Functionality in the ideal BMS?

Discuss hardware related to the VESC such as the NRF nunchuk.
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arvidb
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Functionality in the ideal BMS?

Postby arvidb » 26 May 2017, 13:05


rew
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Re: Functionality in the ideal BMS?

Postby rew » 27 May 2017, 08:06

IMHO, the BMS should keep the battery safe under all circumstances. This means a forced cutoff if things are getting out of hand.

However, an interface that indicates to a consumer: "only 50% of normal load allowed" is something that oldfashioned BMSes don't have, but would be very useful if it is standardized so that more consumers can "consume" such a hint from the BMS.

On my bike with second-hand cells, the internal resistance is so high that even though it has about 930kJ of energy, after only 150kJ I can drive the VESC into lower-performance mode due to sagging voltages. An oldfashioned BMS will cut the VESC off and I don't have any assistance until I get home...

My proposal for such a signal is simply a 0-3.3V voltage where 3.3V indicates "full power allowed", 2.2V is "only 66% allowed" etc.

Similarly, there should be a throttling output for the charger. 3.3V means "you can go as fast as you want", and lower voltages means the charger has to throttle down. So, when you connect a 200W charger to a small battery, the BMS will quickly start to throttle the charger to something that it can handle. But similarly, the balancing circuits will kick in once a cell reaches 4.20V. But above 4.25V the BMS should start indicating that throttling the charge current is the way-to-go.

To achieve this, the BMS will need some inputs. The cell voltages are one. I have a 1P arrangement, so a single bad cell could require me to throttle down. (in practise, I trust the VESC to do the right thing, but in reality I'm probably overstressing the cells that are already the worst.)

Also the cell temperature can be monitored. When things get too hot, time to throttle down.

At this stage, you mentioning "cylindrical" and "not prismatic" cells is IMHO not useful. You could say: Of course we're probably thinking about Lithium cells.

FYI, after NOT balancing my bike-pack for like three months, I checked the balance yesterday. About three (out of ten) cells were above 4.2V when the whole pack was at 41.5. So I took out 20-30 mAh from those cells to make them 4.2V.

So, balancing "every time" means that you occasionally hit say 4.22V, this normally happens in the "current taper, CV" stage, and within 10 minutes the voltage of the cell will be back at 4.2V even when you only balance at 20mA. Most balancing circuits will balance at 150mA and achieve balance even quicker.

I did charge at around 200W yesterday. I forgot to put the battery on the charger, and there was little time left before I had to leave. Although the lab powersupply has a 600W limit, at my batteries voltage it can only produce about 400W.

arvidb
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Re: Functionality in the ideal BMS?

Postby arvidb » 27 May 2017, 12:37


rew
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Re: Functionality in the ideal BMS?

Postby rew » 28 May 2017, 08:01


TilmanBaumann
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Re: Functionality in the ideal BMS?

Postby TilmanBaumann » 15 Feb 2018, 16:57

While recently reading through the code I also noticed that VESC doesn't seem to have battery protection in mind when it comes to regen braking.

Ideally, it would interact with the BMS for such matters. But since it has voltage sensing for battery voltage, I would say a max cutoff voltage over which no regen should happen would be a very useful thing indeed. (Ideally, it would communicate this issue via CAN and serial to any potential display unit)

Ideally, it would achieve to not drive the brake voltage over a certain maximum to comply with CV charging regime in the top end. (Current limit is effectively already in place)
If that is not easily achievable, the brake could just be deactivated at a certain pack voltage.

Mind you this condition would only exist in the first few minutes of a drive after a full charge. It sucks to live on a hill. :)

TilmanBaumann
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Re: Functionality in the ideal BMS?

Postby TilmanBaumann » 15 Feb 2018, 17:03

Ah, I see the issue has been addressed in this topic too. viewtopic.php?f=15&t=586

Perhaps not a bad idea actually. Just dump any over-voltge into a resistor. Easy low tech. Leaves us only with a thermal management issue.


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