HPI S2 BATTERY HOLDER MOD for USVTA/ROAR Approved HARD CASE LiPos
The size of the files to accomplish chronicling this mod pictorially exceeds the forum limitations in place for uploads.
I recommend that you copy them to you pc in case an unforeseen circumstance arises.
The clearance from the bottom of the battery holder to the chassis is approximately .90” (22.86mm).
The thirteen batteries I have researched so far range from 22-25.14mm thick. This translates into a ROAR approved Hard Case LiPo battery necessary to run legal in USVTA won’t fit easily - if at all.
The upper deck to chassis clearance is approximately 1.095” (27.813mm).
By relocating the battery holders completely you can install those 25mm thick ROAR Approved 7.4v 2S 5000mAh Hard Case LiPo Batteries that are USVTA legal in the S2. However, you will need to do a BELT LIFT MOD as well. One is in post# of this thread.
The max forward room (battery width) is a about 49mm – this isn’t cast in stone because I didn’t disassemble my S2 to verify it - BUT I had a 49mm soft case stuffed in there… just sayin’.
The 25.14mm thick ROAR approved SKY LiPo will fit as well and the belt should clear with the Belt Lifter Mod in post #2 of this thread. If not, swap out the two 3x6x2.5 bearings and the two M3x8 washers for two 3x8x2.5 bearings and two M3x10 washers.
The waterproof receiver box (if you’re using it) will have to be relocated forward.
Ignore the 8-32 Allen head bolts in the pictures – just pretend they’re not there.
DELRIN 1/4" diameter rod (seller info is included in the attached ZIP file)
2 M3x12 Button head Phillips screws (for the forward posts top)
4 M3x8 Button head Phillips screws (for the battery mount holders – these replace the flat head screws)
2 4-40x1/4 Allen head screws (for the new – to be made –battery mount hold downs)
12 M3x8 OR M3x10 washers (anywhere you see M3x8 washers you can use M3x10 washers)
Team Associated #91186 Saddle Pack Foam
1/4”, #29, #39 drill bits
220 grit sand paper
Modeler’s saw (I use a #43 blade)
Modeler’s flat & round files
X-Acto knife (I use a #11 blade)
Automatic center punch
I have attached a ZIP file that includes the seller info for the delrin rod
Remove receiver box screws, unplug the ESC wire – move the box out of the way for now.
Remove the battery holders = remove the hair pins from the rear plastic posts, swing the holders off & out from the plastic posts, unscrew top & bottom screws from the aluminum posts – slide assemblies out - twist the off holders off of the aluminum posts – replace the aluminum posts with bottom screws only.
Remove the ‘clawed’ holders from the wing plates.
The 'Clawed’ Holders
Slot the battery mount holders – cut a piece of Team Associated #91186 Saddle Pack Foam to fit the inside of the ‘claws’ on the holders with a little overlap on the sides – apply the foam to the inside of the ‘claw’ on the holders = holds the battery more firmly VS not. Trim to final fit with a sharp X-Acto knife.
First row on the left going down to the last picture = When using the Reedy Wolfpack style of wire harness – 4mm bullet connectors for + & - ports and 2mm bullet connector to the balance port = you will have to mod the holder as shown. It’s not an exact science.
With the foam installed on the inside of the ‘claws’, but the wire harness NOT installed, position the holder on the battery as it would be if on the car.
Mark on the where you need to remove material to accommodate the harness wires. I used a Dremel with a routing burr to rough out the material, a rat tail (round) rasp file to fit with minimal wiggle room and 220 grit sand paper wrapped around a round Bic pen to finish.
Do one only holder – in case you decide to change to batteries with a different wire harness configuration later on
The Wing Plates
Drill out the original pivot holes to 1/4” and slightly ream one side of each of the holes. Test fit the rod – if it’s too tight lightly sand the inside of the hole so that the rod slides though easily. You need this clearance for the CA to fill in and affect a good bond.
