Solar Energy:

Intelligent battery optimisation for maximising battery life

Attached to a 12-collector system on a southeast facing pitched roof.

Battery Storage Principle Operation

Storage batteries come in a wide range of technologies and capacities so it is difficult to define “which is the best” Invariably there are compromises between size, charge and discharge rates, cost the KWH of storage and ultimate service life!

We are looking at specifically Solar storage in the UK to maximise self consumption and minimising grid usage. The electricity generated by solar panels is directly proportional to the solar intensity the panel receives, this is fed into the household distribution board feeding any power consumption at that time, excess power will be exported (and eligible for Smart Export Guarantee if signed up for).

If the instantaneous usage exceeds the PV production then the shortfall is taken from the grid. As the imported grid electricity is three times the cost of the exported SEG payments the purpose of the storage battery is to absorb the excess production and release it back into the house when the demand requires.

The storage system uses a current sensor on the incoming supply and when it detects export the system charges and if something is turned on in the house the current sensor detects import and discharges effectively balancing the supply cable to zero all the time there is capacity in the battery.

If the battery is fully charged and there is still excess PV then this is exported, likewise if the battery is fully discharged then the required power in imported as per normal.

The specifications for Storage systems have two important parameters, Power (rate at which the batteries can be charged and discharged in KW, DC coupled systems are limited to the inverter size where as AC coupled have the PV and battery inverter capacities!)
The other factor is usable capacity in KWH. This is where the usable capacity is governed by the depth of discharge.

Battery service life depends on the usage profile of the charge and discharge cycles. Excessively deep discharge levels and high charge and discharge currents adversely decrease service life such that the control systems limit the depth of discharge to increase the service life of the battery (80% DOD (depth of Discharge) is typical for lithium based technologies where as 50% is more typical for Lead Acid technologies.

The Discharge level is adjustable to a fixed level on most systems.

Discharge optimisation is a method of varying the DOD depending on the amount of excess solar charge available.
In winter months there is limited excess solar available so a fixed Discharge level will result in the battery operating close to its discharge level for extended periods thus reducing the service life!

Conversely in the summer months with abundant surplus energy available there is plenty to fully charge batteries so having a high DOD will give maximum self consumption so fixing the DOD at too high or too low a level will reduce service life and limit the self consumption!

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