Lexus IS Battery: Specs, Fitment & Common Faults

A genuinely useful piece of context: if you own the IS300h hybrid specifically, the 12V battery genuinely can't be jump-started conventionally — it needs trickle charging, or the hybrid system's circuits risk damage.

Illustration for editorial purposes

OEM Part Number: 28800-AZC35

Genuinely important to know, IS300h: the 12V battery covered here is a small auxiliary unit, genuinely separate from the hybrid drive battery — a completely different, much larger component. Genuine part numbers below are shared with the closely related RC300h/RC200.

IS300h, current genuine reference (shared with RC300h)	Lexus OEM: 28800-AZC35 — S46B24L, 45Ah (also cross-referenced as 28800-YZZQX, 28800-YZZA1)
Alternative genuine reference (Panasonic-made)	Lexus OEM: 28800-36080 — S46B24L, 49Ah, 325A
Important	Even with the vehicle off and locked, the alarm and electrical system stay connected to this battery and will drain it over time if the car sits unused — worth knowing if diagnosing an unexplained flat battery.

The Part Everyone Blames First, Not Always Correctly

A car that won't start gets blamed on the battery more often than any other component, and it's right often enough to make that assumption a reasonable first check. But it's not always the actual cause, which is worth knowing before spending money on a replacement that doesn't fix anything.

Before you buy: battery size, capacity and type (standard vs AGM/EFB for stop-start systems) vary by trim and engine. Always confirm against your VIN before ordering.

What It Does

The 12V battery provides the power needed to start the engine and runs electrical systems when the engine isn't turning. On cars with stop-start technology, the battery works much harder than on older designs, since the engine switches off and restarts far more frequently — which is exactly why these cars need a specific AGM or EFB battery type rather than a standard unit.

A Genuinely Useful Piece of Context Worth Knowing — Lexus IS Battery

If you own the IS300h hybrid variant specifically, this is genuinely important: because it's a hybrid, the small 12V auxiliary battery genuinely cannot be jump-started in the conventional way. Instead, it needs to be trickle charged, since attempting a standard jump-start risks genuine damage to the hybrid system's circuits. This doesn't apply to the conventionally-engined IS variants, worth confirming which specific version you have before assuming this limitation applies.

A genuinely specific diagnostic finding worth knowing if you're chasing a persistent parasitic drain: documented cases on related Lexus models describe successfully isolating a drain to specific fuses, eventually traced to a specific multiplex network communication unit — worth having a proper fuse-by-fuse test carried out rather than assuming a battery replacement alone will resolve a recurring drain.

It's also genuinely worth knowing that on hybrid variants of related Lexus models, the average price for a 12V battery replacement has been documented at around £247, worth using as a rough benchmark.

Warning Signs, DIY Difficulty & Aftermarket Advice

These are the same regardless of make or model — see our full Battery Guide for warning signs to watch for, confirming fitment against your VIN, and matching the right technology (standard, EFB, AGM) when buying aftermarket.

More Lexus Battery

The same part, covered across other Lexus models.

Lexus IS Battery: Specs, Fitment & Common Faults

OEM Part Number: 28800-AZC35

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A Genuinely Useful Piece of Context Worth Knowing — Lexus IS Battery