9398 Mini motorized crawler

[Alasdair Ryan]

I really wish the rechargeable battery packs weren't so expensive.

You can also get it in a AAA version too.

Electric 9V Battery Box Power Functions (Non-Rechargeable) with Dark Bluish Gray Bottom

[07jkearney]

Does this battery pack make much difference to performance?

[DLuders]

@ 07jkearney: On this TechnicBRICKS article, Fernando Correia (Conchas) noted that the Power Functions AAA Battery Box has an "...auto turn OFF feature, after approx. 2 hours of continuous operation. Only with the 88000 PF AAA Battery Box, the user can override this function - If you press and hold down the green button for more than 3 seconds the green light will blink to indicate that auto turn OFF is disabled. This means that a motor will run until the batteries are empty. To enable again turn the AAA Battery Box OFF and ON again."

According to http://shop.lego.com/ByTheme/Product.aspx?...cn=587&d=70 , the 8878 Power Functions (PF) Rechargable Battery Box only has an output voltage of 7.4V (instead of 9V). So, running a PF Medium or XL Motor with it would not be as good as using 6 ea. AAA batteries in the new PF Battery Box.

According to Philo's "Lego 9V Technic Motors Compared Characteristics" webpage http://philohome.com/motors/motorcomp.htm , the PF Medium Motor's performance at 7.4V is rather weak compared to 9V:

8878 costs USD $50, and it needs the USD $25 8887 10VDC Tranformer to charge it up. The 88000 PF AAA Battery Box costs USD $14. One can buy a LOT of AAA batteries to equal the USD $61 difference in cost between the two setups.

Philo measured the actual (not theoretical) output of the PF Medium and PF XL motors in bench tests. See the bottom of his webpage http://philohome.com/pf/pf.htm :

"PF motors power curves when connected to IR remote receiver -- Since motor driver inside remote receiver has some dropout and current limitation, charts above don’t tell the whole story. The two curves below show the mechanical power of PF motors driven by the remote receiver. These characteristics were measured at 9V (alkaline batteries voltage) and 7.2V (NiMH rechargeable batteries voltage)."

Here's a comparison for the typical output of NiMH rechargable batteries. I own lots of AA Eveready Energizer batteries, and they produce 2450 milliamp-Hours (mAH). 9V Energizer Rechargable batteries only produce 150 mAH, and AAA Energizer Rechargables produce 850 mAH.

So, the moral of the story is that if you HAVE ROOM for the large PF Battery Box in your MOC, you're better off using AA batteries (because they produce more power). If you have limited space (or want to keep weight down), you can use a small PF battery box but your electrical output will be a lot less.

Edited September 22, 2012 by DLuders

[07jkearney]

Wow! Thanks for the comprehensive overview , but this is going slightly off-topic, so I will attempt to bring us back.

The reason I wanted to know this was to find out about performance of the mini 9398 with a standard AA battery box, but I just realised that it was mentioned earlier as poor. What do people think is the best approach to a rock crawler; minimalistic, with low power and low weight, or heavier, with a high power drive of several large or XL motors?

[Zerobricks]

Smaller models are usually more sturdy and faster. While bigger oones may be appelaing and massive, they are usually slower and tend to load the parts more. This results in more broken gears, bricks, twisted axles, etc...

[DLuders]

Zblj's 9398 Mini Motorized Crawler has been added to Rebrickable with a Parts List and Lego Digital Designer (LDD) .lxf file for Building Instructions.