I own a 2016 Macbook (with the USB-C charging) - at the time and in response to all of the reports of poor cables and poor chargers, I bought a meter so I could see what the throughput of my cables/chargers was. However, having run some tests I'm now not sure what to consider normal / what is harmless to the battery.

For all tests I used an original Apple C-C cable - though was able to replicate with two third party cables I own.

I have the original Apple block charging at 14.3 V / 1.99 A and a third party block that charges the same.

Another third party block which advertises USB PD and 3 A is currently charging at 20 V / 1.43 A

I've done some research online but I can't find any articles that indicate what a normal USB PD output is relative to the devices that connect to it and would appreciate any guidance / assistance.

TL;DR: Have Apple charger which charges at one voltage/amperage. Bought several others advertised at PD which are higher voltage, lower amperage. Not sure what is 'safe', or 'fast', or does least overall damage to battery.

  • I'm curious - are all these measurements you're quoting taken from the Multimeter you bought, or from something else? – Monomeeth Nov 4 '17 at 10:37
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    Also, are you taking account of the load when taking measurements? Measuring voltage into zero load will tell you nothing, you need to measure parallel to the loaded circuit, which would require access to the internals, or a spliced cable. Sticking your meter into the end of an unloaded socket doesn't do what you think it will do. – Tetsujin Nov 4 '17 at 11:31
  • Hi both - I misspoke in my original message, and meant a USB C pass-through meter. In connecting it to my Macbook, I can see the voltage and amperage running through the meter into the computer. The reason for my looking is because a charger providing less than ideal input will do long term damage to the battery (AFAIK) and it seems no PD charger I buy can provide as much power as the Apple plug can. This leads to the question - what is 'normal' for USB PD running into a laptop? – George Pearce Nov 4 '17 at 23:24

A good Multimeter is fantastic - I use one all the time - however it's not clear from your question just how you're using a Multimeter to take your measurements?

Considering we're talking about a fairly recent model MacBook Pro, I suspect you haven't opened it up to take these measurements. Instead, I recommend you download and install coconutBattery1. It provides a whole heap of information that you'll find useful, and from memory includes both Discharging and Charging measurements in watts!

Some thoughts

It's not clear from your question why you're investigating all this? Unless you've actually got a problem you're trying to address, using the charger that came with your MBP should be perfectly fine. Also, in relation to you not finding any articles that indicate what a normal USB PD output is relative to the devices they connect to, that's because PD stands for Power Delivery and most of these devices are just designed to allow the power travel through them from the charger to the device being charged.

In other words, you can use a hub with a USB-C PD port with a 2017 MacBook Pro and also use it with a HP Elitebook and the voltage/watts will differ, depending on the device and charger.

1 I am not affiliated with coconutBattery in any way.

  • Hi Monomeeth - thanks for replying. I have a USB C pass-through meter (rather than a multi in this case) and my concern is around the fact that the original charger delivers 2a at 14.5v, but any 'PD' compatible chargers I've found open market seem to deliver wildly variable results - always at a lower amperage than the Apple one. I wish to have a spare to travel with - but don't want to run something with long term negative effects on the battery. Thank you for the coconutBattery recommendation! – George Pearce Nov 4 '17 at 23:27
  • Using a charger that provides a lower amperage than the Apple one is not going to cause any negative impacts on your battery. The only difference as a result of using different chargers (as I guess you'll already know) is that charging times will vary depending on voltage/amperage. The lifespan of a battery is primarily based on the number of battery charge cycles - whether it takes 5hrs to charge from 0% to 100% or 10hrs is irrelevant, as it still counts as one charge cycle. In a nutshell, if you're using a charger that meets the PD specs and it's made by a reputable brand, you'll be fine. – Monomeeth Nov 5 '17 at 2:48
  • Also, your MacBook came with a 29W USB-C Power Adapter (hence the 2A @ 14.5V), and your MacBook battery is rated at 1,000 battery cycles. You'll be fine, so long as you're not using some cheap and nasty no-name charger (primarily because I wouldn't trust the specs nor quality of those). – Monomeeth Nov 5 '17 at 2:52

I'm now not sure what to consider normal / what is harmless to the battery

It doesn't matter. All modern battery powered devices including MacBooks have a charging controller (part of the SMC subsystem, for a lack of a proper word) that regulates the voltage.

