Battery Mounting
Stage 1E · Subsystems — Locating, retaining, and modeling the FRC battery in Fusion 360
The Heaviest Single Part
The MK ES17R battery weighs ~12.4 lbs.
- Roughly 10% of your whole robot's weight.
- Where it sits dominates center of gravity (CG).
- A loose battery = instant match-ending failure.
- Mounting is a real subsystem, not an afterthought.
Open with the scale: a 12-lb lead-acid brick is the single densest thing on the robot. Students underestimate this constantly. Stress that the battery is heavy enough to move CG by inches depending on placement. Mention that a battery that pops out mid-match has lost teams events — this is why robust retention is a rule and a habit.
Know Your Battery
FRC legal: 12V sealed lead-acid (SLA).
- Common: MK ES17R or similar ~18Ah.
- Size roughly 7.1 x 3.0 x 6.6 in.
- Two ~1/4in lug terminals, often with Anderson SB50.
- Model the real envelope — measure or grab a STEP.
Have a physical battery on the table. Pass it around so they feel the weight and see the terminals. Note brands vary slightly — always model the actual battery your team uses. Tell them AndyMark and others sell STEP files of common batteries; otherwise model a simple block at the measured dimensions for packaging.
Where To Put It
Low and centered lowers CG and stays stable.
- On the belly pan, between the drivetrain rails.
- Keep it inside the frame perimeter, never overhanging.
- Balance side-to-side so the robot drives straight.
- Leave clearance for the SB connector and a hand.
Reinforce: low CG prevents tip-overs, especially on swerve where you accelerate hard in any direction. Centered helps the robot drive predictably. Common mistake: jamming the battery in a back corner because there's space — that skews CG and makes the robot pull to one side. Remind them a human must physically swap this battery in under a minute between matches, so access matters.
LOW AND CENTERED WINS MATCHES
The battery is your biggest lever on center of gravity — spend it on stability.
How It Stays Put
Three jobs: locate, cradle, and clamp down.
- Walls or a tray locate it horizontally.
- A strap or bracket clamps it vertically.
- Battery must survive hits, tips, and full rollovers.
- Tool-free swap is a huge pit advantage.
Break retention into the three jobs so it's not mushy. Locating = walls/tray keep it from sliding. Clamping = strap/bracket keeps it from lifting out when the robot tips. Emphasize 'survive a full rollover' — referees and physics don't care that you thought it'd stay. Praise tool-free designs: at competition you swap batteries between every match against the clock.
Strap Vs Bracket
- Fast, tool-free, forgiving fit
- Hook-and-loop or cam-buckle strap
- Anchor to slots or rivets in pan
- Most common FRC choice
- Metal bar or printed clip over top
- Bolts down, very secure
- Less tolerant of battery size variation
- Heavier, slower to swap
Most teams use a strap because it's fast and tolerant. A rigid bracket is bombproof but adds weight and a tool to every swap. Recommend strap for rookies, but show that a hybrid (tray walls + one strap) is the sweet spot. Mention anchoring matters as much as the strap itself — a strap is useless if its anchor rivet tears out.
Set Up A Component
Right-click > New Component, name it Battery Mount.
- Activate it so new bodies land inside it.
- Insert the battery STEP or model a block body.
- Position it on the belly pan, in the frame.
- Use Align or a Joint to seat it precisely.
Demo live: right-click in the browser, New Component, rename it immediately — naming discipline saves you later. Activate the component (double-click) so bodies you create belong to it, not the top level. This is the single biggest Fusion organization mistake students make: modeling everything loose at the root. Insert > Insert McMaster or insert a STEP for the battery, or model a placeholder block.
Build The Tray Walls
Sketch the battery footprint on the pan.
- Offset ~1/16in for fit clearance around it.
- Extrude walls up ~1.5in to cradle it.
- Add a cutout for the SB50 lead exit.
- Fillet or chamfer top edges to ease drop-in.
Show offsetting the sketch profile outward so the battery actually drops in — a zero-clearance tray won't fit a real battery with a manufacturing tolerance. Walls ~1.5in tall locate it without burying the terminals. Don't forget the lead exit notch; students model a perfect box and then can't route the connector. Chamfer the lip so pit crew can drop the battery in fast and angry.
Add The Strap & Anchors
Model the strap as a thin swept body.
- Sweep ~1in wide over the battery top.
- Place anchor slots or rivet holes in walls.
- Use User Parameters for strap width/thickness.
- Keep it as its own body for clarity.
A strap is a sweep: draw a path arcing over the battery, sweep a thin rectangular profile along it. Tie strap width and thickness to User Parameters (Modify > Change Parameters) so you can tune them once. The point isn't a perfect strap model — it's reserving the space and proving the anchors land somewhere solid. Check the anchors aren't floating in mid-air over a hole in the pan.
Check Cg & Clearance
Assign the battery body a real mass.
- Use Modify > Physical Material or Properties.
- Inspect > Center of Mass to see CG.
- Confirm the swap hand-clearance and lead routing.
- Run interference check against nearby parts.
This is the payoff of modeling it properly. Override the battery body's mass (~5.6 kg / 12.4 lb) so the assembly CG is honest — a hollow block reads near-zero otherwise. Inspect > Center of Mass shows whether you actually got it low and centered. Then sanity-check that a hand fits to lift the battery out and the SB50 lead reaches the breaker. Interference check catches the strap clipping the bumper.
Your Task
- New 'Battery Mount' component
- Battery body at real size + mass
- Tray walls with lead notch
- Strap sweep + anchors, driven by parameters
- Verify CG is low and centered
- Run an interference check (clean)
- Fusion Share > Public Link
- Paste the link on AltHub
Set the bar: it must drop a real-mass battery into a tray, be retained by a strap, and pass an interference check. Walk the room and check three things: is it in its own named component, did they override the battery mass, and does the strap actually clamp downward. Submission is a Fusion Share public link pasted on AltHub — remind them to set it public or mentors can't open it.
Battery Mounting Low, Centered, Locked
- 12 lbs is your biggest CG lever — place it low and centered.
- Retention does three jobs: locate, cradle, clamp.
- Model it in its own component with real mass and parameters.
Your Task
- Model what this lesson covers in Fusion 360.
- Use the AltSkripts tools where they apply.
- Save it with a clear name.
- In Fusion: Share → Public Link → Copy.
- Paste the link below.
- A coach reviews it in AltHub.