This means that with a 60W charger, the battery will charge only during light CPU load. Since the machine load takes priority and doesn't shed until there's insufficient power available, the battery will be always discharging whenever the supply can't provide sufficient power to cover the machine's needs. That's why the performance will be poor with an inadequate power supply. So the latter will be throttled to maintain the power balance. The 60W supply, even if connected to a 17 inch MBP, will satisfy all internal and external loads (USB, FW, drives, screen), except for the CPU and GPU. The CPU load shedding will, by design, always manage to balance the power. If there isn't enough power for the machine, the load shedding kicks in and throttles the CPU (and perhaps GPU - I don't recall offhand). Conversely, a fully charged battery will demand a zero charger load, and that's fine. If there isn't enough power left for the charger, the battery is by definition discharging unless it has no charge left.

Thus, given an available input power, the machine load is satisfied first, and any leftover power is provided to the battery charger. The power management system also knows the electronic nameplate of the power supply and thus its rated power.

The loads are prioritized: the machine has priority over the battery charger. The power management must maintain the following inequality balanced, in terms of power: (power adapter + battery as a power source) >= (battery charger + the machine). The battery as a power source is exclusive of the battery charger: a battery may operate as a power source, or the battery charger may operate, but never both at once. the machine (logic, drives, memory, screen, speakers, USB devices, etc.)īoth loads are adjustable and the power management's function is to adjust them as needed.The below explains why.Ī MacBook's power management works very simply: it maintains a balance of power between the power consumed by the loads and the power available from the sources. It will operate safely, but the CPU performance will be diminshed.

The MacBook it's plugged into won't operate in a 'brownout'. It will clearly run at full capacity for longer than a higher wattage adapter or even fail to keep up with the energy demands of a machine using more than 60w giving you a flat battery if you run a deficit of energy “while charging”. So no, a 60W adapter won't overheat when connected to a machine that needs an 85W adapter. This is a given for the systems to receive safety certifications. I will give some facts along with my reasoning.Īll MagSafe adapters, when plugged into a mechanically matching receptacle on a MacBook/MacBook Pro, are designed to run safely. There is a lot of misinformation in some answers.

Power supply for mac book