Li-Ion 18650's VS LiPo , LVC and Max Charge ?

[ScooterMan101]

I have recently talked with a RC Flyer that uses the usual Hobby King Lipo's and here is what he does,

Only charges to Max 4.15-4.2 volts per cell .. just before flying . ( not for storage )
that is usual but here is the interesting part ...
He has a Low Voltage Cutoff at 3.65 per cell ( because he does not want to have the battery sag too low. )

On the 18650 Li-Ion battery's Don't we have a LVC of around 3.2 volts per cell ?

And with such a low LVC that would limit the amount of usable watt hours ... which is find if you have a plane/helicopter right over head and can switch/charge battery for another flight.
However with E-bikes/Scooters/Mc's we want as much range as possible .

Also how you measure the amount of watt hours on hobby lipo's ? I would think it a little different since the result of over charging and over discharging them is much more serious than a 18650 pack with built in BMS.

So What voltage per cell do you stop at while operating a E-Bike/Scooter/MC when using Hobby Lipo's ?

What would be the Watt Hour of a

1) 6s 1p 5800 mAh Hobby Lipo Pack ? Based on what LVC that ( you ) use .

2) 7s 1p 5800 mAh Hobby Lipo Pack ? ^

3) 8s 1p 5800 mAh Hobby Lipo Pack ? ^

The Zippy's are rated at 25c

 

[999zip999]

It depends on your amp draw also.

 

[ScooterMan101]

Motor Q100c CST , up to 15 amps max ( 17 amps for a couple of seconds max)

Motor Mac 6T , 25amps to 30 amps max ( 35-38 amps for a couple of seconds max )

Future Motor , unknown at this time, but do know it will be up to 35 amps to 40 amps max

( not much more than 35-40 amps even for a few seconds, since at that amp draw I can take off from stop and add a little for a climb and stay within those amps for what/where I will be riding )

It depends on your amp draw also.

 

[Sunder]

This will vary massively depending on the pack. Take for example this graph:

If you discharged to 3.6v per cell at 2C, you'd get 40%.

If you discharged to 3.6v per cell at 0.2, you'd get 85%.

Some Hobby lipo can be as low as 3C rated - though usually not RC Flyers. Some can be as high as 135C.

The lower the true rating, the more capacity you lose at higher amp draws. For example, I have two 16Ah batteries in my motorcycle. One is old LiFePo4 which started off with a low C rating, and has dropped further. The other is a brand new Multistar. If I full throttle the bike on the LiFePo4 pack, it drops to 96v and then the BMS "protects" the battery. If I full throttle the Multistar pack, it drops to 135v and I hammer on.

If you are intending to cut off at 3.65v under load, you will want to make sure that you get a very high C rated battery, or a bigger battery, so the relative amp draw is lower.

 

[ScooterMan101]

I will be discharging at more than 2c

On bike ( Q100c would be less than 2c )
But
The Mac motor will / should be at 25-30 amps ( 12s - 14s )
and
A future motor should be at 30-40 amp draw, ( 12s up to 20 s )

The Hobby Lipo's i have now are rated at 25 c 6s 58000 mAh.
and
perhaps future hobby lipo's if I can get good enough performance , will also be at 25c 7s or 8s

I do want to stop at 3.56 or even 3.6 volt per cell , I am wondering what others do here with their Lipo's
( the RC flyer that stopped at 3.65 volts per cell was using massive amps on a Helicopter , I think he had 50c or better batteries, more than a usual under 2kw watt e-bike motor)

Not good at graph's , written word in more understandable to me.

 

[ScooterMan101]

Those of you that use Hobby Lipo's

What voltage , per cell , do you have your LVC set at ?

and/or by looking at a display what volts do you stop your charge at ?

 

[Sunder]

I will be discharging at more than 2c

You'll probably be using a higher C rated battery though. That was just an example. Looking at the sag, it was probably a cheaper, low C rate battery.

Unfortunately there's no free ride. You can buy a battery that has a high cycle life and pay a lot. You can increase cycle life by avoiding top and bottom, but then have to buy a bigger battery to compensate. You can be real gentle on the battery, but then require a high spec battery and pay more.

Sorry mate, the compromise has to come from somewhere.

My voltage depends on what I'm protecting. My 16S 5Ah eBike packs charge to 4.2v per cell, and LVC cuts it at 3.2v per cell under load - usually this means about 3.5-3.6v resting, or about 95% gone.

On my 36S 16Ah pack, I have no BMS, so I charge to 4v per cell, and watch the throttle carefully when it starts sagging below 3.6v per cell on load, and don't let it go below 3.7v per cell resting.

