iPod Battery Pack Tests

After building my Band Aids battery charger for the iPod, I wanted to test how useful it really was. First I ran a baseline test with the iPod fully charged playing through a long playlist of singles from various sources and compression rates, most 128k or greater, and using MP3 or AAC encoding. I played the songs at 50% volume out of the headphone jack and into my stereo. I got 7.8 hours until the battery died completely. This is close to Apple's claim of 8 hours but does not reflect realistic usage where I usually run at 75% volume and often switch songs causing the backlight to light up and making the hard drive run more often. I would say I typically get closer to 4 hours of useful runtime and try never to run the battery all the way down.

Next I wanted to try it while doing occasional recharges with the external batteries. The full iPod charging cycle is 3 hours, but you can get an 85% charge in the first hour, so I decided to do that. Also I didn't like the idea of running the battery down every time so I decided that once the battery got down to one bar I would start recharging. This isn't real scientific because the iPod battery meter isn't that accurate. However, while it is common for the iPod to show itself only about half charged after getting an ample charge, after 10 minutes or so the battery meter gives a much more realistic reading. While still playing music I would recharge for one hour.

The bottom line is I was able to get 27.7 hours of music. I think I could get substantially more time than that using higher capacity NiMH batteries (mine are 1500 milli-amp hour and pretty old). I also think that if I ran the iPod down to no bars I could get at least another hour from each of the 6 run cycles (5 charges cycles in between) and as much as 2 hours if I ran the battery all the way down. So that's 6-12 more hours. Of course maybe the one-hour recharges wouldn't be as effective if I started with a battery that was more depleted (either it would take more power to get the batteries up to 85% or you just wouldn't reach 85% in an hour). Still, 27.7 hours is more than 3 times as much life from the iPod without running it all the way down.

After charging the external batteries I got a reading of 10.9 volts without any load. I didn't run the test continuously, so I usually wound up running the iPod and then charging it at the end of the day and turning it off until the next day. With NiMH batteries I don't think the results would be that different from a continuous test, unlike if I were using alkaline batteries which recover charge when they have a chance to rest.

After each charge the battery indicator would read 4 bars, but would usually drop down to the 3 bars within an hour. It would run at 3 bars for a couple of hours and then at 2 bars for a couple of more hours. When I noticed it was down to one bar I would recharge it.

During the fifth charge the voltage had gone done to 8.7 volts under load and eventually started having problems and causing static in the music and the the charge indicator got erratic (moved very slowly). At that point I measured only 7.8 volts (meaning the minimum to charge a 3G iPod is at least 7.1 volts, though less will work for minis and 4G iPods). So the last charge was only about 30 minutes before I unplugged the charger. Still I got a few more hours of playing after I disconnected it.

Time shown below is the time between charges. Each charge was one hour (except the last one) so there was 4.5 hours of charging time during which I was playing music. Unfortunately I changed how I measured the voltage part way through the test. So I go from measuring before or after the charge to measuring while the charging is taking place. To be really accurate I think you'd have to measure the voltage under load at a certain point in the charge cycle, like say 15 minutes into it. At some point the trickle charge starts which causes less load on the battery.

ChargeRuntimeBattery pack voltage
Initial4.8 hours10.9 volts fully charged without load
1st charge (1.0 hour)4.3 hours10.4 volts without load
2nd charge (1.0 hour)3.9 hours10.15 volts under load
3rd charge (1.0 hour)4.6 hours9.5 volts under load
4th charge (1.0 hour)3.5 hours9.2 volts under load
5th charge (0.5 hours)2.1 hours8.75-7.8 volts under load

Second test confirms the first

About six months later I decided to run another test. I had upgraded the software on the iPod in the meantime so instead of four distinct bars in the battery meter it was just continuous now. I decided that I would start the one-hour charges now when the battery meter got down to a little less than one bar, which looks like it is about 3 or 4 pixels on the screen. The results were very similar although on the last run cycle I ran the battery all the way down and it ran two hours past the point I would have started charging (so it might have been possible to gotten 2 additional hours of runtime in each of the previous four cycles). This time I only measured battery pack voltage under load a couple of minutes into the charge cycle, but I would check again every now and then. I noticed the voltage would go down during the initial charge but then the voltage would go back up again towards the end during the trickle charge. I also measured the initial voltage of the pack without any load (11.1 volts) and at the firewire port on the PowerPod. I measured a drop of 0.7 volts across the PowerPod (careful: I blew the fuse when my probe touched the contact and casing at the same time, but I had some extra fuses and they are easy to replace).

Anyway here are the results of the second test. The first voltage number is taken at the battery pack (not after the PowerPod) and the second is the lowest I saw during charging. This test took place over four days so the iPod was off (asleep, it never had to be rebooted) all night and while I was at work for part of the time and even when the iPod is asleep it using some power. The total was 27.7 hours (exactly what I got last time). I was surprised how similar the results were, though maybe I shouldn't have been since I was using the same set of batteries, same iPod, and same songs.

ChargeRuntimeBattery pack voltage
Initial4:4811.1 volts without load
1st charge 1:034:3410.35
2nd charge 0:553:109.78
3rd charge 1:003:359.63 - 9.50
4th charge 1:053:059.50 - 9.13
5th charge 0:353:509.22 - 7.68

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Last modified: August 15, 2005