# Looking for a current multiplier circuit - if possible

I used a very inexpensive circuit based on the TP4056 chip and built a Lithium Ion battery charger for 18650 batteries.

The problem is, the batteries I use have so much capacity that the circuit, which is limited to only charging at 1 amp, takes many hours to charge my batteries.

What I would like to do, is build a circuit that I can add to the one I purchased. This new circuit would (ideally) sample the current flow from the charger circuit to the battery, and multiply it by two. My batteries can handle a charge current of 3 amps safely. All I want to do is charge them at 2 amps.

I have attached a diagram that I made hopefully clearly illustrating what I am looking for. Obviously any and all of it is subject to modification based on new information, knowledge etc.

• If you double the current, you double the power. Where does the extra power come from? Jul 17 '16 at 11:47
• To clarify Dave's comment, you are drawing P = V x I watts from the charger and putting V x 2 x I watts into the battery. Where does the power come from? Jul 17 '16 at 14:35
• @transistor I think tp4056 has a max current limit of 1.4A. Jul 17 '16 at 16:15

Theoretically TP4056 is capable of charging at 1.4A. Practically your design is limited to the capability of the USB source par the picture or your external source. You need to change the resistor on Program pin on the TP4056 to the desired charge current

Per the circuit above the resistor is R4.

Also from the LTC4056 data sheet:

PROG (Pin 5): Charge Current Programming Pin. Provides a virtual reference voltage of 1V for an external resistor (RPROG) connected between this pin and ground to program the battery charge current. In constant current mode the typical charge current is 915 times the current through this resistor (ICHG = 915V/RPROG). Current is limited to approximately 1.4mA (ICHG of approximately 1.4A).

Based on the maximum available Li-Ion battery capacity you should be able fully charge the battery under 4hrs.

Word of caution: Although you might want to charge at 2A the battery chemistry might not recommend such a high charge rate. Read the battery chemistry specification very carefully. Improper use can be dangerous. For example LG Chem 18650 MH1 has a standard charge current of 1.5A and Max charge current of 3.1A.

Below is a chart on charging Li-Ion batteries. In most cases it is recommend that the battery be charged 0.8C. Also as the battery reaches full capacity the charge rate also drops as described by the chart below

The advised charge rate of an Energy Cell is between 0.5C and 1C; the complete charge time is about 2–3 hours. Manufacturers of these cells recommend charging at 0.8C or less to prolong battery life. Most Power Cells can take a higher charger. Charge efficiency is about 99 percent and the cell remains cool during charge.

Reference:

• Thank you for the very thorough answer. This is excellent information. I'm currently working on making my own LiIon charger using an Arduino and your response here has helped me a lot! Dec 2 '17 at 13:15

What you ask for won't work.

First, if you were to create a current amplifier as you describe, it would require a separate power source. You can't cheat physics. Twice the current out at the same voltage means twice the power. The additional power has to come from somewhere other than the input current.

If you have additional power available, it would be better to use it directly to create a higher current charger than trying to amplify the output of a low current charger.

Second, the charger is managing the battery by creating a particular current and voltage profile over time. The charger will get the wrong idea of what the battery is doing with a current amplifier between it and the battery.