batteries for robot

[ltmhall]

Can I power two servo motors with the same battery supply or will I need to use seperate supplies

[JesseWelling]

Depends on a number of things.
How much torque will each motor need?
How long do you want the motors to run?
How fast do you want the motors to spin?

Generally with servos you want 4.8- 6 volts and generally have about 1.5 too 3 amps available (depends on the size). Once you decide on how many volts and amps you need you can start shopping for batteries and pick one that suits your price range (that has a decent amount of storage). Remember that if you have a battery that has 5000mah and you draw 1amps per hour it will last 5 hours (roughly).

For a hobby type project I reccomend Nickle Metal Hydride. They offer the best performance and safety value for hobby applications. Unless it's carefully controlled environment or you have some hardware to monitor them, I wouldn't get lipoly....you don't want to fry your project because you accidentally drew to many amps or killed the battery because you let the voltage drop to far.

[megaman935]

I think wat he means is can you power 2 servos on 1 4.8 battery pack or do you need 2? If you use 1 doesn't that make your batteries last 1/2 the time it would if u had 2?

[ed1380]

If you have them in parallel then yes.

[ComputerGeek]

Parallel or Series would give about the same life (parallel=Vconst, Iadd. series=Vadd, Iconst). The issue is that in series the first unit will probably starve the second for voltage. Torque and speed can be fine tuned with simple gear ratios. If your using Li-on batts. then use two their light weight and the energy is easy to recharge. I your using Metal Hydride try for one, you need to bleed them dry to prevent the 'memory effect' anyway. If your using Lead Acid...pack your bags and go home before you blow one up and end up in the hospital  . The choice doesn't make too much difference past these considerations, but if you do decide on using two isolate the common/ground/black lead to make sure there is no feed back. P.S. if you run one in parallel make sure the load provided by your motors is about the same or your amplifiers may play favorites.

[ed1380]

I though that if you had 2 batteries in parrallel and the current draw stays the same, they would last longer.

[dunk]

hi ComputerGeek,
are you sure you have all that right?

If your using Li-on batts. then use two their light weight and the energy is easy to recharge.

lithium-ion batteries are notoriously difficult to recharge. if you buy an off the shelf battery charger meant for this battery type you should be all right but it's kind of complicated to build your own.

The issue is that in series the first unit will probably starve the second for voltage.

what do you mean here? connecting batteries in series is how you build a battery pack.
i'm presuming you mean it is not a good idea to connect cells of different value together in series as when the lower rated one runs out of power current will be forced through it backwards, destroying it.
as long as you are using batteries of the same type and charge level, connecting them in series is the standard way of building a battery pack of higher voltage than a single cell.

If your using Metal Hydride try for one, you need to bleed them dry to prevent the 'memory effect'

while early nickel metal hydride batteries had a few issues with memory effect, any you buy today are really easy to look after. i have never found any of my packs to need any special care.
while it may be a good for single cells, "bleed[ing] them dry" is a very risky tactic if you have several of these cells connected in serial in a battery pack. if one runs out of power before the rest it will be damaged by the other cells. in general it is not good practice to completely drain any battery that is made of multiple cells.

as for lead acid batteries, they are perfectly safe as long as you don't over charge them. there are probably not the best choice for a bot though unless you need to be able to suck all the power out of your battery in a short space of time. (this is why they are still used to start cars. you can draw a huge amount of current from lead acid batteries without doing them damage.)

sorry to disagree.
batteries is one of these areas that every one has their own opinions about.

hey ed1380,

I though that if you had 2 batteries in parallel and the current draw stays the same, they would last longer.

yup. that's what ComputerGeek's saying, as the voltage stays the same, your equipment will draw the same current.
what does change is the number of Ah (Amp hours) goes up. in this case you could draw the same current (Amps) for twice as long (hours).

dunk.

[ComputerGeek]

In Order of appearance:

Recharge: Easy is a relative term. What I meant was you don't have to deal with 'memory' as you do with older rechargeable designs. As for charging them I've never had an issue using recycled cell phone batts in my projects after you determine what poles go where. I'll try to get my design onto the computer and make a post for the Li-On charger

Starve the second unit: I should have been more clear, I was referring to the motors or what ever is being driven. In other words if you run two motors in series the current is up but the voltage would drop across the first unit leaving only what was left for the second motor.

M.H Batts: I was unaware that they've improved on the Nickel Metal Hydride design, this may make them as much of a viable choice as Li-on units but I must do more research before recomending them.

Lead Acid: I still don't trust the buggers and even when you don't consider weight either of the other options are safer (relatively speaking) and would give the same result. P.S. when your trying to dump a large amount of power quickly use a capacitor.

I think you said it best

"batteries is one of these areas that every one has their own opinions about"

but I do appreciate the attention given to my post  .

[dunk]

hey ComputerGeek,
interesting to hear about your experience with Li-On batteries.
i must try them some time.
how are you determining when the cell is fully charged? are you just timing the charge cycle and measuring the current flowing or testing the cell voltage or something else?

i'm a big fan of Nickel Metal Hydride batteries because they have reasonable power to weight ratio (although not nearly as good as Li-On) and the best bit in my opinion is how easy they are to charge.
it is possible to leave them connected to a trickle charger without doing them damage greatly simplifying the charging circuit.
i have never really found performance to degrade over time as long as you don't drain them until totally flat. (i usually monitor the battery voltage from a microcotroller and power the project down when the voltage drops to 80% of the rated level.)

dunk.