PF idea: 8x AA battery for 9v with NiMH?

[andythenorth]

So I use 1.2v NiMH batteries with PF. 6x 1.2v gives 7.2v. (6x 1.5v alkaline gives 9v, but alkalines are expensive and I don't like the waste).

Philo's page shows that some PF motors deliver significantly less mechanical power at 7.2v than at 9v Makes sense.

http://www.philohome...ec/pfcurves.htm

TJ Avery figured out a PF hack using commodity 3rd party battery box for 8x AA with a PF plug, providing ~9.6v.

http://texbrick.com/tjab/?p=127

3rd party battery holders for 8x AA are cheap and easy to obtain. Like this one:

http://www.conrad-el...x-29-mm-x-59-mm

That one fits in the same dimensioned volume as the battery compartment of the current Lego PF AA battery box, which is 64mm x 32mm x 56mm (but 64mm including the volume occupied by the switch, so the 3rd part part fits).

This volume usefully doesn't include the pin connection structures (BI points) on the Lego part.

A 3rd party battery box part for integrating 8 AA batteries would be awesome. For bonus points, it would integrate a switch (use a commercial toggle switch, no BI point).

A voltage limiter to 9v might also be a bonus (in case 8x 1.5v Alkalines are added, I don't know if PF parts have over-voltage protection, or if the magic smoke will escape at ~12v...).

Shapeways? Kickstarter? Efferman?

Edited January 4, 2015 by andythenorth

[efferman]

It should be possible to make a battery box, without soldering, under using the parts of 2 standard battery boxes.

[1974]

6x 1.2v gives 9v.

Maybe on your planet, but not on mine ;)

You need to keep the voltage below 11V. Getting rid of the current limiter is also a good idea, you need watts/power (v x A = W) to those XLs, not just voltage

Edit :

Philo has all the info, forgot that!

The output driver is the DRV8833. Maximum input voltage is 10.8V, maximum output current is 1,5A/2A peak but there's a fuse that really needs to be eliminated. The chip has it's own internal protection

It's also possible to current limit the outputs but I do not know if TLG have done that? Triple (actually quad with the polarity bridge) protection seems to be overkill

Cheers,

Ole

Edited January 4, 2015 by 1974

[andythenorth]

Maybe on your planet, but not on mine ;)

Ha ha, oops. I fixed that typo

So if max input is 10.8v, someone who connects 8x 1.5v alkalines lets the magic smoke out? "Just don't do that" is one answer. Is it simple to cap voltage? (I studied electrical physics in school, but am not good at it).

Edited January 4, 2015 by andythenorth

[1974]

It's not that easy to make a precise cutoff at say 10.5V. Simple overvoltage protectors (VDRs etc) have a too high tolerance

If you hack it just don't use 1,5V cells

In that other PF thread (well one of them, there seems to be quite a lot of them recently, maybe even enough to have a general "LEGO Electricy" forum? <-- I'd support that!) I saw mentioned that the reciever does not care about polarity. That means there's an internal diode bridge with a voltagedrop. Eliminate that and motors will spin even faster

Has anyone see a pic of the V2 internals?

Edit : Philo has loads of info, my posts have been edited

Edited January 4, 2015 by 1974

[Brickthus]

I would assume that any H-bridge driver chip has flywheel diodes across the transistors to cope with inductive discharge from the motors when the stop command is given.

This would mean reverse polarity would be shorted through 2 series diodes (with 2 parallel paths).

The PF system would not have reverse polarity by default - it would only be by a hack like using another power source and an empty PF battery box to connect it to the 9V and 0V rails.

If you use that method I recommend adding a baulking feature to stop you putting the battery box switch in the reverse polarity position by mistake.

Mark

[1974]

I would assume that any H-bridge driver chip has flywheel diodes across the transistors to cope with inductive discharge from the motors when the stop command is given.

This would mean reverse polarity would be shorted through 2 series diodes (with 2 parallel paths).

It does, so no need for diodes at the power input of the reciever

[Blakbird]

So I use 1.2v NiMH batteries with PF. 6x 1.2v gives 7.2v. (6x 1.5v alkaline gives 9v, but alkalines are expensive and I don't like the waste).

Philo's page shows that some PF motors deliver significantly less mechanical power at 7.2v than at 9v Makes sense.

Something else Philo's pages show is that battery voltage is not always what you'd expect. Yes, the nominal voltage of a set of alkaline batteries is 9V versus 7.2V for NimH batteries, but in practice this difference does not exist. The alkalines' voltage falls off very quickly as they discharge and as you draw higher current. Therefore the actual amount of motor power you get from each kind of battery is actually about the same. This is all to say that you don't really need to use 8 cells to match alkaline performance; you only need 8 cells to exceed alkaline performance. You'll run into the current limit though.

[andythenorth]

Eh, that's a good point. If I was using alkalines I'd keep running them until they were useless (because of cost & waste). With the NiMH I swap them out for a fresh-charged set as soon as they show signs of fading.

So conclusion....everyone here is 'meh' about an 8x AA option?

[Philo]

'Meh' here too... There is also the problem of NiMH voltage right after charging that can reach 1.4V per cell, meaning you are above the rating of motor driver chips. And you add sifnificant weight... Imho the real issue is the low current limit in battery packs!

[Doc_Brown]

All you need to get is 6x Energizer lithium batteries!

These are around 1.75V new and kick asss!

After an hour of driving my Deadly Nightshade buggy their voltage was around 1.72 and still going strong! I still have not made them close to flat as they are now being used in experiments.

They are more expensive yes, but it looks like you will get 3-4 times the run time of cheapies, and with the added bonus of more speed!

[andythenorth]

@philo I'm going to defer to you on that My electrical physics goes about as far as V = IR.

Weight tends not to be a problem in the things I build, they're mostly heavy trucks. Weight aids traction. I'm actually splitting motors across multiple battery boxes and IR receivers, so using 12 NiMH batteries where 8 might be sufficient. But then again....by splitting I also reduce the current limit issues.

@Doc, yair, useful info cheers. I prefer rechargeables myself, but for other people those Energizers sound like a top tip.