Backing Up a Battery

How much do you know about battery run time?

Do you know how long your backup batteries will last?

When SHTF, it’s important to know that you’ll have power to get through the dark times, as well as how to get power when the grid goes down. That’s why it’s important to know the battery run time of your backups. Check out the article on battery run time by energy expert Robert Brenner below.


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Battery Run Time: How Long Will That Backup Work?

The 12-volt battery provided DC to my inverter causing its cooling fan to hum nicely as it converted DC to AC for the DVD player running a recent movie. Watching the player operate, I began to wonder how long my charged battery would continue providing power like this. Soon I was deep in research on battery run times and visited a number of fascinating and interesting websites. Along the way I found Peukert’s Law, Ragone and the reality of battery backup.

Backing Up a Battery

With my creative and inquisitive juices active and eager, I shut down the system and replaced the 12 volt 35Ah battery with two 6-volt 3.5Ah batteries connected in series to supply 12Vdc. Then I energized the 400W inverter and smiled as its internal fan immediately started up. Plugging in the portable DVD player into one of the inverter’s AC output sockets I was pleased to see the DVD player energize and begin playing a test sample DVD movie.

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Then I sat back and monitored the operation of the player as DC power was draining from the battery and being converted to AC to run the player.

Current in—current out. Or so I thought. At face value a 3.5Ah 12 volt battery system should produce 1 amp of DC for 3.5 hours before the battery becomes discharged. Instead, after 2 hours and 45 minutes a low power note appeared on the screen and the inverter shut down. What happened? Obviously the charge in the dual battery system had decreased to a point where the battery could no longer drive the inverter. Perhaps I just needed a beefier battery.

I replaced the two 6V batteries with the 35Ah 12V standalone battery. Connecting the clips from the inverter input to the battery posts I was immediately rewarded with inverter activation. The DVD player was soon restarted and I began playing a DVD movie as shown in Figure 1.

Battery Run Time: How Long Will That Backup Work? by Survival Life at http://survivallife.staging.wpengine.com/2015/05/27/battery-run-time-backup/

DVD player operating off a 12V battery and an inverter.

Now how long will this configuration run? I decided to run the player in continuous replay so the movie would repeat. Then I began logging the operation so I could develop a time line of system performance. The player operated perfectly until the 13th hour. Then the inverter shutdown with 9.75 volts measured across the battery terminals. When I disconnected the inverter removing this load, the battery voltage read 10.75 volts.

The question remained. Why did a 3.5Ah battery run the DVD player for 2.75 hours and a 35Ah battery drive the DVD player for 13 hours. I needed more information.

I had just purchased a new 35Ah 12-volt battery so I decided to see how this device would perform right out of the box (like I did with the LED flashlights in an earlier article). I connected the new standalone battery in place of the old standalone battery—same maker, same size. Then I restarted the continuous operation test. This time I monitored both the time and the battery voltage. Figure 2 shows how voltage decreased over time as the battery discharged.

Battery Run Time: How Long Will That Backup Work? by Survival Life at http://survivallife.staging.wpengine.com/2015/05/27/battery-run-time-backup/

Battery discharge voltage curve.

Wow! This was interesting! The new battery in Test 2 ran continuously for 21 hours. At shutdown, the battery voltage had reduced to about 10.5 volts like the first standalone battery. But why did the old 35Ah battery run for 13 hours and the new 35Ah battery run for 21 hours? I suspected charge level. The old battery must not have been charged to the same voltage level as the new battery.

As I continued my research, I discovered that battery discharge depends on a number of factors including the age of the battery, the number of discharge-charge cycles it had already experienced, the materials used to make the battery, ambient temperature and the load placed on the device. All of these can affect the efficiency of battery performance—the ability to provide DC power for driving electrical devices (including a DC-to-AC inverter and the DVD player). A helpful and informative site is http://batteryuniversity.com

Battery manufacturers have studied battery capacity for years and developed yardsticks for evaluating battery performance and explanations for why a 35Ah battery doesn’t provide 1 amp of DC current for 35 hours before becoming fully discharged. They settled on Amp-hour, a capacity rating defining full battery discharge in 20 hours based on the power withdrawn.

Each battery sold has an Amp-hour rating stamped on its case. But actual performance will be less based on a constant used in calculations for capacity and discharge time. This constant is called Peukert. It’s an exponent in a mathematical equation. An ideal battery with a Peukert factor of 1 should operate according to the Ah rating, but in real life, the Peukert constant, k, is between 1.1 and 1.6 depending on the type of battery. Lead acid batteries typically have Peukert values between 1.3 and 1.4 so discharge time is less than the ideal condition.

Battery specs often include a chart comparing Amp-hours and Amps based on the Peukert constant. The curve in Figure 3 shows the true battery capacity.

Battery Run Time: How Long Will That Backup Work? by Survival Life at http://survivallife.staging.wpengine.com/2015/05/27/battery-run-time-backup/

Battery capacity curve based on the Peukert constant.

You technogeeks can calculate the Peukert constant by accessing these three sites: http://green-trust.org/peukert/, http://www.smartgauge.co.uk/peukert2.html, and http://www.smartgauge.co.uk/peukert_depth.html

Another chart that helps understand run time is the Ragone plot shown in Figure 4. This compares discharge power in watts with watt-hours of runtime. Figure 4contains diagonal lines that represent various runtimes in minutes or hours.

Battery Run Time: How Long Will That Backup Work? by Survival Life at http://survivallife.staging.wpengine.com/2015/05/27/battery-run-time-backup/

Ragone plot of battery run time.

Without going into the complexities of battery construction and operation, my actual operational measurements gave me enough run time information to move forward. Getting continuous power out of a 12-volt battery for 13-to-21 hours was not too shabby. I could have recharged the original battery and run the test again. Nevertheless I now had a range of actual run times that I could use in my backup power plan. In addition, the more charged I kept my batteries, the longer they would serve my energy needs.

Since my notebook computer draws about the same current as the DVD player, the battery-inverter-AC load design is an ideal solution for continuing my computer work while grid power is out. And it’s a simple solution to backup electricity.

The next sunny day, I recharged the batteries using solar panels and a charge controller. Then I was ready for the next power outage.

This article was not written to show you how to calculate battery values including Peukert constants. It was written to show you that you can conduct your own battery runtime tests so you know how long your own battery backup system will likely work if the grid goes down in your area. Figure 5 shows how your laptop could be configured to determine runtime.

Battery Run Time: How Long Will That Backup Work? by Survival Life at http://survivallife.staging.wpengine.com/2015/05/27/battery-run-time-backup/

System configuration for determining your own battery run time.

You could connect your computer or other device to an inverter fed by your battery and see for yourself how long your backup battery will keep your system functioning. This then defines how long you can be productive during a power outage. And you’ll never again worry about a grid-down work loss. Here’s a situation where knowledge is power—electrical power. Next, let’s look at charge time and how long it takes to recharge your battery.

Want to know more? Check out these related articles:

Make Batteries Last Longer

Get Your Home Off the Grid with This New Battery

How to Charge Your Phone with a 9V Battery

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