detecting life in batteries

[wkethman]

I want to be able to determine the life left in a batter. I am not sure if it is best to know current or voltage and if there is a way to predict how a motor will react. Say if you want to operate a vehicle over a certain speed but when the batteries are close to now being able to support that speed a warning is given or a system shutoff instead of dropping below that speed. the speed the motors can't go under would be the set point but you have to predict when its going to go below that speed so using maybe a voltage or current sensor woudl be best. I am really unsure about this so if i said something wrong dont hold me to it. please list specific sensors or link to them. does need to sense anything above 24 v but maybe a safe band so 32v or something. thanks guys

[The Lucas]

I'm still not sure what exactly you want.
What you might want is a battery load tester. It tests the battery's ability to maintain voltage under a load. It is the easiest way to determine if a battery is defective.
If you are talking about testing a good battery while you are using it, a voltage sensor would tell you when the battery is discharged.

[KenWittlief]

I think the application being described is a battery charge meter. The best way to do this is to measure the current being supplied by the battery along with the voltage. Think of it more as an electron counter. For any given capacity battery you can 'pump' a fixed number of electrons into it. If you then keep track of the current it is supplying while being used you have a pretty good idea of how much power the battery has left.

This is very tricky stuff if you want to do it well. Some of the things that make it complicated are:

1. The voltage across the battery terminals is a function of both the amount of charge remaining and the current being drawn at that instant. The more current you draw the lower the voltage across the terminals. When you stop drawing current the voltage will go back up! So a simple voltmeter is not enough to tell the whole story.

2. The temperature of the battery affects its capacity. At -20F a lead acid battery looses half its capacity. Depending on the current level the battery will heat itself up to some degree. This makes it very difficult to measure or calculate how much energy remains in the battery.

3. Every time you charge and discharge a battery its voltage curves drop slightly. As the battery gets older its capacity drops, and the voltage at each point of its discharge curve will be lower. This also makes it difficult to know how much energy is remaining.

As you might have guessed by now, a good battery charge 'meter' is not a trivial design. In fact, for things like cell phones and laptop computers a lot of design effort goes into that little icon on the corner of the screen to be able to accurately say "you have 29% remaining".

[wkethman]

wow. so no one uses a sensor like this on their robots to automatically power down certain systems that are less critical on their bot when the battery is almost gone?

[Mike AA]

Originally Posted by wkethman

wow. so no one uses a sensor like this on their robots to automatically power down certain systems that are less critical on their bot when the battery is almost gone?

Speaking from experience and from hearing many other teams, most swap out a battery after one or two rounds as to not worry about impairing the robot capability. While there is an ability to measure with a custom circuit and tell the controller there is low power turn off this device, I don't recall hearing of anyone using such a device. This would be a very simple circuit if you wanted to make it, but build at your own risk. you would have to take the power at the breaker panel (fed off a 20 breaker of course!) and feed to a circuit that will send a signal to the controller telling a relay or victor to turn off if voltage gets to say, below 11.2 volts. But the biggest problem you may have is the battery can take deep discharges for a second or two then be fine for the rest of the match.

As I said, an easier solution is just to swap out batteries each match with a freshly charged battery.

-Mike

[Al Skierkiewicz]

William,
There is a method to this madness when it comes to our robots. The dashboard port already gives you the battery voltage as it is read at the RC terminals. If you coupled that to current monitors for the various high current draw motors in your robot you could correlate what systems are drawing high current and when is the RC in danger of cutting out (<8 volts). You could then either automatically shut certain systems down or alert the drive team that they are dangerously close to shutting down and let them determine what to do. We used this method a number of years ago with StangSense, a custom circuit that monitored current in our drive motors and fed the data to a color PDA. The software guys wrote a graphic of the robot for the PDA showing the four drive motors and they went from green to yellow to red as the current went from low to high. Another flashing indicator then showed that the battery was getting close to the level at which the RC would drop out. Remember this was in the days before the backup battery was used.