Minimum cell voltage - opinions and facts please

[auraslip]

I'm doing some testing on 4s hardpacks.

What voltage would be good for capacity testing? I'm thinking 4.15v down to 3.6v as this is what I feel is a happy place for them. But I need to compensate for the sag of my 10a load because if it shuts off at 3.6v, then it will rise up to a higher voltage when the load goes away. I need to find a OCV discharge graph I guess?

 

[neptronix]

I'm doing some testing on 4s hardpacks.

What voltage would be good for capacity testing? I'm thinking 4.15v down to 3.6v as this is what I feel is a happy place for them. But I need to compensate for the sag of my 10a load because if it shuts off at 3.6v, then it will rise up to a higher voltage when the load goes away. I need to find a OCV discharge graph I guess?

Balance charge to the full 4.2v.
Discharge until the first cell hits 3.0v. ( you can use a celllog 8 hooked into a PC with logview via USB to watch this happen in real time.... and produce graphs like i have plastered all over this board. )

The sag of the load is problematic. Ultimately though, this will give you a "real life" 2C test, which is what you want to know when you actually run these on the bike.... you're not gonna run lipo at 1C are ya?

If you've seen battery spec sheets, they often have 0.5, 1, 2C tests..

 

[edcastrovalley]

If you're running anything sold at hobbyking in the last 2 years, it's all 3rd generation chemistry (or nano-techs), and you will be sacrificing almost no capacity by stopping at 3.5, or even 3.6v.

That is good to know because I've been kind of abusing some lipo like my Turnigy 2s battery that was "left on" by accident and discharged to 1.5 volts. Just for kicks I slow charged at 1/2 amps a couple times and have been using it gingerly and I can't see any problems so far... Amazing battery - it is a hard case lipo so I can't really check for puffyness.

 

[sn0wchyld]

I'm doing some testing on 4s hardpacks.

What voltage would be good for capacity testing? I'm thinking 4.15v down to 3.6v as this is what I feel is a happy place for them. But I need to compensate for the sag of my 10a load because if it shuts off at 3.6v, then it will rise up to a higher voltage when the load goes away. I need to find a OCV discharge graph I guess?

This will hopefully give some idea, this is a 4s 5ah pack at 5.5a CC discharge to 3.45V LVC. you can see that after about 3.6V there's basically nothing left. The discharge was started from 3.8V storage charge.

 

[auraslip]

Could you repost the image for me snowchyld?

thanks in advance

 

[sn0wchyld]

Could you repost the image for me snowchyld?

thanks in advance

picture should be back up shortly... thats what happens when you forget to pay you bills

edit
yep, there the exact same type of hardcase packs, top value huh? 25$ ea delivered, getting a dud pack really doesnt sting at all when they're that cheap! And I just bought 20, all of them are fine, only 1 has a notably high IR (one cell is ~2.9mohm vs ~2 for the rest!!).

 

[neptronix]

Same cliff, different charger..

I notice that FMA sells a 10s charger. I would assume that unit can produce graphs as well. Interesting alternative to the icharger 1010b+, imho.

 

[sn0wchyld]

Same cliff, different charger..

I notice that FMA sells a 10s charger. I would assume that unit can produce graphs as well. Interesting alternative to the icharger 1010b+, imho.

yea I went with the 8s 'cause of the 1400w chargeing/discharging, and better overall reviews. Its not much more than the icarger eq. but the customer support is great and so far the charger has worked like a charm. given the fact ive changed to 4s packs it means I can charge em as a 8s 6p pack at 1c balance charge!! 1 hour 10min to balance all that!

 

[izeman]

i did some charging and decharging, and logging now, and want to share the results. i fully balance charged this 5.200mah 6s pack. then discharged it under "laboratory" conditions with two halogen 12v 50w bulbs in series = 100w and 24v. load = 4a discharge. that's not even 1c, but i couldn't find more unused bulbs . total capacity taken from the pack till shutdown when the first cell reached 3.0v was 4.000mah. (the pack was brand new)

as you can see: as soon as the bulbs were connected there was a voltage sag of 1v. current was 4.2a, and 3.7a due to lower voltage during the end.
cells were perfect balanced until voltage reached 3.65 - 3.60v. then they started drifting apart, but slowly, and totally drifted apart when cell voltage reached 3.50v.

