Shop OBEX P1 Docs P2 Docs Learn Events
Min Voltage for LiPo — Parallax Forums

Min Voltage for LiPo

xanaduxanadu Posts: 3,347
edited 2013-02-26 08:30 in Robotics
I've been running RC lipos on my BOE bot and one of them got really puffy. So puffy I put it in a LiPo bag. I would imagine the battery would have a specific recommended voltage but I cannot find it. I'm used to having an ESC that shuts it off but on the robot it's just plugged into Vin and I guess I got careless.

The pack is 11.1v 3S 1800 mAh 30c/60c burst. To avoid this in the future I got a little battery minder that goes off when any cell gets to a preset voltage. The problem is, what preset voltage per cell?

Let's talk about voltage under load, not resting voltage.
  • My airplane's ESC is set to 70% of turn on voltage so 12.6v to 8.82v = 2.94v per cell
  • The general rule I've been using is 3v per cell
  • Most of the RC forums say 3.7v per cell
So 2.94v seems on the low side and 3.7v seems high. If you didn't have the manufacturer's spec for would you just go with say 3.2v per cell under load?

Thank you.

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2013-02-23 09:55
    See here under "Charging" where it says that most LiPo cells have a minimum voltage of 3.0V. It depends on the type of chemistry of the cell. I've seen higher minimum voltages quoted. You really have to look at the manufacturer's spec.
  • xanaduxanadu Posts: 3,347
    edited 2013-02-23 10:20
    Thanks. Oddly the Wiki hasn't come up in any of my searches. I'm going to go with 3.1v per cell (under load/absolute min). Guess I needed to read up on storage voltage too.

    Parallax should make a LiPo power supply board that works with larger off the shelf LiPo batteries for robots, that would be really cool. Drop the 12.6v down to 7.2v, and 5v rails, maybe an AD per cell with I2C for voltage monitoring. Batteries always seem to be an issue once you get past something the size of a BOE Bot. I'd definitely be using 3S 5000mAh on a Stingray in the near future.
  • Duane DegnDuane Degn Posts: 10,588
    edited 2013-02-23 10:54
    I believe if you let a LiPo drop too low it shortens its life. I think 3.3V is a safer cut off voltage for normal RC LiPos. I generally don't let mine drop below 3.5V. IIRC, once the voltage starts to drop, it drops pretty fast so I don't think there's much capacity lost by using a slightly higher cut off voltage.

    I know a sure fire way to shorten the battery life on my micro helicopter batteries is to land and take off again after the helicopter has started losing power (a signal the battery is getting low). Draining the last bit of charge from the battery is really hard on it.

    You can drop the voltage of LiPo packs pretty easily and inexpensively with these cheap ebay switching regulators. (You might need one for each motor to use with a Stingray.)

    BTW, If you can find the LiPo you want at the HobbyKing USA warehouse, the shipping costs much less and it arrives much soon than from China.
  • xanaduxanadu Posts: 3,347
    edited 2013-02-23 12:07
    Thanks Duane. Those switching regulators are a really good price I will order at least a couple of them.

    I guess 3.5v makes sense, given the load is going to vary and 3.5v under a light load could easily drop lower under a heavier load. For the robot it's not like I have to land or anything though. I think for primarily indoor use I'll go lower, 3.2v. I can see for aircraft 3.5v would make more sense, especially depending on what and how you fly. I guess that is why so many people say so many different voltages. Since nobody really knows how the pack is being abused err... used, it's better to just go with a higher cut off.

    The microcopter I have is a Blade MSRx and it will toast a pack if you let it fly to the ESC min. I just measured that to be 3.0v by hooking the copter up to my bench PSU. The pack goes bad after around 50 cycles if you don't stop flying before the ESC cut off. Lame, but re-enforces the don't hit 3v rule. Those are 1S 3.7v 30mAh E-Flite brand. All this time I was trusting the ESC on it to make the right decision. At least it uses a set reference and not 70% of turn on voltage, that is just a horrible idea...

    I get SkyLipo brand packs, from US based places. Every time I order from HobbyKing, even if it is in China the shipping is still pretty good. I had my Skywalker in 10 business days with the cheapest shipping, I was pretty amazed people said I'd be waiting a month haha.

