88000 AAA Battery Box - 8.4V - 7 Cell Modification

[Costas]

We are all fairly familiar with the 88000 Battery Box - Suits 6x AAA cells for use with various PF components.

 

It is the same form factor as LEGO's 8878 rechargeable battery box which contains a 2S LiPo battery providing a nominal voltage of 7.4VDC.

The 8878 is actually difficult to source here in Australia possibly due to LiPo air freight restrictions these days and I also note that it is listed as 'Retired' on the German LEGO shop: https://shop.lego.com/en-DE/product/LEGO-Power-Functions-Rechargeable-Battery-Box-8878

 

The standard 6-cell AAA battery box allows users to run with 6 cells providing a nominal voltage of 9v (Alkaline) or 7.2V (NiMh). While Alkaline cells can be utilised - many of us typically opt for NiMh cells due to their recharge ability and overall lower cost.

 

When using NiMh cells most of us are aware of the slightly lower voltage which can result in reduced performance in certain applications.

To combat this, some users have utilised their own 3rd party LiPos etc which have their own drawbacks such as: 

• Size - hard to find a LiPo which fits in the AAA battery box which has decent capacity.
• More convoluted charging requirements (cell balancing etc).

 

Taking a closer look at the 88000 it looked like there was enough room to squeeze in an additional AAA cell bringing the cell count up to 7.

This would have the advantage of raising the nominal voltage up to 8.4VDC while still retaining a decent mAh capacity figure. 

This should provide even better performance than a 2S LiPo battery such as the 8878 if using premium AAA NiMh cells.

 

So I decided to modify one of my spare 88000 boxes as a prototype to see if it was feasible to achieve a workable 7-cell solution..

Main criteria for the prototype was to:

• Maintain external dimensions and appearance as per original
• Maintain internal circuitry as is so that original power switch and reversing switch would function as normal
• Maintain compatibility to all other LEGO components as well as 3rd party devices such as S-Brick etc

The AAA battery box is a tad different from the LEGO 6-cell AA box in that the 88000 AAA unit has an internal 'electronic' switch to control the power rather than just relying on a mechanical switch (Uses a small IC and FET as on/off switch).

It is a little more complex than the AA 6-Cell unit and it also has a timed 'Auto Power Off' feature which can be disabled by holding down the power switch until the LED starts to flash rapidly then release.

 

Ok - So below are a few pics of my finished prototype.

Keep in mind that it is a prototype so it could be neater.. :)

I will build a second unit and I will carefully photograph/document the important mod build steps if anyone is interested - This was just knocked up late last night in a rush to see how well it would turn out and not much attention was paid to detail.

 

I chose to utilise Panasonic's Eneloop PRO AAA cells which feature their well known low self discharge technology, a nominal capacity of 950mAh (which is excellent for AAA cells) and high current delivery capability (relatively low internal resistance).

 

*** A couple of important points:

• This modification will of course void your warranty.. :)
• It requires a reasonable level of soldering skill - You need the ability to solder the batteries WITHOUT over heating them otherwise they can be damaged. Using the appropriate soldering tip size (and temp) as well as pre-cleaning cell terminals and utilising additional flux etc is a given.
• De-soldering (and replacing) of one electronic component (22uF Electrolytic Capacitor) on the existing PCB is required.
• Internal 6-cell 'carrier' is removed as is not required - This frees up a fair amount of space.
• Overall space inside the AAA box is very limited so you need to be reasonably precise with assembly otherwise the cells may not quite fit...!
• The main case (DBG base) must be removed to recharge the pack (same goes for original) - The two original screws on the bottom are still utilised to hold the unit together as per the original. 

 

One side of the 88000 - Looks fairly 'stock' in this view.

 

The 'Other' side - Features a short length of silicone wire, equipped with common JST connector - used to recharge the pack. This lead is soldered directly to the pack and tucks in neatly in the available space when you screw the base back on.

 

You can sort of make out 4 cells on this side if you look carefully - 2 of the cells sit roughly offset and in-between the other cells.

 

Here we can see that there is absolutely no clearance between the cells and the PCB.

I even had to move the standard electrolytic capacitor off to the side so that everything would squeeze in. This was done by de-soldering the original capacitor and replacing it with a new one as it requires longer leads to now reach the PCB.

 

So all went well and it works a treat - The extra voltage really boosts performance on my pneumatic pump equipped Xerion as compared to using a standard 6 Cell AA pack for reference.

Will try it out on my 'Ultimate 8043' and compare the 8878 which is already fitted in that model to the 7 Cell AAA pack.

So overall I think (for me) this will suffice as a high performance alternative to the 8878.

 

 

 

 

 

 

Edited June 8, 2019 by Costas