The Honda TRX400EX typically uses a 12-volt lead-acid battery, most commonly in the 6 to 8 amp-hour range. The standard replacement you’ll encounter is the YTX9-BS or an equivalent 12V battery. This guide walks through common sizes, how to pick the right one for your year and climate, and what to check when installing.
Common battery sizes for the Honda 400EX
Below are the batteries riders most often choose when replacing a worn or failing stock unit. Consider capacity, cold-cranking power, and how well the battery will fit in your bike’s tray before purchasing.
- YTX9-BS — 12V, approximately 8 Ah, typically 180–190 CCA; the most common replacement for TRX400EX models. Fits in the standard tray on most years, but verify dimensions and terminal layout.
- YTX7-BS — 12V, approximately 6 Ah, around 110–125 CCA; a lighter option that can work in milder climates or if tray space is tight, though it may be marginal in cold starts.
- AGM equivalents (YTX9- or YTX7-series AGM) — sealed, maintenance-free versions of the same sizes; offer better spill resistance and lower water maintenance, but confirm space and wiring fitment.
For most riders, the YTX9-BS or an AGM equivalent provides reliable starting power across a range of conditions, while the YTX7-BS remains a viable alternative for lighter usage or constrained fitment.
How to choose by year and climate
Model year generally aligns with the same electrical system design on the TRX400EX, but climate and riding habits strongly influence the right choice. Use the guidance below to match a battery to your conditions and maintenance routine.
- Most 400EX units from the early 2000s accept a 12V battery in the 6–8 Ah range; for all-around use, the YTX9-BS is typically recommended.
- In very cold climates or for riders who frequently start the engine cold, opt for a higher cold-cranking ability (CCA) within the 180–190 CCA range if possible.
- If space in the battery tray is tight, measure the tray before purchasing and consider a compact 6–7 Ah option (like a YTX7-BS) only if you’re certain it fits and meets starting needs.
- Maintenance-free AGM batteries are a popular choice for convenience and reliability, but ensure the charging system and terminals are compatible.
In practice, selecting a battery in the 8 Ah class (or an AGM equivalent) provides robust performance in a variety of climates, while the smaller 6 Ah option should be reserved for specifically constrained installations or light-duty use.
Installation and maintenance tips
Before you install a new battery, double-check fitment, wiring, and charging compatibility. Simple steps can improve longevity and starting reliability.
- Verify terminal orientation and cable lengths to ensure proper connections and avoid crowding the positive/negative posts.
- Choose a battery with adequate CCA for your climate; higher CCA helps cold starts in winter.
- Use a maintenance charger or trickle charger when the ATV is stored for extended periods to preserve battery health.
- Apply a light coating of dielectric grease to terminals to prevent corrosion and improve electrical contact.
- Keep the battery tray clean and dry; ensure venting (for flooded types) is unobstructed.
With the right size and proper installation, the 400EX will start reliably in a wide range of conditions, and maintenance practices will help extend battery life.
Summary
For a Honda 400EX, the standard battery is a 12-volt lead-acid unit in the 6–8 Ah range, with the YTX9-BS being the most common replacement. In cold climates or when high starting power is needed, choose a battery with higher CCA (around 180–190 CCA). If space is limited, a compact 6 Ah option might fit, but verify fitment and performance. AGM variants offer maintenance-free convenience. Always confirm tray dimensions, terminal layout, and compatibility with your charging system, and consider using a maintenance charger to prolong life.