Battery questions

Big Tom

10 W
Joined
Apr 6, 2011
Messages
78
Location
Ontario, Canada
I have 2 questions.

First is, from most of my reading, it is recommended to only charge the battery to 80%. The battery maker (em3ev) says to charge to 90% for longer battery life and lower Cut Off to be 43v (52v 23 AH). Is one better then the other and why.

The second is watt/hours & amp hours. I can figure out the total watt hours & amp hours for the battery but how can I figure out what is available to me with a 43v LVC.

Thanks, Tom
 
Truly accurate measurement of the correspondence between Voltage (isolated at rest) and SoC% is both very challenging both in skills and kit required,

and not really necessary, just use voltage.

For those li-ion chemistries with 3.6 - 3.7 Vnominal

charge to between 4.05V and 4.15V, lower the better for longevity, but a lot depends on your charger's stop-charge logic - HVC CC only, then setpoint (capacity utilisation) depends on the initial C-rate, or

hold CV for an Absorb cycle, how low is trailing current allowed to get?

In any case, getting that last 5-15% CU will be at the cost of lots of lifespan cycles, stupid IMO.

But that longevity extension only comes if other factors - e.g. discharge C-rate, average DoD% at LVC - are also conducive to longevity - no point coddling the charge cycle if you're murdering the pack discharging

some people are happy getting 80 cycles, others want to try for 1000+

_____
Actual capacity needs to be measured empirically, using the same parameters as your real-life use case.

A 0.05C discharge will show lots more capacity than 2C average.

To be accurate, CC dummy load and accurate timer.

Or just use a coulomb-counting "wattmeter" and accept it won't be as theoretically accurate but give ongoing feedback in actual use.

As the battery wears out, capacity will drop. EoL is arbitrary, I use 70-75 SoH%

_______
For 14S, 42V LVC shows lots more capacity utilization than 47V average, but the latter usage may triple cycle lifespan, up to you! very hard to quantify beside WAG ballparking.


 
Yes good example of the curve slopes

but actual numbers look like LFP, a scale of magnitude longer lasting than li-ion and much lower energy density

each chemistry even model battery will be different
 
Big Tom said:
First is, from most of my reading, it is recommended to only charge the battery to 80%. The battery maker (em3ev) says to charge to 90% for longer battery life and lower Cut Off to be 43v (52v 23 AH). Is one better then the other and why.
Neither is better. Charging to 80% extends cycle life, and charging to 90% increases energy and power.
The only actual automotive EV spec I've seen for enhanced cycle life is 4.15V/cell, so charge a 14s pack to 58.1V, and 3.0V/cell limit, so 42V lower cut off -- but this probably assumes active balancing.
If you want the best of both worlds, top-balance charge to 4.15V on every charge.

Big Tom said:
The second is watt/hours & amp hours. I can figure out the total watt hours & amp hours for the battery but how can I figure out what is available to me with a 43v LVC.
Test and measure.
Or estimate: 4.15V/cell is at least 95%, so 58.1V to 42V is at least 95% of 23Ah, or 22Ah.
 
Real life Ah will be lower.

Even those top shelf marques that underestimate capacity om the nameplate

base it on drawing down to the spec sheet minimum (often 2.5V) and at a crazy slow rate like 0.1C

The generic lower quality makers routinely exaggerate even then.

20Ah might be a safer assumption for such cheap units.

They may even be Grade B or lower, QA rejects so much more variable, would need to be binned / matched in like sets.
 
Big Tom thanks.

Tom's dilemma is indicative of what plagues us noobs, lack of knowledge and practical experience.

There is a problem in figuring out facts. Physics is facts.
Eighty percent, 90 percent pack charge - versus advice from makers. For example, do we fully charge cells?

Basic? Yes!

What is bogus? What works vs advice, the pragmatic the case example...

Spinning Magnets (Ron), Agnius (N.E.S.E.), John61, AngryBob, Dark Angel, Tomjaz, and a very few other dodgers here, have got it right, as do friends at Grin Tech.

Others seem to be engaged in guesses or self-service of one kind or another. Intent means nada. Results need to be consistent.

So, how does the curious dodger know?

Being a noob, here, I now have now one pack I had to acquire because of dual failues, plus some 60 cells and two BMS's which have thusly given only grief. Ag's pack may have fixed some slights...we will see.

"Caviat Emptor" applies not only to objects but to advice also...and painting with a broad brush is not so wrong, it seems.

Irony, in the end, rules...! Bull walks...always, except in politics but that tells us something about us...

