# Having issues with using Lithium Ion cells on a small quadcopter

I just built a quadcopter with the following specs, as seen here on eCalc.

eCalc says this build should work, however when I take off on a fully charged battery, the voltage immediately drops from 12.3V down to around 11.1V, and then continues to drop down to 10V before the quad falls out of the sky. I am using 21700-40T cells in a 3S1P configuration, which should be able to discharge fast enough for my build, so Iβm not sure what the issue is.

One thought I had is that it could be related to the BMS I have attached to the battery. I am using this BMS which says it has a max working current of 6-8A, so it should be fine, considering my quadcopter pulls about 6A to hover. However, after the failed flight, the BMS was extremely hot.

Hereβs a dataflash log of the failed flight: 32 12-31-1969 7-00-01 PM.bin - Google Drive

remove the BMS and try without it.

According to the eCalc screenshot your drone pulls about 3.29A per motor to hover, so 3.29A * 4 = 13.16A is a bit (about 65 - 120%) over the BMSβs working current.

Ah that makes more sense, I thought it meant 3.29A total for all 4 motors to hover. So if I use a 40A BMS I should be good for everything except motor @ maximum?

I just tried it, and it does indeed work without the BMS. I guess I will need to get a bigger BMS. On the other hand, how necessary is it to have a BMS on the lithium ion battery?

two points are mandatory for LiIon battery packs.

1. never charge each single cells over the maximum charge level
2. never discharge each single cell below minimum discharge level

Point 1 can be controlled by the LiIon charger if it has contact to each pole of each cell.
Point 2 can be controlled by the FC and/or PDB by setting a min value clearly above the cumulative discharge level together with a good balacing of the cells during each charge. This also can managed by the charger.
So if you maintain your LiIon carefully with a good charger than a BMS is not always necessary

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Additionally a BMS limits the charge/discharge rate to protect the cells. (thatβs the β8C/12Cβ in eCalc: the maximum allowable discharge rate)
Charge rate is controlled by the charger, discharge rate is, in your case, limited by the maximum current the motors can pull.
So another reason why you donβt need a BMS.

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Unless you have a specific use case, then Itβs generally not recommended using a BMS. Monitoring the total battery voltage is enough.

Its generally a bad idea on aircraft because the cost of the battery is usually nothing compared to the cost of the vehicle so most would rather damage a battery running it down to nothing to save the aircraft rather than having the BMS cut it off when it got too low and have it fall out the sky.

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I do actually have a rather specific use case. I am building a drone that will automatically dock on a charging pad. The feet of the drone have magnets through which the battery will be charged. There will be a relay on the drone to disconnect the battery from the flight electronics and switch it over to the charger, which is on the ground. Therefore, I need a BMS actually on the drone, otherwise I would have to also connect the balancing wires to the charger on the ground, which would be difficult.

what you can do is have separate charge and discharge connections, keep the BMS connection for charging but add another for discharge that is connected directly to the battery, so the BMS will be bypassed for discharge.

So if you had a standard battery you would wire the balance leads to the BMS for charging then use the regular high current wires for discharge.

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Based on your link to the BMS I am not sure that this modules are only have protection function or also balacing function. In the description on amazon balacing is not shown.
The balacing function is really necessary for good maintining your battery specially on cells with high currend usage.
So take care that the BMS you select is really doing this.

That sounds like a great solution but Iβm not quite sure how to incorporate it into my relay for switching power between the flight electronics and charger. It seems easier to do that if both charge and discharge use the same wires.

why do you need to turn it off when charging? its not going to do anything when disarmed and I would be surprised if it used more than 5 watts when idle.

Lithium batteries canβt be charged and discharged at the same time, as far as I know. If there is any amount of current being pulled from the battery, it messes with the charging cycle of the battery, which requires stages of constant current and constant voltage.

as long as you dont go over the max voltage or rated current you can charge them at any rate you want, thats how they can be charged with solar.

so you have a 1000ma charger and it takes 100ma to run the vehicle then 900ma will go into the battery so it will charge it a little slower, once its full the charger will just hold the battery at full charge while delivering enough power for the electronics. The BMS wont let it get overcharged.

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Thatβs not exactly what Iβm talking about. I was using this Boston University project as a reference. Here is what they say:

There is a relay system to switch between the flight controller and ESCs and the battery charger. This is an essential part of the system, as otherwise the battery charger gets fooled by the constant current being consumed by the flight electronics.

I dont see why that would be an issue unless you have a capacity limit set on the charger. Itβs no different to solar powering a vehicle. I looked into something like this a long time ago for flying around with an advertisement banner.