Question about 8878 circuit

[Rob Klingberg]

Hello all--

I searched online for details on this, so apologies if I missed it somewhere. I'm using the 8878 Rechargeable Battery Box in a lighting circuit intended to run for a long time (e.g., stay on until the LiPo battery is depleted). In my testing, it seems the battery box is auto powering off after about two hours. Does anyone know if this is a function of the circuit inside the battery box, or should it be staying on permanently?

Any insight anyone could provide into how the 8878 circuit works would be much appreciated.

Thank you!

--Rob

[DLuders]

In this TechnicBRICKS article, Conchas (Fernando Correia) wrote,

"An undesired feature, from AFOLs perspective I would say, is a built-in power-off timer which actuates after 2h without any interaction with the battery (speed change command or charging). Any of these actions will immediately reset the timer and are the ways to make it run longer without interruption, which is of particular importance at exhibition displays."

In another TechnicBRICKS article, Conchas discussed the work of Philo (Phillippe Hurbain) in dissecting the inside of the Lego 8878 Power Functions Rechargeable Battery. More appears on Philo's Brickshelf folder. Conchas wrote, "•Modifying the electronics board to override the 2h time-out function, seems now a vanished dream since all the control might be inside the Chip-on-Board circuitry (the black kind of a bubble in the PCB left side)."

Edited May 17, 2012 by DLuders

[sqiddster]

Could you not plug it in?

"An undesired feature, from AFOLs perspective I would say, is a built-in power-off timer which actuates after 2h without any interaction with the battery (speed change command or charging)."

[Rob Klingberg]

Thanks DLuders, those were very helpful articles. And sqiddster, to answer your question, ideally I'm aiming to have a setup that will be portable and enclosed (one that will not require plugging in).

So it looks like the battery pack needs some "keepalive" signal from its load. If I don't have the battery plugged in and charging, and I also don't have it connected to an IR receiver (to send speed commands, etc.), what's the easiest way to simulate these keepalive signals on the bus? I'm assuming this would need to be some sort of signal sent on either the C1 or C2 lines (per http://philohome.com/pf/pf.htm). Could I just keep C1 or C2 connected to Vcc full time? Would this be enough to continually "reset" the 2-hour counter, or would this damage the unit?

Again, the goal is to use the battery pack to power LED lights alone, without any additional PF motors or IR controllers, for as long as the battery pack will last (certainly more than 2 hours).

Thanks!

--Rob

[DLuders]

@ Rob Klingberg: There is another option. Instead of using the (expensive) Lego 8878 Rechargeable Power Functions (PF) Battery Box (shown on the left below), you could use the (less expensive), similarly-sized 88000 Power Functions AAA Battery Box shown on the right. You could use regular Alkaline AAA batteries, or rechargeable ones. The AAA PF Battery Box does not have internal shutoff circuitry. The only drawback is that the AAA Battery Box + AAA batteries WEIGH MORE than the LiPo 8878 Rechargeable PF Battery Box.

If you don't mind having an electrical cord plugged into your lights, you could use a 2868b "Electric, Train Speed Regulator 9V" with a transformer suited for your country. You will also need an 8871 Power Functions Extension Wire (to enable you to connect to your Power Functions elements.