Wowza 20Ah Multistar 6s !

[miuan]

mmm I found up to 10% difference between the 16000mah and the 20000 mah, the 16000 really hits a good price point. but yeh not enough to make a huge difference. I sometimes think more smaller packs in parallel are nicer, if you get a bad cell its far cheaper to fix. those 6s 20ah would hurt if you got one bad cell.

That would be the only issue that holds me back. Saying that, I've not heard any issues regarding the multistar packs being DOA. I think lico manufacturing has improved since some of the horror stories of yesteryear, although maybe someone else can set me straight on that.

it's still a topic no matter how much it's improved. bad lipo cells can develop their symptoms after 1 year of usage, as happened to me.

 

[Jestronix]

Out of the 10 or so packs I've bought all have been fine.

 

[miuan]

Out of the 10 or so packs I've bought all have been fine.

I've bought about 100 different lipo packs at hobbyking. Several had this problem, but with mere 10 packs, you may as well be lucky - hopefully for entire cells' lifetime. Most recently the QC before shipping apparently got much better as I stopped receiveng DOA cells altogether. But I doubt the manufacturer has much (if any) better resources/QC, so the less obvious mfg defects still get thru to the customers, as long as the cells receive and keep the initial charge for a reasonable amount of time.

 

[neptronix]

The only problem with big lipo brcks, is heat. The middle cells don't have much chance to cool. I did compare 6s and 8s bricks of the same cells, on the same bike, and the 8s lost capacity much sooner. This might be negligible with lower power systems, I am discharging and charging very fast, and that is hard on any battery life.

This will happen in 18650 packs too.

It is important to not get any battery substantially hot. Either lower your amps, or add more batteries in parallel.

The harder you push them, the less cycles and actual delivered voltage you get out of them anyway.

My rule is to run batteries at a fourth of their maximum c continuously, for best battery efficiency and life.

Therefore, i would run these at an average of 50A continuous. So, set your controller to 75A and choose your wind to draw no more than 50A while cruising.

 

[neptronix]

it's still a topic no matter how much it's improved. bad lipo cells can develop their symptoms after 1 year of usage, as happened to me.

I have owned 28 lipo packs over a 5 year timespan.

I weeded out all the bad packs ( there were 3 that had a wonky cell ) and have used the good packs with zero issues ever developing. One time, a series brick went bad due to me draining it down to 3.4v and sitting it in the garage for a year.. it leaked electrolyte a year later.. my bad.

As long as you cycle graph and toss or return the occasional dude you get, you should be fine. If you run your packs lightly ( half, or preferably 1/4th the C rate they specify ), they will last a damn long time. 300-600 cycles with meticulous care is not unusual. 100-300 cycles is what you get when you run a bum pack or beat the hell out of it ( ie run a 20C pack at 10-15-20C.. )

 

[dequinox]

it's still a topic no matter how much it's improved. bad lipo cells can develop their symptoms after 1 year of usage, as happened to me.

I have owned 28 lipo packs over a 5 year timespan.

I weeded out all the bad packs ( there were 3 that had a wonky cell ) and have used the good packs with zero issues ever developing. One time, a series brick went bad due to me draining it down to 3.4v and sitting it in the garage for a year.. it leaked electrolyte a year later.. my bad.

As long as you cycle graph and toss or return the occasional dude you get, you should be fine. If you run your packs lightly ( half, or preferably 1/4th the C rate they specify ), they will last a damn long time. 300-600 cycles with meticulous care is not unusual. 100-300 cycles is what you get when you run a bum pack or beat the hell out of it ( ie run a 20C pack at 10-15-20C.. )

If you're already aware, I apologize: Justin had been doing research, while developing the cycle satiator, about the effect of the charge (or float) voltage on charging lithium. He found, as some others have, that reducing the charge voltage to 4v instead of 4.2v increased the lifespan dramatically. Have you or anyone else experimented with this?

 

[neptronix]

If you're already aware, I apologize: Justin had been doing research, while developing the cycle satiator, about the effect of the charge (or float) voltage on charging lithium. He found, as some others have, that reducing the charge voltage to 4v instead of 4.2v increased the lifespan dramatically. Have you or anyone else experimented with this?

I charge to 4.15 and always let my batteries sit unused around 3.7-3.9v. I also stop discharging at 3.5v nominal for the pack.. that is part of how my lipos have lasted so long. My oldest pack ( purchased in 2010 ) is still giving me 80-90% capacity after 300 cycles this way.

