80/20 LIPO Rule |VS| LVC cutoff |VS| Ideal V range

[onloop]

This is probably not great "forum etiquette" but I have actually posted this in two places because I wasn't sure if the battery gurus or the e-board gurus where needed to answer my question... Because I can only assume LVC is probably configured very differently depending what electric vehicle/device you use. Such as when your flying an expensive plane as they can fall from sky when there is no power etc.

So to read it on the skateboard section go here: http://endless-sphere.com/forums/viewtopic.php?f=35&t=55931#p833136


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So I know there are lots of existing posts & thread & info about LIPO technology and rules to follow when charging / discharging, but I haven't been able to find the exact answer to this:

NOTE: For reference purpose, I am using an Alien ESC on a skateboard with a 6S 8000mah zippy flightmax 30c battery, powering dual 270kv motors.

SO BASED OFF THIS FAIRLY WIDELY EXCEPTED INFO ON LIPO's:
> You should never use more than 80% of your battery (so 800mah of 1000mah bat - leaving 200mah)
> & if the Ideal charging / discharging range for Lipo is say, 3.65v minimum to 4.2v maximum(or 4.1v as some peeps rcm'd)
> And you should NEVER GO ABOVE 4.3V NEVER BELOW 2.7V (i'm assuming for safety reasons)



What should I set my ESC Low Voltage cut-off at?

Some people say (ESC supplier also) 3.3 or some say 3.5 but this still seems very low because at this voltage you are way past the 80% mark & also well out of the Ideal voltage range..... Is this because sometimes voltage can drop down then come back under heavy strain?

So when taking into account the "approximate" info available from these various "Capacity VS voltage charts", such as:
4.20v = 100%
4.03v = 76%
3.86v = 52%
3.83v = 42%
3.79v = 30%
3.70v = 11%
3.6?v = 0%

Should I Set my LVC at say approx 3.75v? therefore hopefully not breaking the 80/20 rule. Or would this cause problems when heavily drawing energy from battery?

But then what the hell is the ideal voltage range referring to then? Because 3.65 is probably like 5% capacity left in the battery, which based on the 80/20 rule will reduce battery life.

Based off the existing information I'm confused? :?

So should I?
A) rely on my LVC set at a 3.75 voltage to obey the 80/20 rule
B) or use a voltage alarm sounding at 3.75 (obey the 80/20 rule) and leave the LVC at 3.3
C) or use voltage alarm sounding at 3.65 and leave the LVC at 3.3
D) or do I need to literally do my own calculations and measure the voltage level at (or record how long it takes in minutes to get to) 80% capacity of the battery energy.

 

[wesnewell]

For a typical ebike/scooter application setting LVC at 3.5V is perfect imo. But the setting should really depend on the sag you get.If you have a high amp draw the battery will sag more and you may want to set LVC lower. Then again, if your application has close to 0V sag then you may want to set LVC to 3.6V or even higher. I've set mine to 88V, 3.66V per cell and I can still run my battery down to about 10% soc if I baby it when it gets low. At 3.5V per cell I can use ~96% of the charge, down to 4% soc. About the only way to get what you want in the end is by starting with a higher LVC and then adjust down if it doesn't give you the usage you want. I really never run my battery down to LVC, since I monitor the voltage with a volt meter and usually recharge when soc is at 20-30%. If I was looking for max range, I'd set it 3.5V per cell so I could get more range out of it if I needed to.

 

[onloop]

thanks for your advice wesnewell,

so what is SOC.... does it mean 'State of Charge' or somthing?

The lowest i got my battery was like 3.55 volt .... and during recharge i ended up putting 7700mah back in.... so i guess that's not good for my battery??. leaving 300mah in my LIPO is using like 97%

so I need to be more careful, can this totally stuff the battery?

 

[dogman dan]

I think you are slightly on the wrong track from step one. Let me explain.

In the so should I section, item A. Should I rely on my LVC.... No, you should rely on yourself, by having some type of pack monitoring. I understand with a board this is less convenient. No handlebars to mount stuff.

