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Thread: Robot vac battery gone silly

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    Default Robot vac battery gone silly

    Haven't used the robot vac for quite a while so I thought it would be a good idea to get it charged up to keep the Lithium battery going.
    I put it on charge and noticed the charging light running up the bars as they do, then around 15 mins later saw that the bars were all on solid. That says it's charged (or so I thought). I was a little concerned that it charged so quick because when a battery charges quickly generally means it will also discharge quickly and may be coming to the end of its life.
    Set it to work in a room and it run around for about 5 mins then went into looking for the charger mode. That tells me it definitely isn't holding much of a charge. I wasn't too surprised at that considering how long it hasn't been used for but usually 2-3 charges from flat will bring em back to near normal.

    This case seems a bit different. An attempt at a second charge saw it flashing lights for only a few seconds then going solid, but after that the vac wouldn't even turn on.
    I pulled the battery and found something like 0.02 volts on the battery connector. Very strange since it was working ok not long before that.
    I removed the shrink wrap cover from the battery pack to have a look. Nothing abnormal looking here. After fiddling with it for a while I see 14.4 volts are suddenly back on the terminals. Ok so I put the battery back in and set it to go on the floor again, turned it on ok but as soon as I hit the start button on the remote it lurched forward about and inch then completely turned off.

    Ok so now to the photos. The pack is 4 x 18650's in series. At the 2 terminals marked B+ and B(-) there is constantly 14.4 volts. (Batteries can't be too bad) At the terminals leading to the connector is almost nothing or like .1 volt or so.
    RSE2 ( which looks like a resetable fuse) appears ok and has continuity so B(-) and P(-) are connected. The problem is between B+ and P+.
    What am I looking at ?



    Last edited by loopyloo; 24-04-19 at 08:56 PM.



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    Looks like Q1,2,3,4 are mosfets connecting P+ and B+ controlled by U5 which stop charging when the battery voltage hits the threshold. B1,2 3 are for balancing the cell voltages. If the overall battery starting voltage is too high or too low the controller wouldn't be switching the FETs on and those thresholds for max and min voltage would be set by the resistors around U5 (at a guess)

    Have you checked the individual cell voltages?

    LiPos are usually 4.2V/cell at max (16.8V for 4 cells in series) and the 14.4V is a bit low at less than 3.7V/cell.

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    Lithium ion so only .4 of a volt down.
    Individual cells vary a little but not too much. I think I remember 3.8 and 3.6.
    The fact that it worked fine when it was going tells me it should have enough left in it to charge ok. Even if it needs a charge at that point it shouldn't just switch off completely.

    I did wonder if that RSE2 (R008) could be dicky. If it is I'm not sure how to match up a new one.

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    RSE2 looks like a current sensing resistor to me IE 0.008ohm for the controller to monitor the load current and it could cause problems if it's gone high in value. I'd be looking for a datasheet for U5 to get some ideas on what to check.

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    Ah current sensing ... yep that figures.
    I just tried a test. Stuck a 12v tail light globe across the battery contacts.
    It lit up like a beauty and nearly blinded me.

    Then I went looking and found this other battery from a robot vac I threw out about 3 years ago.
    It also had 14.4 volt still in it, and lit up the bulb brightly too.. I'm amazed it still had any charge.
    The controller is a little different. I wonder if I could just swap the controllers over.
    I might try the blue battery in the vac so I'm currently looking for something I can rip the pins off cos it's got the wrong gender in the plug.


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    One of those things that might be useful some day has actually served its purpose
    Circuit looks extremely similar so shouldn't be a problem to wire it in even if only temporarily for testing

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    These controllers are meant to match up with the battery, not the device which in this case is a vacuum cleaner, so it appears I have 2 good batteries but one having a bad controller. The blue set had a fair bit of use so may not run as long as the green set. Both sets would be around the same capacity rating of 2200mA. Both vacs use a power pack rated 24v @ 1A so a match there too. I think it shouldn't matter which battery I use, they would probably perform the same except one will run for longer and I'll have spare batteries for later.

    My microscope shows that resistor as eroded or burnt.
    Checked in circuit it is not open but low ohms and pretty much impossible to measure even with my ESR meter. I can get 5 of them from RS for just under $4 so might be worth replacing for a fix.

    Last edited by loopyloo; 25-04-19 at 04:19 PM.

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    Better check that the motor doesn't have a problem like a tight bearing, impeller jammed with sucked in junk etc causing it to take excessive current.

    That R008 resistor looks like a 1W rating which equates to a maximum current of around 10A so it would need to have experienced some prolonged loading in excess of that 10A figure to be showing significant signs of heat damage.
    Last edited by Skepticist; 25-04-19 at 07:38 PM.

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    When I run it the first time for that 5 mins, it was working really well and picked up stuff off the floor as good as a normal vac so I don't think it would have any overload problem.
    Interesting thing is if anything burns the smell will stay in the product for months, this one has absolutely no smell not even the battery board.
    The resistor does have continuity and low ohms, I'm just guessing maybe it's dicky and going open on load or something.

    Late last night I cut the green battery lead and swapped it to the blue battery. The vac worked perfectly and demonstrated it will go into charge mode with no problem. Only tested it for a few minutes though.

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    After a couple of full charges and drains the vac only runs for around 15 mins with the blue battery in it.
    I ended up taking the controller from the old blue battery and soldering in to the green battery.
    The vac now runs for 40 mins and working fine.
    I'm hoping a few charges will bring the battery up some more but I guess for now at least I've got it going again.

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