[HELP] Battery Pack Options

[aval5]

So i am currently looking for new ways on battery packs for lego technics and I dont want to continuously go out and buy batteries and i dont want to spend alot of money to get a rechargable pack but i do have some li-po batteries sitting around for my RC planes, is it possible i can use these? If you have any ideas please comment about them, thanks!

[zux]

Sure you can use rechargeable AA or AAA bateries (depends on the BB you are going to use). Normaly rechargeable batteries have lower voltage, so full BB will have 7.2V instead of 9V.

[bonox]

The PF gear runs very happily on 7V (as attested to by the factory rechargable battery box). You could cut up an extension cable to plug into the pack you've already got, or wire it into the standard AA battery box if you wish. There are plenty of guides on this site + google if you need help with it.

[aval5]

Thanks bonox! I'll check the voltages on my li-pos see if they can do 7V.

I saw my smallest one and it's about 11 volts, do I need to use a resistor or anything on it, or can I just plug it in?

[bonox]

That's up to you.

The PF and older 9V gear is specified for 9V only. Having said that, some people (http://www.philohome...s/motorcomp.htm) have run them happily at 12V, however all long term motor survival bets are off if you do this. You'll also have to accept significantly higher speeds as well with some motors.

You can use resistors as voltage droppers, however you need to be aware that they really only work well for constant current draw. As a general rule, DC motors will draw different current depending on how heavily they are loaded. Different current means different voltage drop following the DC Voltage=CurrentxResistance rule. You could happily find a midpoint if you wish as long as the motor is running under a constant load. Also keep in mind that a 2V drop at the peak current of an M motor for example (800mA) will require about a 2watt resistor, which are harder to find than typical electronic 1/4W units. Otherwise, you'd be better off looking for a cheap voltage regulator that will accept 10V+ input and output 9V.

Here's a simple example of a Do-it-yourself

http://www.learninga...or-in-a-circuit

or you could use an off the shelf example like

http://www.aliexpres.../509138006.html

with suitable voltages for what you want.

Pragmatically though, I would suggest trying it hooked up directly and seeing if you're happy with it before spending time and money to get it back to 9V.

Edited October 21, 2014 by bonox

[aval5]

That's up to you.

The PF and older 9V gear is specified for 9V only. Having said that, some people (http://www.philohome...s/motorcomp.htm) have run them happily at 12V, however all long term motor survival bets are off if you do this. You'll also have to accept significantly higher speeds as well with some motors.

You can use resistors as voltage droppers, however you need to be aware that they really only work well for constant current draw. As a general rule, DC motors will draw different current depending on how heavily they are loaded. Different current means different voltage drop following the DC Voltage=CurrentxResistance rule. You could happily find a midpoint if you wish as long as the motor is running under a constant load. Also keep in mind that a 2V drop at the peak current of an M motor for example (800mA) will require about a 2watt resistor, which are harder to find than typical electronic 1/4W units. Otherwise, you'd be better off looking for a cheap voltage regulator that will accept 10V+ input and output 9V.

Here's a simple example of a Do-it-yourself

http://www.learninga...or-in-a-circuit

or you could use an off the shelf example like

http://www.aliexpres.../509138006.html

with suitable voltages for what you want.

Pragmatically though, I would suggest trying it hooked up directly and seeing if you're happy with it before spending time and money to get it back to 9V.

Thank you very much Bonox! I'll reply when ever I get around to do this!

Also, my battery says that it has a "High Discharge Rate" which i assume is a feature of the battery, do we need to worry about this?

Edited October 21, 2014 by aval5

[bonox]

High Discharge rate?Not sure what this means as stamped on the battery with no other info, however batteries have a property related to how much current they can deliver (or how much power they can deliver). Since electric supplies will only deliver as much current as is being asked for, a higher capacity to deliver isn't an issue. This is why you can run a 1amp device using a (regulated) 10amp supply.As a general rule, you can get high energy capacity cells that can give that energy back to you at some particular low to moderate rate. You can get lower capacity cells that can give their energy back to you at a high rate, but they don't have the same total to give. Older style Nickel Cadmium batteries used to be great at high current discharge (which is why the lunatic fringe high power remote control car/aircraft crowd liked them), but they wouldn't run as long as a NiMH cell of the same size. You can see this in practice as AA NiCd batteries really only being available in less than 1000mAh capacity, but lithium cells of same size being of 2000mAh or more. The flip side is that the NiCd might be able to deliver 5 amps for a short while, which the lithium one couldn't do more than 2 amps (made up examples only).

