136 Amp alternator upgrade for a 2000 XJ

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Judging from below you really don't.

The PCM will try to attain 14 volts or close to that depending on battery temp, load and other factors. When the engine is off the battery should rest at ~12 volts. When the engine is running the voltage should be ~14 volts. When the accessories are pulling more amps than the alternator can provide the accessory starts pulling juice off the battery. For example. You have a 90 amp alt. You then add a bunch of accessories, lights, audio, compressor, etc. When combined they total out to 130 amps. Now, that is quite a bit higher than the alternator can provide so it will go to the battery for the extra power needed.

Thanks for clearing that up for me. I should have said that I have a very basic understanding of the two.