Forum

BMS Low Voltage Cut Off  

  RSS

OutdoorAcorn
New Member
Joined:4 months  ago
Posts: 1
20/09/2017 9:35 am  

Hi,

First off I love the idea of this kit. I say "idea" because it's not arrived yet but hopefully I'll love it just as much when it arrives! 😉 It should make it a lot easier to prototype a battery pack build before having to commit to a configuration and buy/build a spot welder.

You say in your DIY Lithium Batteries book that even though you can discharge a Li-ion cell as low as 2.5V it significantly reduces their life and that you shouldn't discharge the cells below 3V. However, for most of the BMSs I have seen the over discharge detection voltage is 2.7±0.08V some even as low as 2.5±0.1V and 2.4±0.05V. The BMSs you sell don't have any information about this so I'm going off the specs listed on ebay items. Why is it that BMSs have a lower than recommended cut off and do you know of any that have a higher cut off closer to the recommended 3V?

Thanks


ReplyQuote
Micah (Vruzend Rep)
Street cred = 1 Ah Admin
Joined:9 months  ago
Posts: 243
25/09/2017 4:12 am  

BMSs usually have the minimum possible safe cutoff voltage, which is usually 2.7V. This allows every user to be able to get nearly 100% of the energy out of their battery.

If you'd like to avoid discharging that low, then its on your to watch your discharge voltage. For example, on my ebike, I have a cycle analyst which is a glorified watt meter. It tells me my voltage, and I can stop riding when my voltage gets below a level I'm comfortable with. An added bonus of the cycle analyst is that it also has a programmable LVC so I can force it to cut power at a certain voltage.

There are also user programmable BMSs out there, but they are usually 3-4x the price.


garolittle liked
ReplyQuote
NealXu
New Member
Joined:1 month  ago
Posts: 1
07/12/2017 12:29 pm  

Hi...i am a new user here. As per my knowledge if you have a VESC, BMS for discharging is pointless. The only thing it would be good for is short circuit protection. But you can use fuse for this. Some BMSs offer e-switch as well, so that may be the reason to buy one.


ReplyQuote
garolittle
Street cred = 100 mAh
Joined:4 months  ago
Posts: 38
09/12/2017 10:10 pm  

“I have a cycle analyst which is a glorified watt meter”.... would this work for an electric go kart?  If so, where can we get this product. Could me in as a potential buyer. 


ReplyQuote
Micah (Vruzend Rep)
Street cred = 1 Ah Admin
Joined:9 months  ago
Posts: 243
16/12/2017 10:15 am  
Posted by: NealXu

Hi...i am a new user here. As per my knowledge if you have a VESC, BMS for discharging is pointless. The only thing it would be good for is short circuit protection. But you can use fuse for this. Some BMSs offer e-switch as well, so that may be the reason to buy one.

This is not true. The only protection a VESC is going to give that a BMS also gives is LVC cutoff for the entire pack. But without a BMS, you could have some cell groups go low and your VESC would never know it until the entire pack voltage is low enough to hit the LVC. But by then you've damaged the low cells in your pack because you didn't have a BMS to cut off power when the first cells got low.


ReplyQuote
Micah (Vruzend Rep)
Street cred = 1 Ah Admin
Joined:9 months  ago
Posts: 243
16/12/2017 10:16 am  
Posted by: garolittle

“I have a cycle analyst which is a glorified watt meter”.... would this work for an electric go kart?  If so, where can we get this product. Could me in as a potential buyer. 

The cycle analyst is from Ebikes.ca in Canada. Great product. Yea, a standalone Cycle Analyst with an external shunt would work for an electric go kart. It's one of the versions they offer at Ebikes.ca. It's pricy, but a nice product. There's other much cheaper watt meters out there though.


ReplyQuote
  
Working

Please Login or Register