How many watts would a milliamp charge if a milliamp could charge watts?
September 5, 2022 4:25 PM Subscribe
Apple specifies device battery capacity in watt-hours. Power bank capacity is usually specified in milliamp hours. To convert between the two requires voltage, which I don't know. How can I tell approximately how many recharges I will get on an Apple device from a given power bank?
For example, the iPad Pro 10.5 inch has a 30.4‐watt‐hour. This Anker power bank has 6700 milliampere hour battery. Will the power bank only be able to charge the iPad from 0% to 10%, or will it be able to fully charge the iPad 5 times? Or something in between?
I know that power gets lost during transfer. I know that it is there are factors that will make it hard to come up with an accurate number. I'm happy to settle for a ballpark number.
Bonus question: will the ballpark calculation be different for an iPhone, an iPad, and a MacBook Air?
Summary: I'd appreciate any tips for guesstimating the capacity relationships between Apple devices and power banks.
For example, the iPad Pro 10.5 inch has a 30.4‐watt‐hour. This Anker power bank has 6700 milliampere hour battery. Will the power bank only be able to charge the iPad from 0% to 10%, or will it be able to fully charge the iPad 5 times? Or something in between?
I know that power gets lost during transfer. I know that it is there are factors that will make it hard to come up with an accurate number. I'm happy to settle for a ballpark number.
Bonus question: will the ballpark calculation be different for an iPhone, an iPad, and a MacBook Air?
Summary: I'd appreciate any tips for guesstimating the capacity relationships between Apple devices and power banks.
As you are aware, to get Watt-hours for a battery from the amp-hours, you have to multiply by the voltage of the battery. The nominal voltage of a single lithium-ion cell is 3.7 V. If a battery bank is wired in parallel, the voltage will match the voltage of a single cell. At the spec page for the Anker power bank you mention it lists the capacity as 5200 mAh / 19.24 Wh, which matches 3.7 Volts * 5.2 Amp-hours .
posted by RichardP at 4:37 PM on September 5, 2022 [9 favorites]
posted by RichardP at 4:37 PM on September 5, 2022 [9 favorites]
Charging is not 100% efficient. For your powerbank, probably around 85% efficiency. This is because the powerbank has to adjust its voltage output (which varies as the batteries discharge) to match what the iPad is expecting (5V). This conversion efficiency should be the same for all devices you plug into it, assuming they all charge at the same rate.
Batteries will drop in voltage as you discharge them. That’s why it’s common to state their capacity in milliamphours (1 mAh is the same measure of capacity whether or not the battery is at 3V or 4.2V). As others have mentioned, the iPad capacity is expressed in the energy units of Wh, which is the capacity in Ah times the average voltage of the battery.
posted by beepbeepboopboop at 11:31 PM on September 5, 2022
Batteries will drop in voltage as you discharge them. That’s why it’s common to state their capacity in milliamphours (1 mAh is the same measure of capacity whether or not the battery is at 3V or 4.2V). As others have mentioned, the iPad capacity is expressed in the energy units of Wh, which is the capacity in Ah times the average voltage of the battery.
posted by beepbeepboopboop at 11:31 PM on September 5, 2022
Batteries will drop in voltage as you discharge them. That’s why it’s common to state their capacity in milliamphours (1 mAh is the same measure of capacity whether or not the battery is at 3V or 4.2V).
No, what one would actually be interested in is indeed the battery's energy capacity. Which is calculated using a battery's nominal voltage, times the capacity in amp-hours. Also, those values depend *strongly* on the rate of discharge, and of course that calculation is done using optimal conditions.
The reason why powerbank capacities are commonly specified in mAh (and not in Ah) is because they result in Big Numbers, and Bigger Numbers are clearly better. Never mind that for battery packs such as for electric kickbikes, which have several strings of batteries in series, the amp-hour rating is still given as cell capacity times number of cells (not cell capacity times number of strings, as would be technically correct*), again because that yields a Bigger Number.
* if you have a cell with a 1Ah capacity, you can draw 1A for one hour and it'll be drained. Putting, say, ten of these cells in series and again drawing 1A they'll all be empty after one hour. But in the second case you have a ten times higher string voltage, and thus your 1A current carries ten times as much power, and over that one hour delivers ten times as much energy.
posted by Stoneshop at 1:42 AM on September 6, 2022
No, what one would actually be interested in is indeed the battery's energy capacity. Which is calculated using a battery's nominal voltage, times the capacity in amp-hours. Also, those values depend *strongly* on the rate of discharge, and of course that calculation is done using optimal conditions.
The reason why powerbank capacities are commonly specified in mAh (and not in Ah) is because they result in Big Numbers, and Bigger Numbers are clearly better. Never mind that for battery packs such as for electric kickbikes, which have several strings of batteries in series, the amp-hour rating is still given as cell capacity times number of cells (not cell capacity times number of strings, as would be technically correct*), again because that yields a Bigger Number.
* if you have a cell with a 1Ah capacity, you can draw 1A for one hour and it'll be drained. Putting, say, ten of these cells in series and again drawing 1A they'll all be empty after one hour. But in the second case you have a ten times higher string voltage, and thus your 1A current carries ten times as much power, and over that one hour delivers ten times as much energy.
posted by Stoneshop at 1:42 AM on September 6, 2022
For most phones and iPads the rated voltage is 5v (actually 4.3v but they round it up because big numbers are always better), for the iPad Pro it is 14.5 volts though the Apple charger provides 12v.
The way that Apple chargers and (some) power banks negotiate voltage is complex, if you have a simple/old unit which just outputs 5v it will take foreeeever to charge an iPad Pro or may not work at all.
What you need to look for is the latest Power Delivery standard, which is currently USB-C PD 3.0.
Looking at compatible power banks 20W seems to be a popular size, but if you are travelling with this be aware that some airlines only allow up to 10,000mah for carry on board lithium batteries, in the US 20,000mah.
posted by Lanark at 4:58 AM on September 6, 2022
The way that Apple chargers and (some) power banks negotiate voltage is complex, if you have a simple/old unit which just outputs 5v it will take foreeeever to charge an iPad Pro or may not work at all.
What you need to look for is the latest Power Delivery standard, which is currently USB-C PD 3.0.
Looking at compatible power banks 20W seems to be a popular size, but if you are travelling with this be aware that some airlines only allow up to 10,000mah for carry on board lithium batteries, in the US 20,000mah.
posted by Lanark at 4:58 AM on September 6, 2022
@lanark - charging speed and pd protocols have nothing to do with the op's question which is about nominal voltage and energy capacity which Richard has already answered correctly.
posted by turkeyphant at 5:08 PM on September 7, 2022
posted by turkeyphant at 5:08 PM on September 7, 2022
Sorry I was assuming he would plug the iPad into the power bank, not disassemble both devices and directly wire the batteries together, my bad.
posted by Lanark at 4:08 AM on September 12, 2022
posted by Lanark at 4:08 AM on September 12, 2022
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posted by rodlymight at 4:34 PM on September 5, 2022 [3 favorites]