VEX Power Accessories to Transition to Nickel-Metal Hydride (NiMH) based Batteries

VEX News
1

The VEX Robotics Design System is beginning a transition to Nickel-Metal Hydride (NiMH) based batteries for its Power subsytem. With this transition comes the addition of (3) new products: the smaller 2000 mAh 7.2V NiMH battery, the standard size 3000 mAh 7.2V NiMH battery and the VEX Smart Charger.

http://content.vexrobotics.com/images/products/276-1456.jpghttp://content.vexrobotics.com/images/products/276-1491.jpg

The new 2000 mAh 7.2V NiMH battery (above top left) has the same capacity as the 7.2V NiCad battery in a much smaller form factor. The new 3000 mAh 7.2V NiMH battery (above top right) offers 50% more capacity in the same form factor as the previous model 7.2V NiCad battery.

The transition to NiMH based products from the current Nickel Cadmium (NiCad) ones is necessary for several reasons. NiMH batteries consistently outperform NiCad batteries, as they deliver more power, last longer and can be put through more charge cycles.

In addition to the performance enhancements offered by the NiMH batteries, the fact that they have no Cadmium added is good news for the environment. Cadmium is a toxic heavy metal and can cause substantial pollution when disposed of improperly.

The VEX Smart Charger (above bottom center), with (4) different regional power cord options, is capable of charging ALL VEX rechargeable power packs (7.2V and 9.6V, NiCad and NiMH). The charger features dual charge rates and an automatic shutoff.

**WARNING: The new NiMH batteries must be charged on the VEX Smart Charger. DO NOT use the VEX Fast Charger to charge NiMH batteries. **

Follow these Smart Charger settings for each battery:

Fast or Safe Setting:
7.2V, 3000 MAh NiMH (standard)
7.2V, 2000 MAh NiCad (old model)

Safe Setting Only:
7.2V, 2000 MAh NiMH (small)
9.6V, 960 MAh NiCad (transmitter)

7.2V NiCad batteries will be discontinued as soon as our current stock is depleted. There are no plans to discontinue the 9.6V NiCad Transmitter battery in the near future.

The VEX Power Pack and Fast Battery Charger have both been discontinued.

Any VEX branded 7.2V battery (NiMH or NiCad) will be legal for use on robots in the VEX Robotics Competition.

The new batteries will be available on VEXrobotics.com in the coming weeks.

The VEX Robotics Team

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We bought a few of the 2000 mAh NiMH batteries at worlds and made a few notes.

Firstly, one experience I had during the competition.

We charged them until the lights were green using the safe profile, tried running them during a match at worlds and by the end of the match, our robot was running noticeably slower with six drive motors connected to the power expander, powered by the NiMH batteries. The light on the power expander had turned orange by the end of the match.

I’ve also noticed other strange behavior at worlds. Immediately after charging the NiMH’s the first day we received them, we measured the voltage at 8.50v +/- 0.05v (I checked this after I came home after worlds). We didn’t use them the first day but rather used them the second day. After the situation described above, I checked the other charged NiMH’s we had and found that the voltage had dropped to between 8.0 and 8.1v. Is this normal behavior? Should they be left in the charger until they’re required to be used?

Also, the manual said the fast setting was not for transmitter batteries, but nothing about the 2000 mAh batteries. We have set it to fast using those batteries and all we’ve noticed is that they heat up but it didn’t appear to damage the batteries in any way. I’d imagine that doing that repeatedly would cause some long term degradation but is there any actual danger of damaging the battery by setting it to fast?

3

I would be very interested in weight specs of the batteries. I have been really into the new weight specs for some of the products on the site and batteries seem to be a big portion of the robot weight. Does anybody know the weight of the new small batteries as well as the weight of the current batteries (the standard ones). I have the standard batteries now but no way of accurately weighing them.

~DK

4

I don’t have the exact numbers but I remember saying to team members that using two NiMH batteries saves almost a pound of weight. I don’t have the exact numbers with me right now though.

