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  • 18v drill battery rebuild questions

    Hey guys, I've got a DeWalt 18v drill with tired battery packs. They will run the drill but run time is way down. I charged one up and then took it apart and here is what I found:
    -Most cells at 1.3 v.
    -Two at 1.28
    -Two a 1.29
    -One at 1.4

    -Total voltage charged: 19.5
    -Voltage when drill is running but not under load: 18.4
    -After running drill for 60 seconds voltage while running drops to 17
    -Battery voltage no load after above run: 18.8

    Most of the websites I've checked say to find the one or two bad cells and replace them but I don't see any individual bad cells. So is the whole set over the hill? Any rebuild options or should I just go for new packs? These are about 4 years old and only get part-time use.

    I hate to drop a C-note on a pair of packs if these are salvageable.


    THANKS!!

  • #2
    Try recovering them instead.. Try this.

    Find a REALLY small load. Lets say 100mA.
    R=V/I, 180ohms for 18v.
    60W 120v bulb = About that. (240ohms on resistance. .less when cold). Less is OK, More is not. (less wattage that is)

    Connect it across the 18v drill pack once its discharged enough to no longer spin your drill, And measure the voltage now and then. I believe those 18v packs are 15 cells, So you want to wait untill it hits 0.4v per cell, or 0.4v * 15, or about 6v

    Once thats done, Recharge it fully, And repeat. Your charger may initial refuse to charge it due to low voltage, but most trickle charge it in that state and it should quickly perk up and start charging, Failing that find a 24v+ supply and hook the battery to it in series with the light bulb, untill the pack voltage exceeds 15v. (Should only take a minute or two)

    Discharge to 0.4v per cell has been shown to actualy recover Nicad capacity. The only problem is discharge below 1v per cell can easily reverse some cells as the internal resistance is VERY high below 1v per cell, and any high current reverse is very damageing, Hence the use of very low current.
    Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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    • #3
      Test each cell individually under load, a 6V lantern bulb makes a good test light. I bet you find a few of them dropping down in voltage while the test light is hooked up.

      While I am not an authority on "dry cell" nicads, I have been maintaining aircraft nicads for many years, and I assume the chemistry is similar. I have seen "reversed" cells in a 24V battery go as high as 6v. This is when the nominal voltage is about 1.3V.

      What this means, is that assuming a 20 cell battery, you might have 19 cells at + 1.3vdc, and one cell at - 6.0vdc, for all intents & purposes, you now have a 18.7 vdc battery!

      As for as nicads not developing a "memory", I wish someone would notify Saft America and Marathon, (the manufacturers of Aircraft Nicad batteries) so they can amend their maintenance & inspection schedules. Currently, the batteries require a full discharge, cell shorting, and recharge to 120% of capacity every 100 hrs of aircraft operation.

      http://www.mptc.com/

      http://www.saftbatteries.com/SAFT/Up...ft/PDF/tn5.pdf
      Last edited by Thruthefence; 12-12-2011, 03:11 PM.

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      • #4
        When my packs die I just buy all new cells. I don't mess around trying to find the weak ones. If you are capable and shop around, they can be rebuilt for about half of what a new pack costs.

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        • #5
          THANKS for the fast response, guys. Back to the shop to try your suggestions. Boostinjdm, you may be right in the end but my inherent cheapness won't let me do that until I've spent more than $100 worth of my time trying to save these!

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          • #6
            I have bought rebuilt cores from here:

            http://mrwindystbs.com/index.php

            The price per 18v core is $42. I'm not associated with them, just a satisfied customer.

            Bill

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            • #7
              I've wondered if buying one of these and stripping the batteries out would be a practical source for rebuild.

              http://www.harborfreight.com/catalog...ord=battery+18

              Pops

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              • #8
                You didn't say what type of cells they are.
                Free software for calculating bolt circles and similar: Click Here

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                • #9
                  Originally posted by armedandsafe
                  I've wondered if buying one of these and stripping the batteries out would be a practical source for rebuild.

                  http://www.harborfreight.com/catalog...ord=battery+18

                  Pops
                  I've done that, or something like it, in the past, buying leftover/overstocked etc. battery packs and rebuilding others with the contents. It can work, but difficulty varies considerably with layout. I rebuilt a few batteries for an old Milwaukee, and extended its life for years. But when I opened up a DeWalt 18 volt battery, I found the way they're stacked rather intimidating, and I think it might be difficult to roll your own. You may find it less daunting, but I suggest anyone planning this open up a pack first and see what you're getting into.

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                  • #10
                    Bruto's right. It's not a quickie job, as you have to do some planning on how you're going to lay out the cells and the connections. I suggest some close color prints of the existing one, so you have a guide as you go along.

                    One of the main incentives for rebuilding your own is that you can usually get Sub-C's with a considerably higher capacity than the original cells. I remember my first rebuild came with 1800 ma cells while 15 3300 ma cells could be had for an amount that was still far less than a whole replacement battery.....and if you opt for the replacement battery, you're likely to get the same low capacity cells as the original.

                    Like most things we venture into, the first time can be a bit of a challenge, but after that first one, it's a breeze.....plus you're going to wind up with a battery pack that is far superior than the original.
                    Wayne

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                    • #11
                      If your going to pull it apart replace all of them. I think my Milwaukee 14.4 volt packs were sub C type nicads. I replaced them with nickel metel type. The cost of rebuilding each pack was less than half of a new one.

                      JL....................

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                      • #12
                        Replace all the cells together. The cells as they age change impedance. You want them as matched as possible. Putting two new ones in will not be a fruitful exercise. As Evan said you have not told us the chemistry? I expect NiMH. Around Vancouver there is a couple places that will rebuild your pack. I expect there will be lots of options on something as common.
                        Is there an option for getting a Lithium Ion replacement pack. For performance they kick NiMH big time. You do need a proper charger for them. I know there are some brands of drills that will take both types.

                        Dave

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                        • #13
                          As a subnote, My discharge to 0.4 recommendation is only valid for Nicad.
                          Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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                          • #14
                            Originally posted by JoeLee
                            If your going to pull it apart replace all of them. I think my Milwaukee 14.4 volt packs were sub C type nicads. I replaced them with nickel metel type. The cost of rebuilding each pack was less than half of a new one.

                            JL....................
                            To my knowledge, Dewalt only used NICAD and Lithium cells. I tried rebuilding a pack with NIMH cells before and it was pretty much a failure. It wouldn't charge right, it got hot when charging, and it didn't last as long per charge as the NICADs.

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                            • #15
                              I have gotten some very good deals on brand new battery packs from Canadian Tire here. When a product is discontinued they will often have left over packs that only fit the discontinued product. They clear them out for sometimes 75% off or even cheaper for high quality units. I have bought quite a few that originally retailed for around $50 for as low as $10 bucks. All you need to do is move the batteries to the old pack, sometimes with minimal fuss.
                              Free software for calculating bolt circles and similar: Click Here

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