18.5V Li-Ion battery charger circuit.
Fig. 1a

Charging Multi-Cell Lithium-Ion Battery Packs

by Lewis Loflin


The topics covered here are testing and charging salvaged Li-Ion cells and batteries.

Related YouTube video TL431A Lithium-Ion Cell Charging Circuits

Fig. 1 is identical to charging a single Li-Ion cell other than setting a different charge voltage. With two cells in series and both being charged at 4.2V, the charge voltage must be 8.4V. Use a 12V or greater supply for this.

To determine the input voltage, multiply the number of cells by charge voltage and add 5 volts.

Setup: disconnect BATT2 or BATT3. Connect the voltmeter from TP1A to TP2A, connect power, and adjust R2 for 8.4V. Disregard the LED. Remove power and reconnect the cell. Apply power.

When the two cells are charged the LED will turn on.

Simple enough, but what about large numbers of cells in series? What about voltage variations between individual cells? Or series-parallel combinations? Those issues will be explored here.

See part 1 TL431A Lithium-Ion Cell Charging Circuits for the TL431A circuit operation.

Once again a safety warming:

Over-charging or shorting out lithium-ion batteries can result in fire or even explosions! What I present here is, for information purposes, use at one's own risk. Fire is a serious issue for lithium cells due to their high energy density!

3 lithium-ion cells connected in parallel. Current capacity adds.
Fig. 2a

Three lithium-ion cells connected in parallel. Current capacity adds. The charging circuits will sense a single cell.

Uniden power pack has 2 lithium-ion cells. This consists 2 parallel connected Li-Ion cells and a small charging circuit.
Fig. 3a

Uniden power pack has 2 lithium-ion cells. This consists 2 parallel connected Li-Ion cells and a small charging circuit.

The unit can supply 1A max with a capacity of 3000mAh.

Two lithium-ion cells connected in parallel. Current capacity may add but degraded cell produces less overall power.
Fig. 4a

(Above) Two lithium-ion cells connected in parallel. Current capacity may add, but degraded or dead (or open) cells produce less overall power. The charging circuit based on voltage alone can miss this problem.

These cells must be tested under load based on a known charge voltage. Salvage Li-Ion cells, for example, from laptop batteries, may have no part numbers or identifying information.

In the case of Fig. 1a with two cells in series, after charged disconnect from the charging circuit. Place a 10-Ohm 10-watt resistor across the series pair. Measure each cell with a voltmeter. The voltage drop difference between individual cells should be a few milli volts. A difference of more than 5% could cause future problems.

18 volt lithium-ion battery removed from cordless tool power pack.
Fig. 5a

Above is an 18 volt lithium-ion battery removed from cordless tool power pack. This was discarded from a work site. Internal to the power pack was a "charge balancing" circuit board that failed. All the Li-Ion cells still had some charge.

If any the cells are below 2.5-3V they could be bad. I charged one cell (not part of this battery pack) from nearly dead and it worked fine.

There were no part numbers on the cells, so I don't know the current capacity.

10 Lithium-ion cells connected in series-parallel.
Fig. 6a

The schematic of the battery is shown in Fig. 6a. This consists of 10 Li-Ion cells connected as five parallel sets wired in series. There were six solder connections to the failed charge balancing circuit board, as shown.

Each cell pair, when measured with a voltmeter, read over 3V. This number is good news. But should one try to charge the battery as a single 18.5 battery?

Because I had access to the six connections for all cell pairs, I charged each pair to 4.2V at 200mA separately. See TL431A Li-Ion Cell Charging Circuits

As for milli-ampere-hour (mAh) the definition is,

It is a measure of battery capacity. Electronics use, or draw, energy that's measured in amperes(A). So a 1500mAh battery can power an electronic device with a draw of 1mA for 1500 hours, or a device that draws 1.5A for one hour.

Power load test of a salvage lithium battery pack.
Fig. 7a

In Fig. 7a, the battery was tested using a 20-Ohm load. The amp meter indicated 0.94A current. I let this operate for several minutes, and the resistors got hot. All voltage drops across the five pairs of cells indicated a difference of no more than 0.04V. This reading indicated that the battery is good.

The battery is charged as a single 18.5V battery through a constant current source. The charge circuit is set for ~19-21V while connecting the CCS to a 24V supply.

The circuit in Fig. 1a can be used; I tested it at 19.5V, but had to change the 33K resistor to 10K. In addition add a 220 Ohm resistor from Q1 base to the TL431. This limits Ika to under 100mA

Calculating Charge Voltage Resistors

The input voltage Vin must be 4V-5V or greater above the charge voltage as a general rule.

R1 is the 100K + 4.7K, R2 sets the upper and lower range. 4.7K was added to limit the TL431 Ref current to under 10mA at 24V: 24 / 4.7K = 5.1mA. VRef = 2.5V.

Note: due to VRef = 2.5V and circuit voltage being low this circuit doesn't work under ~3.2V.

The formula using 33K for R2 is Vout MAX = (1 + (104.7K / 33K)) * 2.5V = (1 + ~3.17) * 2.5 = 4.17 * 2.5 = ~10.43V.

For Vout MIN with 33K = (1 + (4.7K / 133K)) * 2.5V = (1) * 2.5 = ~2.5V.

So our voltage range is 2.5V to ~10V. If one desires a charging voltage over 10V then R2 must be changed to a lower value.

The formula using 10K Vout MAX = (1 + (104.7K / 10K)) * 2.5V = (1 + 10.47) * 2.5 = ~28.7V.

For Vout MIN with 10K = (1 + (4.7K /110K)) * 2.5 = 1 * 2.5V = ~2.5V.

Note: due to VRef = 2.5V and circuit voltage being low this circuit doesn't work under ~3.2V.

So the range with 10K is ~2.5V to ~28.7V. I won't use a power supply above 25.2V so this value is fine.

Note: the power supply says on the label 24V but measures 25.2V.

In this case I'll use a 10K for R2.

Disconnect a battery connection. Apply 24V, adjust R2 for 19-20V. Disconnect power, connect the battery, and apply power.

This is fine as long as all cell pairs are identical and in the same electrical condition. This series configuration is how car batteries (6, 2.2V cells in series) and cordless phone batteries (6, 1.2V cells in series) are charged.

TL431A 2 cell charge balancing circuit.
Fig. 8a

Fig. 8a is a two-cell charge balancing circuit. Each cell has its individual charge circuit set to 4.2V.

In the case of the battery in Fig. 6a would require one charge controller per cell pair, or five units in this case.

Example lithium battery charge balancing circuit.
Fig. 9a

Fig. 9a illustrates a 4-cell charge controller from Ebay.

Example lithium battery charge balancing circuit.
Fig. 10a

Fig. 10a illustrates a Li-Ion cell charge controller in strip form. Rated at 2.5A, five would be required for the battery pack in 6a.

I tested these, and they work as advertised. They are marked "+" and "-" and can be soldered directly to the cell.





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