Taranis - setting up low voltage warnings

[Geoff Gardiner]

Hi All.

Just a quick question:

I am setting up some low voltage alarms, for my flight battery, on my Taranis.

I am setting one for the whole pack and one for the lowest cell.

I am wondering what settings people use for these voltages - I have googled it and opinion is varied. The measurement is under load so I am thinking.

3.7 Volts for each cell (4.2 Volts fully charged).

14.8 Volts for a 4S pack (16.8 Volts fully charged).

Is this too conservative?

Geoff...

[Masher]

That sounds good to me Geoff, 3.72 equates to 80% discharge which is a safe level

[Frank Skilbeck]

Yes, too conservative as the voltage will sag under load, so a 2200mah battery which is only 50% discharged could well see just over 3v per cell at say 30 amps.

I use an amp sensor so can alarm when xxxx meh have been used, but if I have a low voltage alarm then this is usually set at 3.1v per cell

[flight1]

remember at 3.4v the battery is dead and at 3.0v you are gettting beyond recovery you need to check the battery with a good multi meter and compare readings and then set the offset to adjust, not much usually.

3.7 v is the bateries nominal voltage so as a setting is to conservative

[Gary Manuel]

Posted by flight1 on 14/05/2016 20:32:37:

remember at 3.4v the battery is dead and at 3.0v you are gettting beyond recovery you need to check the battery with a good multi meter and compare readings and then set the offset to adjust, not much usually.

These are no-load voltages.

Under heavy load, the voltages of a perfectly healthy battery could drop below these voltages. An alarm at these voltages at full throttle doesn't necessarily mean there is a problem.

[Andy48]

I use 3.4v for low and 3.2 for critical, but I also use current sensing. Current sensing is the best way to see how much power you've used, and you can set a verbal warning using consumption at whatever point(s) you want. The low voltage warning (single cell) only ever triggers with a failing battery, and when it happens on take off with a fully charged battery I know it is time to retire the battery. This is a far better indicator of a dying battery than measuring internal resistance.

[PatMc]

Posted by flight1 on 14/05/2016 20:32:37:

remember at 3.4v the battery is dead and at 3.0v you are gettting beyond recovery you need to check the battery with a good multi meter and compare readings and then set the offset to adjust, not much usually.

3.7 v is the bateries nominal voltage so as a setting is to conservative

LVC for many ESCs is typically - user programable from 2.7v/cell to 3.3v/cell.

Even under no load around 2.8v - 3v/cell, whilst not a good idea, is still normaly capable of being re-charged.

[Bob Cotsford]

I use 3.2 for low and 3.0 for 'throttle back NOW!'. If you throttle off from 3.0v the battery voltage will rise by half a volt or more, as has been said it drops a lot under load.

I like current sensors to tot up mAH used, but it soon gets expensive if you fit them on all the cheapo foamies that seem to appear in the model room aka hangar.

[John Dennier]

Perhaps you should consider the liklihood of your final circuit becoming a go around and set voltage warning at a level that will keep you in the comfort zone to handle a missed approach. If you do arrive with charge remaining your next battery recharge will be quicker.

[Frank Skilbeck]

Just to illustrate the points above, here is a log file from my Eagle Tree E logger monitoring Volts, amps and mah used on a 4s 3300 mah lipo. Note even with a fully charged battery 100% throttle draws the pack volts down below 14.7v.

[Andy48]

Posted by Bob Cotsford on 14/05/2016 23:18:46:

I use 3.2 for low and 3.0 for 'throttle back NOW!'. If you throttle off from 3.0v the battery voltage will rise by half a volt or more, as has been said it drops a lot under load.

I like current sensors to tot up mAH used, but it soon gets expensive if you fit them on all the cheapo foamies that seem to appear in the model room aka hangar.

You don't need to have one in each model. Simply keep a Smart Port lead connected to each receiver in each model and then its easy to move sensors between models. I have three of each sensor, as I generally never take more than two or three models down to the field at once.

Mentioning ESCs. It is important to program the ESC to have a lower cut off/reduce voltage than your OpenTX warning, otherwise the latter is pointless.

 

Edited By Andy48 on 15/05/2016 11:46:48

[Bob Cotsford]

Posted by Andy48 on 15/05/2016 11:44:42:

You don't need to have one in each model. Simply keep a Smart Port lead connected to each receiver in each model and then its easy to move sensors between models. I have three of each sensor, as I generally never take more than two or three models down to the field at once.

Mentioning ESCs. It is important to program the ESC to have a lower cut off/reduce voltage than your OpenTX warning, otherwise the latter is pointless.

 

Edited By Andy48 on 15/05/2016 11:46:48

Andy, that makes sense with the 40A units, but to do that with the 150A ones means adding another plug/socket pair with XT, EC or Deans connectors. Maybe that makes a point in favour of 4 or 6mm bullets where the sensor could be slipped over a lead easily?

Or maybe I'm too paranoid about adding extra connectors.

I wonder what the actual max rating of the 40A unit is?  I've seen 70A registered when testing a d/f without any smoke escaping,

Edited By Bob Cotsford on 15/05/2016 12:05:16

[Andy48]

I've used a bullet with the 150amp sensor. Think you're being paranoid. So long as you use good quality connectors that are a reasonable fit then it would seem to be OK. What's an odd amp amongst 100? I wondered about making a very short extension lead with the sensor fitted between. Might do a test or two on this to see just what effect the extra couple of plugs and sockets make in terms of power.

The 40amp ones will certainly take over 40amps, but for how long I don't know. Many of my 40 sized planes would draw up to 50amps with a fully charged battery, and all I did was dropped the prop a size down to reduce that to about 45 amp max. Never noticed the difference in performance, but certainly got a bit longer flight time.