Test car battery with oscilloscope during cranking

[TaylorD93]

I have a 2016 Toyota Auris, and the workshop manual actually quotes a minimum permissable voltage during cranking. Certainly with Toyota UK you can pay a few £ to access the entire workshop manuals online for a few minutes.

Another thing to be aware of when looking at the cranking voltage however is the "relative compression test". It is a test where you use an oscilloscope to measure the battery voltage during cranking (usually done with the injectors disabled), as the engine rises on the compression stroke, the starter motor is put under more load and therefore the battery voltage will sag at each compression stroke. so if you have a cylinder which is lower compression relative to the other cylinders, you would see a smaller drop in battery voltage on that compression stroke.

If the 2nd channel of the scope is set to monitor a coil pack pulse (often can be held on the back of the coil pack, if coil-on-plug setup) you can then find which cylinder is actually low.

My point being, engine mechanical health will impact your readings.

[ugawoxoto61]

My car battery (12V lead acid) is around 7 years old, and I'd like to check its health.

I plan to measure the voltage at the battery terminals with an oscilloscope while starting the car.  No plan to measure current because my biggest current clamp tops out at 150A.

Does anyone have recommendations / rules of thumb for how low the voltage can go during cranking and still be ok?

The car is a 2017 Toyota Yaris if that makes any difference.

EDIT: The cold and warm crank voltage profiles at https://www.monolithicpower.com/en/learning/resources/from-cold-crank-to-load-dump-a-primer-on-automotive-transients seem instructive.  I'm guessing they're worst-case low voltage since they come from industry standards for automotive electronics.

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Typically, the voltage should not drop below 9.6V during cranking, even under load. A healthy battery usually stays between 10V-11V during a cold start, depending on factors like temperature and the starter motor’s demand.

[Psi]

Yeah, there's so much variation that will affect the current.
- Cold vs warm state.
- The ambient temp in your part of the world.
- The size and compression of the engine
- The thickness of the engine oil your using. (also temp related)
- The number of accessories you have attached to the engine with belts.