Mindboards

I have been considering adding building external, switchable power packs consisting of 2 sets of rechargeable AA batteries. I use NiMH, so I am not getting the full 9 volts available in the NXT when using conventional batteries.

I have been looking at boxes which hold 8 batteries at a time but do not want to risk damaging the NXT. This led me to wonder what the maximum supply voltage for the system is? I have been searching everywhere and the only comment I could find was from Philo who said :- "....make sure you don't exceed 9V. The motor drivers which are connected directly to that supply are specified for 9V supply and should never be powered at more than 10.5V."

I read an advert for NXT solar power supplies which stated that they provided up to 10.5 volts yet these appeared to be connected directly to the NXT in place of the batteries.

I could not even get a definitive answer on what the voltage of an AA cell was! Some sites said a conventional battery could be up to 1.6 volts which would mean the system had to be designed to take 9.6 volts plus a bit for luck. Some of the newer batteries go up to 1.65 v, making 9.9v in total.

Now I remember reading an article on the RCX 1.0 controllers a while ago and the author was happily running his off a 12V power supply but I don't want to go that far!

I could always measure the output using a fresh set of batteries then short out one of the bays and just insert 7 batteries if it is too high, but wondered if anyone regularly used higher voltages? Or does anyone know somewhere I could find details of the tolerance on voltage?

I have used up to 10.2-5 without issues. This from 6 E2 energizer's(1.70 new Lithium) that is just stock batteries(but thet are very expensive). So as long as you use say 1.2v AA in say an 8 cell(9.6-10.5) at full charge you should be ok. Though for most things a 6 cell works fine. Using an external battrey pack I do alot these days in X2, be very sure to get the polarity correct.

NO way is 12v going to be safe. The NXT would be way over volted. The motors may work BUT....they will wear out very fast. These motors have brushes made of wire, not a carbon brush. 12v will make them ark ALOT. once you burn out a brush they are done.

Doc

12v is way too much for the motor drivers in the NXT. 10.5v is the LIMIT. The RCX was designed much differently, and I think 12v would be okay. I wouldn't try anything over 9.6v for the NXT.

Just the other day, I got to thinking that I should use H-Bridges to power the motors. That way I could use a separate power supply of my choice, and not have an absolute max voltage of 10.5v.

Thank you for such quick and helpful answers.
I will now order the 8-cell boxes.

I have some rechargeable NiMH Energizer AA batteries. I like them quite a bit, even if they do have a rather high self discharge rate. I have noticed though, that even though they're rechargeable, they are about 8.7V when fully charged, and drop to about 5V fully discharged (that's six of them). So, even if the nominal voltage per AA is 1.2V, it will still fluctuate quite a bit.