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Fire up your cordless drill WITH A LITHIUM BATTERY PACK Do you have a perfectly good cordless drill or other tool with a dead battery pack? Repacking it with sub-C Nicad cells used to be the standard approach to recycling but now you can do much better – use a lithium polymer battery pack. You’ll get more battery capacity for less weight and you can discharge them more deeply. by LEO SIMPSON B ack in December 2006 we featured an article on how to bring dead battery packs for cordless power tools back to life by re-packing them with sub-C Nicad cells. Fast forward seven years and the problem still exists, although the cost of sub-C cells 12  Silicon Chip makes the proposition not as attractive as it was. And if you don’t upgrade the charging circuit with our Cordless Power Tool Charge Controller, you can look forward to the same battery pack failure in a few years time. But now there is a better way – up- grade to a lithium polymer battery pack. With the rising popularity of radio-controlled helicopters, Li-Po battery packs have become a relatively cheap and very potent power source. They are far more energy dense than anything based on Nicad or nickel siliconchip.com.au metal hydride cells and don’t suffer from “memory effect” which is a (bad!) feature of Nicad-based battery packs. This trend has been reflected in the rising sales of Li-Po cordless drills. Almost all 18V, they are lighter and more powerful than the superseded Nicad-powered drills but there is a big drawback – they are expensive. And what do you do about a perfectly good old drill with a dead or dying battery pack? This problem confronted me recently when I went to use a 24V cordless drill which I purchased some years ago from Dick Smith Electronics. It’s a powerful but heavy beast which can also be used as a hammer drill and indeed I have used it many times with masonry bits. Sadly, its battery pack has been gradually losing capacity, to the point where it would only drill a few holes in timber after a recharge, whereupon it would just wimp out. Clearly, a replacement battery pack for this drill is highly unlikely to be available now and even if it were, it is likely to cost more than the original price of the drill plus its battery pack, carrying case etc. Yes, you can get battery packs re-packed but again, the cost would be very unattractive! However, the drill is quite ruggedly built and has a gearbox with metal gears rather than plastic so I was loath to discard it. OK. So I decided to follow the lead The original dead (and as-yet unmodified) 24V Nicad battery pack from my cordless drill with the more-powerful 22.2V Li-Po replacement alongside, showing the relative sizes. The Li-Po is an easy fit inside the old battery case. given in the June 2013 issue of SILICON CHIP in an article on Lithium batteries. In fact, that article was devoted to LiFePO4 (lithium iron phosphate) rechargeable batteries but this article is devoted to the more potent lithium polymer batteries used by aero-modellers which are available in a wide range of capacities and in 11.1V and 22.2V packs. In my case, I wanted to replace the original 24V 1.2 amp-hour Nicad battery pack with an equivalent or better Undoing the (in this case four) screws allows the top of the battery to be lifted off, revealing the 20 Nicad cells packed inside (20 x 1.2V = 24V). Note the two cells on top which hold the battery connector – this part is important! siliconchip.com.au Li-Po battery. To cut to the chase, the one I chose is a Zippy 35C series 6-cell 22.2V pack with a capacity of 2450mAh, almost double the energy rating of the original battery pack. As an aside, to calculate the energy rating, just multiply the battery’s nominal voltage by its amp-hour rating. Multiplying 22V by 2450mAh gives an answer of 54.39 watt-hours. Even better, the nominated Li-Po battery pack weighs 390 grams which is significantly less than the original The original cells are now effectively garbage (but note the symbol telling you not to dispose of them in the trash). However, you do need to carefully remove the battery connector from the top two cells, noting + and – connections. October 2013  13 DRILL CONNECTOR BATTERY LIFESAVER PCB – Fig.1: using the Battery Lifesaver makes connection pretty straightforward. Make sure you use heavy duty cable to cope with the high charging currents and even higher discharge currents B+ involved. 11108131 10V + B– L– + CHARGING CONNECTOR Reproduced from our September 2013 issue, this is the tiny “Battery Lifesaver”, a very worthwhile addon for any battery powered tool. It prevents the battery being permanently damaged by over-discharging. and its discharge rating is very high: 35C constant; 45C burst. This means that it is rated to deliver a current of 85 amps with a peak of 110 amps! Now it is highly questionable whether the output leads of the battery would survive more than a few seconds at such huge currents but does indicate that it would be more than adequate to cope with the discharge current of a 24V cordless drill, even if you stalled it. So why did I choose that particular model? The main reason is that it will fit into the battery holder of the drill – pretty important. A smaller battery could have been fitted but that would mean less battery capacity. Now a search will quickly reveal that there are numerous sources of these batteries via the internet but many of these are a questionable proposition. There are plenty of reports where people have