Battery Dynamic State-Of-Wear (SOW) contradiction

[julmou]

Good morning,

As I was designing a stand-alone system, I noticed a trend that I can't quite explain... regarding the dynamic ageing of the battery (wear-and-tear due to number of cycles).

I understand that, the deeper the DOD (Depth-Of-Discharge), the more degradation, as is explained in this section of the Help : https://www.pvsyst.com/help/battery_ageing.htm

However, I noticed in my simulations, that I was getting the opposite. The more I had a system with a high SOCmean (battery around 60-100% most of the time) the more I had dynamic wear (SOWCycl around 89%), and the opposite is true: in a system with deep DOD (SOC around 20-30-40% most of the time), my dynamic wear is much better (SOWCycl around 94%). I can't really explain it... Please find details about the said simulations below:

Site: Dakar, Orientation: 35° tilt (optimized for winter), User's Needs: Load a CSV file, Household_BDEW template, re-scaled to 7000 kWh/year

Battery set: Sonnenschein 12V 190Ah Pb Sealed Gel Solar S12/230A, PV module: SunPower 400W 55V SPR-MAX-400, Universal Controller Direct Coupling

Case 01 -> Autonomy 3.4 days: 4x10=40 batteries, 1900Ah ; PV array oversized on purpose to illustrate the issue: 16 modules 6.4kWp

Case 02 -> same system but with much smaller array of 8 modules, 3.2 kWp (and deeper DOD as a result)

Case 03: without going to the extreme, a medium-sized 4kWp array, gives us SOWCycl 92.6% and SOCmean 43.3%

6.4 kWp -> SOWCycl 89.1%, SOCmean 82.3%

4.0 kWp -> SOWCycl 92.6%, SOCmean 43.3%

3.2 kWp -> SOWCycl 93.9%, SOCmean 33.9%

As you can see, we have less wear on the battery when the DOD is deeper... I don't understand this. Would you mind helping me figure out what's going on?

Thanks so much,

Julien

[André Mermoud]

45 minutes ago, André Mermoud said:

The wearing dependency on the SOC depth is indeed a marginal effect. The main  source of wearing is the number of cycles, i.e. the total Charging/Discharging energy. Please have a look on the energy real use on your Loss diagram. The "battery use" is probably higher in the first conditions.

This may be explained by the next plot, which shows the total number of cycles "allowed" as a function of the SOC (or DOD) without degradation and with a degradation I have set the model to 40% additional degradation for 100% DOD  (blue line),  just for illustrating the phenomenon.  But you see that the real additional degradation of your battery (orange dots) is almost independent on the DOD (close to the dotted line). 

 

 

 

 

[julmou]

Quote

The main  source of wearing is the number of cycles, i.e. the total Charging/Discharging energy.

Hmm, I didn't realize we could "count" the number of cycles through the energy used by the battery.

When you say the "battery use" energy in the loss diagram, do you mean this one (circled in red) ?

 

In that case, we have:

8.0 kWp -> SOWCycl 88.2%, SOCmean 82.4%, Battery Charging/Discharging energy 5540 kWh (7611 kWh eff. array energy, 72.8% stored, 27.2% direct use), SF = 1

6.4 kWp -> SOWCycl 89.1%, SOCmean 82.3%, Battery Charging/Discharging energy 5088 kWh (7527 kWh eff. array energy, 67.6% stored, 32.4% direct use), SF = 1

4.0 kWp -> SOWCycl 92.6%, SOCmean 43.3%, Battery Charging/Discharging energy 3733 kWh (6799 kWh eff. array energy, 54.9% stored, 45.1% direct use), SF = 0.915

3.2 kWp -> SOWCycl 93.9%, SOCmean 33.9%, Battery Charging/Discharging energy 3070 kWh (7611 kWh eff. array energy, 56.4% stored, 43.6% direct use), SF = 0.728

=> for the same given yearly load, and with the same battery size, the bigger the PVarray the higher the part of the energy transiting through the battery (not sure I understand why that is btw, especially between the 8 and 6.4 kWp, the part of direct use decreasing as we increase the Pnom...) and thus the more wear in the battery.

I guess where I'm coming from is this part of the Help telling us that a low average State of Charge could be damaging for the battery (snapshot below). So in my mind, it was better to have a SOCmean of around 60% than one of 30%. Which is why I was totally surprised to see the opposite in the simulations, the lower the SOCmean, the less wear/damage I have on the battery... (SOCmean circa 30% -> SOWCycl 94% !)

It definitely feels quite contradictory.