FESTO cordless drill CDD 12 battery holder

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This is a replacement LiPo battery holder that can be used in place of the FESTOOL BP12C or BPH12C batteries for the FESTOOL CDD 12 ES or CDD 12 FX cordeless drills.The holder also includes a voltmeter to monitor the battery state and 4 magnetic bit holders. #History I have used several cordless drills but my favorite is still the FESTOOL CDD12 because of its ergonomic design which I have had for 17 years. I have replaced the batteries several times over the years but, somehow, they do not last as long as the originals. When it was time for another battery replacement, I decided to convert it to lithium. #Design Considerations The original batteries were NiCAD's with a nominal voltage of 12V. LiPO's are nominally 3.6V per cell but can go as high as 4.2V. That leaves you with a choice of either 3 of 4 LiPO batteries. I chose to go with the 4 which gives a maximum of 16.8V and a nominal voltage of 14.4V. This is on the high side but I think that the engineer probably used at least 20V components any way and (hopefully) this should be fine. The easy way to do this is to simply rip out the existing batteries and replace them with LiPo's (they should fit in quite easily since you replace the existing 10 batteries with only 4). but decided against this for the following reasons: - you need a spot welder to connect them together (you can solder them but it is not a good idea) - you need to add a connector to balance charge the batteries - somebody might think that this is a normal NiCAD and place the LiPO's in the NiCAD charger. - it is more difficult to replace individual cells - you can use your normal LiPo charger - you can share the batteries between different devices So I opted for standard size 18650 that can fit into a 3D printed battery holder. #Battery Choice 18650 LiPo batteries are easy to find. Unfortunately you can not use any 18650's because most of them will not work on a drill due to the high current needed. For a drill you need at least 15A peak and about 5A nominal. There are several different 18650 that meets this requirement, I chose the SAMSUNG INR18650-30Q's. Note that there are a lot of fakes that will not deliver the needed current, so buy from a reputable supplier! Update: May 2019 I was kindly gifted a second drill of the same model (because the batteries failed) for which I printed a battery holder as well. This was done in black. An alternative way of mounting the back battery contact is also shown. This is basically a flap contact attached with tape to the battery holder and 2 layers of foam tape to ensure pressure on the contacts. The contacts must be domed with a pnuch to ensure good contact.