The other trick is the 1/4th C rate discharge rule.. thus if i have a 20C pack, i discharge it at around 5C continuous.

The battery will not be even warm to the touch if you operate it like this.. so, the middle cells will never be in danger.

The recordholder for lipo life is el_steak.. he got 600 cycles out of the pack doing the same thing. They just got too saggy at that point

 

[danielrlee]

If you're already aware, I apologize: Justin had been doing research, while developing the cycle satiator, about the effect of the charge (or float) voltage on charging lithium. He found, as some others have, that reducing the charge voltage to 4v instead of 4.2v increased the lifespan dramatically. Have you or anyone else experimented with this?

I think this information has been superseded somewhat. More recent evidence suggests that it's not the charge voltage per se that causes degradation of the cell, but the amount of time spent sitting at that voltage. Charging to storage voltage, then topping off immediately before use is now recommended practice.

If this is too much of a faff, then Justin's advice is still good - the lower the voltage a cell is left to sit at, the slower the degradation.

 

[sn0wchyld]

I've put together a brief spreadsheet of the Multistar packs for comparison if anyone's interested:

https://docs.google.com/spreadsheets/d/1W41rjVDKoyWpvP4ztPbINtFTxXSp3tgTxIbXBsZdlos/edit?usp=sharing

It's read-only, so let me know if you think of any worthwhile additions or adjustments.

Dan

wh/liter would be good too...

 

[dequinox]

I think this information has been superseded somewhat. More recent evidence suggests that it's not the charge voltage per se that causes degradation of the cell, but the amount of time spent sitting at that voltage. Charging to storage voltage, then topping off immediately before use is now recommended practice.

If this is too much of a faff, then Justin's advice is still good - the lower the voltage a cell is left to sit at, the slower the degradation.

Ah I wasn't aware of that newer info, and that is an interesting point. Other chemistries (Nimh for ex.) have a similar problem when left at high V, and can grow crystals (as I recall anyway) that reduce the usable surface area... I've read they can even puncture the lining and cause and internal short.

 

[Jestronix]

Yeh i wasn't aware of this until watching the Grin tour last week (great tour BTW) I'm actually going to only charge to 4.1 now. Its worth buying slightly bigger pack and using the 20% to up 80% range. 1c charge is also easy on the batteries, however what are people doing with their regen settings ?

 

[neptronix]

I think this information has been superseded somewhat. More recent evidence suggests that it's not the charge voltage per se that causes degradation of the cell, but the amount of time spent sitting at that voltage. Charging to storage voltage, then topping off immediately before use is now recommended practice.

If this is too much of a faff, then Justin's advice is still good - the lower the voltage a cell is left to sit at, the slower the degradation.

Ah I wasn't aware of that newer info, and that is an interesting point. Other chemistries (Nimh for ex.) have a similar problem when left at high V, and can grow crystals (as I recall anyway) that reduce the usable surface area... I've read they can even puncture the lining and cause and internal short.

NASA prove that around 5 years ago when testing lithium batteries. I saw the paper in 2011. Wish i could find it.

 

[danielrlee]

wh/liter would be good too...

Spreadsheet updated.

https://docs.google.com/spreadsheets/d/1W41rjVDKoyWpvP4ztPbINtFTxXSp3tgTxIbXBsZdlos/

 

[sn0wchyld]

I think this information has been superseded somewhat. More recent evidence suggests that it's not the charge voltage per se that causes degradation of the cell, but the amount of time spent sitting at that voltage. Charging to storage voltage, then topping off immediately before use is now recommended practice.

If this is too much of a faff, then Justin's advice is still good - the lower the voltage a cell is left to sit at, the slower the degradation.

Ah I wasn't aware of that newer info, and that is an interesting point. Other chemistries (Nimh for ex.) have a similar problem when left at high V, and can grow crystals (as I recall anyway) that reduce the usable surface area... I've read they can even puncture the lining and cause and internal short.

NASA prove that around 5 years ago when testing lithium batteries. I saw the paper in 2011. Wish i could find it.

ive also seen some limited data (cant find it now damnit!) that pulse charging (same average C rate of charge, but very high, short pulses - an aprox 30% duty cycle) are also very beneficial to cycle life...

 

[Punx0r]

Luke (liveforphysics) recently made comments to that effect regarding ripple current in ghetto bulk chargers.

 

[Jestronix]

wh/liter would be good too...