Listen to politicians, especially to American republicans now.

If you do not know facts, keep silent.

"Bogus" seems to include some advice even here, sadly. Tom ya got right!
 
Charge it 100% and go further. I’d rather have the extra range and power then longevity. If I had a 40 amp hour battery yeah 80% all day. Smaller batteries with less range I prefer 100% charge.

This really depends on what you’re doing and how far your commute or traveling is. If you have a short commute and you don’t need all the battery then 80%
 
I am done wearing out batteries quickly. Long commute 200+ days per year and I want a few years out of my batteries. Using principles outlined here and elsewhere they are sized for 80-40% within weight limits set for myself. It also gives margin for wear. I can't always charge at the other end so I need batteries for the complete trip. Works out at about 0.1C discharge. Very low stress. 70% charge on shorter runs.
Oh, and I use chargers specifically designed for the purpose.
 
East, and TDB Thanks! It is all so relative, which might be the reason we have so many options.
Come to think of it I had to rethink some of my e-minding since physics classes...the basics remain, but applicaitons vary for a jillion reasons. They came to me slowly.
Obviously, I prefer long-life cell packs, but have been shocked into careful dealings, both physically and in theories.
Some here have plodded alsong and have not a care, others, dispite care and hope, have encountered the opposite events, and have absorbed the cost. No more!

Still, if one intends or needs to add cells to pack already extant, what are criterial...same mAmp limits, brand and voltage, or simply same voltages...

I have had some hot spots, and learned about respect for power and house is still standing. But, Caveat Emptor remains a high priority.

Many commutes are not my thing, but with these cells.cell packs, all comes back to roost...doesn't it?
 
If you really value longevity go to LFP chemistry

all thing equal 10x the life cycles easy.

or LTO even better, 10x again.

Yes lower energy density, but then so is intentionally not utilizing full capacity.

Higher Ah cells thus fewer cells per group less packaging overhead might even make for **higher** energy density at the whole-pack level.

Same guidelines for avoiding the extreme data sheet "shoulders", but when you are starting from 3,000 or 10,000 cycles as your baseline spec no need to be too stringent about it.

Plus much safer wrt thermal risks. . .
 
My point was to size conservatively for the required distance to get the best life out of whatever chemistry is chosen.

In my case I have plenty of margin for weather and for longer days (once or twice a month) where I go from 100% to 20%.
 
Big Tom said:
I have 2 questions.

First is, from most of my reading, it is recommended to only charge the battery to 80%. The battery maker (em3ev) says to charge to 90% for longer battery life and lower Cut Off to be 43v (52v 23 AH). Is one better then the other and why.

The second is watt/hours & amp hours. I can figure out the total watt hours & amp hours for the battery but how can I figure out what is available to me with a 43v LVC.

Thanks, Tom


To answer your second question pull out test data for the cells in your pack. It is just an indicator, not gospel.

I have two EM3EV 14S7P 52V 23Ah packs with Samsung INR18650-35E cells. Current draw in my application is 5A or about 0.37A per cell maximum. Yes, very low (legal) power.

43V cut off is 3.07V per cell. Select the load line closest to your typical use from test data and look at where it meets 3.07V. In my case about 240Wh or 7% remaining.

FWIW, I set the lower limit on mine at a conservative 46V or 3.3V cell. That is about 15% +/-.

If you have the BT BMS the phone app (BMS Doctor works with mine) is a useful tool.

inr1865035e.png

IMG_009_2.jpg
 
Normal improvments notwithstanding. the above sort of proves my point.

Bet I have three (3) cell voltage requiremnts referncess for cell to pack install...the confusion costs in terms of dollars and time. For example, I have one commercial pack and some 60 loose cells and after two Avruzend failed packs. Now, am assembling a fourth and fifth pack to try again...mind over matter anal me. All this outstrips previous bike gas power builds in complexisty...not a good thing, reallly.
 
Start with top quality brand new Grade A cells, all from the same recommended trustworthy vendor

and ideally all from the same production run.

Otherwise you just won't the results you need, and will have to redo all that work, sooner rather than later
 
John, I know what you are saying...start clean!
but that is one of my points. Too spendy and chancy. .cannot afford time and money. Just want to ride bike...Gas job was simple afast and ran fine and long. Got to be a netter way.

I have one commercial pack and 60 high end cells, and twi BMS boards, Vruzend BMS,,,and plan to insert into Agnius' NESE pack modules...after I match them and get 19 new cells "in case."

Previous builds failed (Vruzend)

Money/time hole is too deep at the outset.
 
Back
Top