There are two scenarios to consider when charging the battery while the inverter generates alternating current to the loads connected to the inverter. A solar panel array can charge the battery via a charge controller, or the battery can be charged by a battery charger connected to the grid.
When connected to a solar panel via a charge controller, the inverter can draw DC from the battery bank for as long as the DC input for the solar panel is sufficient to maintain the battery state of charge. The inverter will stop working when the battery has reached its disconnect state of charge.
Charging the battery from grid AC while using the inverter to generate AC to power the connected devices is possible. Still, caution should be taken not to allow the charger to overheat. Let’s consider all the possible permutations:
- The inverter is running from a battery being charged by a solar panel via a charge controller.
- The inverter runs from a battery being charged by an AC grid-powered battery charger/rectifier. Input current to the battery is equal to inverter current draw.
- The inverter runs from a battery being charged by an AC grid-powered battery charger/rectifier. Input current to the battery is lower than the inverter current draw.
- The inverter runs from a battery being charged by an AC grid-powered battery charger/rectifier. Input current to the battery is higher than the inverter current draw.
Let’s look at all four scenarios and discuss what would happen in each case.
Solar Powered Battery Charging While The Inverter Is On
Solar Power Systems are designed to allow the inverter to be running while the battery bank is being charged via the charge controller.
If the battery bank is large enough to house sufficient Watt Hours (Wh) of power and the solar array is large enough to build up and maintain a sufficient state of charge during the day to supply power to the inverter for powering loads during the charge cycle and after that.
The inverter can produce AC from the battery for as long as the battery state of charge can be maintained between the low voltage disconnect charge and near full charge. Lead-acid batteries can only be discharged to a 50% state of charge to avoid damage to the battery chemistry.
Li-ion batteries have a much larger effective operational state of charge range and should ideally be run between 15% to 85% state of charge to obtain the best possible battery life.
Grid AC Powered Battery Charging While The Inverter Is On
In this case, there are three possible scenarios that all require special attention to be given to the battery charger. The inverter will happily generate AC power drawn from the DC battery bank while an AC grid-powered battery charger is charging the battery bank.
Scenatio1: Battery Charger Amp Input To Battery Is Equal To Inverter Amp Draw
Assume you have a 500W inverter connected to a 105 Ah 12V battery, and the inverter supplies the maximum 400W to the AC-powered devices (400W/120V=3.33A). The battery can supply this 3.33A of 120V AC for a total of 15.76 hours before the battery state of charge reaches the cutoff level of 50%.
If we can charge the battery with 3.33A of 120V AC via a battery charger, we will maintain the battery state of charge at a constant level within the operational state of charge range between 50% and 100%.
The problem with this scenario is that the battery charger will be running continuously and will most likely overheat and fail.
Scenario 2: Battery Charger Amp Input To Battery Is Greater Than Inverter Amp Draw
Assume we are charging the battery with 5.00A of 120 V AC via the battery charger while the current draw from the inverter is only 3.33A. Then we are recharging the battery faster than the inverter is depleting it.
The battery charger will maintain the battery charge at a near full level but will never switch off as the battery will never be fully charged due to the continuous draw of current from the battery.
The battery charger will slow down the charge rate as the battery nears full charge to prevent overcharging and damage, and hence the battery will never become fully charged.
Similar to scenario 1, the battery charged will eventually overheat and fail.
Scenario 3: Battery Charger Amp Input To Battery Is Less Than Inverter Amp Draw
In this scenario, we are only charging the battery with 3.00A while the inverter continuously draws 3.33A to power the connected AC devices. Similar to scenario 1, the battery charge will deplete until the 50% state of discharge cutoff is reached and will switch off the inverter.
The battery charger will continue to charge the now depleted battery. Still, you will have to wait until the battery is again sufficiently charged for switching the inverter back on again.
You will have AC power from the inverter disrupted and potentially also run the battery charger to overheat.
Does It Make Sense To Charge A Battery With The Inverter On?
When you are using an Inverter Battery system as an Uninterruptible Power Supply (UPS) to protect your AC-powered appliances from power spikes and disruptions from grid power or to overcome short duration power outages, it makes a lot of sense.
The AC grid power is used to charge the battery via a rectifier circuit to maintain the battery charge with the battery’s operational state of charge range. The inverter then used the DC supplied by the battery to produce pure sine wave AC of a constant voltage and frequency.
The Inverter Battery acts as a filter to clean up the “dirty” grid-supplied AC and build up a reserve of backup power in the battery bank if the grid AC is completely disrupted.
The battery bank size will determine how much backup power the system will have to maintain AC power output during the grid outage. In such applications, the modern Lithium-Iron-Phosphate (LFP) batteries with a broad operational state of charge range will be able to do the job.
Lead-acid batteries only have an effective 50% depth of discharge range and will not be capable of maintaining such a UPS without requiring a lot of battery maintenance.