Balance plug for Mavic pack

[KDan]

I'm new here, but not new to RC, Mutirotors, Drones, or LiPo batteries. Been flying electric for 20+ years and have been around the block more than a few times building and flying LiPoly packs.

I recently bought a Mavic from eBay and it came with 4 batteries and a ton of other stuff. Seemed like a great deal and in fact it flies great! I also own a couple of 3DR Solos and the Mavic is a clear winner when it comes to gimbal smoothness and flying range.

Anyway - One of the Mavic batteries was giving a battery error message and was not flyable. Looking in the Go 4 app I noticed that the middle cell was about 0.15 volts lower than the other two. It seemed to be be fairly consistent in its offset from the other two cells until almost completely discharged. This looked like a classic case of cell imbalance. I believe these smart batteries are supposed to have a built-in balancing circuit, but for some reason it didn't seem to be doing its job.

Since the battery was unflyable, I had nothing to loose by cracking it open and trying to get access to the individual cells so that I could try to re-balance it. With considerable effort I finally got it open (it seemed to be super glued in more places than others have reported) and installed a balance plug. I cut a small window in the front of the pack to allow the plug to be tucked in there when it wasn't being used.

The results so far look promising. After running it on a true balance charger that charges cells independently it is now flyable and has a much smaller offset between cell voltages. Now I'm waiting for another day of good weather so I can take it out and do some cautious testing. Hopefully that middle cell is still healthy enough that it won't significantly sag below the other two in flight.

More to come...

 

[Deadleg]

I'm new here, but not new to RC, Mutirotors, Drones, or LiPo batteries. Been flying electric for 20+ years and have been around the block more than a few times building and flying LiPoly packs.

I recently bought a Mavic from eBay and it came with 4 batteries and a ton of other stuff. Seemed like a great deal and in fact it flies great! I also own a couple of 3DR Solos and the Mavic is a clear winner when it comes to gimbal smoothness and flying range.

Anyway - One of the Mavic batteries was giving a battery error message and was not flyable. Looking in the Go 4 app I noticed that the middle cell was about 0.15 volts lower than the other two. It seemed to be be fairly consistent in its offset from the other two cells until almost completely discharged. This looked like a classic case of cell imbalance. I believe these smart batteries are supposed to have a built-in balancing circuit, but for some reason it didn't seem to be doing its job.

Since the battery was unflyable, I had nothing to loose by cracking it open and trying to get access to the individual cells so that I could try to re-balance it. With considerable effort I finally got it open (it seemed to be super glued in more places than others have reported) and installed a balance plug. I cut a small window in the front of the pack to allow the plug to be tucked in there when it wasn't being used.

The results so far look promising. After running it on a true balance charger that charges cells independently it is now flyable and has a much smaller offset between cell voltages. Now I'm waiting for another day of good weather so I can take it out and do some cautious testing. Hopefully that middle cell is still healthy enough that it won't significantly sag below the other two in flight.

More to come...

Apologies . . this is my very first reply, and can't use the system yet. Thanks for describing your very neat modification. It's good to have a way to bring the battery back into balance. I am wondering if balancing in this way will have an effect on the mAh readings.

 

[KDan]

Apologies . . this is my very first reply, and can't use the system yet. Thanks for describing your very neat modification. It's good to have a way to bring the battery back into balance. I am wondering if balancing in this way will have an effect on the mAh readings.

I have found that when balancing with my balancing charger it does seem to affect the calibration. You will probably have to do the slow discharge to cutoff and then full recharge with the original charger if you want accurate percentage remaining readouts.
The really good news is that my battery pack is flyable again and seems to be in almost perfect balance. I've only flown the Mavic once with the re-balanced pack so far because of windy conditions, but so far so good!

I know from reading the forums here that other people have encountered the same problem. If their battery was still in warranty they could sent it back. However, anyone who was beyond 6 months on their battery may have thought their battery was permanently "broken". Clearly this is not always the case.

