Batteries and Chargers

I am currently building a competitor for the uk heavyweights; (motors are here, transmission is being built, weapon is in development, frame and body under construction) but having some difficulty when it comes to batteries.

We have three 24V (newer bosch GPA style) motors in our bot, (they run no load of 30A but under load can spike upto 120A); looking at various opti-power batteries - no configuration seems to fit the 24V?

Are we going to have to use a voltage regulator of some kind?

Or do we run over voltage as when under load the output usually dips?

Any pointers would be much appreciated! :?

Run the machine on 6s lipo packs. Just a touch over at 25.2v but your motors will happily run on it. Or 3s packs in series.

24V , give or take a few volts isn't a problem. Most heavies are running 24V motors on 36-37V.

Yes, you'll need a kind of variable voltage regulator. Called an electronic speed controller.
That makes it possible to vary the voltage to the motors from 0 to full voltage from the batteries.

But 30 amps no load. That's a lot. Is that the 3 together or just 1?

A manufacturer's motor voltage recommendation is more of a recommendation than an absolute maximum and in any case, a 6S Lipo pack will drop to about 22.2V (3.7V per cell) under load.

Like Mario said, 30A is a very high current for the motors to be drawing under no load - there must be a lot of friction in the drive system to require 720 watts of power just to spin the wheels.

Also: three motors for drive, what's that all about?

If you're not keen on carrying around volatile Lipo packs, LiFePO4 batteries are also an option. Slightly heavier but a lot safer to use. Quite a few heavies use them. Two of these in series will power the drive just fine. http://www.hobbyking.co.uk/hobbyking...dProduct=32206

Luckily we are using Vyper speed controllers (if they ever arrive - it takes ages to get items shipped in from over the pond!) so they are able to handle anything from 7-40VDC.

Saw a pretty intense lipo fire in the arena on the latest series of robot wars filming - it was so bad they stopped filming and had to vent the hangar out!

However lipos do seem to be preferable nowdays due to their weight/power output ratio.

Thank-you for your comments as always chaps!

I believe my team-mate read the wrong section of the data sheet, they draw little over 2A whilst under no load!

Quote:

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Originally Posted by overkill

A manufacturer's motor voltage recommendation is more of a recommendation than an absolute maximum and in any case, a 6S Lipo pack will drop to about 22.2V (3.7V per cell) under load.

Like Mario said, 30A is a very high current for the motors to be drawing under no load - there must be a lot of friction in the drive system to require 720 watts of power just to spin the wheels.

Also: three motors for drive, what's that all about?

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Three motors? Only two are the drive - the other is hopefully for thrashing metal!

What does the 30A represent then? Nominal or peak power?

I've got a couple of Vypers on order too since mid April - supposed to finally be arriving at the end of this week though!

We have done some testing with the Vypers - very impressive results I must say