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View Full Version : Battery re-charging: Removable or in-place?



jwatte
01-28-2013, 11:28 PM
I need to make a decision on whether to make my battery easily removable or not, with an engineering trade-off involved. I'd *love* some advice from those who have been to this rodeo before!

Right now, I'm making my body plates with reinforcing ridges (L-beam style.) Those ridges make the plates light and strong -- I can cut large holes in the plates (saving weight) and still have a very stiff frame.

The L-ridges also "lock" my battery pack in place. The pack is as thick as the distance between the mounting flanges of the servos. Thus, it fits very nicely... but it's not removable. This means I have to charge my robot in-place. Charging a LiPo shouldn't be done over about 1C rate, and as it gets fuller, you will get less than that, so a full charge from empty will take about 90-120 minutes. The robot idles at just below 1C as well, so it can idle for over an hour -- more like 20 minutes of runtime if it's running flat out with guns blazing all the time. Except I'll run out of ammo before then :-)

How will scheduling take this into account? Assuming my robot advances from the first bracket, will there be 90 minutes between match for a particular entrant? I could see pessimal scheduling letting me win the last match of the first bracket, only to be matched up in the first match in the second bracket.

Another alternative is to re-design the brackets or mounting to make the battery replaceable. I already have two packs of the same kind, and milling down the ridges is reasonably simple, at some cost in stiffness. Right now, the play in the MX servos is my main source of give, so I probably have some margin.

Slugman
01-29-2013, 12:31 AM
Apart from scheduling, locking in the battery during charging is like 'buying' an evil lottery ticket. It's chance vs consequences.
Chance of LiPo bursting into flame during charging = Low.
Consequences of afore mentioned LiPo/flame event = Ohmygodthehorrorwheresthepliersfekitfekitfekitnooo oooooooooooo.

Th232
01-29-2013, 01:06 AM
Slugman's observation is precisely why I'm going for a detachable battery pack for my robot. Slide out tray + a plug that requires a low insertion/removal force so I can just yank it out in one motion.

byi
01-29-2013, 06:12 AM
i would go with removable. less worries. i know personally i killed on of my batteries last time. i would have been screwed if it wasnt detachable.

Gertlex
01-29-2013, 11:51 AM
I'm all for removable batteries...

Remember matches are not 20 minutes long (8? too lazy to look :D).

I would personally design for the battery opening to have a different approach to re-inforcing the body plates. E.g. run a strip, or X of aluminum between the nearest servos' mounting holes as a structural brace. I could be imagining the setup completley wrong, though :)

Upgrayd
01-29-2013, 12:35 PM
Having an easily replaceable battery is a must. We run matches back to back and if you are in the finals you will be playing in very quick succession without much time to charge up a single battery.

jwatte
01-29-2013, 12:44 PM
I do have a second battery, and I can swap the batteries with about 15 minutes of screwdriver usage. I basically have to detach all of the hip servos to be able to pull the plates apart. The simplest solution to being able to slide out the battery is simply to turn the bracing ridges the other way, as long as they don't interfere with the movement of the hip leg brackets (should be fine.)

If so, I need to find another way of actually keeping the battery stuck in there, though. Wouldn't want it sliding out during a match :-) On my rover, I use velcro, which is great, but adds 1/4" or more of height, which I don't have.

To describe what the setup looks like: think of 2mm aluminum plates, with the corners notched to fit the servos, and holes drilled. Now, add ridges around the outside of this plate, that stick up from the bottom plate, and down from the top plate. I make this whole thing out of 6.35mm thick aluminum, milling it down to 2mm (or through, for holes,) so the ridges are 4.35mm high. The battery runs the length of the body (it's slightly below 5" long, and the body is 5"x5") between the servos, so the only place to put reinforcing ridges between the two front servos, (and between the two back servos,) is right at the front/rear edge of the plate.

I'm actually considering making the plate thicker where the servos mount (say, 4mm instead of 2mm) which will give me a total of 4mm of additional spacing. I can then shave a few mm off the ridges (perhaps as little as 1.5mm each) and get a hole big enough to slide the battery out.

This really needs pictures or sketches, doesn't it? :-)

I'm using Deans style connectors, which are not super easy to disconnect, because they are rated for 50A. Allegedly, XT-60 is newer, just as good, and easier to disconnect/connect. I'm not sure it's worth upgrading everything right now, though.

jwatte
01-29-2013, 12:46 PM
Having an easily replaceable battery is a must.

Well, that clears that one up :-)

Gertlex
01-29-2013, 12:56 PM
Jwatte, put some holes at the battery exit on both plates. Then put these (http://www.amazon.com/s?ie=UTF8&field-keywords=hair%20pins) through the holes. :cool:

jwatte
01-29-2013, 02:45 PM
Actually, I was going to go with these, (http://www.amazon.com/gp/product/B00ART2X1I/ref=as_li_ss_tl?ie=UTF8&linkCode=as2&tag=enchage-20) but the effect is the same :-)