a few battery questions

[Rebelgium]

Hi
I'm building a robot base which i'll use later on to hold other robotics projects.
I've got 4 separate motors (one for each wheel) with a stall current of 4A, and a voltage of 6V.

Now I'm deciding what batteries to order.
I'll order 2 batteries, on for my electronics, and one for all motors.

*For the electronics i'll need a stabilised 5V voltage, provided by a LM7805.
But what is the minimum input voltage? I think it's 7V, is this true?

*And do I also need a higher voltage than 6V(=operating voltage) for my motors?
To compensate losses in my electronics?

*I also have an RC car with an old 9.6 NiCad battery which could use replacement, so I'd like to buy a 9.6V battery for my robot so I can also use it for the RC car...
Is this possible?

[HDL_CinC_Dragon]

*I couldnt find the minimum operation voltage of the LM7805 in the datasheet But you would be fine with a 7.2v battery pack or a 6v battery pack(I use a KA7805 vreg and I use a 6v 1400mAh NiMH battery pack and get 4.98v regulated but the direct voltage coming from the battery appears to be 7v when fully charged.) Personally Im going to recommend you get the 6v battery for the circuitry.

*You should be fine with a second 6v battery pack or you could definitely get a 7.2v battery pack.

*Technically you COULD do that but the keep this in mind:
1) If you used that to take the regulated 5v from it, that means the vreg is taking 4 volts and converting it to heat (so that only 5 volts get through to the regulated pin) which means that thing is going to get HOT! You will most definitely need a pretty good sized heat sink on your vreg so it doesnt fry or anything. Not to mention your likely to touch it while its hot which needless to say hurts a lot heh.

2) If you used it for the motors, chances are you would fry the motors pretty quickly... unless of course its internal wires were a higher gauge and could take higher amps. Ive got a DC motor right here that I took out of an old power drill that looks like its core wires are about 1.15-1.2mm in diameter which means it can take some pretty decent amperage, im going to guess maybe 4amp before it blows, whereas I have some hobby motors with core wires of about 5 human hairs diameter? lol which means anything over like 250mA would probably fry it.

So you really have to be careful in making a decision like that. You need to watch the amps and the volts depending on what you want to apply them to.


Hope this helped at all.

[Rebelgium]

And what capacity should I choose for the motor battery?
total stall current is 16A, but this is highly unlikely to happen.
Or should keep it in account anyway?

[Nyx]

You could run several small battery packs in parallel... Or get something more powerful, like a 12AH SLA battery... I personally don't like SLA batteries because they die if you discharge them too much.

[Rebelgium]

Assuming SLA stands for "something lead acid", Lead acid batteries don't have a max discharge current that is as high as other, more modern battery types.
And I'm definitely not using lead acid batteries due to the weight...

i'm planning on using NiMh battery packs.


But anyway, i'm suddenly doubting if the stall current actually is 4A.

The motor is related to the "Mabuchi RS 380" motors, and I'm starting to think that 4A might be the normal current (at maximum efficiency, as they call it) And that the stall current might be as high as 20A !! 
The motor is used in a cheap (no-brand-chinese-crap) wireless screwdriver. Here's a pic of it:
http://www1.conrad.de/m/4000_4999/4800/4810/4815/481576_LB_00_FB.EPS.jpg
it normally runs on 4 AA batteries. And i'm planning on using one of these for each wheel.

I'm not sure about all this, can anyone with more experience in small motors confirm this plz?

And if this is the case, what capacity of battery pack should I choose then?

[Nyx]

There is no way a screwdriver 4AA batteries will need 20 amps. The batteries would melt... Probably something like 1-2A at most.

Best bet: hook a multimeter in there and take readings yourself

I'm personally using two motors that have a stall current of about 5A... They are powerful enough to move a 60 pound robot at human walking speed.

[Rebelgium]

That's what I thought and hoped !
I would have measured it already, but i don't hve the screwdrivers yet... :/

btw: 60 pounds is about 27kilograms (for who wondered like me)

[Rebelgium]

I'm pretty convinced now that the stall current is 4A.
*Then what will the current be under "normal" stress? I know you guys can't know this for sure, but I'd like to hear some estimations by people with experience in this matter.
Will it be like 1A or more like 500mA ? ...

