There is nothing worse in the RC hobby than "Delivery Day Disappointment."
You waited a week for that new high-performance 6S LiPo. You rip open the box, admire the shiny label, and go to slide it into your truck... clunk. It doesn't fit. It’s 2mm too long. Or worse, it fits, but the body shell won't close because the battery is 5mm too tall.
Now you are stuck paying return shipping and restocking fees, all because of a few millimeters.
Measuring a battery tray seems simple-Length x Width x Height, right? Wrong. Most manufacturers list "max battery dimensions" in their manuals, but these numbers are often optimistic or flat-out wrong. They don't account for the screw head sticking out of the chassis, the thickness of the velcro strap, or the weird angle of the battery wires.
In this guide, we are going to teach you how to measure your tray like a machinist, so you never have to return a battery again.
The Easy Way: Check FitMyRC First
Before you break out the calipers, have you checked our database?
We might have done the work for you.
FitMyRC has thousands of confirmed "Safe Fit" combinations. Go to our Fit Finder, select your car, and see a list of guaranteed batteries.
If your car isn't listed, or you have a custom chassis, read on.
Tools You Need
Digital Calipers
(Highly recommended)
A Metric Ruler
(If you don't have calipers)
An Old Battery
(For test fitting)
Step 1: Length (The easy part)
Measure the distance between the front and rear bulkheads of the tray.
Pro Tip: The Foam Spacer Rule
Most manufacturers include a foam block in the tray. REMOVE IT before measuring. That block is there to let you run shorter batteries. If you are buying a big pack, you want the full length available.
However, look for obstructions at the ends. Are there screw heads protruding from the bulkheads? If so, measure from screw-head to screw-head, not plastic-to-plastic.
Step 2: Width (The tricky part)
This is where most people fail.
Battery trays often have "lips" or "tabs" at the bottom to hold the battery in place. You must measure the narrowest point at the bottom of the tray.
The Velcro Strap Trap
Look at where the velcro straps thread through the chassis.
The plastic loop where the strap turns 90 degrees often intrudes into the tray by 2-3mm on each side. If you measure the top of the tray, it might say "50mm wide." But at the bottom, between the strap loops, it might only be "46mm wide."
Always measure the width at the very bottom of the tray.
Step 3: Height (The dealbreaker)
Height is not just about the battery strap reaching. It's about the body shell.
1 The Wire Exit
Hardcase batteries have wires coming out of the top or the side.
Top Exit: You need an extra 10-15mm of clearance above the battery for the wires to bend 90 degrees without kinking.
Side Exit: Check where your ESC wires are. If you buy a battery with the terminals on the left but your ESC wires are short and on the right, you might have to stretch wires across the chassis, interfering with the center driveshaft.
2 The "Puff" Factor
LiPos expand slightly as they age. A brand new pack might be 50mm wide. After 50 cycles, it might be 52mm wide. Never aim for a "press fit." Always leave 2-3mm of wiggle room.
Step 4: Comparing to Manufacturer Specs
Now you have your "Safe Dimensions."
- Max Length: (Tray Length) - (15mm for wires)
- Max Width: (Tray Width) - (1mm for fit)
- Max Height: (Tray Height) - (5mm for body clearance)
Go to the manufacturer's website and check the "Specs" tab.
WARNING
Manufacturer specs are nominal. A "138mm" battery might actually be 139.5mm due to the plastic wrapping or the way the wires are soldered. Always assume the battery is 1-2mm larger than the website claims.
Summary
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Measuring takes 5 minutes. Returning a battery takes 5 days.
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Use calipers, not a tape measure.
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Account for the bend radius of the wires (subtract 15mm from length).
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Watch out for the velcro strap loops narrowing the width.