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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:

  1. The relative power (as available for driving motors; and
  2. The relative battery life

For this purpose, I wrote 2 fairly simple NBC programs, you can download them here.


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.

Energizers (ms)
Rechargeable (ms)
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

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.


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.

Energizers (ms)
Rechargeable (ms)
Retrieved (ms) 607533 607522
Add Power (ms) 70134 110850
Total (ms) 677667 718372

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.

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.

13 Responses to “NXT battery use comparison”

  1. Philo Says:

    Excellent job, Ross… We just need to complete it with some NiMH data!

  2. Anonomous Says:

    the nxt rechargable is nimh

  3. KK Says:

    I like what you did. BUT!!!!!!! you may want to try different manufactures too it may help just a little. TRUST ME! ! ! ! 🙂

  4. Alex Says:

    Has someone done this test with the new rechargeable battery?
    What about Li disposables?

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