Most batteries used for remote power and off grid applications are of the lead acid 'wet' type. The main reason for this is economy, capacity and ruggedness. Lead acids can take the kind of abuse that would destroy lithium ion, or other more exotic chemistries.
In an off grid application weight and space are generally not an issue, so the lower power density of lead acid batteries is not a problem. Off grid battery storage is usually arranged in 'banks' of batteries, and each bank connected to produce either a high voltage or high current output.
But why not have just one big battery?
Batteries have traditionally been supplied with voltages of 12 & 24 volts. This is convenient for trucks, caravans and cars – but not so good when you have a charger that operates at 48V on your renewable energy system.
Each battery is made up of cells, and a convenient physical size has resulted in a voltage of 2.2Volts per cell typically. A 6Volt battery will have 3 cells, so the usual fully charged voltage will be 6.6 Volts. A car battery will have 6 cells giving a fully charged voltage of 13.2 Volts
Now back to our 48V charger, to get 48V we can divide by our 'badged' battery voltage (12V).
48 / 12 gives us 4 batteries. If these batteries are connected in series this will give us our 48Volts. But you don't often see battery systems consisting of just 4 batteries – so what's going on there?
If we saw a system with 12 x 12V batteries then the voltage could be 144V. We know the charger operates at 48V so these batteries must be arranged in groups of 4. Each of these groups of 4 batteries will be connected in parallel to each other and then to the charger. When you connect batteries in parallel the voltage stays the same, but the current is added together.
Let's look at our 3 groups of 4 x 12V batteries rated at 60Amps per hour (Ah).
The voltage is still 48V but the available current is 60 + 60 + 60, in other words we now have 180Ah available – or to put it another way 8.6kWh @ 100% DOD (depth of discharge) however you should never discharge a lead acid battery beyond 50% DOD on a regular basis so in reality our 8.6kWh is now 4.3kWh, probably more like 4kWh once we have taken system efficiency losses into account but that is for another time.
An important consideration when installing batteries is to make sure they are all fully charged before they are connected together. If the voltages are unequal current will flow between the batteries causing inefficiency and possible damage to the battery with the lowest state of charge. This is because most chargers treat the battery store as one big battery and are unable to tell the 'state' of individual batteries. Some chargers will however be able to monitor battery temperature and modify charge rates to the whole battery bank accordingly.
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