Batteries
Batteries come in many shapes, sizes and technologies, however the chemical reactions that generate the electrical currents are normally limited to 2 or 3 volts (cell voltage). So, to make up useful power voltages (12V, 24V and 48V are the most common), the cells are connected together in series, to make a battery.
The chemicals involved in the reactions are pretty much unlimited with each combination offering different performance advantages and disadvantages. Lead acid and lithium are probably the most well-known although there are around seven different types of “lithium” battery chemical combinations, and so not all lithium batteries are alike.
Details on individual battery chemistries are best sourced elsewhere.
Battery Monitoring Systems (BMS)
As mentioned above, batteries are made up of arrays of cells linked in combinations of parallel and series connections. The individual cells have very low internal resistances hence can deliver high currents, however very small difference in internal resistance between cells can result in large current differences and thus some cells work much harder than others.
This can lead to premature cell failure so for example a 12V lead acid car battery is made of 6 cells. A single cell failure gives a 10V battery which is no good for a 12V system, and so, even though 5 cells are still perfectly usable, the battery has failed.
A BMS system monitors and adjusts the currents between each cell thus the cells work more evenly significantly reducing the premature failure of a single cell thus increasing service life.
The BMS can also help protect against overcharging and deep discharging, which also helps to prolong battery life. Some BMS’ have communication options to allow monitoring of battery condition and charge.
Cables, isolators, meters MCB’s RCBO’s ancillary hardware
To make up an installed PV/battery system there are other components for connections, isolations and current protection and if you are interested in these aspects, please do not hesitate to contact Solar UK Limited for details.
Battery Storage Systems
Principal operation
The current flow, to and from the grid, is monitored by the amp meter. If PV power is being generated above what the household is consuming, the excess would normally flow back to the grid. However, the amp meter detects this and instructs the battery system inverter to charge the battery bank. Conversely, should the amp meter detect current flowing from the grid, it will instruct the battery system inverter to discharge the batteries.
If the household load exceeds what the PV and batteries can supply, power will be drawn from the grid in the normal manner, likewise if the batteries are fully charged excess PV will go back to the grid.
In the event of grid failure some battery system inverters can isolate the non-priority load but use the batteries to keep the priority loads (UPS – uninterrupted power supply) “live”. This means any household load on the protected supply will continue as long as there is battery power, which, with the right hardware options, can also still be topped up from the PV.
This will give you an advantage of using your PV system to generate power during a power cut and also power critical loads i.e. freezer, heating controls, wifi, lights etc.
Battery system power and capacity
A battery storage system has two critical sizing parameters, one is the power of the system. This is the amount of power the system can charge and discharge at, measured in kW or AMPS, for example a system utilising a standard 13 Amp plug connection is limited to 2 kWp. So if the household is drawing 3kW of power i.e., 300 Watts of parasitic power for TV, fridge, WiFi router etc., and someone turns on a 2.7kW kettle, then the system can only supply 2kW even if the batteries are fully charged. In that instance the additional kW needs to come from another source, which could be PV if it is available or from the grid. However, a 3kWp battery system would still be able to provide ALL the energy thus saving the expense of the grid top up.
The same principle applies for charging the batteries, if the household has a 3kW PV system and a 2kW battery power package then during full sunshine the battery can only charge at the 2kW rate and so, potentially, 1kW of solar energy is lost to the grid whereas the 3kW battery solution would still save ALL the PV energy for use in the household.
The second critical parameter is the battery storage capacity. This is a measure of the amount of energy that can be stored in kWhours or AmpHours. This value can be difficult to compare as different battery technologies and different manufacturers use different specification measurements.
The charge and discharge rate, the temperature and the battery type all affect the absolute capacity, and the usable capacity is always less as fully discharging (known as flattening) a battery does significant damage, and the lower the discharge on each charge/discharge cycle reduces the battery life. This also affects the warranty periods, and the life of a battery is nominally determined as when the available capacity has reached 50% of the original usable capacity.
DC-coupled (hybrid inverter) battery system
Because the PV panels output DC and the batteries are DC, a hybrid inverter can take power from both PV and batteries thus eliminating the need for a separate battery inverter. This gives a cost saving however it does limit you to a string inverter PV system and restrict upgrade options.
Also being at the budget end of the market some hybrid inverters do not offer UPS supplies or grid charging options. You are also limited to the power output of the inverter so even in bright sun and fully charged batteries, you can only pull the rated power of the inverter, which means that any additional household loads will have to pull grid power.
AC-coupled inverter battery system
An AC-coupled battery system is a standalone battery and inverter combination, you don’t actually need to have a PV system as you can charge off the grid using cheap rate and discharge to offset peak rate costs. Obviously, using PV energy is the best option and the PV system isn’t limited to a string inverter.
Because you can pull power from the solar as well as the battery the maximum load the household can pull before resorting to grid power is the total of both inverters, so the short-term switching-on of a kettle, with dishwasher already running, is still on free energy.
Systems are much more flexible and normally offer grid charging, UPS outputs and upgrading of inverter or battery capacity as standard.
Uninterrupted Power Supply (UPS)
In normal household electrical supplies every single socket is connected to the incoming supply so putting power in would allow power to go out onto the grid and in the event of a power cut this would cause massive safety issues, particularly for the engineer up the pole twisting wires back together and the sun comes out; they’re not going to thank you.
Hence all grid connected PV inverters shut down with loss of mains power. Any household device you wish to power “off grid” will need approved isolation, hence will need to be separated from other household circuits.
Some battery storage options have this separation either built in or available via additional hardware and so offer a permanently live supply (all the time there is either mains and/or power in the battery) The switchover is normally seamless but is limited to the power rating of the inverter.
A typical PV and battery schematic
