Battery Parameters

Voltage

This is simply the voltage the battery supplies. It is common to find 6 V and 12 V batteries, but they can be various voltages.  This is important to us because the battery voltage will tell us how many batteries we need to hook in series to achieve the input voltage of the inverter. 

If you buy 12 volt batteries, and you are supplying 48 volts to the inverter input, you will need to hook 4 batteries in series to get the proper input voltage.  You will also have to add batteries to the array in groups of 4.

Suppose we find a battery such that 6 of them would meet our storage needs.  It is a 12 volt battery.  Eventhough we only need 6 batteries for storage purposes, we would have to install 8 batteries because we can only install batteries in groups of 4.

`"Bn" = "Vin"/"Vb"`

`"Bn" = (48 "volts")/(12 "volts")`

`Bn = 4`

Energy

This is how much energy the battery holds when fully charged.  It is often expressed in amp-hours, but watt-hours is a more appropriate measurement.  It is easy to find the Wh rating of a battery by multiply its Ah rating by its voltage.

A 100 Ah battery has enough energy to supply its output voltage to a load drawing 10 amps for 10 hours.  It can also provide its output voltage to a load that draws 100 amps for an hour, or 1 amp for 10 hours.

A 12 volt, 100 amp-hour battery can supply 1200 watts for an hour, or 100 watts for 12 hours.

A 12 volt, 100 amp-hour battery is also a 1200 watt-hour battery.

Depth of Discharge (DoD)

With every discharge of a battery, its lifetime is shortened.  The deeper the discharge, the less the battery can store efficiently.  This is important because discharging a battery too much can rapidly destroy its life.   

For instance, a lead-acid battery that is operated above 25% discharge has a life expectancy of  900-1000 discharge cycles.  In solar energy storage, that amounts to 900-1000 days since the batteries are charged once per day.

If you increase the discharge depth to 50%, those same batteries are only good for 350-450 cycles.

If you increase it even further to 100% discharge depth, those batteries are only good for 150 cycles or less.

These DoD numbers are typical for flooded lead-acid batteries.

By the same token, Lithium Ion batteries have a different DoD curve.  

Different battery technologies have different curves and different prices etc.

For this reason, selecting batteries can complicate the design of the battery array.