The Sparkfun videos seem to indicate that you can use NiMH batteries. When I tried that, it did not provide enough voltage to run the RPi. Regular Alkaline batteries work fine. What is the minimum voltage required? I have my XRP set up in WPILib mode. Any ideas? Thanks!
Yes, NiMH batteries are fine to use, I use them on my XRP without issue. In my testing, the Control Board will operate down to about 3.5V on the barrel connector before browning out. NiMH batteries are nominally 1.2V each, but get down to about 1V when fully discharged. With 4 batteries in series, that should be at least 4.0V, or 4.8V nominally, which is plenty for the Control Board to turn on. (FYI fully charged NiMH batteries are typically closer to 1.5V, so 6V total when in series, just like Alkaline batteries)
When you insert your NiMH batteries, could you please measure the voltage at the barrel connector with a voltmeter? If it’s much lower than expected, double check that all batteries are inserted in the correct orientation, and check the voltage on each individual battery to ensure they’re charged. Also make sure none of the batteries are completely dead and blocking all current somehow.
Something else to note - the internal resistance of the batteries can cause the output voltage to drop when more current is drawn from them. This could cause brownouts if your batteries are almost fully discharged and you create a high load, such as suddenly reversing or stalling the drive motors or servo. However this is usually only temporary (very brief, on the order of milliseconds), and should not be an issue when the motors and servo are inactive.
Hope this helps!
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An additional point/question.
With a custom mount, could a pair of 18650 Lion batteries work? At nominal/storage voltage they will be 3.8v each, thus they would be 7.6v. At full charge they would provide 8.4v for a set in series. Is that too much?
Yep! Any voltage up to 11V is supported by the barrel connector on the control board, as indicated by the silkscreen next to it. Just be aware that the motors are driven by the input voltage, so providing a higher voltage will have a few effects:
- The motors will spin faster.
- The motors will produce more torque.
- The motors will consume more current. This will lead to increased heating, especially in high torque/stall scenarios, which could lead to them burning out if you’re not careful. So be careful
Recently, I purchased the following rechargeable batteries for our XRP robots. During our classes, their robots are constantly in motion as they are completing assignments. The longest class time was 4 hours.
Rechargeable AA Lithium 1.5V Batteries - Full-Recharged 4 Pack with 2H Ultra Fast Charger for Household Office Devices, Long-Lasting High-Capacity 3500mWh, Cycle Times up to 1300x, Constant Output
$43.99 CAD
https://www.amazon.ca/dp/B09R46MMZY?ref=ppx_yo2ov_dt_b_product_details&th=1
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I have had a student running ours on the worlds cheapest ni-mh batteries from Amazon and it seems to be working fine. The batteries are charged each night but certainly run enough for a class period and after school (one hour plus due to US FIRST robotics season)
The students reported that the 6v vs 4.8v that we were using with the NMIH batteries did make a significant difference so we did it to all 10 of our XRPs.
We secured the new battery holder with 3M VHB trim tape and while we were at it secured the 4 AA holder into the chassis with a strip of VHB tape.
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