Minimising power loss and the best ways of powering PF motors.

[beelzibus]

I wondered if I might ask the collective for some advise;

My current and most complex construction yet currently utilises 6x medium motors (M), 1x large (L) and 2x Extra large (XL), plus 3 pairs of the old style 1x4 electric light bricks. In addition if I get around to it there will also be a couple or three pairs of PF LED lights added along with an old style micro motor.

Of course not all are in use at the same time, the light bricks are the only things that are on at all times.

The two XL motors are the main drive motors and they are working together and working hard due to the sheer weight of the model, if they are being used then there is the likelihood of one other hard worked medium motor also being in use intermittently. The rest of the motors tend to work singularly or in pairs but only when the aforementioned drive motors are inactive.

Currently partly due to a lack of other options and space constraints within the construction, everything is run from one 8881 battery box.

As you can imagine I am caning batteries like there is no tomorrow.

I've considered adding another battery box the same, physically fitting it in could be an issue, but will spreading the load over two batteries make that much difference, and if I do go that route is there any advantage other than packaging for using the 88000 battery box? I'd have to buy one or two as I don't own any, but are they any better than the 8881.

Lastly is there anything I can actually do to reduce the power being used? Currently most things are remote controlled using the first generation RC receivers, 8884, Typically I have connected function to these in pairs that work together, and all are operated by one remote control 8885. I'm not sure if I can wire things differently, but if I could what could I hope to achieve, are there any ways of minimising consumption, I did contemplate two paired function from separate receivers on separate battery boxes, but this doesn't seem obviously workable.

Your thoughts would be very much appreciated.

[beelzibus]

Wow! 201 views over just 24 hours.

But no replies.

Am I on the wrong forum?

[Block_Smarts Lego Technic]

Wow! 201 views over just 24 hours.

But no replies.

Am I on the wrong forum?

No, were just thinking...

[bonox]

All else being equal, doubling your number of batteries will get you twice the run time.

You need to think about application though - you'd need to spread the two driving motors across both battery packs for example, or you'll flatten one and not the other which is pointless. You could also link the two boxes to achieve the same result, at the cost of another cable run.

In general, short direct power runs are best. Where possible, avoid gearing, lots of 'bearings' and especially changes of angle like universal and CV joints.

to be honest, you'd probably get better results from either:

1. A custom LIPO battery pack that you make fit into where ever you have space or

2. My favourite, use a power cable. Not realistic for general outdoor RC use, but I only run my stuff around on the desk or floor. This isn't as silly as it sounds though. If you're building a truck/train etc, then the power cable can run to a trailer, tender, next carriage etc where you can put a very large battery if you want.

Other things to think about:

use wheels instead of tracks

drive 1 fixed axle instead of multiple

drive a fixed axle instead of a steering axle.

use a motor with speed as close as possible to your desired speed, so you don't need intermediate gearing.

Get weight out of the structure

Find real high capacity cells if you're currently using worn out yumcha chinese *1 billion mAhr* cells.

Use bigger cells. A AA cell for example generally contains at least double the energy of a AAA cell, often 3 or more times.

[Brickthus]

For a large mobile construction not requiring high speed I would consider using D-size batteries by getting a 6-cell holder. Those have a lot more capacity - 15.6Ah compared to ~2Ah.

The same technology of batteries should give a similar resistance. Duracells work OK for medium power but may have higher resistance than necessary for high power items.

The AAA battery box would just end up with half the running time before the batteries go flat; AAAs are not much cheaper than AAs!

If the structure were static I would use a LEGO 9V train controller or a dual 30V 3A power supply set to 9V (e.g. for trains powered through the rails, static air compressors or jet engine test rigs).

For a low resistance and least waste of power, the PF LiPo battery is a good option - 800mA before the current trip cuts out but it attempts to restart the model if it does that; I tested that with a train.

It can give about an hour's running of a high power model (longer for low power) and takes about 4 hours to recharge, so getting one or more than one depends how many hours per day you want to run the model.

Alternatively you can get a higher capacity LiPo but be careful as they can overpower LEGO motors; you would need to know what you're doing in radio controlled models to get the right type and get the best from them.

Useful tips from Bonox too, especially minimising gearing and optimising the application to the speed of the motor.

Mark

[beelzibus]

Thank you both for your thoughts.

Sadly I'm not sure it will help much.

The model is mobile, and it's singular in so far as it doesn't have a train, or load or whatever in which to hide additional batteries. Space is getting very tight within it, hence wondering about the other battery box 88000, I genuinely hadn't realised that that is for AAA not AA batteries, I hadn't really given it any thought, so that is clearly out of the question. I may end up rebuilding some of the model to see if I can hide another 8881.

As far as using other non Lego power solutions, that's not me I'm afraid, too much of a purist. Only unmodifed Lego parts in my constructions.

I will also look at the various mechanisms and see if I can improve in any way to minimise their power consumption.

Thanks again for your time and ideas.

[Jurss]

There is only two main ways:

1. Use less motors

2. Minimize mechanical resistance/friction in main mechanisms as described by Bonox

Of course, You should consider second battery or Li-Po.

[beelzibus]

I've spent a few hours today on this, it's amazing how long it takes to effect changes within a structure where everything is so tight for room, and everything has been built around and intertwined with everything else.

Anyway, I've managed to squeeze in a second 8881 battery box, so at least now I can spread the power loading a bit. after a bit of testing of various bits it seems like it might make a difference, but for now it's late so all the batteries are on charge. I'll have another play over the weekend. If I had the finances I'd pick up a pair of the PF rechargeable batteries, but it's cash I just can't drop right now, this model (and others) has way too much invested in it as it is.

Can someone answer me a question; one function in particular really struggles, it's down to the sheer weight of the construction, and there's nothing I can do about that. I started with a M motor, it now has one of the newer L motors, IF I could squeeze in an XL would it make much difference to the torque available and assist this struggling section of the model?

Apart from trying to improve that, it's really just a few details I need to work on. Hopefully with the changes I'm doing I'll be ready to take some pictures soon and post it up.

[bonox]

torque is a twisting 'force'. The slower you make an arbitrary motor spin, the more torque you get out of it for the same input power. In practice this means that if you gear a motor down by 2 input turns to 1 output turn, you'll get twice the twisting force (torque) out of it. There are obviously some losses in there due to friction in the gearing, but the idea is what i'm trying to get across.

To a first approximation, the output power of the L and XL is the same (http://www.philohome...s/motorcomp.htm - about 2W mechanical output at 9V). The L turns twice as fast as the XL and therefore has about half the torque of the XL. That means the XL is about twice as good at moving a heavy truck - but it'll do it at half the speed of the L.

Taken to extremes, you can gear down a tiny motor to produce enough torque to twist a steel bar (or plastic lego axle), but it might take all day to make one revolution. This (and the life of your lego parts) is a tradeoff you're going to have to make.

Edited August 17, 2015 by bonox