Does anyone know of a good battery tester. We’ve had more that one battery cause cortex issues as soon as load is applied (though show fully charged).
We have had luck with a Battery Beak. Unfortunately it is a bit expensive. Don’t forget to buy the Tamiya connector or it won;t be much good to you
Bad news for us, we lost it at our competition this past weekend. 
Yes, those are very good. A cheap alternative is to build your own simple tester with switching battery from the VEX charger to the ~5 ohm passive load and seeing how much voltage will drop as a result of that.
You can build a tester for less than $5 using parts similar to these:
Mini LED voltmeter ($0.99): http://www.ebay.com/itm/171998574682
2x 10A NO+NC Switch ($3.75): http://www.ebay.com/itm/271421895638
2x 20W 5 ohm resistor ($1.57): http://www.ebay.com/itm/171834515283
5x Tamaya Connectors ($3.90): http://www.ebay.com/itm/221130866372
Can you elaborate on this design? Could you provide a sketch of the circuit and a key of what the values mean? From what I read a NiMH battery can drop 1 volt and lose its ability to provide a quality current source, so I would think a voltage drop wouldn’t provide enough information. What I read was that you have to measure the current for a recorded time period and then integrate it since it isn’t a linear function. I only ask because I want to build this myself.
Hey forum, please show your battery charger/tester setup. I have a classroom filled with batteries. Right now, they just used a regular multimeter to test the batteries. I want to create inexpensive charger/tester station for the classroom and a portable one as well. As our rookie season comes to a close, I’m worried about mixing old and new batteries. Please share!!
Use a cortex + a custom program with the battery voltage and lcd
You could put duct tape on the old batteries.
Use one of the old VEX controllers to test batteries
Can you elaborate as to why you would need the resistor? It appears that the Mini LED voltmeter can be hooked directly up to the battery? Wouldn’t a resistor mess with the accuracy of the reading?
The thing about batteries (and the resistor) is that there will be a little voltage drop as soon as some load is applied, so the resistor helps give a better picture of the battery’s capability. A poor battery will suffer a higher voltage drop under load than one in tip-top shape.
As @kmmohn pointed out, immediate battery voltage only tells part of the story. With each recharge cycle electrodes erode, some of their parts becoming thinner while other parts building up unevenly. Eventually, internal resistance of the battery cells grows and accounts for larger and larger voltage drop when it is under load.
Yes, high end testers connect battery to a set of dummy loads and then integrate voltage drop over fixed periods of time. Then they could estimate not only internal battery resistance, but the slope of the discharge curve, and effective power output of the cell. Here are Quazar’s VEX battery test curves (links are broken, click on them for the graphs).
Actually, I wanted to build a charging station with automatic testing, just as you have described, using an old cortex but it became complicated fast and, for now, I opted for a simpler solution.
Since most of the teams intimately know each of their batteries, simulating a light cortex load of about 1.5A with a 5 ohm resistor is quite sufficient. Here is the schematic of a single channel of the six channel charging station pictured below:
You connect VEX chargers to the top (yellow) wires, then batteries to the bottom (red) wires and, by default, they charge and Mini LEDs display voltage. On each battery we write voltage when it is fully charged or discharged and this way you can estimate where you are in the charging process.
If you want to do a load test, you press the button and it disconnects battery from the charger and connects it to the tester (orange wire). In the picture it is a 5 ohm resistor, but could be a fancy tester. All six channels have common ground (G) and test (T) points connected.
Nice thing about this setup is that you could see all voltages at a glance and you don’t have to pull and reconnect batteries every time you want to test them.
I built two of these and field-tested them during several recent competitions. For whatever reason, high school teams would only use two or three channels at most. However, middle schoolers were all over it - running back to the station and pressing test buttons whenever they had a chance.
The manual setup turned out to be quite practical, but automated station idea is still in the running. You can monitor charging voltage with 8 analog ports and then, instead of the buttons, switch batteries with a bank of 10A relays. I tested these and they could be driven by the digital cortex ports:
Once battery is switched to the cortex it could do sophisticated load testing with resistors or motors connected to the motor ports. USB port could both provide idle power to the cortex and be used to send test results back to the computer.
Also, I’ve got an HC05 module and will try to convince somebody from HS teams to write an app that could receive data from the cortex on the smart phone and display status of all batteries, that are charging in the pits, in the real time. Just need to figure out a simple way to relay it, because HC05 has a limited range.
The charging station looks great. I’ll have to incorporate it into our charging setup when it gets built. (if it gets built…going on 2 years of thinking about it now…)
My teammate set up a system with LED’s and the cortex to display the battery status throughout the match. It’s simple as plugging in the battery and powering on the cortex. I’m thinking about expanding this for use with the power expander as well. With some extension cables and fancy zip-ties, we can have status meters next to each of our bot’s batteries that give real-time feedback on battery status. It may be a lot of work, but it seems far cheaper and I would certainly recommend it.
A cortex is like 250$? Also running the cortex actually kills the batteries pretty quickly compared to voltmeter.
But all teams (should) have a cortex… this is on our competition bot. The one we use every day.
I’ve used the “Battery Beak” https://www.ctr-electronics.com/battery-testing/battery-beak.html
It does a good job of giving you a voltage reading and giving you some level of load testing (for the few seconds it puts a load on the battery).
For a more comprehensive load testing I use a version of this http://www.westmountainradio.com/product_info.php?products_id=cba4 I put the battery under test for 3 minutes at 6 amps (kind of worst case for a battery) and plot the curves to see how they are doing.
The CBA4 is pretty pricey, but check around with your local amateur radio club, one of their members most likely has one and the added expertise on how to set it up, run it and analyze the results.