    I have two brand new 3300mAh packs, I'm going to run one down to 3.1v and one down to 3.5v and maybe in six months see how big of a difference there is.
  • shimniokshimniok Posts: 177
    edited 2013-02-23 21:18
    Search on RC forums like RC Groups or ultimaterc.com or rctech.net for their advice. I try not to run below 3.4/cell
  • Cats92Cats92 Posts: 149
    edited 2013-02-24 01:30
    When flying quadcopters (between 0.7 and 1.7 kilo) most of the people i know at my rc group use :

    * 3.5 or 3.6 volts as a first limit (10,7 volts for 3S lipos) : the lipoAlarm beeps

    * 3,3 volts : land without delay

    (If you dont have one , buy a lipoalarm : its cheap about 2 or 3 $ and usefull with leds giving the voltage : total and per cell , and one LED blinking and a loud beep for alarm. Connect it to the balancer connector of the lipo and you are safe)

    If a lipo gets puffy don't use it. May get in fire.

    Jean Paul
  • Duane DegnDuane Degn Posts: 10,588
    edited 2013-02-24 12:33
    Cats92 wrote: »
    (If you dont have one , buy a lipoalarm : its cheap about 2 or 3 $ and usefull with leds giving the voltage : total and per cell , and one LED blinking and a loud beep for alarm. Connect it to the balancer connector of the lipo and you are safe)

    Agreed. Erco just posted a link to a dual voltage meter / alarm.
  • xanaduxanadu Posts: 3,347
    edited 2013-02-24 19:21
    Thanks for the answers. I have a nice voltage alarm that does per cell, two actually since I run two packs. Wow it is the same one Erco posted lol, small world. My LHS sells them for $9.99.

    I've been playing around with this and found that the cut off voltage depends a lot on how much current you consume and the condition of the battery.

    If your battery is in bad shape, it can drop as much as .5v under load, in good shape close to .1v. I have some LiPos in bad shape and did a lot of flying (with my big rc plane this weekend) and fully charged the alarm would instantly go off at full throttle. The voltage depends a lot on the condition of the pack.

    I'm sure this is pretty much common sense to electronics savvy people, however I have never given it this much thought.
  • photomankcphotomankc Posts: 943
    edited 2013-02-24 20:40
    I have been using 3.4V per cell as the "cut the cord" line for my LiPo projects. So far I have been having good luck with my packs lasting a long time like that. As mentioned earlier, one the voltage per cell starts to really drop the curve gets steep quick. Pushing it to the very edge really nets very little additional time.
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2013-02-25 06:27
    I suspect that a lot of users are confused and think that all Lithium ion cells are the same, but LiMn0 and LiFePO4 have different low cutoff voltages as well as different nominal voltages and different maximum charge voltage.

    It is all about chemical differences in the ions involved. BTW, Lithium ION cells are the rechargeable lithium cells.

    I have strong doubts that people that refer to LiPO really know which chemistry they really are using. Many retailers are confused or are just making claims that they are providing the type that is most popular at the moment.

    In general, there are Lithium ion cells that provide a steady 3.2 volts per cell and cells that provide a steady 3.7 volts per cell. They have different cutoffs and the 3.2 volt cells are always lower settings than the 3.7 cells.

    http://en.wikipedia.org/wiki/Lithium-ion_battery
  • Duane DegnDuane Degn Posts: 10,588
    edited 2013-02-25 08:51
    Loopy Byteloose wrote: »
    I have strong doubts that people that refer to LiPO really know which chemistry they really are using. Many retailers are confused or are just making claims that they are providing the type that is most popular at the moment.

    Having LiFePO4 as a possible chemistry could confuse the issue. The PO in LiFePO4 are the symbols for the elements Phosphorous and Oxygen. The "Po" in LiPo is short for "polymer".

    I''ve always seen "LiPo" used to describe Lithium Polymer cells commonly used in RC vehicles. The voltage cutoff levels suggested by most posters in this thread have pertained to this type of battery.
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2013-02-26 08:30
    At 11.1 for 3 cells, the battery is definitely NOT a LiFePO4. 11.1/3 = 3.7 volts nominal output.

    Just look here for a list of all sorts of different voltages ===>http://en.wikipedia.org/wiki/Lithium_battery

    Set your cut off higher 3.2 volts or so.

    That is likely your problem as the LiFePO4 is a 3.2 volts nominal output and tolerates a much lower cutoff before failure.

    Yes LiPO and LiFePO4 are easily confused. The rc users are likely to explain how to avoid this minefield of confusion. But the nominal voltage labled on the the battery should be the best clue.

    http://en.wikipedia.org/wiki/Lithium_polymer_battery
    http://www.rcgroups.com/forums/showthread.php?t=209187
Sign In or Register to comment.