; the difference is that both its connections are brought out to a 3.5mm socket with a suitable lead connecting the thermistor to the charger. Mount the thermistor on the cell without insulation (it can be glued on with a drop of super glue) and find a suitable place to mount the 3.5mm socket. In most batteries, space is at a premium but as the cells are round, there should be some gaps somewhere big enough to house the socket. Make sure you don’t mount the socket where it fouls either the charger or the power tool when the battery is inserted. If there is simply NO space to mount the socket, it may be necessary to bring the leads out through a hole in the battery case to an external 3.5mm socket. But this really would be the worst-case scenario because there would be a real danger of Here’s our finished battery, complete with terminals, ready to be placed back into the case. We haven’t as yet fitted the thermistor or its socket (that is done at final assembly), although we have removed the insulation on one cell to accommodate it. This particular battery is 12V (10 cells x 1.2V) siliconchip.com.au Higher rated cells – or perhaps NiMHs? catching the leads or socket as you work! Battery connections You are going to need the connections from the original battery, along with any hardware which holds them in place. So they will have to be carefully removed. Before doing this, make sure that you know which one is the positive terminal and which is negative (use your multimeter; hopefully there will be some charge in the old pack to help you). If the cells are so dead that there isn’t enough charge to measure, examine the charger to work out how the pack went into it – this will allow you to determine polarity. Again, use your multimeter if there are no markings on the charger terminals. The cells we used in our reconstruction were pretty much the same as the dead’uns which came out – 1.8Ah nickel cadmiums. We did that deliberately, if for no other reason than wanting to maintain the “originality” of the cordless tool as much as possible and use the supplied charger. This would keep the charging times and currents pretty much identical. But could we – should we – have taken the opportunity to put higher rated cells in to get more usable life? And what about using nickelmetal-hydride (NiMH) cells instead of nicads? To answer the first part, yes, we could have upgraded to higher capacity nicads – but as hinted at above, this would probably have altered charging times. Remember that the charger supplied with most cheap cordless tools has little more than a current-limiting resistor, which would almost certainly prevent the charger delivering the higher charging current required. As far as NiMH cells go, they’re becoming even more readily available than nicads and they don’t develop the dreaded “memory effect” that nicads do. More to the point, they are now available in much higher capacities: 2500, 3000 and even 3500mAh are common (we’ve actually seen 4000mAh sub-C NiMH cells advertised on the ’net, although that could be a marketer taking a bit of licence). And prices have dropped, too. However, most of the information we have seen claims that nicads are better than NiMH for short term, high current drains such as cordless tools. To counter that, we’ve also seen high-capacity NiMH cells with a 36A short-term rating, which are claimed to be “ideal for cordless drills etc”. So it’s up to you which way to go. Remember that if you do elect to increase the battery capacity, you’ll have to adjust charging times to compensate, especially using the original charger. December 2006  29 Remember that the charger “+” terminal connects to the battery “+” terminal. To avoid any possibility of mistake, we normally mark the battery terminals with a “+” and “-” and mark the terminals of our new pack the same way. Once again, on an original battery the terminals are normally welded to the cells; by far the easiest way to remove them is to cut the cell tags with a pair of fine snips or even scissors as close as possible to the top of the cells. Sometimes the weld is not particularly strong and you can remove the terminals with a pair of pliers. Even if the metal tears (and it often will) this can be repaired during soldering. When completed, you should have a cell pack that is as near a duplicate of the original as possible. Reassemble it back into the original battery case, making sure the terminals emerge in the right places. Virtually all battery cases have a keyway to stop you putting the battery in back to front but you must ensure that the battery pack is in the right way in the case. On a drill, reversed connection normally wouldn’t be too much of 30  Silicon Chip a problem (simply flip the reversing switch!). But on a charger, it would be disastrous. Speaking of chargers . . . Of course, you could use the existing charger with the new battery pack. But with all the problems we talked about before, do you really want to risk damaging the new pack? Elsewhere in this issue, we present a microcontroller-powered charger specifically intended for cordless drills and similar battery-powered devices. John Clarke has designed a beauty: it simply plugs “in line” between your existing plugpack charger and the charging cradle (it basically ignores any built-in charging circuitry) but will monitor temperatures and charging voltage to prevent over-charge – and even turn itself off if the other methods fail. Your new battery pack will last dramatically longer than the old one did and give you much better performance into the bargain. Even if you haven’t repacked your battery cells (perhaps you even have a brand new drill?), we commend this Special Offer from JAYCAR ELECTRONICS Exclusively for SILICON CHIP readers, Jaycar Electronics have a special offer on a pack of ten sub-c 1.8Ah Nicads, as we have used in this article. The pack of ten (for 12V) would normally sell for $79.50 but for December and January, all Jaycar Electronics stores and their online Techstore will have the tenpack for just $62.50 – as long as you tell them you saw the offer in SILICON CHIP! new charger to you. It will keep the battery in tip-top condition and save you the problem (and expense) of having to repack the cells in the future! Just remember that you’ll have to replace the thermistor and fit a 3.5mm socket. SC siliconchip.com.au