Introduction: Dirt Devil M607 Spider Vacuum Li-Ion Battery Conversion

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About: I love to tweak, repair and fly things :)
Disclaimer: At time when I came to idea about writing this instructable, the conversion was already done. My apologies about lack of many colorful photos with same stuff from different angles. I hope that added diagrams and description will compensate that.

My relative asked for help with her Dirt Devil M607 Spider robot vacuum. On full battery charge thing barely went few meters then stopped with low battery warning light. Voltage check revealed that in 5 seconds full battery voltage dropped from 15.2V to 8V. Alas, the battery is dead and new battery is required. Original 1000 mAh NiMH battery pack cost 35 euros + shipping. Although... I have a heap of unprotected 18650 Li-Ion cells, salvaged from old laptop batteries. These cells still have more juice left than brand new original battery had. Nowadays DC converters and BMS are available as cheap and easy to get modules. Sooo... let pimp the little devil, DIY style.

Supplies

Optionally:

Step 1: Design Notes

Vacuum current draw is 1.25A on start and 700..850 mA during work. Thus is compatible with any 18650 cell.

1000 mAh NiMH stock battery pack have 12 AAA size cells, connected in series and built-in NTC 10K ohm thermistor for thermal protection, internally connected to negative battery pole. Fully charged battery have 15.2V voltage. Battery is charged by internal charger, located on main circuit board from external 19V 600 mA power supply. Power supply "-" and battery "-" poles are connected to common ground. Charging socket and power switch are directly connected to onboard charger via cable and have no direct connection to battery. All commutation happen via main board as well. Battery is connected to board via separate cable. Battery compartment internal dimensions match to 18650 cell battery size in 2x2 layout with 2mm headroom at top, 3mm at width and 12mm at length. That free us from tinkering with compartment enlarging. Original battery spring contacts must be removed to free space for BMS board.

To simplify things here, do not mess with internal charger. It is incompatible with lithium battery chemistry anyway. Use separate charger module instead. Obvious solution should have 4 cell lithium battery in serial connection (4S, 16.8V) with DC step-down converter to 15.2V between battery and appliance circuit. However additional testing revealed that this vacuum can work from full 4S battery without issues. Thus additional DC step-down converter module is not required. The remaining parts to have are lithium battery charger module, BMS module for battery, and reverse protection modules or Schottky diodes between charger and BMS and between BMS and main board.

You can keep the bundled 19V power supply that is bundled with vacuum. Charging current then is limited to 650 mA. It is also possible to repurpose old laptop 19V power supply which can support larger charging current. If you are using different 19V power supply, pay attention to charging current supported by your 18650 cells, forward current of reverse current protection solution that you are using and current supported by power supply itself and use the lowest value from both mentioned as charging current for your conversion to avoid smoky surprises. Do not exceed 650 mA current if you are using bundled power supply or it will become damaged.

Battery, BMS and thermal protection

Conversion design assume that new battery is made from four 18650 Li-Ion cells in serial connection (4S). Full battery voltage is 16.8V and 13.2V when empty. All four cells must have the same capacity and internal resistance.

For 4 cell battery obtain 4S Li-Ion BMS module, preferably for 18650 cell based based battery in 2x2 layout. Pay attention to supported constant work current and charge current - must be at least 2A or more if you are using different power supply.

The best thermal protection is achievable if BMS module have built-in NTC thermistor support. it will automatically shutdown the charging process if during charging cells become too hot and also will shut down the battery if cells become too hot during work. Otherwise connect NTC thermistor between brown NTC wire in vacuum battery cable and battery "-" pole. It will not help against battery failures during charging, but at least will shut down the vacuum if battery become too hot during work.

You can keep and reuse thermistor from stock battery pack. It is compatible with BMS boards, mentioned under links in Supplies section. But if your beliefs forbid messing with dead batteries (don't be afraid, dead NiMH battery will not blow), get another one.

Use two 18650 2-cell holders stacked one on another. Or weld cells together at 2x2 layout in serial connection.

I used cells from same laptop battery. Both have 2000 mAh left and 75 ohm internal resistance. SMT mountable 18650 holders was my choice because they was easier in battery assembling and disassembling during conversion testing and still are small enough to fit in vacuum battery compartment. And... I don't have a spot welder.

Charger module

Use dedicated 4S lithium battery charger module or other adjustable CC/CV DC voltage regulator.

