# Introduction

Various resources [1] [2] state that the optimal method of charging a li-ion cell -- such as one found in a mobile phone -- is to charge at a constant current (usually <1C) until a certain voltage threshold is reached, then switch to charging at a constant voltage until the charging current drops to about 0.1C, at which point the battery is fully charged.

The exact target voltage differs slightly for different battery chemistries, but according to National Semiconductor (see figure 5) it's common to switch from CC to CV mode at 4.1V with the CC target voltage being 4.2V.

Many manufacturers sell inexpensive charge-controller ICs that take the guesswork out of charging li-ion cells, but they're not as widely available to hobbyists as, say, an LM317 voltage regulator. Additionally, they tend to be small, SMT devices that are not breadboard friendly.

## Question

Although both constant-current and constant-voltage charger circuits can be built with an LM317, a current-limited, constant-voltage circuit is particularly simple to build.

Is there a downside to charging at a constant voltage (say 4.2V) so long as the maximum current is limited to a reasonable value for the cell (say 0.5C)?

(In this context, consider "downside" to be more along the lines of "cell doesn't fully charge with CV-only, is damaged, explodes, steals your lunch money, burns house down, etc." and less along the lines of "CV-only charging takes longer", though that is also useful information to know.)

## Background

I ask because the internal charger circuit in my mobile phone died unexpectedly and, while waiting for a replacement phone, I constructed a CC-CV charger using an LM317 and successfully charged the battery. This was a bit messier than expected, and I was curious if a simple current-limited constant voltage circuit would also be suitable for occasional use.

## Edits

1. Possible duplicate of this question, which was answered with "If you really want to charge battery from time to time - charging with resistor and constant voltage 4.2V or less will work and battery will not blow up if you choose proper resistor." and a caution to ensure it's OK for the cell to be charged at high current when empty.

2. Fortunately, charger ICs like the TP4056 as well as assembled PCBs with such a chip and the necessary connecting pins are available cheaply on eBay and other vendors. Since I needed to safely charge lithium batteries for other, unrelated, electronics projects I've purchased several of them and keep a few spares around just in case. They work very well.

• On the surface, I don't see a difference between the two options. Whether you call it "current-limited constant-voltage" or "voltage-limited constant-current", you still have two limits, and it will follow the lower of the two at any given time, which makes a sharp corner on a V-I graph. Are you proposing "current-unlimited constant-voltage" plus a resistor? – AaronD Jan 22 '15 at 21:11
• @AaronD Yes, my circuit is just a standard constant-voltage circuit with a resistor. See figure 53 on page 25 of the TI datasheet for the circuit I've constructed, though with different R1/R2 values. – heypete Jan 22 '15 at 22:12
• @AaronD: Also, see the "Internal Current Limit" section of the TI application note for more details about why I wired things that way. It seems to work reasonably well: although the current starts out high and decreases as the battery charges (not constant-current), I can limit the max current to a safe value. The link for the datasheet got lost in my previous comment, so I'm providing the link here. – heypete Jan 22 '15 at 22:33

(the numbers are given for LiCoO2 chemistry, for LiFePO4 they should be lower) Short answer: yes, this is OK. I charge lithium cells with a bench supply set to 4.2 V and whatever C/10 figure is for the cell I'm charging. If you are not in a hurry, slow charging is better, even for cells claimed to be capable of withstanding higher charging currents.

However (quoting you):

charging at a constant voltage (say 4.2V) so long as the maximum current is limited to a reasonable value for the cell

means you will have constant current charger till your cell is at ~95%. Up to this point the voltage across the battery will be less than 4.2V if you measure it. Only when your charger starts outputting 4.2V it will become constant voltage.

What you are about to build is CC/CV charger and this is the right thing to do. "Constant voltage only" charger will be set to 4.2 V with no current limiting and it will charge the lithium cell very slowly. You can check it youself, just construct var.voltage circuit and measure the current into (discharged) cell at 3.5, 3.7, 4.0, 4.2, and 4.5 V. Cheap Chinese chargers are constructed like that, they restrict the voltage to 4.2 V so the cell won't ignite after being charged but the consumer would have to wait longer. I once bought a portable emergency charger which could be emptied in a couple of hours then took 3 days to recharge its internal cells.

There are other precautions to observe while charging lithium, you can learn them from any modern charge controller IC datasheet (my favorite is Linear Tech., their literature is very high quality). If you don't implement these precautions in your design never leave it unattended while charging, otherwise it may ruin your morning one day.

• I don't understand "[CV only] charger will be set to 4.2 V with no current limiting and it will charge the lithium cell very slowly." But when the battery is at 3.7V, does not the current tend to be huge due to low impendance? – thexeno Apr 28 '17 at 15:02
• Where did you get it? – Oleg Mazurov Jul 16 '17 at 1:32
• Sorry, where did I get it what? The huge current assumption? – thexeno Jul 17 '17 at 8:58
• @OlegMazurov: I think there is a problem with this (old) answer as queried in the first comment. S/he's quoting from your fourth paragraph. – Transistor Nov 4 '17 at 0:45

That would be an excellent way to start a fire, so yes, DO limit current to manufacturer recommended values.

But to answer the question: no downside. I prefer to charge up to 4.1v to make my cells live longer but that's unrelated.

The key point you make is: "so long as the maximum current is limited to a reasonable value for the cell (say 0.5C)" That alone will make it so the 4.2v you aim for will be dropped so as to push only 0.5C in the battery at a given instant.

My experience: I actually tried to push a constant 4.2v with NO current limit through a li ion cell.

The amps can reach double digits and the cell hates it so much that it heats up like crazy.