High Lipo voltage sag and when to stop using batteries?

[EdwardNY]

I know this has been discussed probably 100x on the forum but I can't seem to find anything about voltage high voltage sag and when you should stop using your lipo batteries.

With my new 45 amp controller, the voltage sag seems to be alot when going full throttle which I do quite often.

my 18s4p 20c 5000mah packs have a voltage sag of about 2+ volts on full throttle. This makes me unsure about when I should stop using my batteries.

should it be at 3.65 under the full throttle voltage? But with my extreme voltage sag that I seem to be getting I am not sure what is safe to optimize my battery life.
What voltage per cell can I safely do under full throttle?

What should I look at more closely when determining to stop using my battery, full throttle voltage, resting voltage, half voltage?

I have read posts on the forum where people stop using their batteries 3.7 under load. But if I did that, my batteries would have a resting voltage over 3.8. With a 4p setup that seems too high to stop using my ebike as I still have plenty of battery power left at this point.

My last rides with my new controller I have been ending my rides early because I am looking at the voltage under full load, only to drive all the way back home and still have close to 3.8 volts per cell.

 

[ian.mich]

That's high dicharge for you. Imagine what is like at 100V 60A, the voltage is all over the map. The consensus is to stop dicharging at 3.6-7 resting voltage, and disregard what the batteries do while under load if the batteries are in good health.

 

[dogman dan]

I would call that pretty close to normal, for a " 20c :lol: " lipo pack of that size. 2v sag on 18s is not extreme.

I'm assuming you mean 2v sag for the whole 18s, not per cell. 2v sag per cell is not usable of course. If it's sagging 2v per cell, that's a pack that is done for all time, not just done this trip.

Stop based on the resting voltage. 3.65v x 18= 65.7v. So when you see 64.5-65v it's that time.

 

[electr0n]

I'm running 8 bricks of Turnigy lipo 20c, 5ah. Fully charged is 99v. I'm pulling 90amps at 24s. Most of the discharge my batteries are resting between 94v and 92v. When pulling 90 amps I see less than 84 volts! So I'm getting 0.42v sag per cell. Batteries are exactly 9 months old. With somewhere between 200 and 250 cycles.

Anyone else see sag like that? I have a feeling I might be losing some voltage with bad connections or poor wiring. None of the bricks are puffy. They are always cycled between 4.15v and 3.7v. Only ever balance charged at 1C. Cells stay pretty well balanced after every ride.

EdwardNY, your batteries are fine, compared with mine. I retired my last lipo set a little over a year of use. I had some puffing cells and reduced capacity by that point.

 

[Ykick]

I'm running 8 bricks of Turnigy lipo 20c, 5ah. Fully charged is 99v. I'm pulling 90amps at 24s. Most of the discharge my batteries are resting between 94v and 92v. When pulling 90 amps I see less than 84 volts! So I'm getting 0.42v sag per cell. Batteries are exactly 9 months old. With somewhere between 200 and 250 cycles.

Anyone else see sag like that? I have a feeling I might be losing some voltage with bad connections or poor wiring. None of the bricks are puffy. They are always cycled between 4.15v and 3.7v. Only ever balance charged at 1C. Cells stay pretty well balanced after every ride.

EdwardNY, your batteries are fine, compared with mine. I retired my last lipo set a little over a year of use. I had some puffing cells and reduced capacity by that point.

24S1P? Hammering C rate for 20C bricks. Cold temperature plays a big role with regard to sag but 90A outta 1P 20C bricks? That's not gonna hold up very well....

 

[electr0n]

24s 2p - 10 amp hours, forgot to mention.

That's almost 10c still, but dropping almost 10v seems a lot even with the cold. Going to do some tests today at different amp draws to compare. Thinking about grabbing some more batteries soon, not sure if I'm going to get the same ones or try higher C rated stuff this time. I'd like to make the battery last 1.5 to 2 years let's say 500 cycles without a big drop in performance. Maybe A123 are better after all?

 

[Ykick]

24s 2p - 10 amp hours, forgot to mention.

That's almost 10c still, but dropping almost 10v seems a lot even with the cold. Going to do some tests today at different amp draws to compare. Thinking about grabbing some more batteries soon, not sure if I'm going to get the same ones or try higher C rated stuff this time. I'd like to make the battery last 1.5 to 2 years let's say 500 cycles without a big drop in performance. Maybe A123 are better after all?

Even 2P is hammering pretty hard. Especially as they drain and IR increases. Coupled with cold temps you will certainly see/feel a large sag. I know my 3 year old Turnigy 20C are starting to perk back up a bit as warmer weather slowly creeps into my region.

Better is a matter of opinion. A123 can't compete energy density IMO but there are other advantages to be sure.

 

[ZOMGVTEK]

I have 3 year old 20C Turnigy cells that I run around 20C regularly, charge at 2C, and didn't balance in a year. I get about 5V sag at WOT when the pack is cold. They still deliver 4.4-4.5AH, but the IR is a bit higher than new cells.

 

[electr0n]

zomgvtek thanks for that. Good to have some reference point to compare performance.

I think I've eliminated cold and C rate as the cause of the excessive voltage drop in my case. Batteries dropped from 98v to 90v within minutes of testing my room temperature batteries at 90 amps bursts for accelerating. At 50 amps I get close to the same voltage drop as well. I think poor connections are giving me a couple extra volts of sag somewhere. 3 years of use from those Turnigy's is impressive, but how many cycles?

I estimate I'm somewhere between 200 and 250 cycles at 9 months used for commuting through the week, then shopping and fun on weekends. My cycle analyst shows 170 cycles but I put several cycles on there before I had the cycle analyst and I also didn't bother resetting the cycle analyst every cycle.

I've only been hammering 90 amps for the past week or so. 99% of those cycles the amperage was kept between 40 and 65 amps, so between 4c and 7c. I run my bike all winter long so my batteries have seen many days at -10c, not sure if the cold would have any impact on longevity. I know capacity and performance is decreased in the cold.