 

[AnzacJack]

Good job, but I would always be worried about that battery as a weak link. To expensive a toy to take the risk for me.
I would find another use for the battery in all of my projects though. Nice repair

 

[KDan]

Good job, but I would always be worried about that battery as a weak link. To expensive a toy to take the risk for me.
I would find another use for the battery in all of my projects though. Nice repair

I understand that some might be concerned, but in my there are no indications that there is anything at all wrong with the battery cells themselves. The internal resistance of all cells seems well matched, and after 4 flights the battery seems to be staying in exactly the same state of balance. The center cell is now .02 volts higher than cell 1 and 3 (it was low before). Almost perfect. I have one other pack which has a cell that is .07 volts low, and that one is only halfway to the limit. If and when that one reaches the limit, I know what to do now.

I think the issue is with a not-so-smart built-in balancing circuit in the so called smart battery. It's not really up to the task of keeping the Mavic batteries in correct balance. It seems to need a little help sometimes.

While the Mavic isn't an inexpensive toy, the Mavic batteries are definitely not inexpensive either. Quite honestly I believe the batteries are grossly over-priced compared to the Mavic in general. There is no way I am going to toss out a perfectly good battery without good reason.

 

[Memphis3]

I think the issue is with a not-so-smart built-in balancing circuit in the so called smart battery. It's not really up to the task of keeping the Mavic batteries in correct balance. It seems to need a little help sometimes..

Truth
You are doing a balance charge with that 3s connector and DJI charger/battery is not a true balance charge.
Is it nessary?
You can get away with 10 or maybe 20 non balance LiPo charges and never get a cell balance problem. .
Problem
Balance Charging LiPo disadvantage is it takes more time to balance charge a LiPo and is not really necessary every charge cycle
Cost.
You would think how expensive DJI birds DJI battery and DJI charger are that DJI would add a cheep 3s balance lead and let the customer decide how they want to charge the battery.
Last
Really nice mod!
Its really to bad you have to break the battery case apart to add a simple cheep low weight 3s connector.

 

[chazz]

I'm new here, but not new to RC, Mutirotors, Drones, or LiPo batteries. Been flying electric for 20+ years and have been around the block more than a few times building and flying LiPoly packs.

I recently bought a Mavic from eBay and it came with 4 batteries and a ton of other stuff. Seemed like a great deal and in fact it flies great! I also own a couple of 3DR Solos and the Mavic is a clear winner when it comes to gimbal smoothness and flying range.

Anyway - One of the Mavic batteries was giving a battery error message and was not flyable. Looking in the Go 4 app I noticed that the middle cell was about 0.15 volts lower than the other two. It seemed to be be fairly consistent in its offset from the other two cells until almost completely discharged. This looked like a classic case of cell imbalance. I believe these smart batteries are supposed to have a built-in balancing circuit, but for some reason it didn't seem to be doing its job.

Since the battery was unflyable, I had nothing to loose by cracking it open and trying to get access to the individual cells so that I could try to re-balance it. With considerable effort I finally got it open (it seemed to be super glued in more places than others have reported) and installed a balance plug. I cut a small window in the front of the pack to allow the plug to be tucked in there when it wasn't being used.

The results so far look promising. After running it on a true balance charger that charges cells independently it is now flyable and has a much smaller offset between cell voltages. Now I'm waiting for another day of good weather so I can take it out and do some cautious testing. Hopefully that middle cell is still healthy enough that it won't significantly sag below the other two in flight.

More to come...

Good job man. I did this too with the purpose of overcharging the battery to 4.4V but i only ended up gaining maybe 100mah and it prob wasn't worth it with the added weight from the leads...Yours definitely looks better then mine, i didn't split the battery casing open.

 

[KDan]

Truth
You are doing a balance charge with that 3s connector and DJI charger/battery is not a true balance charge.
Is it nessary?
You can get away with 10 or maybe 20 non balance LiPo charges and never get a cell balance problem. .
Problem
Balance Charging LiPo disadvantage is it takes more time to balance charge a LiPo and is not really necessary every charge cycle
Cost.
You would think how expensive DJI birds DJI battery and DJI charger are that DJI would add a cheep 3s balance lead and let the customer decide how they want to charge the battery.
Last
Really nice mod!
Its really to bad you have to break the battery case apart to add a simple cheep low weight 3s connector.

