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Combining DC power rail and battery to power a robot.

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misterT

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Hi, We are building a robot that moves on fixed rails. The robot will have lots of electronics, motors etc. on board. There will be continuous DC (24V) voltage available through the rail system, but it won't deliver more than couple of amps.

Our plan is to put a 24V battery in the robot to provide extra power. What would be a good way to combine the two power sources? I would like the system to:

1) Provide all the power from the rail when consumption is less than 2 amps and at the same time charge the battery.
2) When consumption is more than 2 amps, provide 2 amps from the rail and the rest from the battery.
3) Use only the battery when the rail power is not present.

Are there commercial systems for this kind of setup? I need good advise on how to approach this problem. Lipo batteries would be nice, but I can live with sealed lead acid batteries too. I can also design the system for higher rail voltages (up to 50V). The battery should be ~24V.
 
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I would just connect the battery across the rails so that it is in parallel. I would also diode isolate it so if something shorts the rails or puts some other load across them, it won’t discharge the battery. However, this is not enough voltage to fully recharge the battery. You would need to put a DC to DC converter on the robot to boost the available voltage to 27 volts to fully recharge a 24 volt led acid battery.

The next issue is what does the rail voltage drop down to at 2 amps load?
 
Here is just a quick hack using steering diodes. No charging shown. Also, if you are using an SLA battery, you will have about 27.6V under charge, it will drop to 25.2 under load, and then slowly sag toward 23V over time. Probably means you will have to use FETs to isolate the battery during charge, and reconnect it during load current peaks.
 

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Hi, We are building a robot that moves on fixed rails. The robot will have lots of electronics, motors etc. on board. There will be continuous DC (24V) voltage available through the rail system, but it won't deliver more than couple of amps.
What will be the max load of the robot, 2 amps in the rail is not alot to work with asuming voltage drop in the rails and all.
Can you run your bot off 12 volts or 18 volts then you just need the steering diodes and regulaters.

Andy
 
What will be the max load of the robot, 2 amps in the rail is not alot to work with asuming voltage drop in the rails and all.
Can you run your bot off 12 volts or 18 volts then you just need the steering diodes and regulaters.

Andy

It is a first prototype we are building so all the numbers are just guestimated. Good thing is that we can do major changes to the plans at this stage and bad thing is that we don't know how much power we will need and how much current the rail system can deliver. Max load can be quite a lot because the robot will have a number of motors, a computer, laser scanner, cameras, manipulator etc.. Here is the project website: **broken link removed**

I think the best solution is to go with simple SLA battery workhorse. Maybe the next prototype will have a fancier battery system with LiPo batteries etc.
 
OK I looked at the site abit but could not find why thay are using 24 V at 2 amp for the rail system. Seems to low to do any usefull work on a big bot. More like a charging current. So your bot may have to work off batterys all the time and charge when idle. Can the rail suppy be changed? Andy
 
Cool project. Have you seen the movie "Moon"? :D

The rails big enough to carry your robot will be quite sturdy, you should be able to run a lot more than 2 amps.

The simplest way is to power the rails from regulated 27v with current limiting of 2A (or whatever you set it to) then connect to the battery through a diode. Then the robot runs from the battery.
 
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