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conversion of cordless tool battery charger from AC to DC?

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  • conversion of cordless tool battery charger from AC to DC?

    I have a small solar setup at a cabin; running lights and occasionally a laptop. No big inverter (nor do I want one), just a tiny car-battery-plug-in unit that ups the volts from 12 to 18 (the unit can convert the volts to anything between 13 and 24). For the laptop connection to the unit, I cut a laptop charger connection off of an old power supply I sourced from an appliance recycling place. I have a connector between the 'inverter unit' and the laptop connection so I can change to the appropriate connections for other devices running between 12 and 24V.

    I am thinking of using the same unit to power the charger for a Bosch cordless drill (and soon to be added, cordless jigsaw and circ saw). The charger is labelled "al 1820 cv" and charges 12 volt to 18 volt Bosch batteries.

    My question is this: will it be a simple matter of removing the contents (designed to handle the mains 220 volt AC input here in South Africa and convert it down to 18 volts) of the charger and connecting the positive and negative wires to the tiny inverter unit? I'm assuming that Bosch has arcane circuity in the charger to detect when the battery is fully charged and that I will destroy the charger's ability to display when the battery is fully charged if I just connect two wires where I think they need to be connected (just like with my laptop situation - I can run the laptop from DC, but cannot charge the laptop, because of Dell's specific wiring). Pictures of the charger (linked above) show clearly that it has 4 connections to the battery it charges.

    I am aware that once I pull the 220V circuitry I will lose the ability to charge from mains, but I plan to get a second charger so that one is for mains and the other for charging as I've described above.

    Links to any other similar queries anyone knows of would also be helpful. Thanks.
    Last edited by whazzatt; 05-31-2017, 09:56 AM.

  • #2
    Be careful. The charging circuit is probably more than a simple step down transformer from 220VAC to XX VAC. There will be a rectifier to convert the low voltage AC to the correct DC voltage for charging which will then be controlled through the proper charging parameters. Wiring the charger directly to a DC source may bypass the BMS charging system and over charge the battery.

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    • #3
      If this is the actual circuit diagram ..
      http://pro-radio.ru/user/uploads/152537.gif
      Then yes you can apply the proper DC voltage to the two pins labelled DC Input.
      If you are using an 18 Volt Battery Pack then you will need to supply more than 18 Volts.




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      • #4
        Originally posted by NEOH View Post
        If this is the actual circuit diagram ..
        http://pro-radio.ru/user/uploads/152537.gif
        Then yes you can apply the proper DC voltage to the two pins labelled DC Input.
        If you are using an 18 Volt Battery Pack then you will need to supply more than 18 Volts.
        This circuit uses an LM317 voltage regulator and other circuitry to control charging, which YOU NEED.
        Note the DC INPUT from the 4 rectifier diodes. You should first note the voltage at that point in
        normal 240VAC line operation. If the line is not connected, you could feed THE SAME DC voltage
        in at those 2 nodes, and the charger control will work normally. Too high, and you may overheat the
        LM317; too low and it won't function. No need to disable the original AC source, the rectifier diodes
        will disconnect it when not powered.

        All this is quite inefficient converting up and down, maybe it doesn't matter. Bruce Roe

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        • #5
          Originally posted by whazzatt View Post
          My question is this: will it be a simple matter of removing the contents (designed to handle the mains 220 volt AC input here in South Africa and convert it down to 18 volts) of the charger and connecting the positive and negative wires to the tiny inverter unit?
          No. You will likely damage the battery, the boost converter or both. Boost converters are not designed to charge batteries directly, nor are batteries OK with a moderately regulated DC voltage being applied to them forever. Your choices are:

          1) Find a battery charger with a DC input
          2) Find a battery charger with an AC input, but that uses a wall wart style adapter that outputs some DC voltage. Then replace the wall wart with your supply, assuming the voltages and current demands are the same.
          3) Build your own.

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          • #6
            bosch charger unit.JPGbosch charger circuit.JPG

            I opened up the charger unit and it looks like it's not a simple mission! Can't find a DC charger option for these batteries on the market. Seems ridiculous to have to get an inverter to take my 12V up to 220V only to get back to 18V. Maybe someone can spell out a better approach for this rookie - would love to "build my own" but this is just not an option for me...

