SAG VS performance and battery life.

[forcefed86]

Strictly looking for 10 sec ”burst performance” Drag Race scenario.

Is there a general “acceptable” sag under load without diminishing battery life?

Should I be concerned with short bursts that sag below 10%? As long as I’m well within the manufacturers “C rating”. Should voltage sag below 10% be something to worry about?

My ESC allows me to set a minimum sag voltage. Once that voltage is met it will hold the amp draw it took to drop it to that voltage. I have mine set to 3.4v per cell. But If I drop it to 3v per cell, I pull more amps and go faster. Bats/leads remain slightly warm. How low can I set this “sag limit” on the ESC?

Some of the RC drag racers claim dipping below 3v per cell under load is common practice. Other state never go below 3.4v, even under load. I know some of this will depend on the cell. I’m using 4 10ah tattu 30c/60c packs. 2s2p, Pulling 400a.

On a side not this in scaled up a bit with 24s on a drag mini bike that I get to ride, lol.

Appreciate any info.

 

[dure16]

Interesting - I didn't know drag ESCs allow a sag voltage setting. This is a challenge we also come across in speed running.

My initial reaction is that your understanding is correct on all points. Limiting voltage sag will reduce performance and improve battery longevity.

So the question only you can answer is - which one is more important to you? Squeezing max performance out of the system or maintaining battery health?

P.S. in speed running, the best solution to voltage sag is to buy a higher quality battery.

 

[LibertyMKiii]

The battery has X amount of watt hours.
High amperage will suck that value down reducing battery life.

High amperage eventually will degrade the cells which happens anyway but it accelerates this.

I think the reason some people take different approaches is based on the weight of the battery. You can have a big battery and have less voltage sag, but it comes with the penalty of weight. Generally in drag racing the lighter the better assuming you have enough traction.

 

[forcefed86]

Appreciate the input!

This is an odd ball example as the ESC is FARDRIVER controller for a motorcycle. Problem is most E-bike hobbyists run lith-ion and not LIPO. Also very few drag race for 8-10sec bursts (1/8th mile) So I’m having trouble finding folks with LIPO drag/SAG experience.

The ESC safety is to protect the battery. So as soon as the safety voltage is met it holds/stops additional amp draw. Even though the pack may be more than capable of more amps and the controller is set to higher amps.

The packs are these 12s 10AH Tattu units. At $430 a pack (I run 4 in total 2s2p) I’m really not wanting to damage them. They are about the best I could afford. Theres not a lot out there in the 24s lipo world to choose from. These 12s packs are for agricultural drones. So not like I can run out and buy 10AH+ 12s SMC race packs. I'm limited to 400a with this ESC anyway, so should be more than enough battery running 2s2p.

https://gensacearespammers.com/ta-3...AbQC-8LOt-wzHa6y-4xnTf1dJcH0s4xzfS7u8jwzQNP4h

I’m just unsure what is “safe” for this setting. I don’t want to neuter performance, but I don’t want to damage the packs either. With a setting of 87v (3.6v per cell) I can pull 200a on a fully charged pack before sag safety is hit. If I drop this to 77v (3.2v per cell) I can pull more amps and the performance felt is night/day. I haven’t had a chance to data log it with the 77v sag setting yet. But I assume its puling more amps. Pretty fun as it is, but I don’t want to be overly conservative on the sag. On the 87v setting the bat and cables aren’t even warm after a 10 sec pull. They are warm at the 77v setting, but only slightly. I was mainly curious if dropping below 3v per cell briefly is "ok" under high load. I've heard as high as half the resting voltage with some folk. Others say never below 3v.

Currently 1.8 sec 0-30 and 4 sec 0-60 times with a 220lb rider. 8 sec 1/8th mile

10AH+

 

[Jerold]

Welcome to AF!

There is lots of info on batteries, conditioning and specs.

If you an ESC with data logging it's very helpful.

Yes you can see a HUGE voltage drop. This is an 8S speed run where I saw 17V (2.2V/cell). This was a driver issue not a system issue.

The batteries are still charging and the IR is decent, but I'm sure it takes it's toll if you do this constantly (bashing). For bursts I'm not seeing any issues "yet". It seems like batteries are a wear item and need to be replaced periodically. More often if you abuse them.

Better runs I'm seeing 3.4V/cell (droop).

In my runs, the nominal voltage before/after the run is 32+V. Once the nominal voltage drops below 32V, the battery is done for the day. I use a battery checker after the run.

I have to turn off the LVC or my runs will be cut short. A more normal droop is 27V (3.3V/cell).

Running batteries in parallel help with the droop because each one has less current loading.

 

[LibertyMKiii]

Appreciate the input!

This is an odd ball example as the ESC is FARDRIVER controller for a motorcycle. Problem is most E-bike hobbyists run lith-ion and not LIPO. Also very few drag race for 8-10sec bursts (1/8th mile) So I’m having trouble finding folks with LIPO drag/SAG experience.

The ESC safety is to protect the battery. So as soon as the safety voltage is met it holds/stops additional amp draw. Even though the pack may be more than capable of more amps and the controller is set to higher amps.

