How Long Will a 12V Deep Cycle Battery Last With an Inverter
Deep cycle batteries are commonly used for running power inverters in the off-the-grid applications, in emergencies, on the boats, in cars, trucks, RVs, etc.
Knowing how fast will power inverter drain the battery is very important when planning the installations and sizing the required battery.
Little Bit of Theory
Deep cycle batteries are used to provide required electrical power (DC: 12V, 24V, 36V, rarely more) which is than converted into AC 110/220 volts, although most modern power inverters also feature USB charging ports for direct charging of cell-phones, tablets, and similar 'smart' devices.
When calculating how long will a deep cycle battery power an inverter, there are few things to consider, including:
- power inverter efficiency ranges between 85 and 90%, rarely more. That means that if the inverter is rated at 1000 Watts, and its efficiency is 90%, it requires ~1100 watts of power:
P (input watts) = P (output watts) / Efficiency = 1000 W / 0.90 = 1111.1 W
Now, in order to calculate how many Amps does a 1000 Watt inverter draw, we have to divide input power with input volts:
I (Amps) = P (W) / U (V) = 1100 W / 12 V = 91.66 Amps
So, if we have 92 Ah deep cycle battery, it will be able to power 1000W inverter with 90% efficiency for 1h, right? Wrong! :)
- nominal capacity vs actual capacity: Nominal capacity of the lead-acid batteries is measured when the battery is being discharged for 20h. If the battery is being discharged faster, its actual capacity decreases, and when the lead-acid battery is being discharged for 1h, the actual capacity is around 50-72% of nominal capacity, battery dependent.
Note: most deep-cycle batteries feature 1h actual capacity around 55-60%, only premium lead-acid batteries feature 1h actual capacity of 65-70%, and such batteries tend to be rather costly.
So, if you need a lead-acid battery with actual 92 Ah capacity, go for deep cycle battery with the capacity of at least 135-150 Ah nominal capacity.
- nominal voltage vs actual voltage: nominal voltage of the lead-acid batteries is 12V, but the actual voltage of fully or almost fully charged lead-acid battery is ~13.5 volts. As the battery is being discharged, actual voltage drops, and when it reaches ~10.5 volts, the battery is 'almost discharged'. we say 'almost discharged' because when the lead-acid battery is being discharged faster, it tolerates lower end-voltages.
Note: most power inverters intended for 12V batteries feature cut-off voltage in the 9.6 - 10.2 volts range, with low voltage alarm ins the 9.9 - 10.5 volts range.
At first, ever-changing capacity, voltage, and current complicate things, however, most battery brands provide constant current and constant power discharge tables of their batteries.
Since advanced power inverters accept different voltages thanks to built-in DC/DC and DC/AC inverter circuits, the best option is to check the battery's constant power discharge table and to find out how long it can provide a certain amount power.
For example, very popular Renogy 12V 100Ah Deep Cycle Pure Gel (RNG-BATT-GEL12-100) battery features a nominal capacity of 100Ah, but if it is used to power 1000W inverter with 90% efficiency, it is able to power such unit for at least 30 minutes - the following chart lists constant power discharge values for Renogy RNG-BATT-GEL12-100 battery, given in Watts, measured at 77°F (25°C):
|5 min||10 min||15 min||20 min||30 min||45 min||60 min||90 min||2 hours||3 hours||5 hours||10 hours||20 hours|
As one can see, new, fully charged Renogy RNG-BATT-GEL12-100, measured at 77°F (25°C) is able to provide 1116 watts for 30 minutes down to 10.8 volts - when the battery is being discharged down to 10.8 volts for one hour, it is not being 100% discharged, ensuring a larger number of charging/discharging cycles.
If one has to power, for example, 2000W pure sine inverter with 90% efficiency, it requires ~2200 watts from the battery.
Renogy RNG-BATT-GEL12-100 battery is able to provide 2200 watts for ~10 minutes, down to 10.5 volts.
However, if this 10 minutes is too short, adding the second Renogy RNG-BATT-GEL12-100 battery in parallel will triple the runtime (~1100 watts each battery, ~30 minutes, down to 10.8 volts).
