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Clearing up the question of battery capacity in electronics

By: Goal Zero Editors

Clearing up the question of battery capacity in electronics

Because mobile electronics are so pervasive, it is important for users to understand how to compare batteries. While most people are familiar with, for example, “gigabytes” as a measure of hard drive space and have a more instinctive appreciation for phrases like “4G speeds,” the nomenclature used to gauge the amount of energy capacity in a battery is still a bit of a mystery. This is exacerbated by some outdated industry practices that are throwbacks to when a single battery technology dominated any given energy sector. Most companies use capacity measurements that aren’t consistent from one battery technology to the next. So how do you compare and choose your battery pack? We’ll try to unshroud some of the mystery here.

How industry makes measuring battery capacity confusing

Most products show battery capacity measured in “amp-hours” (Ah) and milliamp-hours (mAh), where 1000mAh = 1Ah. Amp-hours is a measure of “current capacity”, not “energy capacity”. This is a subtlety, but it is important. Amp-hours only tells half the capacity story. What is an amp-hour? An amp is a measure of electrical current, and the hour indicates the length of time that the battery can supply this current. A 2.2Ah battery can supply 2.2 amps for an hour. If the battery must supply more amps, because a bigger device is connected, it will last for a shorter period of time; if the battery is supplying less amps, it will last longer. The problem with this method is that it doesn’t give a complete picture of the total energy stored. It is easy to find cases where two different batteries with the same number of amp-hours will have completely different amounts of total energy.

How Goal Zero measures battery capacity

The correct unit for measuring energy capacity of a battery product is called watt-hours. Watt-hours signifies that a battery can supply an amount of watts for an hour. For example, a 60 watt-hour battery can power a 60 watt light bulb for an hour. The same battery would run a 5W phone for 12 hours and power a 1W LED light bulb for 60 hours. Watt-hours is a simple and consistent way to measure the capacity of any battery pack, whether it’s a Nickel Metal Hydride (NiMH), AA battery, the 12V lead acid battery in your car, or the lithium ion battery pack on your laptop. Goal Zero offers battery packs in all of these configurations, and they are easily comparable by looking at the watt-hours rated to each one. What is the difference between watt-hours (Wh) and amp-hours (Ah)? The equation for power is watts = amps x volts and for energy the equation is watt-hours = amp-hours x volts.

Amps is only half of the equation. Since different batteries may have different voltages, amp-hours is only useful for comparing products that have the same battery configuration. If you want to be sure that you’re getting the right results, use watt-hours for an apples-to-apples comparison.

What if I don’t have watt-hours?

To get a good grasp of the actual energy capacity, without access to the watt-hour rating of a pack, you can do a quick calculation to generate the watt-hours. First, find out the Ah rating of the battery pack (if it’s given in mAh, take that number and divide it by 1000 to get the Ah value). Second, find out the battery type or chemistry and then the typical voltage for that type. Here’s a short list of the most common types of batteries and their voltages: 3.7V per lithium-ion cell (cell phones use a single cell (3.7V), laptops typically use 3 cells in series (11.1V) 3.2V per lithium-ion iron phosphate cell (multi-cell LiFePO packs are typically configured as 4-cell or 12.8V) 1.5V per alkaline cell 1.2V per NiMH and NiCad cell 2.1V per lead-acid cell (car batteries are 6-cell (12.6V) and some golf carts are 3-cell (6.3V)) Finally, multiply the voltage (V) by the amp-hour (Ah) rating of the battery/pack to get watt-hours (Wh) and then compare. For example, the Switch 8 Solar Recharger is 3.6V and 2.2Ah (2200 mAh) By multiplying those two together you get 7.92W. Will the Switch 8 charge your iPhone? Well, the iPhone 5 has a 1440 mAh (1.4Ah) battery at 3.7V for 5.18W, so yes.

Did you know that the number at the end of each Goal Zero battery pack signifies the approximate amount of energy (measured in watt-hours) available in that that specific battery? Guide 10= 10Wh, Yeti 1250=1250 Wh, etc. This makes it easy to find the battery pack for your power needs.

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