Page 2 of 2 FirstFirst 12
Results 11 to 15 of 15

Thread: General Grievous

  1. #11

    Re: General Grievous

    Now, for parallell powering, since I'm gonna use LiPo batteries, wouldn't that cause an issue if I try to charge two 2s batteries connected in series with my 12v power supply? My impression is that LiPo's are somewhat unstable and should be removed and charged separately under close supervision. This is a issue that I've been very unsure on how to deal with in the best way, so any input here is highly appreciated
    Most cell phones or laptops do not actually catch on fire.

    It is true that, when you get it wrong with Lithium batteries, they do spectacularly catch on fire.
    It is also true that, some things that you could do on other batteries (recharge after deep discharge, for example) count as punishing on Lithium batteries.
    However, Lithium batteries are a fine technology, and when treated alright, do not actually spotaneously combust.
    Most of the horror stories come from:
    - people who think Lithium batteries work like NiHM
    - people who think Lithium batteries work like Lead-Acid
    - people who got bad batteries from the factory
    - people who made mistakes and didn't catch themselves

    You can charge a lithium battery with two wires, and it won't immediately catch on fire. However, as the battery goes through charge/discharge cycles, the internal resistance in each cell ("S") will slowly diverge, leading to one cell getting more charge than the other. Therefore, it's useful to make sure you do a balance charge at least once in a while (say, every 10 cycles.)
    In order that you don't forget, it's totally OK to do a balance charge every time, too.

    You can charge a lithium battery pretty fast (at several "C" rate) as long as you make sure it doesn't heat up too much. Slightly warm is OK. Uncomfortable to the touch is not OK. If you charge too fast, too often, you will wear the battery down faster. If you charge WAY too fast, see above about "punishing lipos" and spectacular fires.

    You can NOT recover a lithium battery after it's discharged below 3.0V/cell or so (maybe 2.8V.) The batteries go through a very specific conditioning cycle in the factory to go from 0.0V to full charge, and that cycle actually builds up the oxide layer between the parts of the battery, that makes it actually work. Once you discharge the battery too far, you've irrecoverably damaged this oxide layer. If you somehow manage to "miraculously" "recover" an over-discharged battery from the borderlands, then you will have a battery with worse performance, higher internal resistance, more likely to build up heat, and less able to hold a charge. Use an undervoltage detector/cut-off at all times. (Took me two lost batteries to learn this!)

    You're not supposed to "keep charging" a LiPo battery at full charge, because at the top voltage, the trickle through the internal leakage of the battery builds on to the oxide layer, slowly wearing the battery out. "floating" a battery at below-top charge, is supposed to be less damaging. Some electric car batteries try to keep the pack at 4.05V, for example. When I float, I typically do it at 3.9V-4.0V per cell, and it works OK. I have a LiFePO4 pack that I've done this to for four years (at 3.5V/cell) and it's still going strong. It does need rebalancing at times, though.
    Do not parallel a 2S with a 12V supply, though; that would cause a house fire. 2S need <= 8V for parallel.

    Regarding step-up versus step-down, it's a question of current capacity. The computer will likely draw 3-5A/19V fairly continuously. A step-up converter with 6A capacity will easily be able to meet the needs of the computer. Meanwhile, all of the servos will draw about 1A/12V when idle, but may draw 20A/12V when working hard. Thus, you'd need a much beefier step-down converter to feed the servos, if you set your battery pack at 19V. Also, I agree that NUCs are super finicky about input voltage, I had one (Zotac, AMD based) that only worked with 18.5V-19.5V input range, and no battery pack stays in that range, so it needed a regulator no matter what. The MX-64 will be OK with 10V-16V actual input voltage. You can also turn off the over-voltage alarm for those times when your 4S cell reaches the full 16.8V. That being said, I highly recommend 3S LiPo or 4S LiFePO4 for the reasons Tician suggest. I burned several MX-64 motors at 4S when they moved into interference because of software bugs, and the built-in over-heat protection wasn't fast enough to save them.

    With Onyx X, I went 4S, because I had more of an idea what I was doing, and I started out simulating the software in a custom C++ rig on Windows, and when I finally ran on the bot, I ran with a 20% "Max Torque" limit while holding the bot in the air. I still managed to stall one servo at full power one time, but was sufficiently aware that I immediately (within a second) hit the power switch and the servo seems OK.

