12 Volt and 24 Volt

[kermit911]

Here is the picture, I have 2 - 12V deep cycle batteries. My electric motors need 24V to run and my cameras, video TX, lights, actuator, etc... need 12V. How can I get 12V and 24V out of the same 2 batteries? A fuse block would give me the same Voltage to everything. Do I need a separate battery?

[dasherman]

You can wire up batteries either in parallel or series:

http://www.otherpower...r_battery_wiring.html

[wildwood]

It very much depends on how steady the amperage demand is and if it is constant. Have a read here http://www.physicsfor...ndex.php/t-13934.html

An extra separate battery is probably easier and safer.

[vascop]

You could connect the batteries in series and get 24Volts across these, and then connect across one of the batteries to get 12V

[jomifl]

don't try connecting in parellel and series as the same time as there will be a big flash, a smell of burning and lots of little copper balls will roll all over the floor.
Also it is not a good idea to leave lead acid batteries connected in parallel as the battery with a slightly higher voltage will discharge through the other one.

[jomifl]

You could connect the two batteries in series to provide your 24V and use the 24v to power a 12V voltage regulator which won't cost much, but make sure that it can cope with the current that you want to draw. (obtainable from radiospares and CPC online)

[beso]

Do not attempt to tap twelve volts from the battery. It will inevitably cause the battery to become unbalanced leading to excessive discharge on one half or overcharge on the other.

Buy a 24 to 12 volt switch mode converter. They come in many sizes. Just choose one that can conver more than your toal 12 volt loads.

Lights are available in 24 volts.

[kermit911]

Thanks everyone, looks like the best way to go is a pair of batteries for 24V and a 12V battery.

[beso]

Wrong. The converter combined with 24 volt lighting is the best solution.

[kermit911]

This is going inside a robot, and I am trying to get the most run time out of him I can. Will adding a converter use more power then is necessary? Every thing on the robot is 12V except the motors which are 24V at 30 amp. The converter is still the best choice?

[beso]

Note that the motors although rated at 24 volts will most likely still work at 12 volts 30 Amps. They will turn slower and only do half the power but still produce the same torque. They are quite powerful motors and half power might be just fine. They will require less reduction gearing at the lower speed.

I would try them out at 12 volts at the earliest possible opportunity. How big is this robot?

One of the advantages of using the converter is the power is available for both purposes while there is any charge. Otherwise you might have power left to run motors but the twelve volts might be discharged. Getting the right balance in the sizes could be tricky.

However if the robot spends a lot of time doing nothing the idle current of the converter can add up. You would have to do the maths.

Converters vary a lot in efficiency. Avoid fan cooled units because they are often poorly designed and inefficient. Look for low no load current.

[kermit911]

I heard that running a 24V motor at 12V could damage the motor or the brushes, something like that. My son and I are building Wall-E here is a link to some of the picture. These are not the most current ones. http://www.mixthisup.com/WallE/ When all is said and done he will weight about 450 lbs. and travel at roughly 3.7 mph if I did the math right. He will be about 3 feet tall including his head. His head will have a servo for rotating up, down, left, and right. There will be 2 cameras, also a audio video transmitter, and any kind of lights I'll put on him. There will be 2 motor controllers with daughter boards connected to a RC receiver. I'm limited on space and power consumption. I am trying to get about a 3 hour run time out of him.

[beso]

Running AC induction motors at undervoltage will burn them out but it isn't a problem with brush motors. Indeed their speed is normally controlled by lowering the avarage voltage.

200 kg supplied with an average of 350 Watts will reach 1.6 metres per second from a standing start in less than a second. It will also climb a substantial gradient without slowing.

The motor will do several times its rated current for short bursts so its acceleration will be impressive. I would run your robot motors at 12 volts. Just adjust your gearing to allow for the lower speed.

You will need some electronics to control the power to the motors. This will need to be designed to run motors so as to cope with the surge and the back voltage from the motor.

Include a sensor to limit the tilt and acceleration or it is likely to fall over as it moves off. Also include safety switches to prevent it bulldozing.

Be very worried if you go to switch it off and it says "Sorry Kermit I can't let you do that" ;)

[kermit911]

LOL that was great! That sounds like a plan then, run everything at 12V and adjust my gear ratio. Cool, thanks beso.

[roger]

If you want to use the two batteries in parallel at 12v, you can prevent one from draining the other with a couple of diodes

[kermit911]

Could you explain a little further please? :-)