Schiller: The MacBook Pro uses 16GB of very fast LPDDR memory, up to 2133MHz. To support 32GB of memory would require using DDR memory that is not low power and also require a different design of the logic board which might reduce space for batteries. Both factors would reduce battery life.
Slaney himself wrote an article explaining how the new MacBook Pro uses a low power, enhanced version of DDR3 RAM called LPDDR3E, which maxes out at 16GB. To achieve up to 32GB RAM would have required using DDR4 RAM, but its low-power variant LPDDR4 is not supported by the Intel processors powering the late 2016 models.
Using the iStat Menus tool, Slaney determined that, under normal conditions, the LPDDR3E RAM uses 1.5 watts of power. In comparison, he said the notebooks would use about 3-5 watts if they were using DDR4 memory, although this estimate is rather loosely based on tests of DDR4 RAM on Windows-based notebooks.
Slaney said the 2-5 watts saved translates to 10% of overall power usage being dedicated to RAM versus 20-30% that would be required for DDR4 RAM, which, if accurate, helps justify Apple’s power versus performance tradeoff.
Schiller previously addressed these power concerns in an earlier comment:
To put more than 16GB of fast RAM into a notebook design at this time would require a memory system that consumes much more power and wouldn’t be efficient enough for a notebook. I hope you check out this new generation MacBook Pro, it really is an incredible system.
Apple’s decision is even more justified when considering background power draw, or the energy a notebook uses to go back into sleep mode after regular usage. Slaney said this figure is estimated to be about 50% of overall power draw on an average system when using DDR4 RAM, but only 20% when using LPDDR3 RAM.
Moreover, the new MacBook Pro would get less than 7 days of standby time if it used DDR4 RAM, compared to 30 days with LPDDR3E RAM, he said.
Apple have been using LPDDR for several generations of their notebooks, and it’s part of the way that they get very long standby time on them. Switching to DDR4 would drastically decrease it from the 30 days of standby time that they get now to less than one week. With DDR4 they’d have produced a notebook that would have a completely drained battery if it was at 50% charge and you closed the lid and left it for a few days. Not only would that be annoying, but by running the battery flat often it would end up damaging a percentage of their batteries because they’d frequently get 100% discharged, which puts a lot of stress on them, and sometimes even kills them.
The rest of the article reflects upon poor battery life in several Windows-based notebooks with 32GB RAM, part of which can be blamed on the FAA’s 100-watt-hour limit on notebook batteries brought on airplanes.
Full Article: “Why the MacBook Pro is limited to 16GB of RAM” on MacDaddy