NXT battery use comparison
Since I got my NXT, I’ve wanted to compare the NXT rechargeable battery pack to non-rechargeables. So when Educational Experience kindly agreed to supply me a rechargeable battery pack, I wrote a couple of quick programs to test it. Read on for the results, such as they are…
The non-rechargeables tested were Energizer e2 advanced alkaline batteries, the rechargeable was a brand new LEGO pack supplied by Educational Experience, charged for the first time immediately before this test. After the battery life test, I tried the Energizers in my 8475 Race buggy, and they have kept working for quite some time, so they definitely were nowhere near flat – I assume the rechargeable still had a fair bit left too.
Note also that I used John Hansen’s enhanced firmware for these tests.
I wanted to compare 2 things:
- The relative power (as available for driving motors; and
- The relative battery life
For this purpose, I wrote 2 fairly simple NBC programs, you can download them here.
Power
The first simply raised a fixed weight a specified distance at motor power increments from 10 to 100, and recorded the time taken, very similar to Philo’s motor power tests. The timing was measured in milliseconds, using gettick.
Results
Power
|
Energizers (ms)
|
Rechargeable (ms)
|
Ratio
|
---|---|---|---|
10 | 32262 | 62757 | 1.95 |
20 | 10786 | 14657 | 1.36 |
30 | 6715 | 8604 | 1.28 |
40 | 4777 | 5947 | 1.24 |
50 | 3775 | 4651 | 1.23 |
60 | 3077 | 3750 | 1.22 |
70 | 2631 | 3181 | 1.21 |
80 | 2277 | 2733 | 1.20 |
90 | 2025 | 2431 | 1.20 |
100 | 1809 | 2139 | 1.18 |
Conclusion
Well not much really, except that the Energizers consistently produce more power at the motor, as expected. The ratio works out at around 1.2, disregarding the outliers at low power settings. It just so happens this corresponds pretty closely with the ratio of claimed output voltage (see table below). Whether these observations are related or not I can’t say.
Interestingly, I repeated the test with a fairly heavily used (but just recharged) pack, and it came out slightly ahead of the new one, by a factor of about 1.03.
Life
The second program simply ran each of the 3 motors in a round robin, 30 secs at a time. Approximately every 2 seconds, a duration (in milliseconds) was written alternately to one of 2 files on the NXT, and these values retrieved after the NXT shut down. In both cases, this program was run immediately after the power program ended, so I have added the total times from the power tests to the time retrieved.
Results
Energizers (ms)
|
Rechargeable (ms)
|
|
---|---|---|
Retrieved (ms) | 607533 | 607522 |
Add Power (ms) | 70134 | 110850 |
Total (ms) | 677667 | 718372 |
Conclusion
The rechargeable actually lasted longer than the Energizers, which was a little unexpected. It’s difficult to put a number on it the way I did the test, but I only intended to get a general feel for the difference anyway.
Battery specs
These are data claimed by the manufacturer. I include them here for information only.
Energizers
|
Rechargeable
|
|
---|---|---|
Voltage (V) | 9.0 (6 x 1.5) | 7.4 |
Capacity (mAh) | 2900 | 1400 |
Disclaimer: While I only did a one-off test with each set of batteries, I think at least the power test shows a decent trend. But you should probably take these results pretty much just as a small indication, and definitely not use them as conclusive proof of anything.
December 15th, 2008 at 12:22 am
Excellent job, Ross… We just need to complete it with some NiMH data!
October 12th, 2009 at 4:57 am
the nxt rechargable is nimh
January 22nd, 2010 at 3:26 am
I like what you did. BUT!!!!!!! you may want to try different manufactures too it may help just a little. TRUST ME! ! ! ! 🙂
November 22nd, 2011 at 3:08 am
Has someone done this test with the new rechargeable battery?
What about Li disposables?
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