Energy Storage

Terms and Concepts

Power: the amount of energy that is required for a task - expressed as watts (W) or kilowatts (kW).

Peak power: the maximum amount of power at any one specific time, even if momentarily - expressed as watts (W) or kilowatts (kW).

Capacity: the total amount of energy that can be stored - expressed as watt hours (Wh) or kilowatt hours (kWh).  One kWh is what grid suppliers refer to as a "unit of electricity". When you buy 100 units of electricity for your prepaid meter, you are buying 100 kWh of energy.

C-rating: the charge and discharge rate for the battery - how long it will take to discharge while delivering maximum power and how long it will take to charge up again.

DoD: The Depth of Discharge is the level that the battery can be discharged to.  The percentage is the amount of effective energy that can be delivered compared to the nominal energy for the battery.

Cycles - also equivalent full cycles: The number of times that the battery can be discharged and recharged.  For some batteries the number of cycles is dependent on the DoD, for others it is a fixed amount.

The charge rating, discharge rating, depth of discharge and the number of cycles have to all be taken into consideration when choosing the correct battery for a particular application.

Battery C Rating

The C rating, or discharge and charge rate, directly affects the maximum amount of power that the battery can deliver or be recharged with at any particular moment.  It is an important aspect as it is one of the main differentiators between the different types of batteries.  A LiFePO4 battery has either a 1C or a 0.5C rating.  A 1C battery means that it will release all of its energy at a maximum rate that would be equivalent to emptying it in no less than 1 hour and a 0.5C means that it will release all of its energy at a maximum rate that would be equivalent to emptying it in no less than 2 hours.  However, most 1C batteries have a DoD of 80% at 4000 cycles whereas a 0.5C battery typically has a DoD of 95% with 6000 cycles.  So for a 3500Wh LiFePO4 battery, you cannot draw more than 1750W (there is some wiggle room here - the documentation lists a discharge rate of 1.8kW), but it will let you do that for 2 hours and allow you to do it 6000 times.  [For the more technically minded, there are some caveats to this.  The Pylontech US3000C batteries that we use can supposedly discharge at a rate of 3.55kW (1C) for 5 minutes and at a surge rate of 4.32kW (1.2C) for 15 seconds.  The BMS should manage this, but take care to read the section further down on Storage C Rating - it may be prudent to enforce a lower C rating for the discharge to protect your cables!]

Understanding the C rating is critical.  Most people are used to UPS systems that have a very high discharge rate and then a slow charge rate.  Typically a UPS will drain a battery in 15 minutes.  This is the main reason that UPS batteries do not last - they are designed for infrequent outages of short duration and load shedding is frequent outages of long duration.  So load shedding is a recipe for UPS battery death.

The battery systems for inverters are not just battery cells, they include a battery management system (BMS) that will limit the charge and discharge rates - as well as keep the battery conditioned and do a whole heap of funky things to ensure you have a happy battery.  It is the BMS that results in the Pylontech US3000C batteries (the ones we are using) being able to have a 95% DoD with 6000 equivalent full cycles and be designed for 15 years of operation, guaranteed for 10.  But it is also the BMS that limits the rate of charge and discharge, so even if you have a 3.5kWh battery, you need to realise that it will not deliver more than 1.8kW at any one time.

The final consideration is the number of cycles and how they are calculated.  The Pylontech use equivalent full cycles where the amount discharged in each charge and discharge cycle is added together until it is equal to the full discharge capacity of the battery and that is calculated as a cycle.  So if you use a third of the energy from the battery in 3 successive discharge cycles, that will only count as 1 equivalent full cycle.

Putting together the information about the amount of energy that can be stored at any one time, the depth of discharge and the number of cycles allows you to calculate your energy storage cost over the lifetime of the battery.  This probably best done by way of examples.

Example 1 - 3.55kWh LiFePO4 battery - Purchase price is R20 000 (early 2022 prices). DoD is 95%. Cycles is 6000.  Lifetime storage cost = 20000/(3.55*0.95*6000) = ~R 1/kWh.

Example 2 - 2.4kWh AGM Gel battery - Purchase price is R 5 000 (early 2022 prices). DoD is 50%. Cycles is 2000.  Lifetime storage cost = 5000/(2.4*0.5*2000) = ~R 2/kWh.

This shows that while an AGM Gel battery is initially a cheaper storage solution, it is twice as expensive over the full lifetime.

Our installation has 3 x Pylontech US3000C batteries, giving us approximately 10kWh of available energy and a peak power draw of 5kW.

