Understanding Battery Specifications and How They Apply to your Inverter

posted by Patrick Fallon @ Don Rowe

An inverter needs a battery in order to provide the required AC power for your household devices. There is a wide range of batteries available on the market and they are labeled with a variety of different specifications. These specifications can seem like a mystery and are often misinterpreted, especially in an inverter set up. Let’s take a few moments go over these specifications and explain how to calculate the specifications that apply to an inverter system.

Cold cranking amps are useful, and pretty much the automotive industry standard, for measuring a battery’s capability to start a vehicle. However, the CCA specification cannot be used for determining inverter runtime, and here is why.

Cold cranking amps is a measure of how many amperes a new, fully-charged battery can deliver for 30 seconds, at 0°F, while maintaining a terminal voltage of at least 1.2 volts per cell (7.2 volts total on a 12 volt battery). The first major issue with using this rating for inverter use is the terminal voltage rating of 7.2V. Let’s say that a battery can produce 300 DC amps for 30 seconds, while maintaining 7.2V. While this is great for starting a cold engine, it is not for running an inverter.

An inverter usually shuts down around 10-10.5V, so you can see that 3V is a substantial difference on a 12V system. Second, it is rare that an inverter is only run for 30 seconds between charging. That being said, there is simply no calculation for applying how long your loads will run based off cold cranking amps.

Reserve capacity is the amount of minutes a new, fully charged battery can continually produce 25 amps, at 80°F, until the voltage reaches 10.5 VDC. This specification is more applicable to an inverter installation due to the 10.5 volt cut off.
There are two good ways to go about determining battery needs. First, you can determine how long your appliance will run on your battery. The second method is to figure out how much capacity you will need to run a load for a predetermined amount of hours. The issue with reserve capacity is there is not an efficient way to make either of those calculations, so I do not recommend using that rating.

Amp-hour rating is the best available specification we have to work with, although it is not without special considerations, but more on that later. The first thing that you want to do when using the Ampere-Hour rating is to convert your wattage and runtime requirement into watt-hours, and then to amp-hours. Let’s say that you want to run a 500 watt refrigerator for 10 hours during a power outage. First, multiply your wattage by the required runtime. This gives you 5000 watt-hours. Now, convert that watt-hour number to amp-hours by dividing it by 10 for a 12V system. This comes out to 500 amp-hours. The reason we divide by 10 is that we need to get the amount of amperage at 12V, while accounting for the minimum 10% efficiency loss during the inversion process. Now that we know that the 500 watt refrigerator is going to consume 500 amp-hours during those 10 hours of cooling cycle. Finally we need to multiple that 500 amp-hours by 2 and that will give you your battery need of 1000 amp-hours. We double the original calculation because an inverter is going to shut off around 10.5V, which is roughly 50% discharged. Now we have a method for converting our real world wattage needs to a given amp-hour battery spec, however that is only half the battle.

Take notice that amp-hour ratings have a duration associated with them. Be sure to get that duration before going any further with your battery calculations. An amp-hour rating at 20 hours is the most common rating. That means the rating is good for a discharge taking place over a 20 hour period of time, and your amp-hour capacity is reduced if the discharge occurs in less time. This is referred to as Peukert’s law.

It is important to keep in mind that as useful as these ratings can be if applied correctly, battery capabilities are based on a chemical reaction involving several factors within the battery. In other words, it is not a simple linear calculation. A heavy load can cause an instant low voltage shutdown due to the resulting amount of internal resistance. This instant drop in battery voltage can cause some confusion because after the shutdown occurs, the heavy load is essentially removed and the voltage rises right back up. This creates the illusion that the battery is “fine” and the inverter is the issue, but in reality the battery is incapable of handling such a heavy load. I recommend that if your desired runtime is less than 3 hours you double your amp-hour calculation, and triple it if the runtime is less than 1 hour.

If you do run into problems ALWAYS check your connections, make sure your battery is completely charged and have the health of your battery tested. Battery maintenance is essential for ensuring longevity and peak day-to-day performance. Undersized cables, extreme temperatures, fluid levels and sulfation build-up can all hurt battery performance.

Without the correct amount of battery preparation, an exciting inverter set up can quickly turn into a frustrating experience. Hopefully you are now equipped with a better understanding of how batteries work, and in specific, how battery ratings apply to your inverter. Use the formula “(wattage) x (runtime) / 10 x 2” to get your amp-hour requirements and remember to over calculate for those heavy loads and short run times.

Original Article Link: Don Rowe Blog