I was wondering if Vex would consider making a Battery Discharger (designed to deep cycle a 7.2V 3000mAh NiMH battery). It could have a built in kill switch that would stop draining the battery when it had reached a certain voltage as well as a signal light that would tell you when it was finished. This device would allow VRC teams attending the Worlds and other large 2 to 3 day events to get the most performance out of their batteries by deep cycling them at least once a day during the competition.
I know that for robotics hobbyists it is not always necessary to deep cycle your batteries but in a competition setting this would be very useful. At the Worlds last year many teams had drills and light bulbs to accomplish this task. I did not and after a few days of constantly draining and recharging my batteries without fully killing them they were in pretty bad shape.
I would so buy this and I’m sure that virtually all other teams attending the 2011 VRC Worlds would also.
well that would depend on how much it was…
“we” were one of the teams that had a drill that drained the batteries
we just plugged the battery in and an attached multimeter will tell us when to disconnect it
and after discharge, they were pretty hot so they would need to “cool off” for another 30 mins
btw if batteries were used like you did, can they preform better after you did a few full cycles in the classroom?
or were they just dead forever?
thx
No after I got them home I deep cycled them a few times and they worked just as well as ever. But on the last day of the Worlds my batteries were losing their initial full power very fast.
Full cycles are the best way to use the batteries Vex supplies, but the format of a competition is understandably designed much more for “two minutes, then charge forever”.
Until such a hypothetical product came out, if it ever did, I would think of cool things you could do to discharge said batteries. One really cool idea: Make a rotating sign in your pit out of some Vex parts. It’ll take awhile to burn a battery, but it doubles as a good attention getter.
Optionally, building a little bench-top rig with a few motors geared severely high (1:7, you can go higher but you don’t want to be too unsafe) will draw a respectable amount of current even with small loads.
Personally I think for any single day competition you shouldn’t worry about battery discharges until after your event, but for a multi day event it could potentially be a problem.
Yep - An inexpensive dummy load that combined cheap resisters, variable discharge rates, something to stop the drain before damaging the batteries, and a lamp, LEDs or some other indicator that the battery was discharged would be handy.
My expensive charger has a discharging and/or deep cycling feature, but it is truly awkward and obnoxious to use.
To avoid unnecessarily stressing the battery (by pulling too much current for too long) I suspect that each discharge would take 1-2 hours. So, you would still want to have several batteries on hand during a tournament. Perhaps you do the deep discharge nightly at a multi-day tournament.
Perhaps you could hook up more than one battery to this hypothetical device?
Maybe someone can do a little research to find and test plans for building a device for under $20 in parts costs?
There are obviously many ways to get the job done. What is/are the “best” way(s) (in some specific well-defined sense) to do it?
Battery discharging is something that i think would be a great help at worlds. I think that it would be best performed nightly rather than in the heat of competition. One question that i do have for those of you who have done this before with your own setups, what is the optimum voltage to drain the batteries to before charging them back up and does this voltage change between the 2000mAh NiMH, the 3000mAh NiMH and the 2000mAh NiCad?
I feel like as far as this season goes if you are going to be cycling your batteries yo are going to be doing it with your own setup because to expect a new product to be developed, prototyped and produced by worlds 2010 is a bit over the top.
The care and feeding of rechargeable batteries is an important enough topic that you might consider adding these to the wiki. Possibly on this page, or perhaps a new page dedicated to [dis]charge theory and practice.
I haven’t seen anything from a credible authority (like a battery engineer) that indicates that there is any benefit from intentionally discharging a NiMH battery pack. What is clear is that in a multi-cell pack, you run a really good risk of damaging a single cell if you over-discharge the pack.
Until someone who knows a lot more about batteries (and knows it from industrial experience and isn’t just a hobbyist) than I do comes along, I strongly recommend that you do not discharge your battery packs until they are flat, or nearly flat.
I can’t find the link anymore since they changed their Website, but Dewalt used to have a lengthy document online outlining the risks of discharging their battery packs. Their engineers contend that it is never necessary to discharge a NiCd or NiMH pack, but since I can no longer find it, I can’t post a reference.
You/we don’t need an EE (I are one). You need a chemist. We EE’s know how to draw the symbol for a battery, and we know how to model them in our equations, but we don’t know much about the icky goo inside them or about any crystals that might grow out of the goo.
So, Is there a (al)chemist out there that can help us?
Yep - I learned quite a bit there in just a few minutes. I think I’ll make some time to go through it in its entirety.
Also, I plan to make one of these constant-current dummy loads to help with battery cycling and testing. I’ll post some pictures of what I cobble up, assuming it doesn’t melt down immediately
I’m not an analog power circuit designer, but here are some common sense notes:
a 10A-20A load is about right for 6C discharge of 3AH battery.
one datasheet for the MJ11012 main series pass transistor in that circuit is rated at 30A, 60V, 200W; so it has some margin to this application Not bad for <$5.
a 20A load x 7v = 140 Watts of heat to get rid of, so you need a really big heatsink on that TO-3, maybe an old PC case with heatsink would be appropriate, or big block of aluminum with a water drip or a bucket of ice. The heatsink may be more expensive than the transistor.
If I knew more circuit design, I’d suggest using multiple MJ11012 in series to spread out the heat, or other methods of additional series load resistors. Since this circuit is reported to work down to 3v, If final voltage is 6v, you could afford to put half the voltage drop somewhere other than in that one transistor.
I’ll be using a nice massive heatsink salvaged from some obsolete equipment, and I’ll add a fan to make sure there is air movement over the fins, so I think I’ll be fine.
I’ll be going down to 5.5V, which is the point that the Cortex battery LED goes red. This is about 0.9V per cell, which is a reasonable discharged state for both NiCd and NiMh.
I am going to omit the polarity protection rectifier, since it is pretty much a dunsel anyway (hook a Vex battery pack up backwards and the 6A diode is going to smoke first, followed by the transistor it is there to protect). Instead, I may put a full-wave bridge rectifier on the input. That would guarantee no reverse voltage, and it would provide a few volts of drop as well.
