This post will introduce you to battery charging and tell you how to charge lithium cells using a bench power supply.

Executive summary:

Attach a single lithium cell to the power supply and set it at 4.20 volts and 0.5C. Leave it charging until the current drops to 1/20C. Repeat for all cells in the pack.

I use lithium cells (LiPo, LiIon, etc.) in a lot of my projects – usually surplus cellphone batteries. They have much better energy/volume and energy/weight ratios than other battery types (the next best is NiMH).

But they have a reputation of being “tricky” to charge, and needing a special charger.

Not really.

Put your money into a bench power supply instead of a lithium charger. The power supply will do the charging just as well, and is tremendously useful for other things as well.

You need a constant voltage/constant current (CV/CC) DC power supply. This lets you set a maximum voltage and a maximum current at the same time.

The Mastech 3003D is typical of inexpensive CV/CC bench power supplies. It goes for $110 to $150 online, and is well worth the money. (Mastech is Chinese, but the quality seems OK. Other brands are fine too.)

Batteries 101

Batteries are made up of cells in series. Each cell has a typical voltage that depends on its chemistry – carbon-zinc and alkaline cells are around 1.5 volts, NiCd and NiMH around 1.2 volts, lithium cells around 3.3 to 3.7 volts. Lead-acid cells are around 2 volts.

To get higher voltages, cells are put in series in a battery. Sometimes there’s only one cell in the battery, as with typical lithium “batteries” at 3.6v or so. (Technically these ought to be called just “cells” not “batteries” since there’s only one cell, but people often call them batteries anyway.)

A cell (or battery) has a capacity rated in amp-hours (AH or mAH). For example, an 800 mAH battery can produce 800 mA for 1 hour, or 400 mA for 2 hours, or 8 mA for 100 hours (at the rated voltage) before the battery is drained. In theory.

In practice, cells are usually rated assuming you’ll be draining them over 20 hours. Most batteries produce more energy if you drain them slowly. So, a 5 AH battery can produce something like 250 mA for 20 hours (0.25 * 20 = 5). If you drained it at a rate of 5 amps, it would die before one hour is up.

Also, the voltage gradually drops as you discharge the battery – it starts a little above the rated voltage, then spends most of the time near the rated voltage. When it starts to drop quickly from there, it’s empty.

The current needed to (theoretically) empty the battery in 1 hour is called C.  Charge and discharge currents are given in units of C. For a 800 mAH cellphone battery, C is 800 mA.

Most cells have a maximum discharge current beyond which their performance drops off dramatically. For tiny watch batteries (coin cells), this current is really small – something like 0.002C. (“0.002C” would often be called the “500-hour rate”.) These cells are designed to run a long time at tiny currents. At the opposite extreme are lithium polymer cells designed for high-current uses such as in electric-powered model airplanes. I’ve seen some with a maximum discharge rate of 20C – meaning you can drain them in 1/20 of an hour (3 minutes) reasonably efficiently and without damaging them!

Charging

Most rechargeable battery types can be charged at modest constant currents, without worrying about voltage – something like 0.05 to 0.1C (NiMH, NiCd, lead-acid). So you could charge one of these cell types in 10 or 20 hours with your bench power supply by setting the current to 0.1C or so (500 mA for a 5 AH battery), and just turning the voltage up all the way (so it’s not limited).

(I’m not talking fast charge here – that’s a whole other topic; ask Google about that, not me).

Lithium cells are different. They need to be charged at a constant current until reaching a specific voltage, and then constant voltage. But that’s trivial if you have a CV/CC bench power supply.

Just set the current limit at the initial charge current, and the voltage limit at the ending voltage. When the cell is first connected it won’t reach the voltage limit, so it’ll soak up the full amount of current. As it charges, the voltage will gradually rise until it reaches the voltage limit. Then the power supply will keep the voltage constant, with the current gradually dropping as the cell charges. Once the current flowing into the battery reaches about 1/20C, it’s as full as it’s going to get (no real harm in leaving it longer – the charge current will eventually drop off to nothing, but I wouldn’t leave it there for weeks).

Setting the correct voltage limit when charging is important. If it’s too high, you’ll damage the cell. Too low and you won’t get a full charge. The magic voltage for lithium cells depends on the exact chemistry:

Cell type Charge limit Notes
LiIon – coke anode 4.1 volts Older; rarely seen now
LiIon – graphite anode 4.2 volts Very common; frequently seen in phones, cameras, computers, etc.
Lithium polymer (LiPo) 4.2 volts Can take very high discharge rates; popular for R/C cars & airplanes
LiFePO4 3.6 volts Mostly used in high-current applications; electric bicycles and cars, etc.

If you’re using something exotic, check what the manufacturer says. For all the lithium cells I’ve used, 4.20 volts is plenty close enough.

Suppose you want to charge a 800 mAH lithium cellphone battery. Set the voltage limit on your power supply to 4.2 volts. Set the current limit to 0.5C (400 mA). Hook up the cell (positive output of the power supply to the positive side of the battery, and negative to negative). If the cell was empty, the voltage will start around 3.2 volts and gradually rise, with the current at exactly 400 mA. Once it gets to 4.2 volts, the current will gradually drop. When it drops to 40 mA (0.05C), it’s done. This will take 2 or 3 hours, because a 0.5C charge rate means a charge in 2 hours. But once the current starts dropping, the charge gets slower.

Most lithium cells will charge fine at anywhere from 0.2 to 0.8C; never more than 1C. In my experience 0.5C is always OK; slower is a little less wearing on the cell than faster.

Be careful – lithium cells can catch fire or explode if they’re damaged or mischarged. Always keep an eye on them while charging, and stop charging immediately if they swell, start making noise, or get very hot (> 70 C / 160 F or so; a little warmth is normal).

Also, don’t over-discharge lithium cells. Unlike NiMH or NiCd cells, it is not good for them. Discharging below 3.1 volts or so will greatly shorten the life of the cell. Lithium cells are happiest when charged frequently to keep them “topped-up”.

If you do over-discharge, you can try charging it very slowly (no more than 0.1C). Sometimes this brings them back to life (sometimes not). But watch it very carefully while doing so.

Balancing cells

Some lithium cells in a pack will discharge slightly faster than others, especially as the pack ages. One cell can drop to too low a voltage without this being obvious from looking at the total pack voltage. To avoid this problem, it’s best to charge each cell independently instead of the whole pack in series (at least once in a while). This will ensure that each cell is fully charged, “balancing” the voltage in all the cells.

Note that you can charge lithium cells in series; but it’s best to balance them, at least every so often.