Discharging all energy of one battery into another

Question

I was wondering how discharging all the energy from one 18650 Lithium-Ion battery into another 18650 Lithium-Ion battery would be achieved. So far I’ve consulted using MOSFETs, DC to DC Converters and current sources/sinks without avail.

The whole circuit I’m attempting to achieve is a circuit that can fully charge battery 1, and then transfer all of the energy (less any losses) from battery 1 into battery 2.

Any direction or reference papers would be appreciated. Thank you.

Edit: Will update with more details shortly.
The end goal is to have a battery testing circuit that does not entirely dependent on the energy from a wall adapter. The circuit will also be testing for battery voltage, charging current and temperature among other things.

So far I have a discharging circuit to discharge both batteries and a charging circuit to charge one of the two the two batteries. The transferring of energy from battery 1 to battery 2 is the essential step I’m currently working on.

Thanks again for all the insight it’s has been a huge help.

Answer 1

The way lithium-ion batteries work is by being at full charge at ~4.2V and "no charge" at ~3.0V. The limits on voltage is due to safety, because lithium-ion batteries are unstable outside of this range.

So given this information, and we know from electronics that the higher voltage-potential is the one that will control the way the energy flows, we can see that you would want the "charger battery" to stay at a higher voltage than 4.2V. Of course you aren't going to charge it to a higher voltage, but you can step up the voltage with a buck-boost converter, also called a step-up regulator. Then you could have the voltage at for example 5V, and it would charge the other battery.

If you want to charge the other way again, for whatever reason, then you can use the same step-up regulator and some switching-circuitry with mosfets to control which battery that is stepped up to 5V.

I have drawn a suggested schematic below. You want to research this more of course, but I think this would be at least close to what you want.

^{simulate this circuit – Schematic created using CircuitLab}

My schematic does not include over-charge and under-charge protection. This is of course an important aspect you need to add to ensure you don't charge above 4.2V. That part of the design is up to you to include, but should be simple enough.

Sidenote: 3.7V stated on the batteries in the schematic is because this is the nominal voltage of lithium-ion batteries. They will charge up to 4.2V and discharge as much as you allow it.

Answer 2

If you connect a fully charged (4.2V) battery to a discharged (3.2V) battery then there will be almost unlimited current flowing and maybe a fire or explosion, then if both batteries survive they will each end with 3.7V.

You [b]never[/b] boost the voltage higher than 4.2V to charge a Lithium-Cobalt battery cell. Also you did not mention using a battery charger circuit.