What if the constant power discharge tables are not available?
If the constant power discharge tables are not available, it is safe to assume that the voltage is a constant value of 12 volts and that ~30 min actual capacity of the battery is 50% of the nominal capacity.
That means that the average 12V 100Ah deep-cycle or dual-purpose battery may power 1000W inverter for:
T (h) = 12V * 100Ah * 0.50 / 1100 watts = 0.545 h = ~32 minutes
As one can see, this 32 minutes for 'an average 100 Ah battery' is very close to the 30 minutes for very balanced gel-cell Renogy battery - just note that in this example, Renogy battery is being discharged down to 10.8 volts, and 'an average 100 Ah battery' will probably be discharged to or slightly below 10.5 volts in order to provide ~1100 watts for 30-32 minutes.
For short: Renogy RNG-BATT-GEL12-100 is above the average deep-cycle battery. And again, this was just an approximation - for example, we assumed actual capacity before we new the runtime!
Anyway, here is the list of few more, very popular deep-cycle batteries that may be used as excellent inverter batteries, just be sure to dimension your battery pack properly:
Constant power discharge characteristics of the WindyNation BAT-NSAP12-100 are given in the following chart (in watts, at 77°F/25°C):
|End Battery Voltage||End Cell Voltage||10 min||15 min||20 min||30 min||45 min||1 hour||2 hours||3 hours||5 hours||10 hours||20 hours|
For example, WindyNation BAT-NSAP12-100 is able to power 1000W inverter for 30 minutes, but it will be discharged down to 10.5 volts, which is still acceptable for 1h discharge rate, but not as good as 10.8 volts of Renogy 12V 100Ah gel-cell battery.
Also, it is able to power 2000W inverter for ~10 minutes, but down to 10.2 volts.
Battle Born 100 Ah 12 Volt LiFePO4 Deep Cycle Battery
Battle Born 100 Ah 12 Volt LiFePO4 Deep Cycle Battery is one of the best lithium-ion batteries on the market.
Unlike lead-acid batteries, lithium-ion batteries' capacities don't decrease as rapidly as the capacity of the lead-acid batteries.
For example, Battle Born 100 Ah 12 Volt LiFePO4 Deep Cycle Battery is able to provide 100 Amps constantly for almost one hour, depending on the battery condition.
So, if this battery is connected to 1000W inverter, it will provide 1100 watts (12V, ~92 Amps) for slightly more than 1h, which is practically double the time of the lead-acid batteries of the same 20h capacity.
On the other hand, Battle Born 100 Ah 12 Volt LiFePO4 Deep Cycle Battery costs several times more than lead-acid batteries and tolerates 5-10 more charging/discharging cycles - it costs more at first, but saves money (and weight) in the long run.
Note: Battle Born 100 Ah 12 Volt LiFePO4 Deep Cycle Battery features maximum allowed continuous current of 100 Amps, and the maximum 30 seconds surge current of 200 Amps and as such, it should not be used for power inverters stronger than 1000 watts. Unless connected in parallel, which this lithium battery allows, thanks to its excellent Battery Management System (BMS).
Long Story Short: if you intend to power the power inverters using AGM or Gel-Cell lead-acid batteries for at least 30 minutes, rule of thumb is to use 12V batteries with the capacity in Amp-hours 10x lesser than the power of the inverter, given in watts.
For example: 12V 100Ah battery will be able to power 1000W inverter for ~30 minutes, 12V 200Ah battery will be able to power 2000W inverter for ~30 minutes, etc.
If longer operating times are required, go for either larger battery or connect more batteries in parallel, taking into account how the batteries are connected. Also, if possible, use the same battery model of the same brand from, if possible, the same batch (better safe than sorry!).
When calculating actual runtimes, constant power discharge tables are of great help, but even without them, one can approximate runtime using a few assumptions...
Just be sure not to forget that these ARE the assumptions, while the constant power discharge tables are official data given by the battery manufacturer (and that may change without prior notice!).