@ greg,
You hit the nail right on the head.
My view on "standard" li ion chem is most from battery university.
The majority of the li ion cells (90%) can be discharged to 2.8v and charged to 4.2v.
If you discharge them to 50% (dod) you would have a v span from 3,2v to 3.8 volts.
Meaning in my eyes if you need 12v for your motor and 4 amp to play for four hours, you will need a pretty large pack.
When we look at the curves in varios chards there is not much above 4v or below 3v.
Now 12v with li ion is tricky 3s or 4s? I would take 4s and a voltige regulator.
If you would take 80% dod (3v to 4v) you can run much longer with less cells, but they will have a shorter life.
What could this mean for the cell's cycle life in the long run....probably 400 to 600 cycles extra, in your watt story: absolutely nothing.
Cous if you take more watts out 1 cycle and thus reduce his cycle life, or take less watts out and extend his cycle life, you are absolutely right.
The worst cycle life is if taken the full v span, 2.8v to 4.25v--> 400 to 600 cycles, depending on manufacturer.
From 3.0v to 4.0v would mean 6-700 to 8-900 cycles.
from 3.6 to 3.8 could mean up to 4000 cycles.
But what is the best watt/life/cycle ratio? I think that depends on the type of use you intend to, a 100kwh powerwall for your house is quite a different setup than for a 1.2v 0.2a motor.
If you really want to have a long life span or the most watts out of a device, than there are a lot of other chemistry's.
With li ion in this situation, i think they will rust away before they are giving up.
Lets assume 900 cycles, that would be 3 years every day one complete cycle, if you only drive in the weekends that would be 9 years...
Assuming you can complete one cycle in one driving day assuming your packs are big enough.
I have packs running that reaches there 1500st cycle and still holds 60% (soh)of there original capacity.
But i keep my c rate around 0.5 to max 1.5.
If they drop below 45ish% soh i change the whole string for lifepo.
With this, so sorry to say, i dont agree on your quote:
Discharging down to 3 volts is the WORST battery life. Better to run the batteries "half way down" and charge up twice as often than run them as low as you can go...
This is one of the very big distinguishing factors of Lithium rechargeables.
My answer:
If you run the cells halfway down from 4.2v to 3.6v you will get not even 50% of the total capacity from a cell.
The culprit area in li ion cells are the 4.0v to 4.2 v span and 2.8v to 3v span, if you leave those area's unused you will get a double amount of usable watts back in the same cell life, however, it is very good for the cells health to charge it all the way up and discharge it all the way down once in a while, every 100 cycles or so.
You can charge li ion in every state of discharge, no need for difficult charging.
Run them as low as you can go??? i think you mean: not below 2.8v, i hope
Even if you take some charge out of the cell, it is not necessary to charge it immediately back up, you can drain it all the way down(max 2.8v) and then charge.
For a period of no use (~3 months) it is advisable to put them away around 3.8v, longer period of time 3.6v, but never put them fully charged or discharged away for longer then 3 months.
To keep it simple for people that dont want to dive into the deep like we did:
Charge to max 4v per cell, discharge to max 3v per cell.
Meaning if you have 3 cells in series max charge would be 12v and max discharge would be 9v.
You can charge the battery every time all day long and drain it 5% 50% or even 100%, just after use ---> charge, but...never ever exceed the max volt on a cell.
For discharge they can handle some abuse, but it is very wise to have a protection, like a volt cut off(diode, relais, v regulator ect) or bms.
Also a very important thing: balance.
You can buy very cheap bmses from ali express:
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For charging li ion you can use almost anything, but dont exceed the max volt and as a golden rule dont charge more than 3 times the total ah.
Better is 50% of the ah.
The same applies for discharge.
Example:
If it is 3 cells of each 2000mah in series: 12v max and max 6a charge or 9v discharge at 6a.
Best