purchased batteries online (did someone mention China?) only to find that their stated capacity is mythical, to say the least. You also need a suitable charger and this is where Li-Po batteries are far more demanding than Nicads, although most drills with Nicads would give far better life from their battery packs if they had better designed chargers. There are plenty of chargers for LiPo batteries but most of these are not intended for use with 230VAC mains supplies – this seems to be related to the fact that most of these Li-Po battery packs are used away from mains source. Having said that, most Li-Po chargers are designed for an input of around 11 14  Silicon Chip 30A BLADE FUSEHOLDER CELL BALANCING CONNECTOR to 18V at 5A or more, ie, 85W or more. Which means that these chargers are fine working from 12V batteries in cars or 4WDs but that means they need a charger and a 230VAC supply which can deliver 12 to 18V at about 6A. With all of those questions to be answered, I decided to purchase the battery, a suitable charger and 230VAC switchmode supply from the one source: Hobby King. www.hobbyking. com Refer to the Battery Lifesaver article (Sept 2013) for setup instructions LITHIUM–POLYMER BATTERY PACK To be specific, in addition to the above Li-Po battery, I purchased a Turnigy Accucell 6 charger and a Hobby King 7A power supply ($18.16). The charger is capable of charging Li-Po or LiFe (lithium iron phosphate), Nicad or NiMH plus lead acid batteries. Furthermore, it can charge lithium batteries up to six cells, Nicad/NiMH up to 15 cells and lead acid up to 20V. All up, including accessory leads, the total cost was around $82. It’s not even a real tight fit to get the new Lithium battery, Battery Lifesaver PCB and the cabling inside the old case. At top you can see the power tool connector salvaged from the old cells – it’s glued in position where it would have sat in the old battery. siliconchip.com.au I hasten to say that this is more than I paid for the drill originally but probably less than a replacement Nicad battery pack, if one to suit the drill was actually available. It is also considerably less than the cost of an equivalent new drill with an 18V lithium-polymer battery pack. Fitting the battery Superficially, fitting the lithiumpolymer battery in place of the old Nicad batteries is straightforward; unscrew the battery case, remove old, bung in the new. In practice, it is quite a bit more involved because the new battery pack has nine leads; two for the main battery leads and seven for sensing the individual cell voltages. This last feature is most important because all the cells must be equalised for correct charging, something which the purchased charger will do. I also wanted to fit a high current fuse because lithium-polymer batteries do represent a fire hazard if their output is shorted. Hence I fitted a 30A blade fuse holder and 30A fuse (Jaycar SZ-2040; SF-2139). However the most important part of the new battery installation is a method of avoiding over-discharge. Lithium-polymer batteries will be damaged or destroyed by being overdischarged. In normal applications such as in model aircraft, the speed controllers contain circuitry to prevent overdischarge but in this drill application we have to provide it separately. We assume that all 18V lithium-polymer drills also incorporate over-discharge protection. Fortunately, we have an ideal means of preventing over-discharge in the form of the Battery LifeSaver featured in last month’s issue. This has very small PCB and has a profile so that it can be squeezed into tight battery compartments. You will need to purchase a kit for the LifeSaver (available from Jaycar; Cat KC-5523 <at> $29.95) or otherwise obtain the components and assemble it according to the instructions in last month’s article (September 2013). The PCB is available from SILICON CHIP. drill’s battery pack. In my case, it was pretty straightforward. Just remove four self-tapping screws and the old Nicad cells slip out easily – they are all spot-welded together to make up the 24V pack. Two cells are in the small cylindrical section which carries the contacts for the power connection inside the handle of the drill. Pulling out those last two cells also pulls out the springy contacts which are also spot-welded. Now it is most important that the polarity of the leads connecting drill connector are correct. If the polarity of the supply connecting to the drill is reversed, the drills inbuilt speed controller will not work and there is even the possibility that if will be damaged. So it is important to check the polarity before you make the connections. Have a look at the cylindrical section of the battery pack and you should see + and – symbols moulded into it. These can be clearly seen in a number of the photos in this article. The contacts, still on their moulded plate, must be clipped off and ultimately be connected to two leads which will connect to the Battery Lifesaver, according to the diagram of Fig.1. We used some of our old friend, JB Weld, to ensure the contacts stayed in exactly the right spot. (Just make sure you don’t let any glue get on the outside of the contacts themselves, as this would make a very nice insulator.) Commercial Li-Po Tools: Do they check cell status? When preparing this article, we wondered: do commercial battery-operated tools using Li-Po batteries have the ability to equalise cells or even check individual cell status, as this project does? The easiest way to check this was to scour the