 

[ScooterMan101]

I understand cost, better battery longer life, when combined with not to much of a discharge,
and I do understand for the most part 18650's Li-Ion

However Hobby Lipo's are a puzzle .

When trying to size Hobby Lipo's the ones I am looking at mostly come in 4,000 mAh, 5,000 mAh and 5,800 mAh. ( at 25c discharge rate , although I would strive for 25%-35% of that ) .

Yes I will be getting better C rate batteries this time, ( that is why I am willing to try and put up with all the connect / disconnect, parallel and series plugs, and put on different plugs for charging, using a power supply , charging outside, etc. ) and talk about no free lunch, Hobby Lipo's with all that I just list above are far from being a free lunch .

I will not be going with 36s for a bicycle so I will not pay much attention to that pack of yours.

But I am looking at 12s, or 14s , or 16 s

So

Your 16s 5,000 mAh pack .... are they Hobby Lipo's ? , or 18650's ?
Do you do a parallel series of 4 to get 10 amp hours ?

Do you have a Cycle Analyst to be able to see what your pack is sagging to ?

On my bike with the Q100c cst motor and S-LCD 3 Display, I would have to figure out what voltage figure to stop at, I don't know how to know if it is showing sag voltage or just a average voltage ?

Anyone else that uses Hobby Lipo's , What do you discharge to , with 5,000 mAh Hobby lipo's ?

I will be discharging at more than 2c

You'll probably be using a higher C rated battery though. That was just an example. Looking at the sag, it was probably a cheaper, low C rate battery.

My voltage depends on what I'm protecting. My 16S 5Ah eBike packs charge to 4.2v per cell, and LVC cuts it at 3.2v per cell under load - usually this means about 3.5-3.6v resting, or about 95% gone.

On my 36S 16Ah pack, I have no BMS, so I charge to 4v per cell, and watch the throttle carefully when it starts sagging below 3.6v per cell on load, and don't let it go below 3.7v per cell resting.

 

[Sunder]

I will not be going with 36s for a bicycle so I will not pay much attention to that pack of yours.

A cell is a cell, whether it stands alone, or is in a 1000S pack. Doesn't matter.

From memory, the were originally 8S Zippys I soldered together and attached a dedicated 16S BMS. No adaptors or connectors, no extra wiring, just a black box that gives me 67.2v until it cuts off at 51.2v. Due to the high voltage and low speed winding on that bike, I rarely even draw 3C (programmed the controller to not draw more than 800w), most often cruise on less than 1C.

I don't have a second set to parallel with. Nothing stopping you from doing it though. If you don't like the parallel harness, solder them together.

I have an temporary inline ammeter for testing for bikes. Once I know the bike performs as I expect, there's no need to keep it on there. Everything is managed for me, and I can't accidentally push anything out of design specs.

The motorcycle has a permanently wired Cycle Analyst. For this one, I need to manage everything from instantaneous load, to battery voltage. The controller isn't programmable, and I have no BMS.

It seems you're looking for a broader range of opinions though, so I'll leave you to it.

 

[ScooterMan101]

Sunder,
I have done some more research since starting this thread a couple of hours ago.

At 67.2 volts charged , you are charging to the full 4.2 volts per cell ,

and it looks like since you put on a BMS you are Using a Bulk charger and not the Hobby Chargers , or the JST-XH balance plugs ?

And you are taking your pack down lower ( to 3.2 v per cell ), than what others are saying, not judging/criticizing , in fact since you have done that and the fact they are still working is a good sign for the durability of the Zippy's . Looks like you can do such a low discharge voltage because you use such a low c rate of discharge and at only 800 watts at that , so I am guessing you are not getting much sag .
How many cycles about do you think you have on them ?

More research has shown that even though I plan on using more amps ( watts ) through the motor, I will still be in a low c rate of discharge, since my pack is rated at 25c continuous 35c burst , and it is a 5800 mAh pack, so rated to be able to handle 145 amp draw , so only up to 40 amps up to 30 most of the time, will/should work fine.

Looking for more opinions , although I do like your responses , so as to see what works for people and what has shortened the life of Hobby Lipo's
and
To try and figure out how many watt hours per pack, since I will not be charging them up as much , and , not discharging them as much per cell/pack , as my current 18650 li-ion cells.

This is further complicated by the fact that on the Q100c motor and S-LCD 3 Display, the Display does not show how many watt hours I have used during my ride, or even the amps used, so I can not do the calculation . ( The Cycle Analyst is looking better and better all the time ). now to find a small higher voltage / low amp controller that will work , or I can make work with a Cycle Analyst )

I found a 14 s BMS but it was only 30 amps , so did not get it. will be looking for a 40-50 amp BMS , but might not go that route and study allot and see how to use the Hobby Chargers/Power Supply/Balance wires/harness.

