Low voltage cutoff question

[ebike11]

Hi guys
I have a question about low cutoff voltage. I have a 36V pack and
I set my cycle analyst to 33V LVC.
When the battery has plenty of charge, the voltage drop is at about 2V.
Now say for example on a long trip without access to a charger, my battery voltage is sitting at 34V. Of course this wouldnt happen often but Im just curious regarding how would the battery react, if the battery is at 34V but when the throttle is engaged the voltage drop normally would go to 32 or 31V but it wouldnt be able to because of the 33V LVC. Im guessing its not good for the battery???
Thanks!

 

[justin_le]

Hi guys
I have a question about low cutoff voltage. I have a 36V pack and
I set my cycle analyst to 33V LVC.
When the battery has plenty of charge, the voltage drop is at about 2V.
Now say for example on a long trip without access to a charger, my battery voltage is sitting at 34V. Of course this wouldnt happen often but Im just curious regarding how would the battery react, if the battery is at 34V but when the throttle is engaged the voltage drop normally would go to 32 or 31V but it wouldnt be able to because of the 33V LVC. Im guessing its not good for the battery???
Thanks!

What would happen is that the CA would limit your current draw at full throttle to half your normal max current, so the battery voltage sag would only be 1V instead of 2V and it would sit right at 33V. As you continued to drain the battery, the CA would limit the power more and more so that the pack terminal voltage didn't drop below 33V. There is nothing harmful to the battery about this, and it's a lot more convenient as a rider than having full power and then suddenly no power when the BMS circuit trips. You also (a bit counter-intuitively) get more amp-hours and range from the battery pack by having the CA's LVC higher than the BMS cutoff and doing a rollback like this.

 

[ebike11]

Ok thx for the explanation!

Actually my pack doesnt have BMS so I rely on the Cycle Analyst for the low voltage cut off

 

[dogman dan]

If you set it lower, you still need to use less than full throttle when its that low. Less than half IMO. In practice, setting your LVC 1v lower than the packs suggested lvc should be ok. When it sags to cutoff under load, you will not be able to go very fast without a cutoff, and that will warn you to stop now, or very very soon. Once warned, you could set the cutoff back up one volt to limp home.

But the main thing is not to discharge past the point your cells will be damaged, which varies by chemistry, WHEN ITS RESTING. Its ok to have it lower under load, if the load is very light.

This assumes you do not have a weak cell in the pack that empties long before the others. What to do?

on packs with no bms, I first identify which cell or cell group is weakest. Then I put a low voltage monitor on that cell, If I am pushing the pack to its limit. It beeps if I get that cell too low under load, which makes me react with less throttle till it stops beeping. When it won't stop beeping, you are done using the pack.

You only need to put the monitor on the pack when you are limping home empty, not 24-7.

Of course, to use the monitors, you have to have some jst plugs on your pack. (balance plugs)

Either way you limp home, you only get about a block or two more riding, once the pack is that low.