I've cycled the battery 4 times now without the balance charger. No need - using just the DJI charger is faster so long as the cells stay in balance.
DJI would not have to include a balance lead on the battery if they simply had a more effective built-in balancer. A really good smart battery design would have this. I question whether or not the DJI smart battery actually has such a circuit. If it does, it's certainly not very effective.

This is very curious in light of the fact that the battery will actually ground itself and render itself useless if one cell drops 0.15v below the others. Had this not happened, I would have never needed to brute force my way into the battery. Busting it open damaged it cosmetically.

 

[GTMonster]

Does anyone know what all the terminals on the battery case actually do?

 

[GTMonster]

How do you open the battery without damage to the case please? I really like the idea of being able to charge the batteries with my balance charger. Thanks

 

[Memphis3]

How do you open the battery without damage to the case please? Thanks

Heat gun sharp thin metal putty knife will help
but
Odds are before you completely open the battery case it will crack somewhere. If it is super glued which I do not think it is then heat and super glue de bond liquid will completely dismantle the battery shell with out any cracks.

 

[KDan]

Heat gun sharp thin metal putty knife will help
but
Odds are before you completely open the battery case it will crack somewhere. If it is super glued which I do not think it is then heat and super glue de bond liquid will completely dismantle the battery shell with out any cracks.

Opening the pack without some cosmetic damage is very difficult. I have used super glue debonder and "glue-buster" on other drone smart batteries (not Mavic) and it does make it much easier to get the cases apart. Unfortunately, both of them will attack the plastic and will crack and weaken it. You'll likely end up with pieces breaking off. I'd recommend against using them if you want to have any chance of being able to put the pack back together again.

 

[KDan]

How do you open the battery without damage to the case please? I really like the idea of being able to charge the batteries with my balance charger. Thanks

It sounds like a great idea except for one thing... When you charge the battery cells directly, rather than charging through the BMS (battery management system), the BMS has no measure of how much charge has been put into the battery by the balance charger. This will screw up the battery calibration data and cause it to report incorrect remaining battery capacity.

The way to fix this is to *slowly* discharge the battery to the point where it shuts down. SLOW is critical here.Just let it sit powered up, but no props spinning. Trying to draw to much current from a nearly depleted LiPo battery will cause it to heat up drastically and will almost certainly damage it.
Once it shuts down, let it sit for awhile to be sure the battery is at room temperature and then charge it uninterrupted with the DJI charger completely.
This re-calibrates the battery BMS by allowing it to actually measure the charge acceptance. This provides an accurate measure of the battery capacity which in turn allows the calculation of an accurate % charge remaining that is displayed on the controller and the app.

It is not recommended to do this any more than necessary because even discharging the battery to cuttoff very slowly still stresses it a bit.

The only time you would want to do this when the battery calibration drifts far from the actual battery charge state. This can happen over time, especially as the cells age. Inaccurate calibration can cause the reported remaining capacity to be too high or too low. Both can be a problem, but over-estimating the remaining capacity is worse (and much more likely). It can lead you into a situation where you believe you have sufficient capacity to get back home, but in reality your battery is really critically low.

 

[Memphis3]

FYI
It is impossible to damage a DJI battery/LiPo by charging it with a balance lead and a LiPo charger.
and
IMO if there is a DJI indicator of over charge then the DJI system is in error but with any LiPo charger you can limit the charge or even discharge or give the DJI LiPo a storage charge to eliminate any DJI readings that are in error.
Again
DJI LiPo with a 3s balance lead using a LiPo charger will never damage a DJI LiPo.

