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Thread: Uninterrupted hot swap Lipo battery with other DC source.

  1. #1
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    Uninterrupted hot swap Lipo battery with other DC source.

    My robot runs Linux and it takes amount a minute to reboot. It's also probably not a good idea to instantly remove power from a Linux machine.

    What I want to be able to do is to have my robot powered via a DC power brick source, but be able to detach from the that DC source whenever I put my robot on the floor. Since my battery is a Lipo it's not exactly trivial to build a charger, so I'm fine ( and feel better ) with having charging a separate commercial charger.

    So kinda like what you would do with a laptop. You can have it plugged into the wall, but if you want you can unplug it and run off the battery.

    Any ideas?

    One idea I had was to just have my Lipo charger ( Cellpro 4S ) plugged into the battery balance connector while at my desk. The idea being that the robot would run off of battery power at all times, but the charger could charge the battery faster than the robot would drain it ( my main CPU only pulls 5 watts. ) I'm wondering if this will dramatically lower battery life or maybe it's just not a good idea to do such things in general.

  2. Re: Uninterrupted hot swap Lipo battery with other DC source.

    I'm working on just such a board, and should have it ready sometime in the next few weeks. It will do the auto-disconnect, and it will also have an on-board Lipo voltage monitor that will cut off the battery if the voltage gets too low.

    I'm planning on selling the board, so I'll let you know when its ready...

    - Jon
    "The best way to predict the future is to invent it." - Alan Kay, inventor of Smalltalk

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    Re: Uninterrupted hot swap Lipo battery with other DC source.

    Another thing I'm looking to do is to have the ( separate ) ability to toggle power to the AX-12 bus. Idle current on the 18 AX-12's current is like 0.5 amp which is annoying. That'd also be useful to reset the AX-12's after they decide to stop working and need to be reset.

  4. #4

    Re: Uninterrupted hot swap Lipo battery with other DC source.

    Any chance the board will support the cutoff for two packs? I want to use a 3S for my dynamixel bus, and use another 2S with a regulator for electronics...

  5. Re: Uninterrupted hot swap Lipo battery with other DC source.

    Quote Originally Posted by Adam View Post
    Any chance the board will support the cutoff for two packs? I want to use a 3S for my dynamixel bus, and use another 2S with a regulator for electronics...
    No, this board will just handle one pack, plus wall power. There's no reason you can't power your electronics using the main battery, as long as you use a switching regulator. Even if you just use a linear regulator (like the Arbotix does), it will work fine - it just gets hot, and wastes a small amount of power that way.

    - Jon
    "The best way to predict the future is to invent it." - Alan Kay, inventor of Smalltalk

  6. Re: Uninterrupted hot swap Lipo battery with other DC source.

    Quote Originally Posted by billyzelsnack View Post
    Another thing I'm looking to do is to have the ( separate ) ability to toggle power to the AX-12 bus. Idle current on the 18 AX-12's current is like 0.5 amp which is annoying. That'd also be useful to reset the AX-12's after they decide to stop working and need to be reset.
    I typically put one of my power switches on the power line going to the bus plugs, so I can power cycle the bus whenever I want, and keep it off if I'm just doing processor-level stuff.

    I've heard that you can reset the AX-12's in software when they stop working, by disabling and then re-enabling torque. I haven't tried this myself yet, so I can't say for certain that it works.

    - Jon
    "The best way to predict the future is to invent it." - Alan Kay, inventor of Smalltalk

  7. Re: Uninterrupted hot swap Lipo battery with other DC source.

    I don't know what microcontroller you are using, but many popular hobby boards (PIC18xx, AVRs) have a brown-out detect interrupt.

    If you configure it correctly, it will use the A2D peripheral to monitor the voltage on the V++ node and trigger an interrupt if it drops below a threshold. You generally have a few microseconds before the board actually dies, which is just enough time to save some critical data - or in your case, switch to an alternate power source.

    You mentioned that you had linux installed, so it's possible you've surrendered control to the low level hardware and this might not be as easy as it sounds for you. However, all you would have to do is set up a simple mosfet/transistor pair switch connected to one of your output pins. Just drive the pin low for battery mode and high for power pack mode.

    A little extra circuitry/logic can even enable you to recharge the battery when the robot is plugged in!

    I posted a short guide to recharging batteries with an AVR here, but you'll have to look elsewhere to find out about the brown out detect interrupt if your board has one.
    http://www.buildtherobot.blogspot.com - for robot builders and enthusiasts

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    Re: Uninterrupted hot swap Lipo battery with other DC source.

    Quote Originally Posted by JonHylands View Post
    I've heard that you can reset the AX-12's in software when they stop working, by disabling and then re-enabling torque. I haven't tried this myself yet, so I can't say for certain that it works.
    It says in the docs that it'll recover after enabling torque again, but it's not working for me. I have keys setup to disable/enable torque at any time and when I get the shutdown.. toggling torque on and off obviously is working for all the other servos, but not for the one that is shutdown.

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    Re: Uninterrupted hot swap Lipo battery with other DC source.

    Quote Originally Posted by badcommandorfilename View Post
    However, all you would have to do is set up a simple mosfet/transistor pair switch connected to one of your output pins. Just drive the pin low for battery mode and high for power pack mode.
    I might give this a go at some point. I'll report back any success with it.

  10. Re: Uninterrupted hot swap Lipo battery with other DC source.

    Quote Originally Posted by billyzelsnack View Post
    I might give this a go at some point. I'll report back any success with it.
    Its not that simple. That might stop the battery from powering the circuit, but wall power will still feed directly to the battery in that case (MOSFETs are only one-way).

    Having wall power hooked up to your Lipo batteries would be a very big no-no...

    - Jon
    "The best way to predict the future is to invent it." - Alan Kay, inventor of Smalltalk

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