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2021 Jeep Grand Cherokee L Summit
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lights dim on and off when driving. Also radio goes on and off. Sometimes car won’t start. Is that the battery or alternator?
First, wrong forum for Jeep Patriot, but as a guess, likely alternator (internal regulator) if it happens while driving. And it could kill the battery as well. And if original battery, I'd start there maybe, but get the alternator tested.
Get a voltage monitor and watch it as it is running/happening.
 

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First thing i'd do is measure the charging voltage at the battery 'posts' not the battery clamps with engine at idle rpm.
It should be between 13.8 to 14.8 Volts DC.

If not between those two voltage margins then measure the charging voltage at the battery 'clamps' this time.
If that looks good then the battery clamps and posts probably need cleaning.

If all that looks ok then i'd take the battery to an auto parts store and get it load tested.

No matter where you measure the charging voltage it not withing the above spec. it could be a bad alternator, loose alternator connection or a loose or corroded ground wire.
 

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lights dim on and off when driving. Also radio goes on and off. Sometimes car won’t start. Is that the battery or alternator?
HUGE problems with grounds on those vehicles.....there are 2 main ground points for the lighting , one on each side up front on the inner frame rails where I circled in red...check those very well...there's a wire bundle on each stud and they corrode
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