Sample Li-Ion Charger Controller

(This is a generic schematic for a power controller, not for a specific Apple product. The premise and its function are the same).

The point is, it's not going to dump raw power into your battery; it will reject voltage that's too low/high and turn on/off the charging circuit depending on charge status of the battery. Additionally, the battery pack within the MacBook has a protection circuit as a safeguard against over/under voltage conditions.

Have Apple charger which charges at one voltage/amperage. Bought several others advertised at PD which are higher voltage, lower amperage. Not sure what is 'safe', or 'fast', or does least overall damage to battery.

The USB 3.1 Power Delivery specification states that it will support up to 20V at 5A (100W). That doesn't mean all power supplies will provide this. The USB power delivery spec also states that a device will negotiate the power that get's delivered, meaning a charger that's capable of delivering 12-18V at 2A for a MacBook will have no trouble delivering 5V at 1A for a smartphone.

Here are the rules of thumb:

  • A charger with a higher power rating than your device can always be used. Eg. (85W MacBook charger can charge iPad/iPhone that requires 10W, but a 10W iPad charger can't/won't charge a MacBook that requires 85W)
  • Amps: devices can only draw up to their rated amps. A device that requires 1A will work fine on a power supply that supplies at least that. It can supply more (i.e. 3A or 5A) but not less
  • Volts: You must have the right voltage. A device that requires 12V will not function with a power supply of 5V and will blow up on a supply of 24V. Unless, you using a USB 3.1 supply and device; they will negotiate that for you.

As for your cables, they shouldn't matter. Unless you have long runs (and by long I mean 5m/16 ft. or more) or it's super heavy (bigger than 22AWG) your cable should have not have enough resistance to even be a factor.

That said...I would stay away from no-name-brand or generic discount adapters and cables. Their quality control is usually lax and you could do damage because they don't strictly adhere to spec.

  • Hi Allan, thanks for this (and the links to spec). The PD plugs I have on hand seem to be delivering a higher voltage at a lower amperage than the Apple charger - does the lower amperage in particular have long term negative prospects for a battery? Is there any easy way to see what the laptop (MB 2016) 'wants' and what the charger is delivering? My meter is a passthrough USB C model. Thanks! – George Pearce Nov 4 '17 at 23:28

When I was charging my Macbook from a chunky Anker 26800 mah USB brick, it was like filling a swimming pool with a garden hose. So like yourself, I started investigating the power situation in respect to both cables and the USB brick.

To determine what "normal" performance was to establish a baseline, I used the Jokitech USB Type C PowerMeter tester connected to the USB-C/Thunderbolt Cable which was connected to the 87 Watt Apple Power Adapter that shipped with my Macbook Pro 2018.

The Jokitech revealed that 19.8v was the baseline I needed to measure the cables' performance against.

The meter proved my suspicions that the USB cable I was charging my Macbook from a USB powerbrick with WAS indeed a "garden hose": This cable was performing at barely 5v

I then swapped out the garden hose for the "FIRE HOSE" cable, a Anker USB-C to USB-C Thunderbolt 3.0 Fast Charging and Data Transfer Cable

Pictures of each cable will be shown first, followed by ones illustrating each cable's performance as displayed by the Jokitech meter while powering my MacBook using a Zendure X6 45 Watt Power Brick. As you'll see, the Jokitech made it possible to clear-up the question marks about where the weak link in the power chain lie:

"Garden Hose" Cable: Note USB 2.0 one end, USB-C on the other USB_2.0-to-USB-C Cable

"FIRE HOSE" Cable": Anker USB-C to USB-C Thunderbolt 3.0 Fast Charging and Data Transfer Cable

"Garden hose"* with a USB-C adapter: struggling to make 5v "Garden Hose" using a USB-C adaptor

"FIRE HOSE" cable performing at 19.7v: enter image description here

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