I used adjustable LM2596S based CC/CV DC converter module as the most available and easiest in installing. It support charging from any 19V power supply, support current up to 3A and exactly fit inside vacuum case installed vertically in empty space at left side of battery compartment. It require simple voltage and current adjusting before first use.

Reverse current protection

Reverse current protection between BMS and main board is required to avoid 19V from internal NiMH charger going directly back into battery. And reverse current protection between charger module and BMS to avoid discharging the battery through charger when power supply is not connected. Either Schottky diode for 30..40V reverse voltage and 3..5A forward current (diodes with larger current rating stay colder too) - 1N5820, SR540 etc.) or so called "ideal diode" module. Schottky diodes are dirt cheap though not very effective due to voltage drop and subsequent noticeable warming up during work. Fortunately ready to use "ideal diode" modules for reverse current protection are available and should be used at least between battery and device.

I used 1N5819 Schottky diode between charger module and BMS because charging current with original power supply is only 650 mA. And "ideal diode" module between BMS and main board.

Charging current

Regular 18650 Li-Ion cells support charging rates between 0.5-1C where C mean the battery capacity in Ah. For example, largest supported charging current for 2000 mAh 18650 cell with allowed charging rate 1C is 2A (1 * 2000 (mAh) / 1000). Find supported charging current for cells used in your battery and write it down.

Airflow and wiring

In M607 vacuum model air outflow from sucking partially goes through battery compartment and subsequently is used for battery cooling. Thus do not cover rectangular openings at inner side of battery compartment. And try to keep openings in battery compartment cover free as much as possible. Place all added wires at outer side of battery compartment. Plan the wiring before cutting and soldering begins and find required wire length before all is soldered together.

Step 2: Disassembly

If stock battery is still installed, take it out.

Carefully pull out red rubber strip at collision detection sensor cover at front side. Try to avoid protruding holder extension tearing. Unscrew two screws under strip. Take the cover off.

Open dust container cover at top side. Pull the container out.

Unscrew 6 screws located in holes around case bottom cover near cover edge. Do not touch flat head screws at middle of bottom cover. Remove the top cover. Pay attention to label glued under charging port and power switch on both top and bottom case covers. Carefully free only upper label part and take the cover off.

Mark wires in battery cable to knew which wire belong to which battery pole. Pull out all three contact springs from battery compartment. Unsolder springs from wires.

Step 3: Thermistor Reusing

If you are keeping the old battery pack thermistor, cut plastic sleeve around battery and cut metal strips between first and second cell layers. Cut off thermistor from battery pole pads. Dispose battery remains in appropriate way.

Step 4: Installing the Charger Module

Solder charger module input to vacuum charging port socket poles on opposite side of port and power switch board with appropriate length wires. Input "+" must be soldered to socket middle pole. Input "-" to socket outer pole. Choose wire length long enough to place them around battery compartment at outer side.

Adjusting

LM2596S CC/CV module must be adjusted before use. It require a multimeter with current measuring support up to 10A.

Connect the multimeter between module output "+" and output "-" poles. Plug in the power supply. Adjust voltage with "CV Adjust" potentiometer to have 16.8V in output. Unplug the power supply. Switch multimeter to current measuring in 10 A current mode and connect between module "OUT +" and "OUT -" poles. Plug in the power supply. Adjust current with "CC Adjust" potentiometer till multimeter display one of current values below:

  • For bundled 19V power supply - 650 mA.
  • For different 19V power supply - the lowest current between your battery cell charging rate (see above) and current supported by your 19V power supply. Do not exceed 3A current.

Full LM2596S CC/CV module adjusting video instruction in YouTube.

Step 5: Battery Assembling

If your BMS support NTC thermistor, solder thermistor to "NTC" holes or pads on BMS module board (see the picture) and isolate naked end of free NTC sensor wire (brown) in vacuum battery cable with electrician tape. Otherwise solder thermistor between NTC sensor wire (brown) and battery "-" wire (black) in vacuum battery cable. For thermistor soldering use thin and elastic wires. Thermistor wire length must be long enough to reach the middle of battery in assembled state with some headroom to ease the assembling. Usually 5..8 cm.

