BEC on one 6S pack of a two-pack 12S system?

[npomeroy]

Castle Creations give clear instructions on connecting a BEC that requires <8S input to one pack (the one on the "earth" side) of the pair of a 2 x 6S (or whatever) system.

However, Western Robotics warn against this:
Their instructions say:

"Q: Can I install the BEC in a multi-cell pack system such as a 12s battery pack?
A: It is not recommended to install any BEC to a multi-cell battery pack that has a voltage rating higher than the BEC’s maximum voltage input. Due to the different battery chemistries in the market, with the introduction of back electro-magnetic interference (EMI) cause from the main ESC motor, this can cause any cell within the multi-cell pack to reach to the full voltage rating of the entire battery pack. For example, in a 12s battery pack (50.4V Peak) of 2 x 6s (25.2V peak) cells tied in series, once EMI is introduce to the battery, it is possible for either of the 6s cell to reach the full 12s voltage rating (50.4V). Therefore, if you attached a BEC (rated only for 6s or 8s) only on one of the 6s cells and not across the entire 12s pack, this still can cause permanent damage to the BEC because of the potential of 12s pack voltage. It is recommended only to use a BEC that is rated enough to handle the entire multi-cell battery pack regardless of configuration."

Any confirmation or denial of this precaution? Perhaps a capacitor across the BEC input side (or a capacitor pack across the whole 12S) would lessen this danger?

[npomeroy]

Anyone... ?

I would have thought that high voltage transients could be absorbed by suitable capacitors.

[extrapilot]

Nelson

I believe this caution is based on the possibility of significant IR variance between packs. That is, this works as a voltage divider, where the equation is (low side / (high side + low side) * Voltage in. So if you have 10V in, and 1 ohm IR high side, and 1 ohm low side, you get 1/(1+1) * 10, or 0.5 * 10 = 5v. But if the lower pack was 10 ohm, you get 10/(10+1) * 10, or 0.909 * 10 = 9.09v, which is very near full pack voltage.

Maybe there is something else going on, but I tested this on a small pack hit with a very high peak current at 2x rated voltage for a few mS, and you get would you would expect- a small pack voltage rise due to its IR, and a division of voltage along the cell chain (i.e. half the pack rise at the mid-point takeoff).

I think you will find published docs from many BEC providers on how to tie into the lower pack in a 2-pack system. So, it seems that they, and many people who fly that config, have not seen whatever electrical event these guys warn about.

[npomeroy]

Thanks extrapilot. I had expected that since the use of the lower pack of a pair was widespread the warning was perhaps an OTT precaution. Can you see any advantage of placing good quality 440uF 63v electrolytic across the BEC input?

[KSHeli]

Originally Posted by npomeroy

Thanks extrapilot. I had expected that since the use of the lower pack of a pair was widespread the warning was perhaps an OTT precaution. Can you see any advantage of placing good quality 440uF 63v electrolytic across the BEC input?

There's an aweful lot of CCPro becs flying successfully under the instructions provided by CC with the CCPro. I wouldn't worry about it, unless you think your the one in a whatever ?million that the overvoltage would hit.

[npomeroy]

Originally Posted by KSHeli

There's an aweful lot of CCPro becs flying successfully under the instructions provided by CC with the CCPro. I wouldn't worry about it, unless you think your the one in a whatever ?million that the overvoltage would hit.

Unless WR use different circuitry than CC. Both get fried occasionally and it's not always obvious why.

[extrapilot]

Hard to see how that can hurt- as long as they have some ESR. Ultra-low ESR tends to ring on front ends of power switching devices without snubbing. Some ESR in the cap helps (as long as you don’t have too much ripple, where the cap will get hot…).

I think it would make good sense to write the manufacturer, and ask specifically what event they believe could cause this scenario where you have 12S voltage across a 6S pack. At the end of the day, if the scenario requires 50% overvoltage at the 12S high side, at high current, it is an overvoltage there also, with smilar margin % required. The only differences I can think of in this scenario is the possibility for a raised ground (bad wiring), or substantial difference in pack IRs. The IR thing may be easier than it seems- where it may well be that you throw a 40mOhm lower pack and a 6mOhm upper pack in series, and end up with 80% of the pack voltage seen at the 6S terminal when you have a BEMF event charging the pack…

Regards

[R_Lefebvre]

Sounds to me like somebody trying to sell their very high voltage rated BEC.

I suppose it's possible to have a real problem related to this, but probably quite unlikely.

[npomeroy]

Originally Posted by extrapilot

... I think it would make good sense to write the manufacturer, and ask specifically what event they believe could cause this scenario where you have 12S voltage across a 6S pack. …

Regards

Well I asked them for more clarification and received a reiteration of what they write in their instructions:

Thank you for our inquiry for Western Robotics’ products.

It is not recommended to perform a configuration as you mentioned and as mentioned on our Q&A explanation. What we have found possibly due to the different battery chemistry and/or possible faulty battery packs, that voltages have fluctuated within the 6s pack and have caused damage to BEC’s that are not rated more than 6s/8s application. Initially when configured it might work, but the long term effects of these voltage fluctuations are unknown and have resulted in failures for any BEC. As a high reliability and durability issue we will not recommend and guarantee this connection.
However, we do recommend to use BEC’s that are rated for12s or more for these types of applications, such as our WRL-HBECSP line that are rated up to 18s.

I'm not sure what they mean by "different battery chemistry".
I've no reason to believe my two 6S packs are damaged or have wildly different IR, and the BEC is already rated up to 8S or 150% of the nominal supply so I'm probably fairly safe. I think I will use the capacitor - surely that's a way to absorb voltage spikes.

[Smoggie]

Bunch of total nonsense if you ask me. There is no way that a 6s pack can produce 12s voltage, if it did the pack itself would explode.

They talk in vague non-specific terms about faulty packs causing fluctuating voltage. Even if that was so, a faulty pack might cause fluctuating voltage regardless of if it's connected in series with another pack or not.

As for 'different battery chemistry'... This makes no sense at all, no one is going to be connecting different chemistry batteries in series, we are talking about two identical LiPos here.

Total BS.

[extrapilot]

I don’t see it as total BS. BEMF is externally generated, and you can have a cell fault in the lower stack that takes that out of the resistor/divider mode, etc. Maybe a more probable scenario is just a wiring fault, where you have the BEC high side connected to the upper stack-side link to the lower stack, where if that connector fails in flight, and you have BEMF as a result, you would see high voltage across the BEC.

Seems possible also that you might have a crappy pack and a decent one in series- doent do anything terrible on the power to ESC side, but on the BEC side- the high IR on the lower stack could certainly see higher voltage than you would expect if the IRs were equivalent.

Seems plausible as a warning, but maybe a bit unlikely in reality given proper wiring and reasonably-matched packs. I sure hope their 6S pack does not have a 1% over-voltage margin- that would be a bad thing in a 6S install, where BEMF can come into play just as easily-

[Smoggie]

Originally Posted by extrapilot

Seems possible also that you might have a crappy pack and a decent one in series- doent do anything terrible on the power to ESC side, but on the BEC side- the high IR on the lower stack could certainly see higher voltage than you would expect if the IRs were equivalent.

Can you explain how high (or low) cell resistance would cause the voltage accross the terminals of one 6s pack to reach 12s voltage (which was the claim)?

I can see that inductance in the wiring will cause momentary voltage spikes, but that's why the BEC has capacitors between the input wires, and you get inductance even with a single 6s pack.

Bearing in mind that they claim full 12s voltage over a 6s battery, I'm still calling it BS unless someone can come up with a some evidence or even a thorough technical explanation rather than woolly techno-babble with vague references to 'BEMF' or 'different chemistry' or whatever.

[extrapilot]

BEMF is not techno-babble. It is key to the motor KV, where the motor acts as a generator whenever it is spinning. If you fail the connection between the two packs, you kill the power to the motor- and it now is generating (for some short period of time) significant power due to its angular momentum.

So now the motor is acting like a charging station, at 12S or whatever. Draw out a typical voltage divider network. The upper resistor is the high-side pack, and the lower, the low-side pack. You get a voltage value at the takeoff between the two- where that voltage is a function of the supply, and the relative resistance of the 2 resistors. If the resistance is equal, you get ½ the supply voltage.

If you just kill the low-side pack (i.e. set the resistance to infinity), the voltage at the takeoff becomes the supply voltage.

Depending on the ESC design, the voltage would typically be seen on the positive rail, and if the lower pack is disconnected, the route for that power is via the high side, through the BEC, to ground. Likewise, if the lower pack IR is very high relative to the upper, the divider voltage would be more than ½ the supply voltage, etc.

[neosunrise]

Sorry to bump up this old thread but do you guys think using a mini is safe in this case? I was going to do the same thing - wiring a mini g2 to one 6S in a 12S setup but hesitating to do so.