Battery capacity explained - Plus how to calculate how much you really need

It would seem like you can never have too much capacity in a battery, however, increased capacity will often also increase size, weight and cost. So how much energy do you truly need to store without overkilling your needs and/or breaking the budget?

Introduction:

Let's start with a foundation of battery capacity terms and what they mean.

Amp hours (Ah) is a measure of how many amps a battery can supply for how many hours.

Watt hours (Wh) is the TOTAL amount of energy that a battery can hold.

Seems like sort of the same thing just written differently, right? Not exactly. The reason you need to know each of these terms is Ah's will vary depending on the nominal voltage of the battery. Where Wh's being a total energy measure, will be the same across the board, so this can be used to compare the total energy storage of any battery regardless of voltage.

If your battery is only rated in Wh's, you can calculate the Ah's using some simple math. Wh / Volts = Ah

On the other hand, if your battery only has Ah's listed and you want to know the Wh's, the math is just as simple. Ah x Volts = Wh

Example 1: A 12 volt, 100Ah battery would have 1,200Wh of capacity.

Example 2: A 24 volt, 50Ah battery would also have 1,200Wh of capacity.

But you say, "at 50Ah, the second example is half the capacity since it's half the Ah". Not really, the voltage doubled so based on the math, each of these batteries holds the exact same amount of energy. That's why needing to know how to calculate the Wh is so important if you are comparing batteries of different voltages. This situation is extremely common in RV's when people are comparing 6V batteries in series vs regular 12V batteries. Also very common in trolling motor or golf cart applications where you can series connect 12V batteries up to 24V, 36V or 48V. If you're comparing your options in any of these scenarios', you must understand these formulas to make your comparisons fair.

Example: Your bass boat currently has three 54Ah batteries connected in series to power the trolling motor. You are debating simplifying your battery compartment and using just one 36V battery instead. You see on our website that a common 36V option is rated at 63Ah. Those three 54Ah batteries must have more energy than just one 63Ah, right?

The math will tell us that 12V x 54Ah x 3 = 1,944Wh and 36V x 63Ah = 2,268Wh

This single 36V battery still holds more total energy than those three 12V batteries. Don't let the numbers fool you when comparing batteries for RV's either. Do the math and be sure you're making the right choice.

Finding the minimum Ah:

Now that we're all on the same page with capacity terms and the math needed, lets get back to the original question of how to determine how long a battery will last in your application so you can make an educated decision when making your next purchase.

We'll need two pieces of information to make the calculations. The amount of energy the devices you will be running consume, and how long you'll need them to run for between charges. This information is almost always published for electronic devices but it may be listed in amps or watts. Either is fine since we know that watts / voltage = amps.

Basic example:

Let's start with a very simple but common example, a fishfinder that you want to install on a kayak. It's the only device that will be drawing power but you want to make at least 3 fishing trips that will be about 4 hours each before you'll be able to charge the battery again. The fishfinder I Iooked up has a standard current draw of .75 amps with a max consumption of 1.5 amps. Since I want to error on the side of caution, I'm going to use the max draw (1.5A) in my calculation to make sure I don't come up short on my trip.

The math: 1.5A x 12 hours of fishing = 18Ah's required

I calculated a little bit of cushion by using the "max" amp draw for the device but I would still consider this a bare minimum capacity to ensure you're happy with your decision. We do offer an 18Ah battery, but for not much more you can get a 23Ah version and have a little extra juice in case the fishing is good and you want to stay a bit longer. This is where opinion comes in to play, the math will give you a "minimum", you can decide what's right for you from there. Also keep in mind that the deeper you discharge any type of battery, the shorter the lifespan of the battery will be. If you can avoid depleting a battery below 50% (even a deep cycle), it's lifespan will be greatly increased.

Next level example:

You want to boondock in your RV and be able to run the air conditioner without the generator. For this example, we are going to assume you already have a properly sized inverter installed and we are only going to focus on the determining your battery capacity needs. You want to be able to run the A/C unit for 4 hours each evening and then let your solar system charge the batteries the following day. This is an average A/C unit with a 1,500 watt consumption rating. This A/C unit may be running at 115 volts but it will be drawing these watts originally from a 12 volt source so we have to calculate the amps at the 12 volt level.

Step 1: 1,500 watts / 12 volts = 125 amps

Step 2: 125 amps x 4 hours = 500Ah's

Since 500Ah's is the calculated minimum and running any battery to zero on a daily basis would be detrimental to it's longevity, I would want to double that Ah requirement to ensure we aren't taking the batteries past a 50% depth of discharge (DOD) very often. This is of course on the dramatic end of storage needs for an RV but this is primarily an mathematical exercise, not necessarily something that would be right for everybody.

Hopefully this information enables you to make an educated decision on your next battery purchase but if there is anything we can further assist you with, please feel free to contact us with all the details of your scenario and we will be happy to help with the calculations.

GMI Energy

support@gmienergy.com