The battery chemistry

Virtually all Home Storage Batteries are made from Lithium-ion.

Nominal capacity (kWh)

Battery capacity shows how much energy the battery can theoretically deliver from fully charged, under optimum operating conditions. The key conditions are discharge current and operating temperature. Varying either of these can really impact performance, changing the capacity of the battery.

Battery capacity is normally given in kWh (Kilo Watt hours) at a certain discharge current (A). The higher the discharge current, the quicker the discharge and the lower the overall capacity (Ah).

Big Discharge Current = High Discharge Rate = Lower Overall Capacity

It is very important to look at the capacity of the battery in kWh and the discharge current in A.

Depth of discharge

The Depth of Discharge (DoD) refers to how much energy is cycled into and out of the battery on a given cycle, expressed as a percentage of the total capacity of the battery.

Lithium-ion batteries can typically be discharged between 80-90% before reaching a potentially harmful state of “deep discharge”. They have a battery management system to prevent deep discharge (and over-charge).

For example, if you have a 10kWh battery, and you discharge 9Kwh, then the DoD is 90%.

Usable capacity

By programming the system to limit the Depth of Discharge, the effective usable capacity of the system is limited.

In other words, you cannot use that final 10-20% of the battery capacity.

The reason for this is that it protects the battery lifespan – the lower the programmed depth of discharge, the longer the battery will last.

What is a “cycle”?

A full battery cycle is the process of your battery going from the state of fully charged, to fully depleted, and back again to a full charge.

Batteries will have a warranty in place for anywhere between 6000-10000 cycles. For most users, one cycle will correspond to one day. If choosing a smaller capacity battery, it would be possible to use the battery more intensively, where you are using 1.5 – 2 cycles per day

The battery will actually function better and last longer when it cycles between a 30% – 70% state of charge, rather than from a state of fully charged to fully depleted.

The expected lifespan with up to 300 discharge cycles per year is 20 years. At the end of it’s life, the battery can be expected to have between 60 – 70% of its original capacity

System efficiency

Electrical energy is lost as it is moved, stored and converted from AC to DC and back again.

The round-trip efficiency of the combined charger and battery is usually in the order of 83% but can be somewhat higher.

Self-discharge

Batteries gradually lose charge over time although in lithium-ion batteries this is relatively limited.

Battery management system

Battery Management Systems are essential for lithium-ion batteries. They perform several functions:

• Ensure safe system start up and shut down
• Ensure the battery operates in optimum conditions, controlling voltage, current, and temperature
• Indicate the available charge and discharge power and current
• Balance the charge across cells
• Provide information to the user about State of Charge (SoC) and State of Health (SoH) and operating parameters
• Send alerts and if necessary put the battery into safe mode if certain critical thresholds (e.g. over-charge, deep discharge, operating temperature limits) are breached.

Battery module monitoring and balancing

It is common to have more than one battery module connected together. In this configuration, there will be a “master” battery module and one or more “slave” battery modules. The master battery has the Battery Management System incorporated into it. All the different battery modules that are connected together must operate as a single battery. Therefore, the power will be drawn equally from all the connected batteries, ensuring that they all have the same state of charge at any one time. To fail to do this will dramatically shorten the lifespan of the individual modules.

Where should I install my battery?

The battery unit should be installed in a ventilated area with nothing that will restrict airflow around the unit. This is to ensure optimal temperature and prevent overheating. Typically, they are installed in utility rooms, hallways and garages. We do not install batteries in loft spaces for practical and safety reasons – a typical battery can weigh somewhere between 50 – 100 kg.

Typically, a battery will operate best at a temperature range of 15°C to 30°C. Batteries don’t like extremes of temperature.

Can I install my battery outside?

Yes. Some batteries are rated IP65, which means that they are suitable for external locations. IP65 specifically, means that its able to protect against water jets. Water projected by a nozzle (6.3mm) against the enclosure in any direction will have no harmful effects.

However, many still manufacturers state that the battery needs to be placed in a “suitable” external location. Undercover, but still outside is highly suitable, whereas a location fully exposed to the elements is unlikely to be suitable.

Note that the operating temperature of the battery needs to be adhered to. In sub-zero temperatures the battery will stop functioning to protect itself. Some batteries can still be used in low temperatures, where they have a self-heating mode (which obviously uses power).

Ultimately installing the battery externally is not suitable everywhere, and always leads to a loss of performance compared to an installation in a dry, ambient temperature room.

UPS – Uninterruptible Power Supply

Some Home Storage Battery systems offer UPS as a feature. UPS means that in the event of a power cut, the inverter will automatically kick in to provide power to your home. The systems will rarely provide power to the whole home. It will probably require additional electrical work to move certain power circuits on your consumer unit onto a new supply, so in the event that the UPS is triggered, that new circuit receives power. Normally you might choose to place some lighting circuits and some plug sockets onto the new consumer unit.

In some areas of the UK, you might be more prone to power outages. Perhaps you live in a rural area, and in winter your power supply might be vulnerable to disruption. In this case, a UPS arrangement might be very welcome. When tied to a solar array, it will enable you to be partially off-grid, even in winter.

However, if you live somewhere which rarely succumbs to power outages, then the UPS feature is unlikely to be warranted.

At Brimstone Energy, we’re here with you every step of the way, as we enter the Green Energy Revolution.

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