shelves at a couple of major hardware stores – and we have to report that we didn’t find a single tool that had any form of charger multi-connector, as would be required if it was capable of monitoring and equalising individual cells. That’s not to say none have such a feature – but we couldn’t find any! Therefore, we have to assume that the battery life of most Lithium battery power tools, even those in the stratospheric price range, may be just as compromised as would much cheaper/older tools using Nicad or NiMH batteries. All it takes is one cell below par – and that is not just a possibility, it’s a certainty as the tool ages – and you would find that battery life will very quickly start to fall. And without individual cell monitoring, there’s nothing you can do to eliminate this or even minimise the problem. A stupid boo-boo to be avoided Before going any further, you need to check how the new battery pack and other components will fit inside the case. Now there is a trap here and I fell right into it. The completed “new” battery, offering much more “grunt” and with individual cell monitoring/equalising, should last much longer than the original Nicad pack. If there is enough space in your battery, you might even be able to glue the two sockets virtually inside the case for an even neater finish. Assembling the new battery pack The first step in the process is to remove the old Nicad cells from the siliconchip.com.au October 2013  15 You may have noticed that there is a difference between the photo in the opening shot (and the one below) and the photos on pages 14 and 15. Haven’t twigged to it yet? Look at where the charging and monitoring sockets emerge from the battery pack . . . I did a trial assembly to see how it all fitted together comfortably. I (foolishly) reasoned that it would be best to have the two sockets emerging from the back of the battery because that seemed to be the logical place for them. So I cut two slots in the back of the compartment; one slot for the 2-way lead for the battery charging connector and the other for the 7-way lead from the lithium-polymer battery pack which is necessary for monitoring and equalising the cells during charging. I then assembled everything, including anchoring the leads with silicone and screwed it up. The two battery connecting leads need to hang out from the finished battery pack so that it can be charged when necessary. (Your battery might have the room to mount them deeper inside and glue them in place, which would look a little neater). After charging the new battery pack, I duly clipped it into the drill, switched it on and it all worked. Beauty! Later that day I went to put the drill into its carrying case and then my boo-boo became abundantly clear. As the two connecting leads exited from the rear of the assembled battery pack, they effectively stopped me closing the lid of the carrying case. Naughty words were uttered. So I had to do it all again, with the leads correctly exiting from the front of the case. Most drills of this type come in a carry case so would need to be treated in the same way. Learn from my mistake. You can see how the various wires are tucked into the case. You need to use fairly thick hook-up wire to ensure good current carrying capacity. However, if they are too thick it becomes difficult to make them sit inside the case while you clip it together. In use As our photo on p12 shows, charging a Lithium-Polymer battery properly is a little more complicated than bunging the battery in the charger, as you would have done originally. Apart from the obvious need to plug in both the charging socket and balance socket Hobby King’s $15 “LiPro Balance Charger” will also handle Nicads, NIMHs, SLAs, and more. to the charger, you need a charger designed specifically for the purpose of charging LiPos. And while this might set you back a few bob, once you’ve switched to LiPo batteries you’ll wonder why you ever persevered with Nicad or NiMH. After we purchased our setup, we even found a cheaper charger (~$15 at Hobby King). They even admit it is a copy of the real thing but we bought one anyway and found it worked just as we would have expected. It will charge LiIon, LiPo/LiFe (1-6 cells), Nicad/NiMH (1-15 cells) and even lead-acid (2-20V) with charge rates from 0.1 - 6A, from a DC input of 11-16V. Hobby King “CELL CHECKER”: a great investment! While gathering together the bits’n’pieces for this article, we happened to notice this USB flash-drive-sized cell checker, also at Hobby King. It certainly looked interesting from the description, so we added it to our order. We’ve got to say it is simply brilliant for checking LiPo cells with a multi-pin monitoring “port”. And at the price – a princely $3.15 (plus P&P) if you use Lithium Polymer batteries, you cannot afford NOT to have one of these in your toolkit. It’s an incredibly cheap investment for what amount to relatively expensive batteries. 16  Silicon Chip All you need to do is connect the integral plug on the Cell Checker to the balance socket of your battery pack. The Cell Checker then steps through each cell (up to six in the battery), displaying the individual cell voltage on the digital readout then the total voltage of the battery. If you check the battery before and after charging, it will give you a very good indication on the state of all cells. Weighing just 13g and measuring 70 x 24 x 13mm, it can attach to your keyring so you’ll always have it handy. Visit www.hobbyking.com for details. SC siliconchip.com.au