Fool A Drone With A Fixed Battery

Lithium-ion and lithium-polymer rechargeable batteries have given us previously impossible heights of electronics power and miniaturization, but there’s a downside they have brought along with them. When a battery pack has to contain electronics for balancing cells, it’s very easy for a manufacturer to include extra functions such as locking down the battery. Repair a battery, replace cells, or use a third-party battery, and it won’t work. [Zolly] has this with a DJI Mavic Mini pack, and shares with us a method for bypassing it.

The pack talks to the multi-rotor with a serial line, and the hack involves interrupting that line at the opportune moment to stop it telling its host that things are amiss. Which is a good start — but we can’t help hacing some misgivings around the rest of the work. Disconnecting the balance line between the two cells and fooling the Battery Management System (BMS) with a resistive divider seems to us like a recipe of disaster, as does bypassing the protection MOSFETs with a piece of wire. It may work, and in theory the cells can be charged safely with an external balance charger, but we’re not sure we’d like to have a pack thus modified lying around the shop.

It does serve as a reminder that BMS boards can sometimes infuriatingly lock their owners out. We once encountered this with a second-generation iBook battery that came back to life after a BMS reset, but it’s still not something to go into unwarily. Read our guide to battery packs and BMS boards to know more.

21 thoughts on “Fool A Drone With A Fixed Battery

  1. Do I have new years Flashbacks or wasn’t the real hack to apply phantom voltage during cell exchange?

    BTW: I stick with my warthog Y6 in acro mode. I can repair it quiet well, maybe I learn how to land this later >}
    Who else still owns a Shrediquette IMU cube here?

    1. He has a video on that as well. He solders replacement cells that are the same voltage to 2 decimal places. The video linked is for cases where the battery has self-bricked, or you have forgotten to always apply the phantom voltage, thus causing a bricking.

  2. “Disconnecting the balance line between the two cells and fooling the Battery Management System (BMS) with a resistive divider seems to us like a recipe of disaster,”

    Well if there is a disaster blame the companies trying to screw the consumer.

  3. Any Lithium Ion battery that does not have some sort of charge/discharge control in it is recipe for disaster. Packing large amounts of energy in a very small volume is literally the definition of a bomb. I was an operations manager for a large semiconductor company and we sold a device that went into many Lithium Ion batteries to prevent discharge currents above a certain level. The euphemism we used to describe the potential behavior of a battery without it was that it could “rapidly disassemble”. DJI may have gone overboard with their version of intelligent control, but anyone who thinks Lithium Ion batteries shouldn’t have some sort of current control function simply doesn’t know what they are talking about.

    1. I suppose the current control system isn’t what people upsets, but rather the use of chips. It brings back bad memories of dongle technology from the past (like these plugs that went into LPT port on a PC). The types of dongles that contain serial numbers or store license information. The type of data that can be used to artificially limit functionality of a device.

      In the past, charger technology was entirely analogue, also.

      Or it used digital technology in the form of TTL chips, without any actual program code, just logic circuits.

      This technology is fine to most users, I assume. Some sort of watchdog or “power good” signal could be implemented instead of using actual serial data:

      As a protocol, just send some pulses over a line that differ in lenght/speed. A timer IC will the decode the information and a logic circuit will use that information to make assumptions about the battery state.

      That being said, I’m just a layman here, of course. I have no idea to which high standards these control systems must comply to. But generally speaking, 70s/80s technology was much more predictable and open by design. I wouldn’t want to trust a proprietary control mechanism I cannot understand.

  4. DJI have their own care package in case the drone flies away and a lot of protection features that I just couldn’t implement myself on a home built drone. And considering the regulations now around drone pilot certification I’m fairly happy to be locked into their ecosystem for peace of mind and to know my certification is kosher and the risk of flying within a certain distance of people is acceptable.

    I’ve seen 3rd party batteries for makitas that were terrible, imagine putting a terrible 3rd party battery into a drone and flying it 120meteres above traffic or a crowd of people. I wouldn’t trust it, if you do that’s fine, but that’s a risk you’re willing to take.

    If I could swap out the cells, great but part of the battery safety is to limit each battery to 200 full charge and discharge cycles to ensure it remains at a decent capacity. If you could refurb them cheap enough with cells I know are decent, that’s fine, but there are many who would pop in depleted or less than quality cells and pass them off as A+ refurbs. So why mess with something that works for the sake of few quid that could cause an accident or injury to a flying object?

    1. I see. You think like a consumer, though. Not like a pilot or a tinkerer.
      A pilot takes responsibility for his/her actions, knows about his profession, his equipment and has the competence to perform a checkup before flight.

      1. I agree absolutely. I trust everything on my drone and I have insurance for that just in case scenario. And on the same note I wouldn’t put some dodgy fuel I got for cheap in my plane!

        Tinkering is fine if I trust everything there and it holds up to regulations. I’ve no problem tinkering and modifying any number of things. But drone batteries for me this is the same as getting cheap brake pads from china for my car. I’ve no problem fitting good ones myself, I’ve done it before. But I need to trust the quality of the item before I put it into a system that dramatically increases the risk of injury.

        1. You the man. I thoroughly subscribe to every point you made. I’m kinda man crushing on you right now because there’s too damn few of us out here today and I wish I could make your acquaintance sir. 😂 🤏🙏 Happy New Year.🤝

      2. One big difference is that a pilot is inside his creation, which is a high barrier of entry the helps prevent sloppy thinkering.

        But it should be possible to make a BMS that tests the cells (maybe do a full cycle or something, IDK) so it could be reused with new good cells.

      3. Okay, just about everything you said is wildly incorrect, and ignorant, all for the sake of trying to undermine the opinion of someone erring on the side of caution.

        Pilots certainly don’t take responsibility for all of their actions. Otherwise, you’s never see someone fired for drinking or using drugs on the job, and Pilots insurance would be optional.

        The pilot is also not the only one doing a pre-flight check of the equipment on the plane. Mechanics and other crew also take part in this, and to make your analogy work it would be a mechanic that would be checking the power systems on the plane to make sure that they’re working properly, not the pilot.

        Lastly, if I ever see a pilot “tinkering” with a plane before takeoff, I can tell you what plane I’m not boarding, a d I really hope nobody with your mentality is flying over traffic or near people.

        1. Not entirely correct. If we are talking about smaller aircraft then the pilot may very well be the mechanic too. This “can” be true with not only kit airplanes and home built aircraft but also with single and twin engine prop and turbo prop planes, small helicopters and the like. I have not yet met anyone who is both the pilot and mechanic on a 746 though. Now unless the pilot is also an A&P licensed mechanic certian work (other than preventative maintenance) needs to be looked over and signed off by an A&P. And unless the pilot also works as an A&P and has obtained an IA certification then he cannot do the 100 hour or annual inspection either. The 100 hour inspection is only required for planes that carry passengers for hire. I think everything i just said is true but I could be off a little. Lol. Also homebuilts etc will still need an airworthiness certificate once complete.

  5. IBM did this on some ThinkPad batteries. They put a charge counter in them so it would quit charging after a certain number of cycles. The fix was simple, a bit of tape over one of the contacts.

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