RC12L3 Resto: '87 Buick GNX Build: Finished

[askbob]

That looks fantastic! Love the GN.

[Coelacanth]

So considering a battery for this car, and suggestions to use a 1S LiPo with a 13.5T motor, I browsed about HobbyPartz.com and had this idea...I could pick up a saddle-pack like this, but only use one of the cells. That way I'd essentially be getting 2 batteries for the car, for $39.95...only about $10 more than the cost of buying a single battery. I suppose using only half the pack would mean the capacity would be 2550 mAh instead of 5100 mAh, but that should be pretty sufficient run-time with a 13.5T motor.

http://www.hobbypartz.com/98p-25c-5100-2s3p-hardcase-saddle.html

Does this sound like a good idea?

[Phin]

One battery out of the saddlepack should stay at 5100mAh and have a voltage that gets cut in half to 3.7V. Which is what you'd be looking for in a 1/12 battery.

If it was mAh that got dropped to half then voltage would stay at 7.4V and that would be no good.

[Coelacanth]

One battery out of the saddlepack should stay at 5100mAh and have a voltage that gets cut in half to 3.7V. Which is what you'd be looking for in a 1/12 battery.

If it was mAh that got dropped to half then voltage would stay at 7.4V and that would be no good.

Thanks Phin...I would've thought that both voltage & capacity would be 50% if only using one half of the saddle pack...

[jwscab]

no, in series the batteries must have the same capacity, which is what the pack will be rated at.

Say you had some weird 7.4V battery series setup where one 3.7v cell was 2500mAh, and one 3.7V cell was 5000mAh. Now pull 2500mA out of the pack; it will take 1 hour to discharge the first cell, but in series, it will start dropping rapidly, while the second pack is humming along with the capacity to run for another hour at 2500mAh. the pack would essentially die even though the second pack would have capacity. the first cell drops, the second is still at 4.3v or whatever, so the pack voltage overall will decline.

this is why it is necessary to balance lipo packs, because in that scenario, the first cell will discharge below safe levels and essentially be killed.

the old saying, the chain is only as strong as it's weakest link holds true in multiple cell packs built to increase voltages. Cells in SERIES plus to minus <> plus to minus add up to desired voltage and must have matched capacity, this is why all the old nicd cells were sorted and matched together and you paid the price for the best packs.

NOW, you could take those cells, put them plus to plus, minus to minus, and then wire them to the load, and the capacity will add, not the voltage, therefore, you will have achieved a 7500mAh 3.7V pack.

clear as mud.

[Coelacanth]

That makes sense, thanks for the explanation. This will be the first time I'm dealing with a 1S LiPo, let alone break up a saddle pack into 2 individual batteries. Usually I'm good with the math.

[Phin]

It's not a bad idea to buy a saddle pack to get two 3.7v batteries. People do that a lot.

Just gotta make sure you're not buying two 7.4v shorty packs that are wired parallel to increase mAh.

[Coelacanth]

It seems my early-version Castle Sidewinder ESC may have a compatibility issue with 1S LiPo. It was working fine in another car on 2S LiPo, makes beeping noises and flashing lights as expected when turned on, but I get no beeps at all when starting up this car and one 3.7V LiPo (half of a saddle pack). It *does* flash all the lights and I can calibrate it just using the lights, but there's no beeps. After calibrating, the correct lights function when I give throttle or reverse, but nothing outputs to the brushed motor. I tried a different brushed motor with the same results, so it's unlikely that the motors are the culprit.

I've also played around with settings with the Castle Link software and another thing that makes me think 'compatibility issue' is when I select "Custom" for low-voltage cut-off, the lowest value I can choose is 4V, which is way too high for 1S LiPo. (The default is "Auto LiPo", which is supposed to use 3.2V per cell, but I think this ESC is expecting at least a 2S LiPo to be connected.) I even tried no cut-off but still no output to the motor.

Do any of you Castle gurus know if this is the case? I have another ESC I can try but I have no idea what the make/model is; it's a sensored unit but I'm running unsensored & brushed motors, I don't know if it'll even work.