Spreadsheet updated.

https://docs.google.com/spreadsheets/d/1W41rjVDKoyWpvP4ztPbINtFTxXSp3tgTxIbXBsZdlos/

Nice

 

[Jestronix]

I did a hard 8km ride today and did lots of wot, 60amps out of my 6.6ah packs , oh so warm. I think heat wise these packs are better rated to say 5c cont.

 

[danielrlee]

I did a hard 8km ride today and did lots of wot, 60amps out of my 6.6ah packs , oh so warm. I think heat wise these packs are better rated to say 5c cont.

I've read that even 5C is pushing it and 2.5C is recommended. What kind of voltage drop do you see at 10C?

 

[Jestronix]

That's a god question, I'll have a look next time on full charge and end of the ride. Yeh time for a bigger pack or nano

 

[danielrlee]

That's a god question, I'll have a look next time on full charge and end of the ride. Yeh time for a bigger pack or nano

Just to clarify my question a little, I'm talking about immediate voltage drop at full power, not pack voltage swing between start and end of the run.

 

[Jestronix]

yep I'm talking sag not swing from full to empty, but sag amount when it's full charge and sag when it's near empty. Sag is bigger and more of a danger when near empty ?

 

[danielrlee]

yep I'm talking sag not swing from full to empty, but sag amount when it's full charge and sag when it's near empty. Sag is bigger and more of a danger when near empty ?

Once you're down to the bottom of the charge, voltage drop really is the least of your worries. At that point, any additional drain is likely causing irreversible cell damage. I wouldn't really want to be taking it down that far in the first place.

 

[magna_503]

I've been using mine at 18s 32ah at 2c 60ah. About to go 48ah and run at 30ah. Anyhow, then lowest voltage I've reached so far is 69.2 v for a 43miles run. 66v = 3.6v nominal, and the lowest i want to run is 68v. I want a range of closer to 100miles at 35mph cruise. 48ah should get me there and keeping the battery above 70v. Easy battery life for long life. Hopefully 48ah would do it.

Once these packs are done, i'll probably see if I can run the 6s 20ah pack to make it 72v40ah. 6 packs vs the planNed 9 packs.

 

[neptronix]

yep I'm talking sag not swing from full to empty, but sag amount when it's full charge and sag when it's near empty. Sag is bigger and more of a danger when near empty ?

Once you're down to the bottom of the charge, voltage drop really is the least of your worries. At that point, any additional drain is likely causing irreversible cell damage. I wouldn't really want to be taking it down that far in the first place.

Internal Resistance ( IR ) is not a constant. It is at it's least when the battery is at it's nominal voltage ( 3.6v-3.9v ), and highest when it's near full charge and near it's low voltage cutoff point. It exponentially ramps upwards as you go below 3.6v. That point is where you will make the most battery heat, if you are pushing the battery hard.

If you run this battery at 10C continuous, it is going to heat up so much that it will be damaged, just like any other lithium battery. Sag will be extreme. It will have the nominal voltage of a lifepo4 cell ( 3.2-3.3ah ) and you will waste around 10% of the battery's capacity to heat.

Always cut the maximum c rate by 1/4th to get the happy point where you are seeing really low voltage drop, high discharge efficiency, unnoticeable levels of heat generation, and long cell life.
Thus, 2.5C or 50A ( 20ah x 2.5 ) continuous is really the ideal continuous discharge for these packs. Run a 2P configuration, and you've bumped that up to 100A continuous.

 

[bionicdan]

That would be the only issue that holds me back. Saying that, I've not heard any issues regarding the multistar packs being DOA. I think lico manufacturing has improved since some of the horror stories of yesteryear, although maybe someone else can set me straight on that.

Im renowned for awful luck with online purchases but iv had a duff cell on each of my orders with these multistars. The first three had a low cell and all eventually became unusable, puffed/popped or just a total pain.

So when I received one of these 20ah ones with a low cell I was told again they will be fine just use them. I decided to just cut off one balance wire and they instantly sent a replacement. Guess what the replacement also has a low cell but not as bad as the first. All the bad ones had older serials than the others in my order so they most likely have spent time boosting up self discharging cells.

Dont accept packs with low cells I would say. (all cells 3.91 vs the oddity at 3.81)

I wonder if master luke could tell us why this is happening. Bad cell matching. Bad formatting charge at assembly and the best way to try and correct it. bottom balance or boost to 4.25 or something?