It depends on your power settings in the bios. On the older Precisions I've had them setup to use full power and they
refused to even recognize a 65W adapter was plugged in until you turn on all kinds of power saving and throttling options.
However once you shut them down they will charge just fine off a 65W adapter.
There's more to this than 65W vs. 90W. If that had been the only difference, you would not be having a problem. In
selecting a charger unit, you need to make sure several items match up:
1. Is the charger unit supplying AC or DC to the laptop? MUST be same as specified for the original charger.
2. Type of battery in the laptop. Many chargers are designed for a particular battery type and supply exactly the right
voltage and charging rate (current) for that. Some even are "programmed" to alter the charging rate depending on the state
of the battery's charge. All have a limit on the battery's maximum voltage to avoid over-charging. All of these are
important, and that's why using the wrong charger can cause trouble.
3. Polarity of the connector - MUST be the same as original.
4. Voltage MUST be the same.
5. Wattage MUST be at least as much as the original charger, can be higher without problems.
6. Max amps must be at least as much as the original, can be higher.
7. Connector on the end of the charger cord - must be the same size pins and spacing so that it fits the connector in your
laptop without damaging it.