Think about it like trying to stuff a sleeping bag into sack. A bigger sleeping bag takes more time to get into and out of the sack than a smaller sleeping bag. THe bigger sleeping bag is more energy (longer run time) but less power (energy per unit time). If you want high current (power) you have a smaller sleeping bag so it's easier to get it out of the sack fast.

Summary - don't worry about it :)

Edited October 23, 2014 by bonox

[aval5]

Thanks bonox! I did a lot of looking for some discussions about this kind of thing, never finding anything! thanks for helping out :)

[Vorkosigan]

In my experience when I was flying RC stunt helicopters was that "high discharge" is over 25C, but it depends on the manufacturer. "C" is the rated current capacity of a battery, usually in "milliamp hours" (mAh) for small batteries. So if you have a 2000 mAh battery that was rated at 25C discharge you could supposedly discharge it at 50 amps, though the battery would last less than 2 minutes. Anyway it won't be a problem with LEGO power functions. I had 50C and 75C discharge batteries because 3D maneuvers with a helicopter require a crazy amount of current.

One thing to note about using LiPos is that you should never discharge them below 3 volts per cell, so for a 7.2V (2 cell) you shouldn't go below 6V for the health of the battery. A good speed controller will protect against this, but with PF you'll probably just have to learn to judge when things are getting slow and turn it off. You can check the voltage with a multimeter to see what 6V "feels" like. I would say just buy a 2 cell battery instead of messing around with regulators on a 3 cell or risk of burning out the motors over time.

[aval5]

I do have a 2 cell, it's a 30C series though

[bonox]

Perhaps I should point out that if you are using the TLG AA battery box, it's current limited to about a single amp anyway, so any capacity for a cell to deliver more than that is irrelevant; the battery box will cut out if you try to draw more than that.

[aval5]

I have four of those vertical rectangular AA battery boxes and which I'm not familiar with the word "TLG"

[princeofgonville]

Hope you don't mind if I chip in from my experience. If you are considering buying rechargeable AA batteries, there are very good ones and very bad ones. The best seem to be the ones made by "Eneloop". They consistently get good reviews on radio-controlled car and aeroplane forums, and are supposed to be good for several hundred - even thousands - of recharges. They might be a bit more expensive than other rechargeable AA batteries, but it's worth investing. HTH.

[AndyCW]

The upcoming S-Brick is supposed to be good up to a three cell Lipo. The motors can definitely take it, but their lifespan may be impacted. Make an adaptor cable and go to town. Zoom-Zoom.

v/r

Andy

[nerdsforprez]

That's up to you.

The PF and older 9V gear is specified for 9V only. Having said that, some people (http://www.philohome...s/motorcomp.htm) have run them happily at 12V, however all long term motor survival bets are off if you do this. You'll also have to accept significantly higher speeds as well with some motors.

You can use resistors as voltage droppers, however you need to be aware that they really only work well for constant current draw. As a general rule, DC motors will draw different current depending on how heavily they are loaded. Different current means different voltage drop following the DC Voltage=CurrentxResistance rule. You could happily find a midpoint if you wish as long as the motor is running under a constant load. Also keep in mind that a 2V drop at the peak current of an M motor for example (800mA) will require about a 2watt resistor, which are harder to find than typical electronic 1/4W units. Otherwise, you'd be better off looking for a cheap voltage regulator that will accept 10V+ input and output 9V.

Here's a simple example of a Do-it-yourself

http://www.learninga...or-in-a-circuit

or you could use an off the shelf example like

http://www.aliexpres.../509138006.html

with suitable voltages for what you want.

Pragmatically though, I would suggest trying it hooked up directly and seeing if you're happy with it before spending time and money to get it back to 9V.

The philhome site gives figures regarding power, torque, etc. when on 12V. But how do you actually hook things up so that the motors run on 12V? The battery packs only go up to 9V right?

[Boxerlego]

In order to go up to 12V you could use a 12v AA battery case like the one in the picture. Tho I don't recommend going above 10.5 volt on the PF receiver, it could potentially break with the excessive voltage. Other then that If your powering the motor on the table then the 12v could be from a bench power supply or a transformer.

[bonox]

The philhome site gives figures regarding power, torque, etc. when on 12V. But how do you actually hook things up so that the motors run on 12V? The battery packs only go up to 9V right?

he states on his page that he is using a bench supply without the AA battery box (so he's not limited to the ~1 Amp cutout on the battery box). I run my stuff using 9V, but from a bench supply as well. I like to see my crane work, but don't actually want to drive it further than the end of the bench ;)

Edited November 1, 2014 by bonox