[Coelacanth]

Since I seem to have run into an obstacle with ESC & 1S LiPo compatibility, I gave up on finishing the electronics and began fabricating a custom battery hold-down. It has been quite a challenge as this chassis was never intended to house a hard-case LiPo. I found that the battery was taller than the lateral carbon piece by just enough for me to cut out an L-shaped piece of carbon, then bolt it to the lateral piece to keep that corner of the battery positioned. I'll use the closer screw-hole to add a tab over the top of the battery which will screw down with a wing-nut or something.

I decided to butcher an old computer tower I/O slot cover as it had an L-shape already formed into it, and it's very thin, and seems strong enough to not bend too easily. I made corner tabs in each section and used a file to round the corners and smooth all the edges. The battery will fit like a glove. I sanded it down and it's waiting for a coat of SEM Trim Black to dry. The foam tape at the top right corner of the battery is just to protect it from abrasion when it's pressed snugly against the L-shaped piece.

All that's left to figure out is a way to secure the front of the battery.

[Coelacanth]

Progress pics of the custom battery holder. I either have to attach these to the chassis with double-sided tape or epoxy, because using screws would mean the battery couldn't sit flat against the chassis plate. I think the automotive-grade double-sided tape should be secure enough, and if not, isn't permanent...I'll try that out before perma-gluing the holders to the plate with epoxy.

The battery fits snug as a bug in a rug.

Now to figure out a way to secure the front of the battery...one idea is to cut an aluminum or plastic post to the correct length, drill a hole in the chassis plate and screw it in close to the bottom-left corner of the battery, then I could easily make a thin hold-down strip that would run diagonally from the top right (where the L-shaped piece is) to the bottom left. That along with 3 corners being secured by the metal brackets & L-piece should be pretty solid...anybody have other ideas?

[Coelacanth]

Battery hold-down finished. Even though this is my first Associated build, wouldn't you know it...there's a bit of Kyosho blood pumping through the veins. That's right, to fabricate the actual hold-down part, I chopped up a piece of Optima top deck that actually had screw holes in nearly perfect locations already.

[FullThrottleTXOutlaw]

Battery hold-downs for these old associated cars make a dummy out of me. I want to make something that holds the cells firmly in place, is easily removable, looks either trick or OEM, and costs less than $20. I have yet to come up with an idea I would call good.

[Coelacanth]

Battery hold-downs for these old associated cars make a dummy out of me. I want to make something that holds the cells firmly in place, is easily removable, looks either trick or OEM, and costs less than $20. I have yet to come up with an idea I would call good.

Well, this certainly cost less than $20...some leftover sheetmetal, double-sided tape, old Parma body post cut into pieces and a couple body clips...functionally, this definitely secures the battery. For the cheap cost, it took a fair amount of work though, and as for looking trick or OEM and being easily removable...that's up for debate.

[Coelacanth]

Finding one-cell ESCs that function with 1S LiPo turned out to be quite the search. Very few actually exist. And I was unpleasantly surprised to discover that one-cell ESCs typically cost a LOT more than the commonplace two-cell-plus ESCs. This Turnigy Trackstar One Cell (Gen II) was the least expensive I could find. So in a week or so, I should be able to wrap up my first-ever Associated build.

http://www.hobbyking.com/hobbyking/store/__46906__TrackStar_GenII_One_Cell_120A_1_12th_Scale_Sensored_Brushless_ESC_ROAR_BRCA_approved_.html

I'll use my Castle Sidewinder ESC for the "Red Tide" Agitator build instead, which I'll mod to take both cells of the same Gens Ace saddle pack I purchased to use with this 1/12 car.

[Coelacanth]

I needed a bumper to protect the body because the only thing I had for RC12L3 parts were these triangular plastic spacers that mount underneath the body posts and protect the chassis plate corners. I had to fabricate a bumper out of spare Kydex that would mount to the existing chassis plate holes, but also stick down just far enough to protect the chassis plate. I also got around to shortening the body posts to better suit the GNX body. I'm gonna have to struggle to get this completed before the upcoming month's 1/12-scale pan car challenge.

After giving it some thought and some sketches and careful measurement, here's the bumper test-fit:

And the completed bumper, after some minor trimming with a Dremel and lots of filing & sanding the edges, and removing some sections to reduce weight: