# What is the minimum current required to charge a smartphone battery?

I am planning to make a portable charger for my smartphone. My input current is 300 milliamps and I am not sure if that will be enough to charge my phone's battery. What is the minimum current required to charge a typical smartphone battery?

• Li-ion batteries charge in distinct stages; the current drawn varies according to the level of charge in the battery. You might find the background information presented here useful: Developing Affordable Mixed-Signal Power Systems for Battery Charger Applications
– Air
Jul 20, 2015 at 16:43
• @RussellMcMahon - I have cleared out almost all of the comments on this post. I removed the ones that were merely tangentially related to the question itself. Extended conversations and soliloquies are better off in Engineering Chat at The Skunk Works.
– user16
Jul 22, 2015 at 16:46
• I'm interested in the answer to this too, particularly if anyone has done any actual tests to determine how low common phones will go. USB devices are supposed to draw only 100mA before negotiation, I wonder if many phones would charge (or at least no deplete their batteries) from that. Sep 28, 2020 at 10:02

My input current is 300 milliamps and I am not sure if that will be enough to charge my phone's battery.*

It will probably charge acceptably.
This varies with product but in most cases modern cellphones and other products which use 1 or 2 cell LiIon (Lithium Ion) batteries will charge from sources that supply less than maximum rated charge current.

• Rather than considering the literal minimum current which would "just" cause charging to occur, I decided to take your question to mean more like "Can I charge smartphone batteries at some small but not trivial rate without significant problems". ie I did not think you wanted to know if you could charge them at 10 or 100 or even 1000 microamps. The batteries themselves would probably charge at currents somewhat above the self discharge rate, but as that is probably under 100 uA*, it does not have a lot of relevance in most charging scenarios.
**eg 1400 mAh x 10-% say pa = 140 mAh.
140 mAh/8765 hours/year ~= 16 uA self discharge rate. Even at 50% self discharge per year a 1400 mAh battery self discharge rate is under 100 uA.

The reference cited by Air is an excellent one but may be hard to follow if you re not technically experienced. The basic explanation below should tell you the minimum that is needed to meet your requirement.

When there is 'more than enough' charging energy available most devices with LiIon (Lithium Ion) batteries charge at a rate which is set by the charger, not by the battery. The rate used is usually the maximum allowed by the battery manufacturer. As this part of the charge cycle is carried out under constant current conditions it is usually referred to with the abbreviation CC mode (for "constant current). The manufacturer's maximum allowed rate is usually based on battery capacity. Typically the maximum allowed charge rate is the "C/1" rate.
C = charge rate where mA of charge = battery mAh numerically.
So for eg a 1200 mAh battery, C = 1200 mA.

Almost all modern cellphones and many other LiIon powered devices use the "USB" 5 Volt input charging standard. When the available charge current is less than the maximum some devices will not charge, but the majority of "5 volt input" devices will accept whatever current is available.

A typical smartphone battery has a capacity of ABOUT 1500 mAh. This would have C = 1500 mA = max charge current. The phone will charge the battery either at C if ample energy is available or at the lower available rate until a predefined battery voltage is reached (usually 4.2V). It will then usually change to a constant voltage mode and the current will decrease with time under battery chemistry control.

When the battery changes from CC (constant current) mode to CV (constant voltage mode) the battery is usually about 70% to 80% "full" (technically 70% to 80% SOC = State of charge). The exact amount varies with situation - and if charging has been at much less than C (say 300 mA instead of 1200 mA in his case) the SOC (state of charge) will be higher than when charged at C. So, when charging at say 25% of max rate it will take less than 4 times as long to achieve a full charge as the slow final CV rate will have less capacity to input.

It is absolutely vital when making a charger that you do not exceed Vbattery = Vmax - usually 4.2 Volts. If you are providing a 5V feed to the existing charger circuitry (usually inside the phone) then it will handle this. If you are connecting directly to the battery you should either use an 'off the shelf' LiIon charger IC or learn quite a lot about what is required to keep your battery alive when charging.

A low capacity charger may charge a battery to a HIGHER final charge state and somewhat shorten battery life. This is because a charger will usually terminate when Ibatteruy is say at I = C/2 or C/4 or maybe C/10 for road warrior level charging. If the charger expects say a 1000 mAh battery then C/4 = 250 mA.
But if charging a 2000 mAh battery the 250 mA represents C/8 which is perhaps 2-5% higher charge level.

###References:

Battery University

Charging Lithium Ion batteries