What I'm trying to say is the lvc is not to be used to tell you when to shut off. Do not run to lvc every time, regardless of where you set it. Use the lvc setting for a last ditch, oh my god I screwed up cutoff. Even then, if you don't have a well sorted pack with weak cells removed, you may still have a cell go too low.

Set your lvc to whatever works with your sag under load, WHEN THE BATTERY IS GETTING LOW. So if you want to be safe, 3.5v per cell can work ok. Or you may want to go to 3.2-3.3v or so. The difference in distance will be measured in feet, not yards.

So how do you know when to stop? By having self monitoring on board. You might want to have two devices on board. The minimum would be a voltmeter, hopefully one you could read while standing on the board, or at least one on top that you can stop and do a quick look at it. For a time, you could even consider a remote mount, like a wristwatch. Use that till you can say you understand how your batteries drain, and when you can ride with no worries, and when it's time to start looking at the meter a lot.

Then, possibly impossible to see without opening a panel, have a real wattmeter. They only cost about 15-20 bucks. The wattmeter will let you learn what the real capacity of your batteries is, and let you correlate what the wattmeter says about % of real world capacity with what you have been seeing on the voltmeter. In time, you will know just how far you can go, using the discharge profile you like best. For me, it's stopping at about 3.7v per cell. This simply reduces the amount of balancing I need to do.

Bottom line, you need to do one of two things. Learn more about your batteries, and how they discharge rather that " rely on a lvc setting. Or,,, get a bms.

Eventually, you will only need the voltmeter, and only need to glance at it at the start of the ride, and then later near the end. You will gain trust in your batteries because you learned how they respond to how you ride them. Out riding, you will know damn well when you are hammering them and should stop sooner. You will know your battery, instead of just riding to lvc, and remaining ignorant forever to how they respond.

The third device you need is a cellog 8 or something similar. It could be one with an alarm, but I don't recommend using the alarms for long, and only the cellog 8 is very accurate. The thing is, you need to have a quick easy way to check every cell, so you know when to bother with a balance charge, and which pack has a cell you need to throw away. You can ride with confidence with just a voltmeter on the whole pack voltage, ONLY when you know the pack is healthy and more or less in balance.

If you end up with a bad cell in a pack, and you let it discharge below 2.7v, don't beat yourself up about it. It was a frocked up cell you needed to get rid of anyway. It got that low because it was stuffed in the first place.

 

[itchynackers]

I set my LVC at the voltage where the cells start going significantly out of balance. I tested and found that to be at 3.62v/cell. Once the average of my cells hit that, my controller ramps back the load automatically. You'd need to test your own battery to see where that is. Ride until voltage is about 3.7v/cell. Stop. Check cell balance. Ride a bit more. Stop check cell balance, repeat until you find where the cells go out of balance. Note the total voltage.

 

[dogman dan]

That's more or less what I did, but on a more causal basis.

I learned that stopping when I saw about 3.6v under load was good for me, and that was ending up to be about 3.75v resting. I could have gone further without seriously unbalancing the pack. I was stopping more because I was riding dirt, and after about 8 miles of hard riding, I'd crash a lot if I didn't rest. So I'd stop riding, and end up home with about 3.75v in the pack.

Riding dirt less hard now that my health got wrecked, I still ride to about 3.5v resting on the grocery getter bike, without any real issues. On that bike, it means stopping at about 48v under load for a 14s pack.

I don't believe deeper discharges ruin hobby lico cells, but I do believe that stopping at 3.65 or so resulted in a pack that stayed balanced enough for a long time.

Time to define balanced. I was quite happy to call .5v off balanced. No way do I give a shit if a cell is .05v off. When it approaches .1v out, then I bother with balancing. This works for me, because I don't discharge to the point where .05v out of balance matters.

 

[onloop]

ok awesome info... thanks everyone for the help.

So is there such thing as a CHEAP, SMALL/LIGHT, BMS that might be suitable for use on a 6S battery strapped to the bottom of a skate board?

ALSO is a $3 voltage alarm thingo accurate enough? see here: http://tinyurl.com/q5xld6d it gives you the voltage of each cell and alarms when cells drop to a pre-defined voltage. OR do you guys recommend a cell-log 8 as a better alternative.