  2. #12
    Join Date
    Mar 2010
    Location
    Stavanger, Norway
    Posts
    7
    Rep Power
    32

    Re: General Grievous

    OK, so to summarize this is what I should do if I get you guys correct:
    Click image for larger version. 

Name:	Grievous Power Scheme.jpg 
Views:	294 
Size:	64.6 KB 
ID:	7010
    And that will allow me to run from either battery or power supply, with the possibility to charge the battery?
    I added one diode on both battery and psu positive wires.

    I should probably add a On-Off switch or a fuse somewhere?

    Do not parallel a 2S with a 12V supply, though; that would cause a house fire. 2S need <= 8V for parallel.
    No, my intention was to connect the two 2S batteries in series so they make up a 4S and then connect that in parallel.

    "floating" a battery at below-top charge
    Now, how do I do that?

    Also, from a stability point of view, should I go with to (smaller capacity) 3S in parallel to make CG better, or will this give me more problems than benefit?

    Somehow I've had the impression that with the Arbotix Pro I could apply 12-24v and it would give me a stable 12V output so I didn't have to worry about the battery supplying up to 16.8V fully charged...

  3. #13
    Join Date
    Sep 2010
    Location
    ಠ_ಠ
    Posts
    2,313
    Images
    27
    Rep Power
    278

    Re: General Grievous

    I had been wanting to use one or more quarter-brick DC-DC converters in Ripley to turn a 12S/16S LiFePO4 battery pack into a high power 12V output (>17A), but never got beyond early PCB layout. Even with heat sink, they weigh less than one MX-28 while being a little longer (50mm vs 60mm) and much thinner (36mm vs 18mm).
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    [git][mech][hack]
    gives free advice only on public threads

  4. #14
    Join Date
    Jan 2008
    Location
    Norway, Stavanger
    Posts
    790
    Images
    276
    Rep Power
    80

    Re: General Grievous

    Quote Originally Posted by Pengatom View Post
    OK, so to summarize this is what I should do if I get you guys correct:
    Click image for larger version. 

Name:	Grievous Power Scheme.jpg 
Views:	294 
Size:	64.6 KB 
ID:	7010
    I belive you don't need the diode from the battery only the PSU. The PSU would keep the battery floating @ (12v-Vf_diode). I've never tried this myself but I would recommend to make sure the battery have a voltage slightly above or equal to this voltage (about 11,4v), then the PSU should keep the battery charged at a medium state. You need to disconnect the battery before charging to 100% with a balance charger if you plan to run the robot from battery alone.

    Always pay attention when connecting two LiPo's in parallell, make sure they have the same voltage/charge state. Or else there is risk for very high charge current to the battery with lowest voltage. So far I've not done this myself..
    Kåre Halvorsen aka Zenta
    ---------------------------------
    Zenta's YouTube channel
    Zenta's Blog
    Zenta's Instagram

  5. #15

    Re: General Grievous

    The diode from the battery will prevent the PSU from charging the battery.

    Yes, you need an on/off switch; I typically put mine between the "Battery/PSU section" and the "everything else" section.

    A fuse is often a good idea, although sufficiently thin wires or PCB traces will also serve as fuses in a pinch ;-)
    There's the simple blade-type "car fuse" for 20A, which would probably be simple and good for your case.
    There's the "PTC self-resettable fuses" (a k a polyfuse) which come up to 9A capacity or so; you can parallel two of them.
    There's electronic fuses that double as power switches and measure current through a sense resistor; when too big, they immediately turn off.
    Whichever you choose is up to your specific needs and budget!

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. General advice for a good starting kit
    By badpanda in forum Humanoids, Walkers & Crawlers
    Replies: 5
    Last Post: 07-16-2012, 01:53 AM
  2. general question on real time control and continuous motion
    By animaniac in forum Software and Programming
    Replies: 1
    Last Post: 10-17-2011, 03:43 PM
  3. Bioloid General I/O Board
    By JonHylands in forum Humanoids, Walkers & Crawlers
    Replies: 45
    Last Post: 01-11-2011, 07:27 AM
  4. Micro LCD for general use
    By jrowe47 in forum Mechanics / Construction
    Replies: 1
    Last Post: 02-04-2008, 09:47 AM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •