SP140 - Battery low power cut off mechanism & thresold

@Pdwhite: Did these smart BMS ever get implemented?

It is doable, but a lot of work and definitely not without risk.

“Electric Fox” upgraded my Battery pack by replacing the Daly BMS with an BT-enabled ANT BMS, which is also rated for 200A permanent load, not 100A (which I easily exceed on the initial climb with the light trike). In addition I got a button mounted to turn it back on.

  • I can turn off the discharge MOS => the plug is not dangerous any more during normal handling. Reminder: 60+ Volt shocks can be lethal!
  • I have a chance to understand what is going on the inside and e.g. get an early warning when a cell group drops far more than the other cells => less risk of deep cycle damage
  • Now I can also manually trigger balancing at lower voltages and do not need to fully charge first.
  • Being able to turn on/off the BMS does save energy when the battery is in storage
  • Weight change is only a few Gramm, impossible to notice

Unfortunately that did not fully fit in the original housing and thus there is now a little 5mm gap on top. At some point I need to 3D print a kind of ring to close it.



From my perspective this is a no brainer: It should be included by default and I really don’t get why it is not the case.

Very nice. I agree 100% and read a number of times they would be included in this latest batch, which is part of why I was waiting to purchase. Waiting to see if this is the case or not?

I’m not sure if this is the right thread to post to, or if I need to start another one. But I received the new battery with the bluetooth Daly BMS that is the new V2 Battery. But when I plug it into my motor it immediately shuts down, and I see in the bluetooth app that the Discharge MOS is disabled. I have to go in the app and enable the discharge MOS.

I also have noticed that when charging, the battery only charges to 99.8V rather than 100.2-100.8 like my 4kw battery does. It appears this is because the BMS is cutting off charge, but when I connect with the app and watch it during charge, no cell is exceeding the 4.2V.

Is something configured wrong, why would it be doing this?

Yes the new BMS that ships has BT and you can change the cutoff voltages to whatever you like. I wouldn’t go above 4.2v but you can if you think its not charging high enough for your liking.

The MOS is turning off because its thinking there is an overcurrent/short from the esc first getting up to voltage. So plugging in the connector half way then plugging in all the way on the precharge is done will eliminate that start up current. Also can set a higher current discharge cutoff and that should also fix that.

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Just a comment from the EV world… The pack is never disconnected while driving, except in the case of a crash. The BMS will alert the inverter (speed controller) about low state of charge or low cell voltage, and it will reduce power. If you keep going you will get all kinds of warnings, turtle lights, sounds and further power reduction. Eventually it will stop before you hit the low cell limit.

It worries me a bit to have a semiconductor switch in line - in the case of low cell voltage, sorry I’d rather damage the pack than my legs :slight_smile: And there’s extra failure modes - the FET itself could blow or the logic controlling it could cause a false shut-off.

The “pack protects itself” model is what the e-bike world uses. That’s fair enough, you’re never in danger from losing e-assist. The R/C world uses “BMS warns you” strategy - like those screamer BMS circuits. That’s because people don’t want to lose their model plane! And cars, well, you don’t want to stop dead on the highway or in a junction, but the pack is very expensive so you want to protect it also.

IMO when it’s supporting a human, the safety risk of losing power suddenly is even higher than for cars, and we should aim for the safest strategies from that world (accurate warnings and power reduction).

Just my opinion!

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Exactly my line of thinking.

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So what is the status of newly shipped units. Can you adjust LVC on the controller?
The best would be 2-steps
First one could be when you reach around 3V/cell or 72V on the whole pack on controller side. This will cut the throttle under load but will not shut down the system(battery), so you can still spin it up for a landing.
Second stage is that below 3V and will be last cutoff made by BMS itself that will shut down the battery itself.

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For those who uses ant bms on their new packs you can go with this display where it will show you the lowest cell voltage during flight.

Yes you can set the cutoff the what ever voltage you would like with the BT App SP140 Battery BT BMS app - YouTube

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Yup, and I can also see that you can set limit in controller here:

Options are None, Hard Shutdown, Gradual Shutdown, 75% Power Limit, 50% Power Limit and 25% Power Limit.

So perhaps one can set it to either gradual shut down or power limit of 75-50% so you know its time to land when you feel that the limiter is kicking in. Now of course, you could look at the display, but would be good to have that extra “safety alarm” if you get carried away during flight.

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One of the keys to a long battery life cycle is to ensure that the propulsion device does not use the full capacity of the battery. If you want to enjoy the pack for a long period, do not charge to absolute maximum e.g. 4.2V per cell (100.8V). Instead you can use 4.15-4,1 (99.6V-98.4)
Same goes with Discharge. I do not recommend going below 2,8V/cell or 67,2V in a full pack.

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I’ve been using the pack from 100.2V to 80V. I have found that the 70-80V range runs out just about as fast as the 98-100V range. So you get much more out of the pack without stressing it by charging up to full and ending early, rather than charging up to 98V and running down to 70v.

With that said, Allex you are totally right about longevity of the pack.
But lets say you were to use it’s full capacity 4.2 to 2.5V every single flight, under hard load. Strenuous usage of the pack, from the Molicel Datasheet, you can expect 85% pack capacity after 500 cycles, or 500 hours of flight.
That seems like it would take years ( assuming 100 hrs flight per year ) for us fair weather fliers to actually see appreciable capacity loss. The capacity of the pack would degrade over 5 years just as much if not more due to age, than from how you are cycling the pack.
In conclusion, charge it, send it, cool it, store it, forget about it.

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You are probably right that it’s not worth worrying about full cycles, given the use case.

But I would still be careful to use a low voltage cut-off that doesn’t leave the cell OCV < 3.0V afterwards. That seems like a recipe for damage.

I am not so worried about the cycles but rather about capacity, personally. I rather be up in the air longer than more often. While you have around 80% of capacity left after 500 cycles at ~10A discharge rate per cell you may have a lot less capacity when you go with 30A per cell. If I read the chart correctly you are at 80% capcity after 200 cycles when discharging at 20A. And we dont know where we will be at at 30A per cell.
Both HVC/LVC(high voltage cutoff/low voltage cutoff) together with discharge rate has impact on Capacity loss. Thats also why a 4kWh battery will most probably live longer than a 2kWh

Oh and you are correct jsneeb at 3,3V(80V) per cell there is basically nothing left in the tank. Could be a sweet spot to tune in controller for a power limit(?). Will take note of that.

Definitely, something like a 50% power limit at 80V would be a good start point. However, also consider that at lower voltage your max power is already reduced. Then maybe consider going up to 75% power limit, or tweak the voltage down a bit more to accomodate the voltage sag. I find that around 75-76V under cruise power, my OCV is 80V. At 80V OCV I start looking to land. That gives me 2 or 3 go arounds of the field if something isnt right.

only as a hint . the data sheets from the battery manufacturers always refer to a cell in a cooled environment at around 20 degrees celsius. as soon as it’s a battery pack in a housing without cooling, the world is completely different. the number of cycles drops dramatically as soon as you very often get to 3 V per cell under load. there are also many inventors who have tested hundreds of cycles under different conditions. my opinion is that if you stress the molicell too much, a maximum of 150 to 200 cycles are possible or a running time of 2-3 years, with a good 20% of them no longer being usable. Under “stress” I see a continuous discharge of more than 15 amps per cell. Ideally, less than 10 amps are needed and a maximum of 15 amps for a short period of less than 30 seconds. I’ve been flying molicell myself since 2018/19.

Is it 300A the controller is drawing as a max value?

Has anyone played with these settings yet? This is one of my big safety concerns with this setup and I’d love to know it any of these options work and how well.

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I configured these settings for now. But have yet to try them out.