From memory, the were originally 8S Zippys I soldered together and attached a dedicated 16S BMS. No adaptors or connectors, no extra wiring, just a black box that gives me 67.2v until it cuts off at 51.2v. Due to the high voltage and low speed winding on that bike, I rarely even draw 3C (programmed the controller to not draw more than 800w), most often cruise on less than 1C.

I don't have a second set to parallel with. Nothing stopping you from doing it though. If you don't like the parallel harness, solder them together.


It seems you're looking for a broader range of opinions though, so I'll leave you to it.

 

[Sunder]

Correct. I need to charge to the full voltage, as my BMS doesn't truly balance. It shunts off current after 4.20v. I use a bulk charger and any time I think the pack is out of balance, I just leave the charger on overnight - in most cases considered risky, but I check voltages on all 16 cells before going to bed. If the top cell is more than 4.3, (has happened once), I disconnect it, move it somewhere safe, and then resume the next morning.

Just to remind you that it's 3.2v under load. I don't drain it to that low at rest. Also to remind you, LiPos sag badly after 3.65v. So while a 1C draw might barely shift it 0.1v at full charge. Once you hit 3.60v, the same 1C draw will probably keep tripping the BMS.

I haven't made an exact count of cycles, but I would estimate around 80-100 cycles. Used it for half a year, 3-4 times a week. I haven't noticed any aging yet.

Just be aware that when they say a battery is 25C continuous and 35C burst, they mean that it's capable of doing that, not that it will have a decent life doing that. Friends who compete in 1/8th scale RC car racing at the national level (yes, it's a serious sport) say if they run it at the full rating, they get 3-5 cycles of full performance out of it. Then they might get another 20-30 "practice" sessions out of it, and if it hasn't puffed by then, they sag like hell and are useless, except for very low rate energy storage applications.

All this said, every LiPo pack is different, and every manufacturer exaggerates to different level. Best way to test it is to do your normal ride, then as fast as you can, put your hand on the battery. If you can't really tell if it's warm, or not, 100% safe. If it's mildly warm to the touch, probably still okay. If it's no doubt as to whether it's warm or not, you're probably shortening their lives. If it's unpleasantly warm, or hot, you've probably already killed the cells.

 

[izeman]

i'd like to post my long term results with 5000mAh 25s zippys. i bought them exactly 3 years ago.
i use them in a 12s4p configuration. so 8x 6s 5000mAh packs. i bought 26 packs, for two bikes, with 2 packs as at least one pack would come with a single dull cell (which was the case).
so i had 8 packs in my bike for 3 years. used a BMS with 3.0V to 4.2V range. only had the BMS cut power ONCE. most of the time i stopped at 3.5V (under load) minimum. always bulk charged to 4.20V and balancing was done by the BMS in case needed.
the pack never had to deliver more than 2-3C (50A max for a 20.000mAh pack).

now after around 150 cycles some of the batteries are down to 80% capacity. i removed all batteries and did a single cell charge/discharge cycle (10A charge, 20A discharge from 3.0V to 4.2V). this showed the weakest cells at 3.915mAh capacity at 20A discharge, the best cells still have 4.650mAh.

interestingly the other pack was used about 5 times (so practically never). and i was storage charged to 3.85V for 3 years. not a single cell is dead, but they all lost significantly while sitting on the bench. none of them is below 4.500mAh, and most of them are at 4.700mAh, some even at 4.850mAh - the average capacity is 4.674mAh (so still @ 93%) . there is ONE pack that was totally unused (one of the spare ones), this has 5 cells that hold 4.980 - 5.050mAh capacity (one cell was a dead on arrival).

 

[dogman dan]

IMO,, the main advantage of stopping at 3.5 or 3.65, for lipo, is the tendency of them to stay balanced better, than if you try to ride till your weakest cell is at 3.2v.

In terms of capacity,, there really is not a lot of distance left in a lipo cell below 3.5v,, but you are leaving some on the table below 3.65. If you do go below 3.6v or so,, slow down and it will help the pack keep it's cool. Below 3.5v,, pull a big c rate and you'll really heat the pack.

To get range,, carry more. Beats flogging your pack daily for another mile, maybe two. When I want max range, I carry 53 spec ah in total. 60-80 miles worth depending on the speed I ride.

In reality,, likely more like 42 ah of 48v. Call it 2100 wh, but if I was to discharge slow enough,, I'm not sure what my real wh would be. quite possibly closer to 2400 wh. In the spring,, with a bit warmer weather, I'll do a test and see what I have in total for the next summer season.