 

[KDan]

FYI
It is impossible to damage a DJI battery/LiPo by charging it with a balance lead and a LiPo charger.
and
IMO if there is a DJI indicator of over charge then the DJI system is in error but with any LiPo charger you can limit the charge or even discharge or give the DJI LiPo a storage charge to eliminate any DJI readings that are in error.
Again
DJI LiPo with a 3s balance lead using a LiPo charger will never damage a DJI LiPo.

Correct, a properly functioning LiPo charger won't damage it. But charging the battery cells directly (not through the BMS) will screw up the calibration. The battery management circuit monitors current and time (mAh) when the battery is being charged and discharged through it. That is how it keeps track of the current charge state. When you charge/discharge the cells directly via a balance lead, etc, the BMS is unaware of the change in charge state because there is no current flowing through it.

Measuring voltage alone is an unreliable indicator of remaining capacity in the long term because the internal resistance of the cells and their useful capacity decreases over time irrespective of the current cell voltage.

The proof that battery voltage is not a main factor in calculating the percentage of available battery charge can be observed by looking at the voltage under load , and then immediately removing the load by landing. The battery voltage will go up noticeably when the load is removed, but the percentage remaining will not.

LiPo battery voltages sag under load, and it sags more and more as the cells age and their internal resistance goes up. Because of this their useful capacity also decreases with time and usage. This is the justification for "re-calibrating" your smart battery occasionally as they age. If your aged battery capacity has dropped to 3000mAh, but the BMS thinks it is still 3830, it will be giving you falsely high percentage capacity values which can lead you into a situation where you don't have enough power to get home.

 

[Memphis3]

My hope for Mavic 2
DJI stop selling a charger and install a 3s balance connector to the Mavic 2 LiPo.
It would lower entry level Mavic costs
Save DJI tons of money due to less returns on LiPo charging issues.
Customer sanctification IMO would go up 200% if not more.
Using your own LiPo charger you can:
Storage charge
LiPo cycling
LiPo partial charge control.
Eliminate charger warranty issues.
Lower entry level costs
Now
I do not think DJI should go std LiPo but the charger should be customer controlled.

 

[KDan]

My hope for Mavic 2
DJI stop selling a charger and install a 3s balance connector to the Mavic 2 LiPo.
It would lower entry level Mavic costs
Save DJI tons of money due to less returns on LiPo charging issues.
Customer sanctification IMO would go up 200% if not more.
Using your own LiPo charger you can:
Storage charge
LiPo cycling
LiPo partial charge control.
Eliminate charger warranty issues.
Lower entry level costs
Now
I do not think DJI should go std LiPo but the charger should be customer controlled.

I like the idea, but I seriously doubt that DJI would even consider it. The vast majority of people would prefer to get everything they need to fly in one package. Being able to use a separate charger is great of those of us who already have one, but I'd bet we are in a tiny minority.
Considering the fact that most of the consumer drone market is comprised of people with little to no previous RC or LiPo battery experience, the smart battery approach probably makes a lot more sense.

 

[Ill]

I have battery where the cell is broken. Now the cells are balanced. The currently the voltages are ~3.7 V/cell.

If the battery is connected to charger only LED 1 blinking few second and then stops for few second. This interval is repeated and there is no indications that battery is charged. All advises are welcome.

 

[GTMonster]

Does anyone know what all the terminals on the battery case actually do?

After looking, multiple same terminals to handle the current.

 

[Balmybaldwin]

I have battery where the cell is broken. Now the cells are balanced. The currently the voltages are ~3.7 V/cell.

If the battery is connected to charger only LED 1 blinking few second and then stops for few second. This interval is repeated and there is no indications that battery is charged. All advises are welcome.

Not sure mate, but how long has it been on the charger?

I've litterally just done the same mod to bring a battery back, I've put it on the mavic, and I'm no longer getting the battery error preventing flying. As battery is at 3.4V/cell plonked it on charge, I had 2 lights doing a unusual pattern I haven't seen before, after about 40 mins on the charger they have now changed to the cascading 4 leds you normally get from charging a pack to minimum.... I'm hoping it's calibrating and now re charging. will let you know when its finfished