Whats a good voltage cutoff for...... - RC-Monster Forums

notnodak · **Whats a good voltage cutoff for...... -** 01.11.2010, 01:56 PM

I'm running a MM/9xl on two 5200 2cell Li-Po's. What should I set my Voltage cutoff at for this? 6.0v is the safety threshold I believe but setting it at 6.5v or higher is better right? What do you recommend to get the most safely from my batterys?

BrianG · 01.11.2010, 01:59 PM

3.2v/cell is usually the standard. 3.0v/cell is too low IMO.

notnodak · 01.11.2010, 03:56 PM

Thank you sir. I'm all set. Going out this afternoon.

bdebde · 01.11.2010, 04:08 PM

If you are running 2 - 2cell packs in series (4s) you will need 12.8 v cut off.

notnodak · 01.12.2010, 02:33 AM

Right where I put it. Thanks

BrianG · 01.12.2010, 02:36 AM

If you have the newer firmware, I would set it to auto-lipo and then the cutoff to 3.2v/cell. That way, it is ready for anything from 2s to 4s. Some setups I have I run 4s and 5s, and this mode makes it easy to switch without having to change settings at all.

Oh, and I assume you are running an external BEC with the internal one disabled right? If running over 2s, I would definitely do that.

02:40 AM

Many people say 3.2v is too low as the voltage under a load can also dip below the safe 3.0v for a fraction of second and over time and slowly ruin the Lipo cells. I've got mine upped to 3.5v to be on the safe side. I ruined my first set of LIPO's this way

notnodak · 01.12.2010, 02:43 AM

Quote:

Originally Posted by BrianG

If you have the newer firmware, I would set it to auto-lipo and then the cutoff to 3.2v/cell. That way, it is ready for anything from 2s to 4s. Some setups I have I run 4s and 5s, and this mode makes it easy to switch without having to change settings at all.

Oh, and I assume you are running an external BEC with the internal one disabled right? If running over 2s, I would definitely do that.

That's right, external. I'l look into that auto setting. I just upgraded firmware an hour ago and haven't looked at it yet. I didn't use auto with my 3.12 version because auto was 3.0. I like the idea of auto because I have it all setup to run single 2 cell @7.4v, dual 2-cell @ 7.4v and dual 2-cell at 14.8v. Just depends on who I let drive.

notnodak · 02:52 AM

I just checked my firmware and it says that auto is 3.0/cell. I'll try the 3.3= 13.2v for the time being, unless my runtimes aren't great. What does it actually do when it's to low? Just stop, beep or stutter?

notnodak · 03:14 AM

I'm dumb. Found the voltage adjuster for the Auto setting. I haven't changed anything in a while and am afraid all was reset so Brian or Freeze or whoever please give me the nod if I have ESC set right for the following setup.

3905 E-Maxx just for general bash, gravel yard steet racing, snow/ice bash
LST waffle tires on dish wheels
rear spool(snow)
Losi DSM radio system
MM w/ UBEC,
9xl purple HobbyCity motor w/ RCM clamp/heatsink
62t spur/ 18t pinion

Voltage cutoff @ Auto-LiPo - 3.3v/cell
Proportional w/ Reverse Lockout
Motor Direction @ normal
Brake @ 30%
Drag brake @ 0%
Punch control @ 0%
Reverse Throttle @ 100%
Motor type @ brushless
Power-on warning beep enabled
Motor timing @ Low
Arming time @ 3 sec
Throttle Deadband @ Large(0.1500ms)
Start Power @ Low

BrianG · 01.12.2010, 03:15 AM

Quote:

Originally Posted by notnodak

I just checked my firmware and it says that auto is 3.0/cell. I'll try the 3.3= 13.2v for the time being, unless my runtimes aren't great. What does it actually do when it's to low? Just stop, beep or stutter?

3.3v is good. If you set the cutoff too high, the worst that will happen is your runtime will be cut short.