5

They look pretty cool! Have to get some!

6

We also had the same experience running only 4 motors. We went back to the old batteries for the rest of the mathces.

7

we also got the same expericnce as lumi
they were green, but when we used them in competitoin, they were WEAKER than the standard batteries (the power expander light turned orange in the middle of the match)
we didnt use them again at worlds
another note, we took all the batterys and chargers to our hotel so we have fresh batteries the next day
but the members in the room said that they charged the new batteries on “fast” mode, and they were “sizzling”
and they were also too hot to touch (no joke, you couldnt even hole it for a second)

8

What happens to the 9.6V transmitter battery if it is charged in the “Fast” mode? Will it become dangerously hot? Will the battery be destroyed?

Eventually, someone is going to forget to check the switch…
Do I need to put all the chargers in “Safe” mode and tape over the switch?

9

In my personal experience with a third party charger for ni-cads, we went with the Amp-hours*2 rule for amperage limit. We charged transmitter batteries at 2A and robot batteries at 4A, during which neither got excessively warm. I’d imagine that leaving it accidentally in fast for charging batteries wouldn’t cause any immediate noticeable damage but could possibly cause some long term damage.

10

a few comments and questions.

  1. how much will the new 3000 mAh batteries cost and when will they be available for purchase in Canada?
  2. i did notice that the batteries got a little to hot for comfort with the new “smart chargers” are there any solutions?
  3. is it just me or does it seem like all these new batteries and other new parts are giving an edge to teams who have more money than others?
    ex. 4 batteries + 2 chargers + 4 new motors ~ $240 american
  4. because of the increased current with the new batteries does that mean that only the new cortex micro controllers will be able to handle the increase in current? or will the old v.5 micro controllers work?
  5. referring to number 3. I did the math in my head and one of the new batteries is equivalent to 1.5 of the old batteries so with use of the power expander some teams could run the equivalent of 3 old batteries with 2 of the new ones.
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Batteries running out fast
All Ni-MH and Ni-Cd batteries need to be cycled a few times before they will hold their full capacity (especially Ni-MH). The 2Ah Ni-MH batteries can only output a maximum of 6 amps, instead of 10 amps like the Ni-Cd batteries. This is due to the Ni-MH batteries being built from AA cells rather than Sub-Cs and can cause the microcontroller to reset due to voltage drop. It would also cause the power expander light to go orange or red with a lot of current draw.
If cycling the batteries, do not discharge below .8 volts (preferably 1.0 volts) per cell. This will mean 6v for the 7.2v batteries, or 8v for the 9.6v batteries. Discharging below this level will cause the cells to become unbalanced, and some cells can become overcharged, while others are discharged so much they become reverse polarity.
**
Safe/Fast settings**
Most Ni-MH batteries can be charged at 1C (a full charge should take up to 1.5 hours at this rate because Ni-MHs are about 67% efficiency).
The charger outputs 1.8A on fast, or 0.9A on safe, so it seems like it should be fine to charge the 2Ah Ni-MHs or 1Ah transmitter batteries on fast, except that due to ohms law at 7.2 volts it will actually output about 4A on fast or 2A on safe. Charging these batteries on the fast setting will be fine, but repeated charging at this rate will reduce their lifetime. Ni-Cds like the old Vex 2Ah packs are often charged in 15 minutes connected straight to a 12v car battery and still have a reasonable lifetime (electric car racers often use this method). Ni-MH batteries are a lot more sensitive though.
**
Batteries getting hot**
Ni-MHs have a different chemistry than Ni-Cds (obviously), and one of the reactions that happen when the batteries are charged is exothermic instead of endothermic as in Ni-Cds. This is why Ni-Mhs get a lot hotter. The batteries should be fine up to about 60 degrees celsius. Most AAs have safety vents that let electrolyte leak out if there is to much pressure from charging to fast (like 15 minute chargers).