lowest safe voltage per cell on lipo

Hi everyone,

I have a 4S 5000mAh battery on my hexacopter.  What is the lowest safe voltage per cell before I risk losing power to my motors.  I want to know in order to set the value on my battery low voltage alarm.  Also is there a way to program this into APM 2 so that the copter will return home when it hits a certain low voltage?

Thanks,

James

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Replies

  • Hi,

    could you give me the name of a manufacturer of single cell 5000mAh lipo batteries ?

  • It has been recommended in many previous posts that you only power the APM from a single ESC.

    Cut the red wire on on all but one ESC connector if you are going direct to the APM and not via the PDB.

  • You're best off keeping track of your current.  That's the only way to know how much you have left.  As was pointed out LiPos don't have much relation between voltage and remaining current, at least not linear.

  • As a disclaimer and a warning, the specifications of the battery manufacturer should always be followed, and generally speaking, they say 3.0V.  In fact, most chargers will refuse to operate if the static cell voltage is less than that.

    The operating voltage at the load depends on the current being drawn and the rating of the battery.  Much depends on your "C" rating, or the current multiplier.  The higher the "C", the lower the internal resistance, and the higher the voltage at the load.  Only the continuous "C" spec matters -  forget about the "surge" rating, which is meaningless.

    You need to figure out where your discharge plateau voltage is.  You can measure it, if you have a dummy load or an outstanding static test stand for your copter.  You'll notice that the cell voltage drops immediately on the application of load, hovers around some figure between 3.7V and 3.3V for a long time, and then falls like a rock.  Or, if you're lucky, the manufacturer will be able to supply a discharge curve graph something akin to this one:

    3692428930?profile=originalIt's clear that the "plateau" voltage is dependent upon discharge rate.  What this graph can't show is the fact that once you reach the "knee" in the curve at the right, the voltage drops very quickly.  In other words, what it doesn't tell you (and frankly, nobody could without empirical testing) is what voltage threshold will give you enough time to "fly home".  Each pack and copter will have different results.  Even YOUR copter will be different, depending on, for example, if you're carrying a payload or not.

    I'm certain someone could come up with a battery "fuel" gauge mechanism like the one in your laptop or smart phone, but it would have to be comparatively complex to account for all the various hardware, load, and flight dynamic variations it's likely to encounter.  For now, you have some data gathering to do.

    P.S. Or just do what Dean said and play around with it.

  • Moderator

    You shouldn`t go any lower than 3v per cell on lipo or higher than 4.2v I have my alarm set at 13.8v which equates to 3.4v per cell which as Dave states, it gives you about 30 secs to land with a Tricopter or quad around 1kg in weight before you start damaging cells or dont have enough juice to keep it in the air... As you are using 6 motors I would raise it to possibly 3.6v per cell as there is more load on the battery, different setups require different measures depending on weight, number of motors, effieciency of motors, ambient temperature e.t.c. the best thing to do would be to set it high and work your way down until you are satisfied with the amount of time you have since the alarm started. 30 secs is fine for me as it is never too far away that I couldn`t land safely. And I would definately set the esc to NiMH so if you do overun slightly the power to the motors is gradually reduced rather than instantly shut off, as it gives you a few seconds chance to save it.

  • I've got a HK Battery Monitor and it starts with warning beeps at 3.4V per cell and goes crazy beeping at 3.1V.  The ESC's start shutting down very soon after - I'd guess at 3V.

    Some say that you should disable the ESC low voltage shutdown (set it to NiMH) - a ruined battery is better than a ruined Quad.  However, I know that I've got about 30 secs from when the beeps start to get the quad on the ground.

    Don't know about RTL on Low Battery, but it sounds like a good idea.

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