Calculating Tablet Charge Times
For a tablet with 4000 mAh battery like Momo7. Using a 2000 mA / 2A charger.
4000 mAh battery = 4 Ah battery
A 2000 mA charger = 2A (per hour) charger
4 Ah / 2A = 2 hours to charge or 4000 / 2000 = 2 hours
That's based on two assumptions:
- A charger rated X Ampere charges the battery with the same X.
- Charging current remains constant.
Both assumptions are wrong.
However, a calculation like that does indeed give you a rough estimate (at least if you use the correct charging current and not the charger's rated maximum output).
USB 2: 0.5A/hour & USB 3: 0.9A/hour
Charge with USB2 = 4 / 0.5 = 8 hours
Charge with USB3 = 4 / 0.9 = 4 hours 27 mins
That's based on the assumption that the used USB port adheres to the standard limits you quoted (500 mA for USB2 and 900 mA for USB3). Which should be true for USB data connections/ports, but not always.
Nowadays, many USB chargers (AC/DC adapters with USB port) can supply quite a bit more than 500mA without problems (see charging ports
The same goes for powered USB hubs and mainboards.
My LCD monitor has an integrated USB hub, for example, which can provide up to 1000mA even when the USB data connection is active.
Similarly, my PC's mainboard by Gigabyte openly advertises to provide "3x USB power" per port (1500mA, I guess).
A 7200 mAh battery @ 3.7V with 2A charger = 7.2 / 2 = 3 hours 36 mins. Should take same amount of time to charge 3600 mAh battery @ 7.4V.
ie. 3600 mAh battery @ 7.4V = 7200 mAh battery @ 3.7V
Note: Battery has to be converted from 7.4V to 3.7V to get the true storage capacity.
Yes, a 7200mAh 3.7V battery should have the same energy content as a 3600mAh 7.4V battery. Namely 26.64Wh. Watt-hours are a more proper unit for electrical energy than milli-Ampere-hours and you'll find Wh values printed on quite a few batteries (but not all).
PS, I would not use any charger with more than 2A on these tablets because sensitive electronics. 2.5A may work but really pushing it. 3A or higher charger will destroy your tablet sooner or later.
This sounds as if you expect a 3A rated charger to "flood" the tablet with a higher current than a 2A rated charger. A common misconception.
Electrical devices only draw as little or as much power from the supply as they need. If a tablet under maximum load requires 7.5W of power, for example, then it'll draw 1.5A from a 5V power supply, approximately. It doesn't matter if the supply is capable of providing 2A, 3A or more. However, it should of course be able to provide at least 1.5A.
I am sure it isn't a perfect 1:1 as things like the fact that most Chinese chargers are cheap and don't provide consistent current and it seems like the last 5% takes longer than the rest (topping it off so to speak). This is a phenomenal starting point however and it is much appreciated.
No, it's not because Chinese chargers are cheap and don't provide consistent current. You're closer with the topping off...
Lithium-ion batteries are charged in two stages. The first is constant current, the second is constant voltage with declining current. That's why the last X% take longer than the rest, and why a simple calculation like "battery capacity in mAh / charging current in mA = charge time in hours" generally isn't accurate.
For more info, check out Battery University
.And here's a real-life example:
I logged voltages and currents while charging my Ainol Aurora II (13.69Wh, 3.7V battery -> 3700mAh).
The diagram shows nicely that the first charging stage (constant current) took about 02:40. At that time, the battery voltage reached its peak at 4233mV. The current actually started to decline a bit after 02:00, but that's apparently normal.
The second charging stage (saturation / constant voltage) took another hour to complete. I don't know why there are two sudden drops in the current. Charging was terminated when the current dropped below 15% of the initial value (about 190mA).
The blue line is the charged energy in mAh as registered by a Coulomb counter. You'll notice that it goes up to 4000mAh, exceeding the battery's rated capacity of 3700mAh. That's normal. The counter isn't perfect and neither is the battery. You won't be able to get the same amount of energy back out of the battery (maybe about 3500mAh if you're lucky). When the battery was newer, I actually measured it being charged with 4200mAh once.
Now to the simple estimates:
The configured charging current was 1200mA, but as the diagram shows, it's actually closer to 1300mA (if the power managemenut unit's sensors are to be trusted).
3700mAh / 1300mA = 2.8h (02:48)
3700mAh / 1200mA = 3.1h (03:01)
The actual charging time was 3.67h (03:40). And it could have been longer - my battery has already lost a bit capacity due to aging and the threshold for the end of charging was relatively high at 15%.
If you've made it this far, congratulations and thanks for reading.
In summary, the most important lessons I wanted to get across:
- Lithium-ion batteries are not charged with a constant current.
- The Ampere rating of a power supply / charger is the maximum value it can output, how much current it actually outputs at a given time is determined by the consumer (tablet).
- The Ampere rating of a power supply doesn't tell you what current is actually used for charging the battery. The tablet's power management unit determines that.
Edited by Tzul, 20 January 2013 - 05:43 PM.