Robot Power

If your Robot doesn’t move power is simple: just plug it into the wall.

If your Robot does move things are very different. This almost certainly means batteries. Lots of batteries. After all, getting several hundred lbs of Robot moving needs considerable power.

While there are many types of batteries available today (Li-ion, Nickel metal hydride, and many others) it is probably best to stay with a known and well established type of battery, they are much cheaper then more cutting edge technology. In this case this means lead acid batteries just like in your car. But not exactly like car batteries, slightly different.

Conventional lead acid car batteries have liquid acid in them that can, and often does, leak out. Strong acid and its fumes can do major damage to your Robot. Today there are lead acid batteries that are slightly different in that they are sealed and use fiberglass mats to absorb the liquid acid so that there is actually no liquid acid to leak out. They are known as AGM (Absorbed Glass Mat) batteries. There are at least 2 different types of AGM batteries out there: conventional (with flat plates) and Optima (with spiral wound plates).

Optima batteries are a bit large and are expensive so I used just standard flat plate AGM batteries.

AGM batteries come in many different shapes and sizes. I picked the biggest I could fit in the space I had:

These batteries are 7 3/4" x 5" x 7" high and are known in the industry as a U1 size. They weigh about 26 lbs each and put out 12 volts. This particular size is commonly used in electric wheel chairs. With 2 such batteries wired in series there is 24 volts available. Having 24 volts is much better then just 12. There are many electronic devices (PLC’s and sensors) available that run on 24 volts and the drive, rotation and steering motors can use it too. It will also be possible to step down the 24 volts with a PWM (Pulse Width Modulation) circuit to run the motors at any desired speed. Anything that needs 12 volts will be fed from a simple 24 to 12 volt voltage regulator, shown below:

While it might seem ok to connect up to a single battery (which one?) in a dual battery 24 volt system if you needed just 12 volts this is really a Bad Idea! Batteries hooked up in series will feed off of each other if the load is not balanced between them. In the slightly different Robot all power will be taken from both batteries and be available as 24 vdc. A converter (above) will run off this 24 volts and put out up to 10 amps of 12 volts dc. Everything is balanced, everything is happy, the batteries will last a long time.

One last power related detail. It looks like just leaving the 24 to 12 volt converter hooked up constantly to the batteries would drain them in an inconveniently short time. To avoid this the converter will be supplied thru a 24 volt (all the other relays run off of 12 volts) relay controlled by the remote control board. Turn this relay off and all 12 volt power to everything is turned off. But I did not forget about power to the remote control relay board itself! This will be supplied by a tiny 12 volt power supply, a LM7812 connected directly to the 24 volts that draws less then 10 ma at idle or, as the sparkys say, at quiescence. This should be very nice, when in the off state the batteries will last a very long time before recharging. Using the remote to latch the relay will send power to the 24 to 12 volt converter and enable all the motors.