 

[dogman dan]

I found my cellog 8, the one I actually got, to be the most accurate thing I have besides a dvm. More accurate than any of the cheap RC chargers I have.

But the little 3 buck cell checker alarms are kind of nice, and the two I bought are accurate enough for my needs. None are like .1v off or anything that extreme. The ones I bought are adjustable, so that makes it possible to set a pretty extreme conservative stopping point if you would like that.

Set em pretty conservative, and they will beep under load well before you get to 3.5v resting. Or, the other approach, you could have them start screaming only when at 3v, as a holy crap stop right now type warning.

What I didn't like about the alarms, was they will definitely draw enough power to unbalance the pack very quick if used. They run on one or two cells, draining them progressively more out of balance each cycle unless you balance charge every time.

So I like to use mine only if I am going to do a very long run where I definitely expect to run out. Mostly, I just run a voltmeter on the whole pack. That is, after sorting any bad cells out of the batch just purchased. Once sorted, you then know you have a good solid set of cells. I much prefer to not have to balance the pack every charge, since I run a 14s pack, and have only 6s balance chargers.

 

[friendly1uk]

You buy these cells with a spec sheet that presumes 100% discharge. If you want to treat them differently you need to write your own spec sheet, which we simply can't do. If you stick to the spec and use them fully then will you kill them in 2 years? because 100% or 10% matters not, they still die from old age at 2-3 years. So my real issue here is that you don't carry to much weight around in the hope of extending there life, when in fact it will make no difference.

I don't even carry excess weight on my bike, so there is no way I would carry 20% more battery on a board. I just size appropriately and change them at the proper service interval. it is old age that finishes them, not using them properly. There is a movement here where peeps try to extend cell life by pampering them, but these same pioneers are still running cells with just 50% capacity left and singing there praises. There is no way you would want to be using cells with only 50% left is there? That would mean your carrying twice the weight you need to. Be careful who's footsteps you follow in. I strongly recommend using them properly. Fitting a bms (which is for life, not just for christmas) and getting a proper charger that makes it as complex as charging your phone. Also fit a $10 power meter (see the 60a/100a on ebay) as it will serve as a fuel gauge by counting used Ah. Then you know how much you have left, and when it is time to buy new cells. Retiring your old one's to 'standby' status

 

[miuan]

I can't fully endorse what the previous poster suggested. On the contrary, I do agree with everything dogman has outlined. I run a 4yo Zippy pack that may have 80% of its original capacity and 1 dud cell that has a bit less capacity, but it's still far from 50% and the cells are 1 year beyond their calendar life. I pound them with close to 10% sag on acceleration daily, and they barely complain. The voltmeter thing is TRUE. I need voltmeter, cell checker and nothing more. I don't even have a LVC.

 

[Ykick]

Old Turnigy 5Ah 20C 5S faithful's here purchased and placed into 6 day/week service Apr/May 2010:
This is the only series group that hasn't required some rework and it was mostly operated in 3P configuration. Based on a load test today 60% labeled capacity from 4.2V down to 3.3 using 2C load.

In my personal experience simple is often better than complex. I'm not gonna knock those folks that have struck a balance with execution of added complexity but for this "good old boy" simple and watchful is what works best for me.

Hindsight's always nice - back when these were purchased many "goodies" that are available today were not as readily available or understood. We were just moving from SLA with a brief stop in Nickel territory. I've been using LEV's 3-5k miles/year since 2005.

I dealt with it by accepting derated capacity to help prevent failures and that practice caught some problems on more than a few occasions. If carrying a bit extra capacity means I carry 12lbs instead of 8lbs so be it. Not a problem for me but it may be for some people.

 

[dogman dan]

I tend to agree with one previous statement, that being that carrying around batteries that are so old they have 50% capacity sucks. I have a large pile like that, now three years old. In my warmer climate, 2 years seems to be the max before the big slide in capacity starts. I do load em on if I must for a long ride, but I hate to carry a 25 pound 10 ah left in it, 48v pack. Better than nada, but yuk.

