Battery discharge cycle for 3-month maintenance...down to 10%? 8%? 0%?

[Kilrah]

There is no attempt to balance cells during auto discharge.

Do you have proof of this?

 

[scubaddictions]

Therein lies the biggest risk when flying with partially auto-discharged batteries. The cell voltage discrepancies can produce unpredictable results under load. Topping off before flight, if it will allow you to (newer batteries like the M2 and P4 batteries can't be topped off to 100% unless below 97%), or setting a longer auto-discharge time period you know you have not exceeded, protects you from flying with unbalanced cells.

I definitely experienced this last week with my Spark. All four of my batteries had Auto discharged a bit, and I flew them directly to drop their charge further for the three-month cycle. Three of the four batteries had uneven cells, and critical battery level warning popped up during the flight, as quickly as one or two minutes up in the air. They tried to force a landing into trees, had to fight to keep altitude to get it back into a safe area.

After running the batteries down safely on the ground, and a full recharge, all subsequent flights have been flawless.

Lesson learned, I definitely feel that the auto discharge allows the cells to discharge unevenly.

 

[GadgetGuy]

Do you have proof of this?

We have plenty of evidence from the past. Do you have proof of a current change to support the belief that it is no longer true? In the mean time, better to assume nothing has changed, until proof that it has changed.

 

[WithTheBirds]

We have plenty of evidence from the past. Do you have proof of a current change to support the belief that it is no longer true? In the mean time, better to assume nothing has changed, until proof that it has changed.

There may be a recent change with later models however the issue has been, as you pointed out, an all to frequent observation. Rapid voltage collapse of one cell shortly into the flight where auto discharge had commenced (a clear correlation of user testimony and log analysis).

In all cases the Texas Instruments BQ30Z series SOC performed the smarts function for the subject batteries up to and including Mavic Pro, perhaps later models- I don’t know if there is a new BMS chip or if DJI has simply rebadged chip.

It should not be an issue for any operator who follows the fly with a fully charged pack recommendation.

 

[GadgetGuy]

There may be a recent change with later models however the issue has been, as you pointed out, an all to frequent observation. Rapid voltage collapse of one cell shortly into the flight where auto discharge had commenced (a clear correlation of user testimony and log analysis).

In all cases the Texas Instruments BQ30Z series SOC performed the smarts function for the subject batteries up to and including Mavic Pro, perhaps later models- I don’t know if there is a new BMS chip or if DJI has simply rebadged chip.

It should not be an issue for any operator who follows the fly with a fully charged pack recommendation.

To that end, frustrating that the newer batteries (P4 and M2) can't be topped off to 100% until first bled down to 96%, when they are at 97-99%. 100% charge is an extra margin of safety, and ensures all cells are properly rebalanced before flight.

 

[everBit]

Auto discharge switches in the resistors used to balance during charge and,

You are mistaken here. Balance resistors have never been used for Auto-discharge purposes in any of DJI batteries. There is dedicated 100 Ohms DTS (Discharge-To-Storage) resistor and DTS FET controlled by MSP430 uC (or by bq9003 in newer design batts) for that feature. This load is connected directly to top of cells stack and to ground.
It could easy be confirmed by heat emission from DTS resistor when auto-discharge is going On.
Voltage drop on all the balancing resistors at the same time is always zero proving they not used for that purpose.


Just for reference have attached picture of DJI Mavic Pro bms with marked DTS components.

 

[WithTheBirds]

You are mistaken here. Balance resistors have never been used for Auto-discharge purposes in any of DJI batteries. There is dedicated 100 Ohms DTS (Discharge-To-Storage) resistor and DTS FET controlled by MSP430 uC (or by bq9003 in newer design batts) for that feature. This load is connected directly to top of cells stack and to ground.
It could easy be confirmed by heat emission from DTS resistor when auto-discharge is going On.
Voltage drop on all the balancing resistors at the same time is always zero proving they not used for that purpose.


