How to utilize 80% DoD

[EnergyOasis]

I was pouring over the data sheet on LG MH1 3200mAh cells that I will be using in my power wall, and I still have the question How do I utilize only 80% DoD (depth of discharge)???

What curve do I refer to, and what is the best method of calculating DoD? I want these cells to last as long as possible, so I designed the powerwall so each cell will only need to push 300mA MAX.

I appreciate the replies and love learning new things.

[daromer]

80% DOD is 0.8*3200mAh at the current needed to get out 3200mAh. If thats at 300mA then that would be 2500mAh max capacity takeout. Doing this by voltage is not good enough so you need to work with a coloumb counter like a BMS with shunt.

Volgate can be somewhat used but only during resting voltage.

[jdeadman]

Generally you can achieve 80% DOD by not fully charging and not fully Draining. So for instance if the data sheet says the voltage range is 3v to 4.2v and you want 80% of that you cannot just use 80 of the difference due to the way Lithium discharges. So a good rule of Thumb is to stop discharging at 3.4V (at the edge of the downward curve) and stop just before full at 4.1V

[EnergyOasis]

Generally you can achieve 80% DOD by not fully charging and not fully Draining. So for instance if the data sheet says the voltage range is 3v to 4.2v and you want 80% of that you cannot just use 80 of the difference due to the way Lithium discharges. So a good rule of Thumb is to stop discharging at 3.4V (at the edge of the downward curve) and stop just before full at 4.1V

3.4-4.1 is the range I’ll program into my BMS as cutoffs. Thank you for the quick response

[daromer]

The BMS should protect and those should be something else if you ask me. If you plan to run the battery between those 2 values that is set in the charger/inverter.

Note that the Inverter/Charger is what determine the use-voltages. The BMS is the protection voltages if the inverter fails. And if that is not far enough you get false triggers and will disconnect.

As example my inverter top voltage is 4.0V. Balancing at bms is 4.05V and critical is 4.2v.
Lower side my inverter switches to grid if below 3.6v. It shuts of if below 3.2V and no grid.
BMS shuts down the battery as 3.0V.

Margin to work and to be hones i rarely even get close to the low point and thats because i have enough solar and battery to work with it.

[OffGridInTheCity]

Remember that the discharge curve is not exactly a straight line - there is very little power between 4.2 and 4.15 as any draw immediately lowers the voltage a bit. And a sharp drop of in power/voltage-drop after 3.5 or so (it varies). You should be able to find the discharge curve (based on amps drawn) for your particular cell.

In general I would think in terms of 4.2-4.1v high down to 3.3-3.1v low to achieve an 80% range - and that would be 'brand-new' batteries and bit rough on them. It might be better to shoot for 70% or 60% and have a bigger battery in terms of battery life.

[completelycharged]

Read through this thread
https://secondlifestorage.com/showthread.php?tid=7873