Square the end of the 1/4” delrin rod – insert the rod through the hole until it sticks out about a 1/2” on the reamed side - apply a narrow ring of CA around the center of the rod where it will be when drawn back through the hole - pull the rod back through the hole rotating as you go - stopping just short of flush – let the CA set up – cut off the excess rod – sand both sides of the rod plugs flush to the wing plate – this is best accomplished with the sandpaper on a hard flat surface. Next, drill a hole in the center of the delrin plugs with a #29 bit.
Use the four M3x8 Button head Phillips screws with one M3x8 washer each for the slots.
Reassemble the holders and wing plates.
Use one each M3x12 Button head Phillips screws and backed by one M3x8 washer each for the newly plugged pivot hole to install over the upper deck into each aluminum post.
Use just a slight dab of medium Loc-Tite half way up the thread – just enough to grip the screw in the post not enough to lock it so much that you will need jawed pliers & a blow torch to get the darned thing out if necessary. Turn the screws all the way down then back them off about 1/4 turn.
, center punch a pilot hole in the center of the top rear plastic posts – drill a hole with a #39 bit / you can run the bit down slightly past the hair pin holes – just make sure that you go straight / screw in a 4-40X1/4” Allen head until it bottoms / go slow it will self-tap – remove them.
Cut off two pieces of Popsicle stick to size /drill a #39 hole in the center of each / back up the 4-40x1/4 Allen head with an M3x8 washer and screw it into the Popsicle stick – keep screwing until it turns free.
Make two = we will call these battery holder hold downs.
Replace the hair pins into the top holes of the plastic posts.
This provides a solid backing for the battery holder wing plates when the new hold downs are installed – unlike the unsupported OEM design.
Rotate the plastic posts slightly so that you have a flat on the bottom facing the battery. If necessary retighten the plastic post screws in the bottom of the chassis.
Trim the ridge on the chassis so that the battery sits flush to the chassis.
To do this, place the battery in the car holding it tight to the rear and chassis.
Score the ridge where you need to remove material – remove the material and test fit again.
Trim as needed until the battery is flush to the flat of the plastic post and the chassis.
Trim to fit & apply Team Associated #91186 to the bottoms of your hard case battery packs - this reduces battery sliding and provides shock absorbing effect.
Popsicle Stick Shim
This Popsicle stick shim works for 47mm wide hard case battery packs. If your battery is more or less than 47mm then you will have to compensate for that.
Use another Popsicle stick with Team Associated #91186 applied to one side as a shim for the front of the battery – foam side facing the aluminum posts = this holds the battery tight. The foam will take a set to the aluminum posts = this keeps the shim from sliding out and the battery from moving forward.
Currently, the ROAR approved 2C 5000mAh Hard Case LiPos on the (WIP) battery fit list run 45-47mm.
The Popsicle sticks that I have = 2.159mm and the Reedy Wolfpack battery is 47mm wide. T
he foam added to one side causes a press fit – nice and snug.
So if you are using a 45mm wide battery, the easiest method is to just add more foam to that one side to compensate until you have a nice snug fit.
I do not foresee the battery moving upward during normal racing use. However, if it does another shim – perhaps a length of balsa wood (from your LHS) with foam applied press fit between the bottom of the upper deck and the battery should work just fine.
Install the battery. Swing the battery holders into position – install the new hold downs. Add the shim.
Adjust the battery laterally for weight distribution /slide the holders as necessary / tighten the four screws.
Receiver Box Relocation
You will have to reroute the ESC wires back under and through the center of the upper deck making sure you have enough clearance away from the belt.
Using the tilted holes on the box, install the rear screw in that hole on the rear box – screw it into the front mounting stump on the chassis – align the box – locate & mark for a hole on the upper deck / drill hole with #39 bit – trim down the ridge on the upper deck to make clearance for a stack of four M3x8 washers as spacers to shim up the front of the box - install the front screw slowly and straight, it will self-tap, tighten both screws.
delrin copy.jpg - Shortcut.zip
Everyone has their own voodoo. This is my voodoo. This is what works for me