Storage C Rating

When considering the system as a whole, it is important to note that the overall C rating for the full storage solution may be different to the C Rating for the batteries.  This is as a result of limits to the amount of current that can flow through the storage connection cables.

As an example, we use the Pylontech power cables that are rated to a maximum of 115A of current and the recommended maximum current flow is 100A.  Each battery stores 74Ah of energy and as it is a 0.5C battery, it can deliver a maximum of 37A over a 2 hour period (current=energy*Crating, time=1/Crating).  In theory, that should mean that the 3 batteries can deliver 3x37=111A.  However, that would exceed the recommended 100A rating and get very close to the maximum 115A rating of the cables.  Our inverter is set to a maximum DC storage current of 100A and we can now calculate our System Storage C Rating as follows: 1/((3*74Ah)/100A) = 1/2.22 = 0.45.  Therefore our overall system has a Storage C Rating of 0.45 and a minimum charge and discharge time of 2 hours and 15 minutes.

When we add the fourth battery to the system we will have a Storage C Rating of 0.34 and a minimum charge and discharge time of just under 3 hours.  At the moment the highest stage of load shedding in South Africa is Stage 8 where the grid power is off for 4.5 hours and on for 3.5 hours.  So even if there is no sun to charge the batteries from the solar panels, we would still be able to fully recharge between load shedding windows even with a storage C rating of 0.34.

Note that Pylontech does have combiner systems with higher specification cables and current rating if you do need to exceed 100A.  For the majority of residential systems they should not be necessary.  However, for an office or industrial application they might be required.

Note that there are other caveats - not all storage systems have symmetrical C ratings for charge and discharge.  The batteries used in UPS systems typically have a discharge C rating (they can dump their energy in a matter of minutes), but then have a very low charge C rating (for example an APC Back-UPS Pro 650VA supplying a full load will discharge in 2 minutes and 44 seconds, but will take 12 hours to recharge).  This slow recharge rate is one of the reasons why UPS systems are a terrible choice for trying to mitigate against load shedding.

The National or Municipal Grid

The grid is also sometimes used as storage.  However, you need to remember that there is a big difference between what you get paid to provide power to the grid versus what you are charged to consume it.  The storage cost of using the grid is not free as many believe and generally works out more expensive than storing to local battery - for example the storage cost for an LiFePO4 is about R1/kWh.  Additionally, the grid may not be available, for example during load shedding, when you need the storage power.

Maximising the life of your storage system

It must be noted that caring for your storage system is vital in achieving the manufacturer specifications.  Heat, humidity will all play a part in ensuring that you get the most out of the batteries.  For example, the datasheet for the Pylontech US3000C batteries states quite clearly:

Design life: 15+ years (25°C/77°F)

Cycle life: >6000, 25°C

Notes on Pylontech batteries

Adding batteries to a stack

If you are expanding a stack, it is absolutely critical that you charge all the batteries to 100% SOC before combining.  The procedure I have adopted is to first charge the existing stack (or battery if just 1 battery in the initial installation) to 100%.  Then disconnect the stack both electrically and from the communication port.  Connect the new battery (usually ships at 50% SOC) both electrically and via the communication cable and charge to 100%.  Disconnect again, add the new battery to the stack making sure that the new battery is set as the master (for communication cable wiring this is best done by placing it on top) and combine the stack.  Connect the power and communication cables between all the batteries in the stack.  Power up the stack before connecting the stack back to the inverter and make sure that they have all settled and all the leds are reporting that all batteries are at 100% and no faults.  Then reconnect the stack back to the inverter with the power and communication cable.

Creating multiple stacks

If you want to create more than 1 stack, you have 2 options.  If you are communicating via RS485, you have to chain the master battery for each stack.  If you are communicating using CAN, then you will need the LV-HUB to facilitate communication between the stacks.  I am not going into detail here as it is quite technical and all explained in the manual.  If you cannot figure it out from the manual, then you should probably not be trying to do it yourself!

Using buckets of water as an analogy.

I have found that some folk find battery systems easier to understand if they consider the batteries as buckets that can hold water supplying a power system that uses water instead of electricity.  I will attempt to explain some of the concepts outlined above in those terms.  

The depth of discharge (DoD) is where on the side of the bucket the outlet is placed.  For a battery that has a 50% DoD rating, the outlet will be halfway up the side of the bucket and you will only ever be able to release the upper half of the water.  A 95% DoD is equivalent to a 100cm tall cylindrical bucket having its outlet placed 5cm up from the bottom.

The capacity of a battery is the maximum amount of water that the bucket can hold.

The C rating is analogous to the size of the outlet hole and filler hole and will determine how fast the water can flow out and into the bucket.