Battery lifetime expectancy - Total energy stored?

[Sylvai Vitali]

Hello,

PVSyst automatically gives the total amount of energy stored in a battery during its life, which is very interesting (it would be even better with the latest Huawei Luna battery models).

How is battery life defined?
In the example below, can I assume that after 114 MWh, the battery is completely dead?

And are these kWh estimated on the AC side of the battery's inverter, taking into account the inverter's efficiency?

Thanks in advance for your help,
Sylvain Vitali

 

[André Mermoud]

The capacity slightly decreases when using the battery. The battery life (number of cycles specified by the manufacturer) is defined as the situation when the capacity drops below a given threshold.

Traditionnally, this limit was usually 80%. Since some years, several manufacturers consider a drop down to 70% of the initial capacity. This is not always mentioned on the datasheets.

The discrimination between input or output of the battery (efficiency, around 5%) doesn't make much sense as the uncertainty on the number of cycles is much much higher.

[Sylvai Vitali]

Thank you for your feedback,

So in PVSyst the estimated number of cycles at 50% Dod, corresponds to the number of cycles (at 50% DoD) possible before the battery capacity goes from 100% capacity to 80% capacity, Right?Also, the estimated total energy corresponds to the total energy stored until the battery reaches 80% capacity?

Isn't it a bit of a shame to assume that the battery is no longer usable when its capacity drops below 80% or 70%?
Couldn't we define in PVSyst that the battery is discarded when capacity is reduced to 50% or less?

Furthermore, does the value given in the Sankey Diagram correspond to the average kWh stored annually by the battery (with a capacity varying from 100% to 80% during its lifetime), or does it correspond to the state with a new battery (100% capacity), i.e. roughly to the first year of use only?

 

[André Mermoud]

The battery end of life is not a well-established criteria. 

As I said, the number of cycles guaranteed by the manufalctrers is sometimes based on 80% and sometimes on 70%.

And the number of cycles you can accept for your own instalation depends on your requirements (kind or use of your system).

We will consider to provide this as an option in a next version. 

 

 

 

 

 

[Sylvai Vitali]

1. So in PVSyst the estimated number of cycles at 50% Dod, corresponds to the number of cycles (at 50% DoD) possible before the battery capacity goes from 100% capacity to the guaranteed capacity (80% or 70% in general), right?

2. So the Sankey Diagram correspond to the average kWh stored annually by the battery the first year of use, right? 

The self-consumption estimated in the Sankey would therefore be the most optimistic situation (year0). To make a correct profitability calculation over the battery lifetime, we would have to admit a decrease in this self-consumption rate over time, right?

To take this effect in the profitability, we have to do another PVSyst variant with a battery with a nominal capacity of 70%-80% DoD of the first battery AND taking the average stored energy of the 2 batteries? Could it be a good approximation?

 

Thank you. and have a good day,

Sylvain Vitali

[André Mermoud]

The energy stored in the battery during the year - mentioned on this Sankey diagram - is indeed the value calculated by the simulation for the actual battery capacity specified for this simulation.

In the present time, the battery capacity decrease according to the ageing  is not taken into account in the ageing tool (this feature is on our roadmap, but will not be implemented before several months).

Therefore you can indeed perform a simulation for the last year of the battery life, with a battery capacity diminished by 20%. However don't wait for a big difference: the battery use during the simulation is not very sensitive to a diminution of 20%. 

NB: don't confuse the self-consumption with the capacity diminution, they have nothing to do with each other. The self-consumption may also be dependent on the age of the battery, but we don't have any information about this.