Sparks Robotics

With 10 Cortexes and 10 VEX IQ brains being put to use by our As our 60+ 4-H club members build, program, and test their VRC and VEX IQ robots, , batteries and chargers are in constant demand. When treated well, the standard VEX batteries hold up well, but we routinely abuse our batteries.  Replacing them is expensive, so we’ve been looking for alternatives.

It’s not that the VEX and VEX IQ batteries are of poor quality. Actually, they’re pretty solid. The chargers, too, have proven durable, if simple.  However, even though the VRC and VEX IQ batteries supply the same voltage, they’re not compatible.  It’s just their size and form, either.  There are other important differences.

THE SPECS – KNOW THINE BATTERIES

VEX VRC
Material: Nickel Metal Hydride (NiMH)
Voltage: 7.2V
Capacitance: 3000 mAh
Max Output: ~30 Amps
Similar to: (6) “C” NiMH rechargeable batteries

VEX IQ
Material: Nickel Metal Hydride (NiMH)
Voltage: 7.2V
Capacitance: 2000 mAh
Max Output: ~10 Amps
Similar to: (6) “AA” NiMH rechargeable batteries (actually, these batteries _are_ 6AA batteries in a special case)

The key for find a replacement is to match the voltage and also meet, or exceed, both the capacitance and max amp output of each battery type. There is a price to be paid for skimping on these specs. In some cases, it can a very high price.

 

 

[ The guts of the VEX IQ Battery: six “AA” NiMH cells ]

REPLACEMENTS FOR THE VEX VRC BATTERY
The VRC battery is actually a common battery among R/C vehicle enthusiasts. As a result, there are plenty of direct, 1:1, replacements. You can find good quality replacements for as low as $12.00/battery (e.g. https://amzn.to/2HlzjDN). Of course, you can’t use these at official VEX competitions, but you can use them for everything else and save your official VEX batteries for official events.

Because the cortex can handle up to 9V (continuous), there are other battery options. Recently, we did an experiment with two EBL 18650 lithium ion batteries (7.4v, 3000 mAh, ~15A max output) where we powered four 393 motors (with normal gearing), while tethered to the remote through a USB cable. The results were impressive. This simple setup ran continuously for 25 mins with the motor output slammed at 100%, and the ‘bot running on a standard VEX VRC field.

Because the LI batteries are the same voltage and capacitance as the VEX batteries, it’s tempting to think they’re direct replacements. However, because their max amperage is significantly lower than the VEX batteries, the 18650 LI batteries are not recommended for configurations with 5+ motors. They’ll likely overheat, will wear down quickly, and may become damaged as a result. Under extreme conditions, there is also a risk that they’ll explode.

The good news is that the next largest LI battery is an excellent replacement. The EBL 26650 is 7.4V, 5000 mAh, and provides 40 max output. That is _better_ than the VEX battery – both in max output and capacitance. In short, they can power your 10 motor setup without fear of overheating, and will run for 25+ minutes without a drop in voltage. At $10.00 for an LI setup that will replace the standard VEX battery, they’re also less expense than even the most inexpensive 1:1 replacement.

REPLACEMENTS FOR THE VEX IQ BATTERY
Replacing the VEX IQ battery is a little trickier than the VRC. This is due to the fact that the VEX IQ battery has a special shape and special +/- terminals that allow it to slide into the VEX IQ brain and and lock into place.  With a little DIY hacking, we found a way to make the 26650 batteries work with the VEX IQ brain.

To make the 26650 batteries work with the VEX IQ brains, we had to do a little surgery with a dremel tool, a solder gun and some epoxy. Basically, we wired up a female Tamiya connector (the same one as on the cortex) to the terminals of the battery, then used epoxy to hold both the terminals and the female Tamiya connector in place, then re-sealed the case.

This setup requires an extra battery holder, as the old case is essentially an empty shell. This can create problems finding a spot to mount the 26650 batteries, but it’s a minor inconvenience when compared to the benefits. When you consider that the LI batteries are half as expensive as the VEX IQ batteries and provide 2.5x the runtime, I’m confident we can learn to live with this inconvenience.

Of course, you could make similar mods and use standard NiMH AA batteries, too. But with 5000 mAh of capacitance (as compared to 2000 mAh) the LI batteries should be able to power a six motor VEX IQ setup for almost an hour before needing to be replaced with fresh batteries.

TWO AWESOME BENEFITS OF LI BATTERIES
If you’ve used the standard VEX batteries for a competition-ready robot, you undoubtedly noticed one annoying property of NiMH batteries: the slow, steady drop in voltage that means after testing your auton successfully for 10 minutes, your robot suddenly starts exhibiting subtle, but immensely annoying, variances. This is because as NiMH batteries drain, they drop in voltage. When their voltage drops below 6.8 – 7.0V, they simply cannot provide the electrons that your motors and cortex/brain MIPS require.

Both the VEX IQ brain (via the System Info screen) and the cortex (via the RobotC debugger and/or LCD display) can tell you when this has happened, but I don’t know of many teams that actually check these values. Usually, they run their auton using the depleted batteries for a few more runs, all the while muttering some very unpleasant things and mistakenly wondering what crazy quirk of RobotC could cause such symptoms.

LI batteries, on the other hand, don’t suffer from this voltage drop. In fact, LI batteries will deliver the same voltage almost up to the point they are fully drained. That means you can run and re-run (and re-run, and re-run …) your auton without any battery-related variance at all. That’s awesome!

There is one problem with this otherwise awesome property of LI batteries. Because they don’t give you any clue that they’re nearly empty, it’s easy to drain LI batteries below 20% charge. If you repeatedly drain LI batteries below 20%, their lifespan is _greatly_ shortened. Basically, if you use LI batteries with your ‘bot, check the batteries often.

Better yet, swap out batteries often.  LI batteries charge quickly when they still have at least 50% charge, and they’ll last longer, too. As a best practice, don’t top off your LI batteries to 100% either. They’ll have a much greater life span if you pull them off the charger when they reach 80-85% charge.

Another major benefit to LI batteries: at 7.4V (vs 7.2V for the standard VEX batteries), there is no noticeable difference in performance between a freshly charged VEX battery and the LI batteries. That means if your robot behaves well with the LI batteries, it will most likely do well with the VEX batteries.

ONE CAVEAT
The EBL 26650 batteries weigh less than the VRC batteries, and a little bit more than the VEX IQ battery (when you factor in the battery case mods, extra wires, and 26650 battery holder required). This can create small, but important, differences in robot performance and behavior.

SUMMARY
Batteries are expensive, but there are good, useful, less expensive alternatives. We’re excited because we found a way to power both our VRC and VEX IQ robots with the same batteries (and chargers, too!).

We’ll post an update on our LI battery experiences after we’ve had a chance to put this new battery setup through its paces for a while.

Coming soon: DIY for connecting LI batteries to Cortex and VEX IQ brain…