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These days a 500ah battery bank is commonplace in solar powered RVs and homes. But the question though, is how long can you expect that battery to last? Is it all right to use up all the power? In this guide you will learn how to calculate battery life and how long you can expect it to really run.
A 500ah 12V lead acid battery is equal to 6000 watts, but you can only use 3000 watts because it needs a recharge when capacity drops to 50%. In theory this battery can run a 500 watt load for 6 hours (3000 watts / 500 = 6), but due to inverter inefficiency, it will last for around 5 and half hours.
How Long Can You Use a 500ah Battery?
When we talk about 500ah batteries, it can refer to a single 500ah unit or smaller batteries connected to each other. A system consisting of 5 x 100ah 12V Renogy AGM batteries can produce 500ah, so they are applicable in the following discussion.
There are two basic categories of batteries used with solar systems: lithium and lead acid. How long your 500ah battery lasts depends on which you use. Lithium runs appliances longer than lead acid batteries because they have a better depth of discharge rate.
There are two common kinds of lead acid batteries, flooded (FLA) and sealed (SLA). There are two types of SLA batteries, AGM and gel.
Depth of discharge (DOD). Batteries should never be fully discharged because it causes wear and tear, reducing its lifespan. A 500ah 12V lead acid battery has a 50% DOD whereas lithium batteries have a DOD of 80-95%.
So if you have a 500ah 12V battery, you can only use half the capacity before it has to be recharged. Lithium batteries on the other hand, can be used until it is almost empty.
Lead acid batteries are less expensive though which is why they are still around. A compromise between the two is AGM, a sealed lead acid battery with a DOD of 70-80%.
Lithium batteries are still more effective in accepting higher charge rates, but AGM is a good option if you are on a budget. Click here for a detailed comparison of these batteries.
How Long Will a 500ah Battery Run My Appliances?
It depends on the depth of discharge, the power load and inverter efficiency. You can use the following calculations for any appliance or load you want to run.
Suppose you have a 12V 500AH lead acid battery bank and a 1000W power inverter. How many hours can you run a 250W desktop computer on this system?
If you have a 90% efficient inverter, a 250W computer will last for about 10.8 hours on a 500ah 12V lead acid battery. If it is lithium, the runtime is approximately 19.4 hours.
How to calculate 500ah runtime
Step 1. Find the depth discharge and watt capacity of the battery
Step 2. Determine the appliance watts per hour
Step 3. Find your inverter efficiency level
Step 4. Divide appliance watts per hour by inverter efficiency to find the power consumption
Step 5. Divide available battery capacity by power usage
Let us continue using the same example of a 250W computer, a 500ah 12V battery and a 1000W inverter with a 90% efficiency rating.
Battery capacity is measured in amp hours. To find its equivalent in watts, multiply amp hours by voltage. In this case:
500 x 12 = 6000 watts
Because it is a lead acid battery with a 50% DOD, only 3000 watts can be used. You already know the computer consumes 250 watts an hour. On paper it seems straight enough:
3000 / 250 = 12
But here you are running the computer from an inverter, and these are not 100% efficient. This inefficiency increases the power your computer is using. To account for inverter inefficiency:
Appliance power consumption divided by inverter efficiency rating. If your inverter is 90% efficient:
250 watts / .9 = 277.7
Your 250W computer actually draws 277.7 watts from the battery per hour. Now all you have to do is divide the available battery capacity by power consumption.
If your battery is lead acid, its DOD is 50%:
6000 watts x -.5 = 3000 usable watts
3000 / 277.7 = 10.8 (hours)
If your battery is lithium, its DOD is 95-90%. Assume it is 90% in this case:
6000 x .9 = 5400 usable watts
5400 / 277.7 = 19.4 (hours)
How Many Hours Can a 500ah Battery Last?
A 12V 500ah battery should in theory, run a 500W load for 12 hours. However, high power loads cause batteries to lose charge faster. So a 500W load will last less than 12 hours on a 500ah battery.
These are the reasons why. On paper it will look like this:
500ah x 12V = 6000 watts
6000 / 500 = 12
In fact, the more amps drawn the faster a battery will lose power. If an appliance draws 500 watts / 41.6ah an hour for example, the battery will not last 5 hours. It will run out of energy before that.
This can be attributed mainly to Peukert’s Law which states that amp hour capacity depletes faster when higher current is drawn.
The reverse is also true. If the load pulls less than 5 amps an hour, the battery capacity will last longer. As the amp draw increases, capacity drains faster.
We cannot pinpoint exactly how long the battery last. With inverter efficiency, Peukert’s Law and varying battery quality, the energy loss can be anywhere from 2-15%.
The point is, you should always make allowances for battery capacity. You can also take steps to make your battery last longer.
How Can You Make 500ah Batteries Last Longer?
There are three things you can do to make a battery run and be more cost effective. These will work for 500ah and other sizes.
- Use a quality battery
- Get an efficient inverter
- Never run below its recommended DOD (depth discharge)
If you want the best and most efficient battery, get lithium. With a DOD of 90-100% you get maximum mileage every time without worrying about damage. If you have five 100ah 12V Weize batteries, you can run it almost at full capacity.
AGM batteries are a good option too. They are not as efficient as lithium but cost less. The important thing is to buy what fits your power requirement needs.
The biggest difference between lithium and lead acid batteries is the depth discharge. Lead acid batteries have a 50% DOD. That means your 500ah battery bank is only good for 250ah before it has to be recharged.
A lithium battery lasts almost twice that with an 85-95% DOD. Its runtime will be nearly double. If you are going to run a lot of loads on your battery, a lithium or AGM battery is the best option.
One thing you should never do is use the battery beyond its suggested DOD. That will lead to faster wear and tear and could cause other problems.
What Inverter Works Best with 500ah Batteries?
A pure sine wave inverter with 90-95% efficiency is the most ideal. For a 500ah battery bank, the inverter should be at least 1000W.
Inverters have become more efficient so there is no reason to settle for anything less than 85-90%. Some even offer 95% efficiency, and while a 10% difference may not seem much, it can be significant.
For example, you have a 12V 500ah lithium battery bank and want to run a desktop computer and a ceiling fan. The computer is 200W and fan is 60W or 260 watts total.
An 85% efficient inverter can run a 260W computer and ceiling fan for about 16.6 hours on a 500ah lithium battery with an 85% DOD. If the inverter is 95% efficient, the battery will last around 18.6 hours.
For a detailed comparison:
500ah 12V lithium battery with 85% DOD:
500ah x 12V = 6000W
6000 x .85 = 5100 usable watts
For the power load and 85% efficient inverter:
220W computer plus 60W ceiling fan = 260W
260 / .85 = 305.8 watts
On an 85% efficient inverter, a 260 watt load consumes 305.8 watts
5100 / 305.8 = 16.6
The total runtime is around 16.6 hours
If the inverter was 95% efficient, the numbers look like this:
260 / .95 = 273.6 watts
5100 / 273.6 = 18.6 hours
Keep in mind this calculation has not factored in Peukert’s Law, but the point is that a more efficient inverter allows batteries to run faster and longer.