Battery storage for P2V

[Scottrod]

How do you guys leave your batteries over time? I have heard that storing around 40-50% charge in the refrigerator around 32 degrees Fahrenheit is best for long term storage. Anyone know the truth about the best way to store them? I've also heard that letting the charge go below 10% is hard on them. Any truth?

 

[Pull_Up]

If you're going to store for an extended period of time then received wisdom is to leave them at 40-50% charge. If you noticed when you first got your Vision the battery was showing 2 greens - a good visual indicator that you're at or near the optimum.

Opinions vary as to what constitutes an extended period. I've heard some won't keep a full charge in their lipos for more than a week, some don't worry unless it's going to be a couple of months before use.

Again, most long-time RC lipo users seem to advocate not taking your lipos below 20% consistently for maxing their life - I've set my second level battery alarm to 17% and always try and land without that having to kick in (usually between 19 or 21%). A very deep discharge can cause the battery to swell up at best (at which point it should be properly disposed of), and to burst into flames at worst. Plenty of videos on YouTube of lipos going up in smoke...

 

[Scottrod]

Funny you say that. Doesn't DJI suggest going below 7-8% every 20 or so charges so that the battery keeps a long charge?

 

[Pull_Up]

Yes they do. By "deep" I meant lower than 5%. I certainly wouldn't want to do the DJI recommended 8% any more often than the 1 in 20 cycles though...

 

[garygid]

Is it really beneficial to take Lithium Polymer cells down to 8%, like NiCad,
or is this trip down to 8% just to help keep DJI's internal Battery Management
System calibrated?

Is there any easy way to carefully, and sufficiently slowly do this
discharge to "exactly" 8%?

Does one need to use the P2V Assistant program, manually
monitoring the battery and cell voltages?

What minimum cell voltage (since all 3 might be different)
is one trying to achieve, and NOT go below?

Doesn't this DJI battery turn itself off at some low-charge level?

Presumably that is a lowest cell voltage, not a low pack voltage, right?

Under high-load flight, if the lowest cell goes too low due to its
apparent voltage drop due to an increased internal "resistance",
presumably the bsttery turns off, and the P2V falls out of the
sky, right?

Cheers, Gary

 

[Pull_Up]

I like you think it's more to help the firmware keep calibration. No other lipo manufacturers I know of recommend this, so I suspect it's more to do with the smart circuitry than anything else.

If you wanted to run down beyond your second stage warning, just fire it up, props off, and run it until your app says 8%. The smart circuitry should look out for the individual cells. I have no idea what cell voltage we are aiming for, as DJI says just use a percentage value. I'm assuming they built in some margin. The Vision battery shouldn't let itself get to a position where the voltage is so low that there is a danger of damaging the battery, that's going to be somewhere below 8%. Once the Vision goes into low battery auto land you are able to push the throttle up and over ride the descent if you need to for some reason, but there will come a point when there's no more left above the critical point it just shuts down. From the DJI release notes:

A new level of protection is a hidden third low battery voltage in addition to the 1st and 2nd battery
level (percentage) warnings. This uses 10.65V as its threshold. Both this voltage threshold and the 2nd
Level Low Battery Warning will trigger auto-landing. Altitude can be maintained if necessary by pulling
up on the throttle.)

 

[Scottrod]

No. The safeguards would automatically land the P2V long before it ran out of battery unless you're way too high in which case then it might run out of battery before it fully descends.

 

[garygid]

10.65 / 3 = 3.55 volts per cell, IF the cells are balanced.

However, since the cells might not be well balanced, just watching the pack
output voltage is not really a safe method to protect the cells.

However, it might be the only method available if
the data connection to the battery is not functional.

It would be better to do recalibration of the battery management
whenever the pack gets a full charge.

It would be helpful if the DJI-Vision app had a diagnostic pop up screen
that included the three cell voltages, along with the pack voltage.
Also, the present on-screen icons are so tiny, they are difficult for me to read!