Assembly the battery together. Solder cell poles to BMS in order displayed in picture at this section. During battery assembling put thermistor with wires already soldered and isolated at middle of battery from inside and fix it to one of cells with electrician tape. Solder wires to BMS P+ and P- pads. Temporary isolate these wire ends with tape to avoid accidental shorting via something during next assembly tasks.

Isolate BMS from battery with heat shrink tube or wrap it with electrician tape. I wrapped it in kapton tape.

When battery is assembled and isolated, temporary connect it to charger module - charger module output "+" to BMS P+ pad and charger module output "-" to BMS P- pad - plug in the power supply and measure voltage on BMS pads. If voltage is not 16.8V, adjust it in charger module. See notes about LM2596S CC/CV module adjusting adjusting above.

Note about BMS module mentioned under link in Supplies section. When battery cells are connected to BMS, output poles "P+" and "P-" will have 0V. It is normal behavior when cells are connected at first time, after short circuit, battery overheating and when battery is discharged. To start BMS, connect full 4S battery voltage (16.8V) to BMS "P+" and "P-" poles for a second. Eq. when all circuit is assembled, simply plug in the power supply into the vacuum.

Step 6: Reverse Current Protection

Solder reverse current protection (RCP) between charger module to BMS.

  • If RCP is ideal diode module, solder wires between charger module output "+" and RCP input "+" and between charger module output "-" and RCP input "-". Solder wires between RCP output "+" and BMS power "+" (P+) and between RCP output "-" and BMS power "-" (P-). Isolate module under heat shrink tube or wrap it in electrician tape.
  • If RCP is a Schottky diode, solder wires between charger module output "+" and diode anode and between diode cathode and BMS power "+" (P+). Isolate the diode assembly under heat shrink tube. Solder a wire between charger module output "-" and BMS power "-" (P-).

Solder reverse current protection (RCP) between BMS and vacuum battery cable:

  • If RCP is ideal diode module, solder wires between BMS power "+" (P+) and RCP input "+" and between BMS power "-" (P-) and RCP input "-". Solder wires between RCP output "+" and battery cable "+" wire (red) and between RCP output "-" and battery cable "-" wire (black). Isolate module under heat shrink tube or wrap it in electrician tape.
  • If RCP is a Schottky diode, solder wires between BMS power "+" (P+) and diode anode and between diode cathode and battery cable "+" wire (red). Isolate the diode assembly under heat shrink tube. Solder a wire between BMS power "-" (P-) and battery cable "-" wire (black).

Pay attention to wire length. Wires must be long enough to comfortably place them and RCP at outer side of battery compartment.

See attached "Reverse Current Protection.pdf" file for RCP symbols used in conversion circuit diagram.

Attachments

Step 7: Installing the Battery

Put assembled battery with BMS inside vacuum battery compartment. Use hard foam, rubber or two-sided sticky tape to fix battery in place. Do not close air outflow openings inside compartment.

Step 8: Assembly

Assemble the vacuum back in reverse order. Be careful with wires. Pay attention to label around power switch and charging port - do not smash it.

If rubber strip in collision detection cover after assembling is still loose at one end, carefully pull it to side where it is loose and carefully push strip holder sections in cover deeper with flat screwdriver. At end strip must be equally tensioned in whole length.

Step 9: Conclusion

After conversion to repurposed 2000 mAh Li-Ion cell battery work time for M607 robot vacuum increased by 50% in average. From 50..60 minutes with stock battery to 1 hour 30 minutes and more. I assume that brand new 2000 mAh cells will increase work time to 2 hours. And even more with 3000 mAh cells.

With bundled 19V 600 mA power supply charging time from empty battery to full last a little more than 3 hours. You can shorten charging time to 1 hour by using repurposed 19V power supply from old laptop and charging the battery with 1C rate current.

Built-in battery status indication behavior is now a little different. During charging it remains as it is described in user manual (blue LED blinks during charging, become steady lit when battery is full). However when battery is empty, BMS cut it off completely thus vacuum simply stop and no lights are lit. It is normal behavior. Plug the power supply in and vacuum will begin charging with blinking blue LED. Switching the vacuum off is not mandatory. MCU will not switch to work mode while power supply is connected.

Total cost of this conversion for me was around 16 euros (prices + shipping) for things I didn't had - a set of 2 LM2596S CC/CV modules, 4S BMS module and a set of 5 ideal diode modules.

My relative is now happy. Her cat is even more happy because his toy is back.