How Long Will a 100 Amp Hour Battery Last?
Lead-acid and lithium 100 Amp-Hour batteries are commonly used in automotive, marine, industrial, and off-the-grid applications as starting, dual-purpose, and, very often, deep-cycle batteries.
Since they are commonly used in many critical systems (medical, security, etc.), many people wonder how long can a 100Ah battery last under a certain load.
Updated: April 3, 2023.
Number of Cycles vs. Discharge Time
When people wonder how long will a 100Ah battery last, generally, they want to know for how many minutes or hours is a certain battery able to provide power to a specific load.
However, sometimes people also want to know how many charging/discharging cycles can 100Ah battery last with its capacity not dropping below a certain level.
If You wonder about the number of cycles, then:
- Lithium 100 Ah batteries can withstand 2000-7000 charging/discharging cycles when being discharged at 0.1-0.3C rates down to 100-50% DoD.
- Lead-acid 100 Ah batteries can withstand 200-900 charging/discharging cycles when being discharged at 0.05-0.2C rates down to 100-50% DoD.
So, if You are looking for a 12V 100Ah battery that will cycle a lot, go for good Lithium Iron Phosphate (LiFePO4) battery.
When compared with lead-acid batteries, lithium batteries are also lighter, can accept charge faster (model dependent), must be charged using dedicated lithium battery chargers, and also feature Battery Management Systems that protect the batteries from unwanted events like low or high temperature, over-discharge, over-current, over-voltage and similar.
Note: If the lithium battery doesn't come with a reliable BMS, don't buy it no matter how its price may sound acceptable - if mistreated, even a Lithium Iron Phosphate (LiFePO4) battery can overheat, catch fire, or even explode.
On the other hand, if you are looking for a 12V 100Ah battery that will not cycle a lot, its weight is not important, but the price is important, go for a good Absorbent Glass Mat (AGM) or Gel-Cell battery.
100 Ah Batteries Discharge Time
The nominal capacity of the lead-acid batteries is given for the 20h discharge - 100 Ah lead-acid battery can provide 5 Amps for 20 hours with the voltage drop down to 10.5V.
The nominal capacity of the lithium batteries is usually given when the battery is being discharged for 1-5 hours (1h → 1C, 2h → 0.5C, 5h → 0.2C, etc.).
When the lead-acid batteries are discharged with stronger currents, they feature a capacity loss.
For example, a very popular, general-purpose/deep cycle Universal Power Group UB121000 AGM Deep Cycle Battery features a 20h capacity of ~100Ah and a 1h capacity of ~54 Ah.
Note: actual capacity also depends on the battery temperature and acceptable end voltage.
On the other hand, the premium dual-purpose Odyssey 31M-PC2150 AGM Marine Dual Purpose Battery features a 20h capacity of ~100 Ah and a 1h capacity of ~71 Ah.
This 17Ah capacity difference is significant but also is the price difference between these two batteries.
Lithium 12V 100Ah batteries don't suffer from such drastic capacity loss when the discharge current is increased, but they have their maximum continuous currents limited electronically and are usually around 80-100 Amps, with the maximum surge currents around 150-200 Amps for 3-5 seconds (model dependent).
For example, AIMS LFP12V100AB is a 12V 100Ah Lithium Iron Phosphate battery with modern BMS featuring a Bluetooth module for remote monitoring and control.
When AIMS LFP12V100AB and Odyssey 31-PC2150 are discharged for two hours, the AIMS LFP12V100AB battery can provide 50 Amps constantly (~1200 Wh, 100Ah), while Odyssey 31-PC2150 can provide 39.0 Amps (~922 Wh, 78Ah) constantly, down to 10.05 volts.
Note: Odyssey 31-PC2150 battery is also designed to recover well from deep discharges, so when its final discharge voltage in many examples is 10.05V (1.675V per cell) and not 10.5V (1.75V per cell).
General-purpose lead-acid batteries perform even worse. However, if You think that the time of lead-acid batteries is over, wait till You see the price of lithium batteries :)
Reserve Capacity (RC) of the 12V 100Ah Batteries
Reserve Capacity (RC) is the value in minutes that a battery can provide 25 Amps constantly without the voltage dropping below a certain safe voltage (10.5 volts for lead-acid batteries, cutoff voltage for lithium batteries).
The Reserve Capacity of the 12V 100Ah general-purpose lead-acid batteries is usually around 170-190 minutes, and between 190-220 minutes for premium lead-acid batteries.
The Reserve Capacity of the 12V 100Ah deep-cycle lithium batteries is usually around 240 minutes - these batteries are able to provide 25 Amps for 4 hours.
12V 100Ah Batteries as the Power Inverter Batteries
12V 100Ah batteries are often used as power inverter batteries. If they are used to power, for example, 1000W power inverters with 85% efficiency, such batteries must constantly provide ~1180 watts.
In that case:
- UPG UB121000 can power such inverter for almost 30 minutes,
- Odyssey 31-PC2150 battery can power such inverter for 36-38 minutes,
- AIMS LFP12V100AB battery can power such inverter for ~1 hour (60 minutes).
1000W power inverters are able to provide power for various home appliances in the case of emergencies and are very popular in off-the-grid applications.
Also, most of the 1000W power inverters feature surge power in the 1500-2000W range, requiring even more power from the batteries. If you opt for the lithium battery, be sure also to check the surge current rating of the battery.
Even if the 1000W appliance uses a 12V power source, lead-acid 12V 100Ah batteries simply can't provide 1000W for 1.2h since the following example doesn't take into account capacity loss due to the strong discharge current:
T(h) = [U(V) * C(Ah)] / P(W) = [12V * 100Ah] / 1000W = 1200Wh / 1000W = 1.2h = 72 minutes
Again, this is the wrong approach in calculating how long will 12V 100Ah battery last - for actual runtime, always check constant current or constant power discharge tables, or at least check the discharge graphs.
When the required power is decreased, the actual differences between these batteries are not as significant. For example, if the batteries are required to provide ~350 watts (300W inverter with 85% efficiency), then:
- UPG UB121000 is able to power such inverter for ~2 hours easily,
- Odyssey 31-PC2150 battery is able to power such inverter for ~2h 20 min - 2h 30 min, even more since Odyssey 31-PC2150 battery recovers very well from deep discharges (just don't abuse that too often!),
- AIMS LFP12V100AB battery is able to power such inverter for almost 3 hours and 40 minutes.
One of the very important features of lithium batteries is that they keep their output voltage very constant and close to their nominal voltage of 12.8 volts.
Of course, these three batteries are used just as an example - many other batteries on the market have similar features, so be sure to choose according to your own needs and preferences.
Long Story Short: In order to accurately determine how long can your 12V 100Ah battery last, You have to check constant power and constant current discharge charts and/or graphs of the battery model that you have and compare them with your intended use and required power/current.
Also, don't forget that in order to get as accurate time as possible, You have to include battery temperature, age, number of cycles, etc.
And in the end, be sure to have some energy/power left for emergencies, just in case.
Note: whatever You do, stay safe - 12V 100Ah batteries commonly belong to the BCI 27 and BCI 31 battery groups, they are able to store plenty of energy and can provide very strong bursts of electricity ... not something to play with ...