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            • #7
              Originally posted by whazzatt View Post
              bosch charger unit.JPGbosch charger circuit.JPG

              I opened up the charger unit and it looks like it's not a simple mission! Can't find a DC charger option for these batteries on the market. Seems ridiculous to have to get an inverter to take my 12V up to 220V only to get back to 18V. Maybe someone can spell out a better approach for this rookie - would love to "build my own" but this is just not an option for me...
              That unit uses high frequency power supply on 220V side- big capacitor on the right side of the picture probably has 400-450V rating and I'd expect it to be connected to direct AC rectifier consisting of 4 diodes right above it. The DC level there is about 320V. IMO you need to look for other solution and not try to modify this board.

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              • #8
                Originally posted by max2k View Post

                That unit uses high frequency power supply on 220V side- big capacitor on the right side of the picture probably has 400-450V rating and I'd expect it to be connected to direct AC rectifier consisting of 4 diodes right above it. The DC level there is about 320V. IMO you need to look for other solution and not try to modify this board.
                Thanks Max. Looks like I'll just have to get a small 12V inverter, 150W maybe, and do things the madly inefficient way. At least I could use it to power a few other things I was just going to abandon in my 12V life out at the cabin.

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                • #9
                  Originally posted by whazzatt View Post

                  Thanks Max. Looks like I'll just have to get a small 12V inverter, 150W maybe, and do things the madly inefficient way. At least I could use it to power a few other things I was just going to abandon in my 12V life out at the cabin.
                  That would be inefficient but at least it would work. That circuit most likely uses high voltage power transistor on 320V side to regulate its output for charging purposes, basically charge controller IC regulates that high voltage transistor through optocouplers below transformer so this board is one single system, you won't be able to feed 18V somewhere in the middle of it.

                  It would readily accept 250-350V DC on its 'AC' side as input if you could find that level of DC somewhere as it doesn't have transformer on primary side working on 50 / 60Hz. In that case you wouldn't even need to modify the unit- just plug it in to the DC source and the polarity won't even matter as its internal AC rectifier will always switch it to the correct one.

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                  • #10
                    Originally posted by whazzatt View Post
                    Thanks Max. Looks like I'll just have to get a small 12V inverter, 150W maybe, and do things the madly inefficient way.
                    What makes you think it is inefficient. Or I should say more efficient than what you plan doing with linear circuits. A good TSW Inverter is 90% efficient. Second point is you do not need a different gizmos for every power tool ot thingy you want to charge. They all come with a 120 VAC charger made specifically for the gadget they come with. Makes life real simple and easy. A properly designed off grid battery system is almost always going to be more efficient than a DC system.

                    There is one other way to do this and it can charge anything from a 12 or 24 volt DC power source. Use a RC Hobby Charger. Example a iCharger 106B+ will charge any battery type you can throw at it with any algorithm you want at any current you want up to 10 amps (250 watt charger). That would be up to 6S Lithium, 17S NiCd/MiMh or 24 volt lead acid. Some like the Cell Pro Powerlab 8 is a 1300 watt charger and will charge any battery type of any voltage of today and tomorrow. It will charge a single AAA NiCd cell at 1.4 volts @ 10 ma or up to a 8C LiPo (34 volts) @ 40 amps and everything in between.

                    All you need is a panel, charge controller, and a 12 or 24 volt battery to make them work. Even Chi-Coms make them cheap at Hobby King.to fit any size or budget.
                    Last edited by Sunking; 07-16-2017, 04:12 PM.
                    MSEE, PE

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                    • #11
                      OP, why not keep it simple and purchase a Bosch automotive 12v dc supply charger?
                      That's how I charge my dewalt' sin the field from a house 12v battery without a inverter.

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                      • #12
                        That unit is nothing like the one NEOH showed. Its a high frequency switcher and my suggestion doesn't
                        apply here. Bruce Roe

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                        • #13
                          Originally posted by Raul View Post
                          OP, why not keep it simple and purchase a Bosch automotive 12v dc supply charger?
                          That's how I charge my dewalt' sin the field from a house 12v battery without a inverter.
                          Can't find one anywhere. Nothing online. Went to the Bosch supplier in my area and the guy there said he knew of no such option.

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                          • #14
                            Yes they exist; type in Google Bosch 1830W-DC.

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                            • #15
                              Milwaukee makes a model of battery and tools, that has an optional cigarette lighter charger.

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