*And then I need to decide what battery capacity i'll order. Keep in mind though that I'll use 4 of these motors, so i'll have to choose 4 times the capacity that I'd choode for one motor.
What is a good capacity range to power 4 of these motors?

*And to choose a capacity, you should keep in mind the maximum (stall) current, right? Well in this case it's 16A.
Wat is the maximum discharge current of NiMh batteries? 2C, 3C, or more?

tnx in advance

[Rebelgium]

sorry to post three reply in a row,
but does'nt anyone know the answer to my questions stated in the previous reply?

It's not that I'm lazy and not trying myself, I've been trying to google it several times...
I'd even be very pleased with just a site where I could find myself how to select the optimal battery capacity for my robot...

[Ro-Bot-X]

What is the max torque for these screwdrivers?

[Rebelgium]

I have no idea
The motor delivers between 10 to 15W, uses 6V and stall current is 4A. That all I know about these motors...

Its probably going to be quite high torque, considering it's a screwdriver.

[Admin]

If you could find a good datasheet or your motors, we could answer your question real fast . . . but I doubt they exist . . .

So if a motor requires 6A to run but your batteries can only apply 4A, they the motors will just operate at a lower torque. This lower torque may be acceptable to you, in which case there will be no problem.

Look in to getting NiCAD batteries, as they offer a higher current than NiMH. Lithium batteries will give you even more current, but are harder to work with.

The motor delivers between 10 to 15W, uses 6V and stall current is 4A

Power = voltage * current
15W = 6V * current
current = 2.5A

[Rebelgium]

If you could find a good datasheet or your motors, we could answer your question real fast . . . but I doubt they exist . . .

As I don't have the motors yet, I don't know more details then the few I've already posted, so I can't search for a datasheet.

So if a motor requires 6A to run but your batteries can only apply 4A, they the motors will just operate at a lower torque. This lower torque may be acceptable to you, in which case there will be no problem.

Does this also apply to stall currents?

So what if my motor asks 3A, the batteries can give it, all ok.
Then my motor stalls, and asks 16A, but my batteries can and will only give their maximum, being 5A (just a guess).  This will cause the motor to have little torque at that time.

Is this true? because if it is, yippeekajeah no problem.

[Admin]

Does this also apply to stall currents?

If your motor stalls, then this means your motor isnt strong enough. You dont want to run your motor at stall current. Bad things will happen.

Then my motor stalls, and asks 16A, but my batteries can and will only give their maximum, being 5A (just a guess).  This will cause the motor to have little torque at that time.

Correct . . .

You should calculate how much torque your motor will need, and find out how much torque your motor can supply, before being concerned with the motor stalling . . .
This page can help:
http://www.societyofrobots.com/mechanics_statics.shtml
http://www.societyofrobots.com/mechanics_dynamics.shtml

[Rebelgium]

You should calculate how much torque your motor will need, and find out how much torque your motor can supply, before being concerned with the motor stalling . . .
This page can help:
http://www.societyofrobots.com/mechanics_statics.shtml
http://www.societyofrobots.com/mechanics_dynamics.shtml

My robot will weigh in at about 2-3 kg.
With my motors being screwdrivers, I'm pretty confident that even one of these screwdrivers will be able to pull this robot.
I'm going to use 4 screwdrivers.

If your motor stalls, then this means your motor isnt strong enough. You dont want to run your motor at stall current. Bad things will happen.

I don't think my wheels will stall very easily, this would only happen in extreme conditions. And if it does stall, the batteries won't be able to give that stall current. So my motors won't run at stall current and bad things won't happen.

Right?

All I need to know is, what are the typical current values for NiMh batteries?
*Max continous current? ( 1 x Capacity , or maybe 3 x C , ... ?)
*Max peak current? (for a short time)

[Rebelgium]

Or is there no typical max current value for NiMh?
I've also read about new and old types of NiMh, the newer ones are supposed to be able to fast charge right?

[Admin]

So my motors won't run at stall current and bad things won't happen. Right?

Stall current is when there isnt enough current to make the motor go faster. When a motor stops, it is at the stall current. If that current is too high, the motor could overheat and melt . . .

All I need to know is, what are the typical current values for NiMh batteries?
*Max continous current? ( 1 x Capacity , or maybe 3 x C , ... ?)
*Max peak current? (for a short time)

The best way to find out is look at the spec shets of batteries. Id say you would typically see 5A peak for a 6V NiMH battery.