LFP Battery SOC vs DOD and finding a balance

[nvesprini]

My apologies if this has been discussed elsewhere. I see lots of info for both DOD and SOC independently for the longevity of batteries, but having some trouble finding info that looks at both factors together. A high SOC is bad for the battery, but by keeping a lower SOC and charging less frequently you are creating a high DOD which is also bad.

Eg:
1)charging frequently with small top-ups or (as an example) charging once the battery hits 70% makes a low DOD (good) but a high average SOC (bad)

2)charging less frequently or (as an example) charging once the battery hits 30% makes a high DOD (bad) but low average SOC (good)

Which factor is worse for wear and/or what balance should we look to strike between these two factors?

Discussion is specific to the LFP batteries if that helps at all

 

[GtiMart]

The only thing specific to LFP as far as I understood here is that the BMS has a harder time properly estimating the battery's capacity since the voltage doesn't vary as much in the "middle" of the pack. Because of that, it's good to fully charge somewhat regularly so the BMS sees the full pack voltage. The good thing is that fully charging is less damaging for the LFP. As far as I know, the rest is the same, i.e. keep your SOC low, make small depth of discharges as you can etc...

After that, you still need to use the car. Pick something that works for you. Your battery will not self-destruct.

 

[Regaj]

Great question. Much of what we might do to preserve pack life involves balancing competing elements such as this.

Although the general rule of thumb with SOC is that lower is almost always better, it's also true that it's the very upper end that causes the most cell stress. If someone was purposely holding off charging, they are seeing a declining health benefit as their SOC moves into lower ranges.

It also sounds like someone in the position to purposely hold off charging over several driving sessions, so as to capture the benefits of lower average SOC, is probably also in the position to charge more frequently, but to lower SOC. If I consumed 20% per day, I'd rather charge every day from 30-50% than to charge to 90% and let it drop to 30%, charging every three days.

And, of course, all of this is much less an issue for LFP. As @GtiMart notes, you'll want to charge to 100% with some frequency, simply to keep the BMS calibrated.

 

[nvesprini]

If I consumed 20% per day, I'd rather charge every day from 30-50% than to charge to 90% and let it drop to 30%, charging every three days.

apologies if im off here but in your example wouldnt you still have a high DOD if going from 30-50? youve cycled 20% of the battery capacity but the battery is discharged by 70% when you start your charge, then only brought up to 50% (so still half discharged) when youre done charging. it seems the battery would continually have a high DOD?

if the above is correct then this indicates SOC is the more important metric to follow than DOD since the example would favor a low average SOC at the cost of a sort of chronic low DOD. is this correct in that SOC is more impactful than DOD or am I misunderstanding?

 

[GtiMart]

DOD: that is how much you "vary" the charge. Going 50->30 is just 20% delta, and charging 30->50 is just 20% delta. It doesn't matter at what SOC you are in that particular variable.

 

[nvesprini]

so if this is true - where the delta is all that matters for DOD rather than the actual amount a battery is discharged - what causes the damage from a high DOD? is it just the heat from charging, so smaller DODs = less heating of the battery?

if its just the heat from charging how applicable is the concern of DOD to people on lv 2 charging...where the C rate (at least for my car) is 0.1C. That seems crazy low so (im assuming) presumably low or no heat. is it fair to be concerned about DOD if the only source of "DOD damage" comes from heat...which for slow charging isnt really a thing to begin with? not trying to sound argumentative, just better understand where the damage is from and whether any effort to balance DOD vs SOC charge habits is more of an on paper vs in practice effort.

 

[GtiMart]

You will have to go read articles to figure it out, and that's probably described in one of the multiple existing threads on the subject around here. I don't think it's temperature related. C rate is also something else.

 

[jjrandorin]

My apologies if this has been discussed elsewhere. I see lots of info for both DOD and SOC independently for the longevity of batteries, but having some trouble finding info that looks at both factors together. A high SOC is bad for the battery, but by keeping a lower SOC and charging less frequently you are creating a high DOD which is also bad.

Eg:
1)charging frequently with small top-ups or (as an example) charging once the battery hits 70% makes a low DOD (good) but a high average SOC (bad)

2)charging less frequently or (as an example) charging once the battery hits 30% makes a high DOD (bad) but low average SOC (good)

Which factor is worse for wear and/or what balance should we look to strike between these two factors?

Discussion is specific to the LFP batteries if that helps at all

Is this not discussed at all in the large thread specifically on LFP batteries in model 3s? Its 40 pages long, so I would be really surprised if no one ever asked "what should we charge to daily?" in a 40 page thread dedicated to the discussion of that battery type:

Model 3 SR+ LFP Battery Range, Degradation, etc Discussion

Now that LFP batteries have been operating for a couple months in the US, I thought it would be useful to start a thread for people to post their experiences with degradation of LFP batteries. A major benefit of these batteries is supposed to be resistant to degradation, so let's test that claim...

teslamotorsclub.com

 

[yesimon]

A high SOC is bad for the battery, but by keeping a lower SOC and charging less frequently you are creating a high DOD which is also bad.

LFP batteries are resistant to all charging-related degradation. That means it is not adversely affected by high DOD or supercharging. In fact, LFP battery research is conflicting on whether cycling at low SOC or high SOC, low DOD, high DOD is better or worse for degradation. The likely impact for these factors is extremely minimal. Therefore the main consideration is calendar aging: lower storage SOC is always preferred, but there's a slightly bigger decline around 70% SOC so some people choose to charge limit there.

 

[gaswalla]

DOD - Depth of Discharge

 

[Regaj]

so if this is true - where the delta is all that matters for DOD rather than the actual amount a battery is discharged - what causes the damage from a high DOD? is it just the heat from charging, so smaller DODs = less heating of the battery?

if its just the heat from charging how applicable is the concern of DOD to people on lv 2 charging...where the C rate (at least for my car) is 0.1C. That seems crazy low so (im assuming) presumably low or no heat. is it fair to be concerned about DOD if the only source of "DOD damage" comes from heat...which for slow charging isnt really a thing to begin with? not trying to sound argumentative, just better understand where the damage is from and whether any effort to balance DOD vs SOC charge habits is more of an on paper vs in practice effort.

All rechargeable batteries have an inherent useful life, typically stated in full cycles from 100% SOC to 0% SOC. Extending my earlier example of a 20% daily battery consumption, someone who charged their car to 100% and then let it slowly bleed down to 0% before recharging would consume one cycle every five days. The person who plugged in every night, topping back up from 30% to 50%, would also consume one cycle every five days (1/5th of a cycle every day).

However, the battery degradation (measured as remaining capacity) of the first driver would be exponentially worse than that of the second driver, simply because of the stark difference in DOD's.

The anode and cathode of the battery cells expand during charging and discharging, often by more than 10%, and that can cause a variety of issues. Shallower DOD's cause this expansion to be lower and to happen at a slower pace. (Lower C-Rate - a closely related phenomenon - does, as well).

Heat is not the bad actor in this case, although elevated heat levels will sharply amplify and accelerate nearly all the bad things that can happen to a Lithium-ion battery.