But to clarify what I mean by babying the cells, I just mean to say that below 3.65v per cell, there really is only a few blocks of travel distance left. So do size your cells so you don't constantly need that last block. You don't have to leave 20% to make the cell last. I just say leaving 20% results in less needing to rebalance the pack. On the other hand, I think I did prove that you don't want to store the pack ready to ride at 4.2v. Particularly if you live where it's hot. One of my packs wore out, despite having been used only about 20 cycles. I shoulda stuck it in a colder place, at lower voltage. I just didn't believe what they say about long term storage of lico at full voltage. So now my new pack is stored at full voltage only if I know I will use it in the morning. Often I will charge it to 4.1v, then I may or may not bring it up to 4.2 just before I ride. If I know I won't use it, I will leave it discharged, disconnected from everything. Generally this will be at about 3.75v. If lower, then I'll just charge it back into 3.8v range till the weather gets better again.

Also, size your cells so the pack doesn't get all hot as hell every ride. What size that is, depends on the quality you bought, and the load you put on em. But if you run it hard, it will wear out faster. But if you are using a pack that cost's less than $100, that may be fine.

Repeating again, you just have to have some idea what is going on, or use a bms. Hell, even with a bms, it pays to check every cell from time to time. It definitely pays to sort out the bad cell pack right at the beginning. It's cheap RC lico, you know there may be a stinker in there. You can't just put it together, ignore it, and rely on a controller lvc.

Unless you are rich.

 

[onloop]

So it seems that some people are believers and others are not!

Which leaves only one question:

Is there actually any evidence proving that if you follow the 80/20 rule &/OR the don't dip below 3.65v Rule that it may ensure you a healthier, better performing, longer living LIPO battery? Therefore potentially saving you from encountering earlier than expected faults which require replacement?

Or in other words, do you get MORE-BANG-FOR-YA-BUCK in the long run?

And furthermore if such evidence does exist proving these benefits are real, do they only get diluted or nullified by external factors, such as upgrade affordability due to the ever decreasing price and constantly increased battery capacity's, not to mention the inevitable changes in technology which occur every few years, etc.

I would love to see some test data! (It might make a good episode for myth busters?)

 

[dogman dan]

What I'm trying to say is this.

Your pack definitely lasts longer, IF YOU DON'T RUIN IT IN ONE RIDE.

If you don't use a bms, then you have to be sure you don't drive one cell below 2.7v, which I think we all can agree is bad. Take it to 0v, and it gets hot enough to melt the shrink, and maybe even catch fire.

How do you avoid this? Watch 14 voltmeters? No, you just stop soon enough so that your pack which is perhaps .1v out of balance doesn't drive a single cell much past 3v. For me, this means trying hard not to discharge much below 3.7v most rides. That's 3.7v in theory, by looking at whole pack voltage. For this to work, you have to start with a pack that has nothing but cells that are similar, and don't go out of balance in one or two discharges.

But sometimes, you end up at 3.6v, and half a mile from home. That's when that little cellog in your pocket is worth it's weight in gold. With a stop and a fairly quick check, you find out you are ok to do that last half mile, or not.

I found that having a voltmeter on the whole pack is crucial. If you don't have that, you never know when one pack unplugs from a parallel set. Or if you just have a day that capacity is way less, like riding in the cold, or your pack is not fully charged when you leave the house. It's that voltmeter that clues you in that it's time to whip out that cellog and look closer.

 

[wesnewell]

I think the primary reason not to run the cells below 3.65V is because below that much the cells tend to go out of balance. Balancing is a slow process, so expect to take a lot more time to balance charge a pack when they are way out of balance at the start. A good pack will stay in balance to 3.65V After that it's a toss up.
Manufactures state that nor using below 10% soc will extend life. They didn't say how much that I recall. Been a few years since I researched this. Personally, I charge to 4.2V with some cells reaching 4.23V, but I don't discharge past 10% soc ever, and usually recharge at 20-30% soc. I've got about 7000 miles on my current pack that is 18 months old and still in decent shape. I expect to get another 7000 miles out of it. pack cost $275, so about 2 cents per mile.