Setting the right LVC point can take a little trial and error. Set it to a safe high value to start (like 3.5v as Freezebyte suggested) and run it. When the LVC kicks in, disconnect and let the pack rest for a few minutes. Hook it to the charger and see what the voltage is (not charging yet) of the depleted pack. You generally try to shoot for a resting voltage of 3.6-3.7v/cell unloaded. If the voltage is above that, you can decrease the LVC to 3.4v and repeat until you get the desired resting voltage. Having a resting voltage high isn't "bad", but you are losing runtime. If the voltage is still low (like 3.4v), then you need to increase the LVC. Chances are that if you have a 3.2-3.3v/cell cutoff set and the resting voltage is under 3.6v/cell, the pack is probably not adequate for the job and is working too hard. Also, a hot pack (over 120*F) means it is working too hard, and could mean the batteries are no suitable for the setup. Gearing down and programming the ESC with gentler settings (higher punch control) can help reduce current needs and be better for lesser cells.

Freeze, you are right that there are peak currents that may cause excessive voltage drop, but these packs are designed to handle bursts like that. The ESC does seem to absorb those peaks so it doesn't false trigger.

From what I've seen, when the MM/MMM nears the LVC point, you lose a lot of snap, but not sure if that's just the battery getting towards the end, or the cutoff kicking in. Within a minute or less of this, the ESC stops.

BrianG · 01.12.2010, 03:20 AM

Quote:

Originally Posted by notnodak

I'm dumb. Found the voltage adjuster for the Auto setting. I haven't changed anything in a while and am afraid all was reset so Brian or Freeze or whoever please give me the nod if I have ESC set right for the following setup.

3905 E-Maxx just for general bash, gravel yard steet racing, snow/ice bash
LST waffle tires on dish wheels
rear spool(snow)
Losi DSM radio system
MM w/ UBEC,
9xl purple HobbyCity motor w/ RCM clamp/heatsink
62t spur/ 18t pinion

Voltage cutoff @ Auto-LiPo - 3.3v/cell
Proportional w/ Reverse Lockout
Motor Direction @ normal
Brake @ 30%
Drag brake @ 0%
Punch control @ 0%
Reverse Throttle @ 100%
Motor type @ brushless
Power-on warning beep enabled
Motor timing @ Low
Arming time @ 3 sec
Throttle Deadband @ Large(0.1500ms)
Start Power @ Low

The LVC settings look good. If you want a little cooler running setup, I would change the punch control to something like 30%.

And I would adjust the deadband to be smaller. A large deadband makes the neutral range larger, which means you have to press the trigger more from neutral just to get it going. This also means that you have less range between neutral and WOT so the resolution is not as good. The only reason to have a large deadband is if the radio throttle potentiometer drifts a lot when going back to neutral (the ESC would have a harder time finding neutral).

notnodak · 03:36 AM

Changed it. Thanks BrianG.

One more question for you. I'm using a Turnigy Accucell-6 w/ a max of 5amp charge and I believe I can use your diagrams to make a harness to charge two 2-cell or two 3-cell at the same time. My question is the charger will see a 4/6-cell setup but will both batteries be charging at 5amp?

Is there a place to buy the harness or do I need to make my own? I'd like harness' that allow charging two 2-cell and two 3-cell setups and I'd like two of each.

BrianG · 01.12.2010, 03:41 AM

The charger will see the total of your pack. If you have two 2s packs in series, it will see 4s. And it will be able to charge the full 5A as long as the charger has around a 100w or higher power limit for a 4s pack. For 6s, you would need a power limit of around 130w to charge at the full 5A. A lower power limit (like 50w) will mean it won't be able to charge at the full current.

I believe there is a balance board you can buy that will take care of the balancer connections, just make absolutely sure you hook it up right according to the main charging leads. Or, you can build your own using my diagrams. I personally prefer building my own since I can be sure it is done safely, but that's up to you.

notnodak · 01.12.2010, 04:08 AM

Quote:

Originally Posted by BrianG

The charger will see the total of your pack. If you have two 2s packs in series, it will see 4s. And it will be able to charge the full 5A as long as the charger has around a 100w or higher power limit for a 4s pack. For 6s, you would need a power limit of around 130w to charge at the full 5A. A lower power limit (like 50w) will mean it won't be able to charge at the full current.

I believe there is a balance board you can buy that will take care of the balancer connections, just make absolutely sure you hook it up right according to the main charging leads. Or, you can build your own using my diagrams. I personally prefer building my own since I can be sure it is done safely, but that's up to you.

It's a 50w Maybe I'll look into getting the Acucell 8 it'll do 150watts. I'll keep looking