@3.60v there was still 1.800mah in the pack, so nearly HALF the capacity.
@3.50v there was only 190mah in the pack, only a mere 1/20th of it's capacity.

so there are two conclusions:

if you don't want your pack to become unbalanced: DON'T go UNDER 3.65v (or maybe 3.60) under load.
if you don't want to waste any capacity, and still be on the save side: DON'T go UNDER 3.5. NEVER.

there really is NO NEED to go down to 3.0. this gives you not nearly 1/20 of the capacity of your pack - in my case some ONE mile/kilometer.
so i will set my lvc to 3.5v now, and save that last drop in case my feet are too weak to take me home

regards izeman

 

[DrkAngel]

as you can see: as soon as the bulbs were connected there was a voltage sag of 1v. current was 4.2a, and 3.7a due to lower voltage during the end.
cells were perfect balanced until voltage reached 3.65 - 3.60v. then they started drifting apart, but slowly, and totally drifted apart when cell voltage reached 3.50v.

@3.60v there was still 1.800mah in the pack, so nearly HALF the capacity.
@3.50v there was only 190mah in the pack, only a mere 1/20th of it's capacity.

so there are two conclusions:

if you don't want your pack to become unbalanced: DON'T go UNDER 3.65v (or maybe 3.60) under load.
if you don't want to waste any capacity, and still be on the save side: DON'T go UNDER 3.5. NEVER.

there really is NO NEED to go down to 3.0. this gives you not nearly 1/20 of the capacity of your pack - in my case some ONE mile/kilometer.
so i will set my lvc to 3.5v now, and save that last drop in case my feet are too weak to take me home

regards izeman

Alternate procedure ...
Manually balance each cell at 3.5V.
Lower the peak charge voltage towards 4.15v. (down from 4.2v)
To prevent the risk of single cell overcharging.
With the iMax B6 - B8 this accomplished in the "Program" menu, changing from "Lipo" to "LiIo".
Most multi compatible chargers have some type of peak charge voltage adjustment.
Capacity loss near peak voltage is minor.
Especially compared to the capacity-loss, you determined, between 3.6 and 3.5V.

Of course, to maintain this "low balance", you must forgo, "standard", balance charging.
A non-balance charge typically discharges, and recharges, to the same "balanced" voltages, unless,
maximum, or minimum, cell charge voltages are exceeded.

 

[auraslip]

This is called bottom balancing drkangel. In depth discussions here: http://endless-sphere.com/forums/viewtopic.php?f=14&t=27263

 

[DrkAngel]

This is called bottom balancing drkangel. In depth discussions here: http://endless-sphere.com/forums/viewtopic.php?f=14&t=27263

The "bottom balancing" graphs, I just looked at, tended to balance at maximum discharge.
Balancing at 2.25V seems worthless, I recommended balancing at 3.5V, what I consider to be a good, safe, minimum usable voltage.
I tend to set my low voltage alarm at 3.5v, with "voltage sag" that leaves me around 3.6V+, a fair reserve to limp home with.

On my one day - 100 mile run,
I ran my homemade battery 25.9V packs down to 25.2V & 25.3V resting voltage.
By performance, this seems to be a nice limit of reasonable usability.
25.2V / 7cells = 3.6V.

My 26Ah Lipo took 23.7Ah to recharge from 25.2V to 29.1V.
Charged at a moderate .15C To provide 98%+ charging efficiency.
Which is 91% 0f rated.
Which means 3.6V - 4.16V seems to be to cover 91% of "rated" capacity!

Which compels me to raise my low voltage alarm setting?

I believe that I shall make my own discharge chart.
Purpose will be to determine the mAh-capacity for each 1/100th V from 4.25V, down to 3.25v?.
Discharge will be regulated to a very modest .2C?, to help eliminate being skewed by voltage sag.
5 hour+ discharge cycle anticipated.


I will test 2 types:
Li-ion, Li-Co, 18650 2600mAh, recycled cells. (Test as 5200mAh paired cells.)
Li-polymer 2160mAh recycled cells. (Test as 4320mAh paired cells.)

Results will be posted here, and under Homemade Battery Packs!

 

[DrkAngel]

Sorry, this is the raw data of my monitored discharge test.
Will graph later, but wanted to post the results.