The packs are these 12s 10AH Tattu units. At $430 a pack (I run 4 in total 2s2p) I’m really not wanting to damage them. They are about the best I could afford. Theres not a lot out there in the 24s lipo world to choose from. These 12s packs are for agricultural drones. So not like I can run out and buy 10AH+ 12s SMC race packs. I'm limited to 400a with this ESC anyway, so should be more than enough battery running 2s2p.

https://gensacearespammers.com/ta-3...AbQC-8LOt-wzHa6y-4xnTf1dJcH0s4xzfS7u8jwzQNP4h

I’m just unsure what is “safe” for this setting. I don’t want to neuter performance, but I don’t want to damage the packs either. With a setting of 87v (3.6v per cell) I can pull 200a on a fully charged pack before sag safety is hit. If I drop this to 77v (3.2v per cell) I can pull more amps and the performance felt is night/day. I haven’t had a chance to data log it with the 77v sag setting yet. But I assume its puling more amps. Pretty fun as it is, but I don’t want to be overly conservative on the sag. On the 87v setting the bat and cables aren’t even warm after a 10 sec pull. They are warm at the 77v setting, but only slightly. I was mainly curious if dropping below 3v per cell briefly is "ok" under high load. I've heard as high as half the resting voltage with some folk. Others say never below 3v.

Currently 1.8 sec 0-30 and 4 sec 0-60 times with a 220lb rider. 8 sec 1/8th mile

View attachment 420865
10AH+

Yes doing this for just a few seconds will be ok.
Looking back at my data logs I was pulling 650+ Amps and would get down to 3.2v
Viewing your data you can greatly reduce your amperage draw based on how you apply the throttle. The speed at the end of the run will be better if you are more gentle on the throttle at the beginning of the run. Finding that balance is the tricky part....

Are you warming the LiPos? Having the battery pack at 105-115 F. will help its performance and reduce voltage sag.

 

[forcefed86]

Guess I’ll keep the safety at 3.2v per cell here initially and see if it gets me the 400a I’m after on a full charge. Have field weakening to play with after that as well.

Once I get this thing dialed in, I’ll be able to semi “cheat” with the power by RPM settings in the controller. Typically in an 1/8th mile drag race you are wide open the entire time and its over is 8 secs or less. With the instant E-TQ though, I can reduce power at the beginning of the rpm band where I’m traction limited anyway. Then progressively add power as the RPM rises.

I haven’t been heating the packs at all. Not really sure of a decent way to do this as there are 4 packs in a battery box enclosure not easily accessed. Could toss the whole pak in my oven at low temp and try to go for a hail mary record pass I suppose. But not sure of a practical way to heat at the drag strip.

 

[LibertyMKiii]

I haven’t been heating the packs at all. Not really sure of a decent way to do this as there are 4 packs in a battery box enclosure not easily accessed. Could toss the whole pak in my oven at low temp and try to go for a hail mary record pass I suppose. But not sure of a practical way to heat at the drag strip.

Maybe a full size cooler for your needs, but this worked really well.

Thread 'Liberty's LiPo Hot Box build ( LiPo warmer )'

Apr 14, 2023

One thing racers and speed runners both do is warm their LiPos. Warm LiPos flow current better and the internal resistance drops.
In the past I have put LiPos up on the dash and ran my defroster. With summer coming I just cant run the heater/defroster when it is hot outside on a 1hr drive to my run location.

I decided it was time to make a "hot box". When I run multiple setups during a day I might have as many as 8 LiPos that I want to keep warm so this would dictate what size "box" you need. Insulation is a consideration as well. I have seen people use ammo cans and line them with...

• LibertyMKiii
• Replies: 25
• Forum: Electronics

• 
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[Jerold]

Yes doing this for just a few seconds will be ok.
Looking back at my data logs I was pulling 650+ Amps and would get down to 3.2v
Viewing your data you can greatly reduce your amperage draw based on how you apply the throttle. The speed at the end of the run will be better if you are more gentle on the throttle at the beginning of the run. Finding that balance is the tricky part....

Are you warming the LiPos? Having the battery pack at 105-115 F. will help its performance and reduce voltage sag.

That was a particularly bad run, my throttle control needs work. I remember talking with you about it. 3.2-3.4V is more typical. The first few speed runs I was throwing over current warnings.

 

[LibertyMKiii]

That was a particularly bad run, my throttle control needs work. I remember talking with you about it. 3.2-3.4V is more typical. The first few speed runs I was throwing over current warnings.

Yeah if you watch Raz running 170+ mph he is disappointed if his 8s setup sags to below 27v (3.375v per cell)
I think that is an extreme example but it shows that if you want peak speed and performance you need each tenth of a volt.

 

[forcefed86]

Got a play a bit last night. Charged to 4.1v per cell 98.5v resting. Set safety to 76.8v (3.2v per cell) and commanded 400a in ESC. Sagged to 19.7v to 78.8 (3.28 per cell) Pulled a solid 398a on the software and holy hell this things quick for a minibike! Lol

Bat/leads only slightly warm to the touch. Still feel like these cells shouldn’t sag this badly at 400a, but I can’t say I’m disappointed with the bikes performance. No draggy times yet. But I’ll get my gear on and see if I can get a new best this weekend.

Thanks all!

 

[Jerold]

Got a play a bit last night. Charged to 4.1v per cell 98.5v resting. Set safety to 76.8v (3.2v per cell) and commanded 400a in ESC. Sagged to 19.7v to 78.8 (3.28 per cell) Pulled a solid 398a on the software and holy hell this things quick for a minibike! Lol

Bat/leads only slightly warm to the touch. Still feel like these cells shouldn’t sag this badly at 400a, but I can’t say I’m disappointed with the bikes performance. No draggy times yet. But I’ll get my gear on and see if I can get a new best this weekend.

Thanks all!

View attachment 421019

That's a bunch of volts, is that 24S?

It should be quick, you running like 45 hp!

 

[forcefed86]

Yup 2 12s in series and 2 in parallel. Need to work on getting it to leave without blowing the tire off, but its a handful. Have it dumbed down to 70% at 500rpm and slowly bring it up to 100% by 3000rpm. Still blowing the tire off more often than not. Needs traction control, lol.