Just for reference have attached picture of DJI Mavic Pro bms with marked DTS components.

View attachment 96341

Very interesting.

Thank you for the correction. I was looking at the application note for the SOC.

So how do we explain the out of balance cell condition following auto discharge?

 

[everBit]

So how do we explain the out of balance cell condition following auto discharge?

Have observed this if out-of-balance was already present at the begining of auto-discharge.
And after long storage due to different self-discharge rate of individual cells especially for batteries which have worked out several seasons. Due to tepmerature gradient DJI Mavic Pro/Air battery cells degrade differently.
Have never observed out-of-balance issue For new batteries being well balanced before auto-discharge.

Are you observing this repeatedly and on every battery?

 

[DanMan32]

I treat my MM batteries just like I do my lithium-powered car, my lithium-powered tools, my lithium-powered RC aircraft and my lithium-powered robovacuum - I use them and recharge. I trust the makers to program their products to work without me having to constantly worry about if I’m doing everything to prolong the life of their batteries. Are the batteries really at 100% when they quit charging? I doubt it - the maker knows it’s best not to. I quit using a battery when I get a low voltage warning, as with the MM, or it quits, as my cordless drill or leaf blower does. My biggest investment is my car. I charge my Chevy Bolt to 100% when it gets to around 25%, or if I want it full for a longer trip the next day. There is nothing in the owner’s manual about not charging to 100% or periodically discharging to a specific level. Chevy doesn’t want to have to replace a multi-thousand dollar battery under warranty, so it makes sense they set every parameter to make sure, regardless of how the consumer treats it, the battery will last as long as possible. So if you want to get down in the weeds, create a spreadsheet, and develop a charge/usage matrix on each battery, more power to you. Me, I’m just going to fly, rotate my three batteries, charge them when they’re low, and have fun.

Metal can Li-Ion are less susceptible to issues being left near 100% than LiPos.
I suspect the Bolt uses can Li-Ion. Also a vehicle is less likely to remain at 100% for very long. Power tools also likely use metal can Li-Ion. They all likely do go to the battery's 100% without reserve.

A hybrid vehicle however does have an unused margin on the upper and lower scale as well as avoiding reaching the usable upper and lower range since its primary propulsion is fuel, not electric.

Mobile devices and laptops however typically use LiPos. Lately laptops have modes you can set if your laptop is mostly on AC, where it charges only to 80 or so % to prevent premature battery swelling.

The two biggest enemies to lipo batteries is heat and constant charging and remaining near 100%

My HTC phone started cutting out when getting around 30%. Lately I have had it on charge a lot. I seemed to have fixed the issue by getting down below 10%, then charging to full, and not leaving it on charge all the time. To get it down to 10% when it was cutting out around 30 was to power on while on AC until it fully started. Then let it go down further.
The real issue for the HTC may have been the "fuel gauge" being out of calibration where my full swing of discharge and recharge got it recalibrated.

 

[dlmvegas]

This thread and the FAA has made me wish I never bought my M2Pro.

 

[dlmvegas]

With the FAA's rules & BS and reading this thread I wish I had never purchased my overpriced M2P. DJI needs to find another solution to power these drones instead of using these overpriced batteries. Especially with, what appears to be, unclear rules for maintaining them.

 

[DanMan32]

Seems simple to me.
Keep them ideally at room temperature and at 60% charge when not in use. Auto discharge helps with the 60%.

Don't leave them in hot environments, especially when fully charged.

 

[WithTheBirds]

Sell it before a new model is released, maximise your return.

DJI doesn’t develop battery tech. They do use what is currently the best performance/reliability/cost compromise however.

I’m not sure what the FAA has to do with it.

 

[BobbyLashley]

Does anyone have a chart showing the rate of auto discharge on DJI batteries?

Like letting the batteries sit for 1 week, 1 month, 3 months etc. Just want to make sure I don't accidentally reach 0%.