Quick stats on mAh per 100th V.
Li-ion (LiCo) 2600mAh used cells.
Charged to 4.25V
.2C discharge rate
..V... mAh
4.23V .. 4
4.22V .. 5
4.22V .. 6
4.20V .. 9
4.19V .. 18
4.18V .. 37
4.17V .. 38
4.16V .. 38
4.15V .. 38
4.14V .. 38
4.13V .. 38
4.12V .. 38
4.11V .. 40
4.10V .. 40
4.09V .. 42
4.08V .. 43
4.07V .. 44
4.06V .. 44
4.05V .. 44
4.04V .. 42
4.03V .. 43
4.02V .. 42
4.01V .. 43
4.00V .. 44
3.99V .. 44
3.98V .. 43
3.97V .. 45
3.96V .. 46
3.95V .. 45
3.94V .. 45
3.93V .. 47
3.92V .. 48
3.91V .. 47
3.90V .. 45
3.89V .. 43
3.88V .. 42
3.87V .. 44
3.86V .. 45
3.85V .. 57
3.84V .. 57
3.83V .. 78
3.82V .. 82
3.81V .. 86
3.80V .. 88
3.79V .. 89
3.78V .. 86
3.77V .. 94
3.76V .. 98
3.75V .. 103
3.74V .. 102
3.73V .. 105
3.72V .. 120
3.71V .. 125
3.70V .. 100
3.69V .. 100
3.68V .. 100
3.67V .. 78
3.66V .. 74
3.65V .. 72
3.64V .. 74
3.63V .. 72
3.62V .. 71
3.61V .. 70
3.60V .. 45
3.59V .. 43
3.58V .. 43
3.57V .. 24
3.56V .. 20
3.55V .. 20
3.54V .. 20
3.53V .. 17
3.52V .. 14
3.51V .. 12
3.50V .. 10

Output is reasonably steady from about 4.18V till 4.1V.
Slowly increases from 4.1V till 3.85V
Then increases sharply from 3.85V till it peaks around 3.7V.
Below 3.7V output decreases rapidly till it reaches minimum usable at 3.6V.

Based on these figures:
For Li-ion (LiCo),
Charge-discharge voltages should be,
Maximum 4.18V
Minimum 3.6V

3.85V to 3.65V is the muscle bulge of the discharge curve.
To take full advantage of this "bulge" I would recommend "bottom balancing", at 3.65V.

Charging above is 4.2V is worthless, and unnecessarily damaging.
Same for discharging below 3.6V.

These cells, with the largest capacity near the lower voltage and the steep drop off, would be ideal candidates for a bottom balancing ... at 3.6V!

Be aware! These results are for a specific cell type-formulation and might have no relation to other brands-types-formulations!

 

[DrkAngel]

Quick stats on mAh per 100th V.
LiPo 2160mAh used cells.
Charged to 4.2V (charging to 4.25V produced no additional "usable" mAh)
.2C discharge rate
..V... mAh
4.23V ..
4.22V ..
4.21V ..
4.20V ..
4.19V ..
4.18V ..
4.17V .. 1
4.16V .. 1
4.15V .. 2
4.14V .. 4
4.13V .. 4
4.12V .. 7
4.11V .. 10
4.10V .. 14
4.09V .. 17
4.08V .. 21
4.07V .. 31
4.06V .. 39
4.05V .. 40
4.04V .. 59
4.03V .. 68
4.02V .. 82
4.01V .. 45
4.00V .. 75
3.99V .. 70
3.98V .. 60
3.97V .. 90
3.96V .. 70
3.95V .. 70
3.94V .. 75
3.93V .. 75
3.92V .. 70
3.91V .. 66
3.90V .. 68
3.89V .. 66
3.88V .. 70
3.87V .. 80
3.86V .. 100
3.85V .. 110
3.84V .. 120
3.83V .. 130
3.82V .. 75
3.81V .. 80
3.80V .. 75
3.79V .. 75
3.78V .. 75
3.77V .. 65
3.76V .. 55
3.75V .. 55
3.74V .. 45
3.73V .. 45
3.72V .. 45
3.71V .. 36
3.70V .. 39
3.69V .. 35
3.68V .. 34
3.67V .. 29
3.66V .. 28
3.65V .. 27
3.64V .. 26
3.63V .. 25
3.62V .. 29
3.61V .. 26
3.60V .. 27
3.59V .. 24
3.58V .. 24
3.57V .. 27
3.56V .. 24
3.55V .. 21
3.54V .. 20
3.53V .. 20
3.52V .. 16
3.51V .. 17
3.50V .. 14

Output is reasonably good from about 4.05V till 3.75V.
Very noticeable, is a muscular bulge near 3.85V.
Decreasing till it reaches minimum usable near 3.7V.

Based on these figures:
For my recycled LiPo,
Charge-discharge voltages should be,
Maximum 4.10V
Minimum 3.7V

I would recommend "bottom balancing", at 3.75V.

Charging above 4.1V looks worthless, and unnecessarily damaging.
Same for discharging below 3.7V.

Be aware! These results are for a specific cell type-formulation and might have no relation to other brands-types-formulations!

Graph "smoothed", to minimize "human error"?

 

[DrkAngel]

Minimum usable cell voltage varies ... drastically, by type-chemistry, brand, manufacturer.
See Lithium - mAh/100th V - Discharge Test