Charging current/voltage for lipo batteries

Question

I have a 500mah 1S lipo battery. I wanted to charge it with my lab bench power supply.

Easy, I thought: I want to charge at 2C (which is 1A) so I simply set my power supply to 4.20V and 1A limit and done.

What actually happened is that the battery charged about 0.9C and it was slowing down the closer it got to 4.2V. This means that to charge at a constant 2C current the whole way I have to use voltage much higher than 4.2V, but a fully charged lipo cell must not be over 4.2V or it does the FLAME ON thing.

So my question is, how do normal chargers do it? Do they simply bump up the charging voltage until they reach the desired current (so they work in constant current mode) and they every couple of seconds the stop charging and measure the battery voltage to find out if it is 4.2V and if it is, then they stop charging, and if it is not they jump back into constant current mode?

Thanks

Answer 1

To charge any lithium based battery you generally should not be using a bench power supply unless it has a special "Charge lithium based cells" function.

But in a pinch (there's no other option) I would consider slow charging an option, set the voltage to 4.2 V and limit the current to C/10. So for a 1000 Ah battery, charge with 100 mA up to 4.2 V. So on the supply: V = 4.2 V, Imax = 100 mA. That will slowly charge the cell and charging will become slower when the cell reaches its full charge. I would not do this on a regular basis though, only when there's no other option and you must have a charged cell.

To properly charge Lithium based cells, read this article.

Most chargers use either voltage regulation, current regulation or even both at the same time depending on the state of the battery (usually derived from its voltage and temperature). Fast charging gets even more complex as the full charge current should only be applied when the battery is between 30 % and 80 % full (those percentages can vary depending on who you ask).

This means that to charge at a constant 2C current the whole way I have to use voltage much higher than 4.2V

Charging at 2C is fast charging, you should not be doing that "the whole way", see my remark about fast charging. Even when fast charging the current must go down when the battery is getting fully charged. If you don't, the battery will overheat and might start smoking but for sure you will stress it and limit its lifetime.

The "FLAME ON" thing doesn't happen that easily, you can "safely" charge a LiPo battery to slightly more than 4.2 V but that does stress the battery and that will limit its lifetime. The 4.2 V is a decent compromise between lifetime and battery capacitance.

Answer 2

Normal chargers have three phases to charging. A precondition charge is done at a constant current (usually 0.1C or less) if the cell has been discharged below the normal discharge termination voltage.

Once the battery is above the normal discharge termination voltage, the battery is charged at constant current until the voltage reaches the CC termination voltage (4.2V typically but it varies by exact chemistry).

Once the cell reaches 4.2V, the charger switches to constant voltage mode until the charge current goes down to the charge termination point (0.1C or so) and charging stops.

Chargers also implement cell temperature sensing and timers for each of these phases to stop charging if something goes terribly wrong with a cell.

Answer 3

I simply set my power supply to 4.20V and 1A limit and done...

What actually happened is that the battery charged about 0.9C and it was slowing down the closer it got to 4.2V.

If the power supply was truly capable of 4.20 V at up to 1 A then it should have delivered that voltage when the current was less than 1 A. If it could not put out 4.20 V at less than 1 A then it must have poor voltage regulation. If the lower voltage measurement was at the battery (with 4.20 V at the power supply) then there must have been excessive resistance in the wires and/or connectors.

This means that to charge at a constant 2C current the whole way I have to use voltage much higher than 4.2V, but a fully charged lipo cell must not be over 4.2V or it does the FLAME ON thing.

You must not do that. The voltage at the battery must never exceed 4.20 V. When it reaches 4.20 V in constant current mode, the charger must hold the voltage down and let the current taper off, then it should shut off when current drops to ~1/10th of the set current. Current tapers off due to the battery's internal resistance and any resistance in the wiring between the battery and charger.

The equivalent circuit looks like this:-

^{simulate this circuit – Schematic created using CircuitLab}

The charger's current regulation is represented by I1 and voltage regulation by D1. The wiring between charger and battery has a total resistance of 0.1 Ω, and the battery has an internal resistance of 0.1 Ω. In this scenario the battery has already charged up to 4.10 V internally.

So what happens now? The difference between the charger voltage and battery internal voltage is 4.2 - 4.1 = 0.1 V. The total resistance between them is 0.05 Ω + 0.05 Ω + 0.1 Ω = 0.2 Ω. Ohm's law says that current = voltage / resistance, so the charging current is 0.1 V / 0.2 Ω = 0.5 A. Voltage at the charger terminals is 4.20 V. At the battery terminals it is 4.2 V less voltage drop in the wiring, ie. 4.2 - (0.1 Ω * 0.5 A) = 4.15 V.

As the battery continues to charge the voltage difference between charger and battery reduces, which causes the current to taper off. If left on charge for long enough the battery will eventually reach 4.20 V when the current drops to zero.

Answer 4

Everyone seems to want to put their own spin on things. Life as they know it I suppose. Lithium Phosphate batteries and many others are charged with a constant current and a constant voltage . CC/CV

Its important to recognise that lithium technology will take all the current you throw at it and it will burn out some supplies. The manufacturer recommends a current maximum and if you are sure that figure comes from the manufacturer then use that as a max. Note that many battery sellers increase figures to look better . It makes no difference to them.

I have found with LFP that .2C is safe. Thats 20% of the cells Amphour rating. Going to .5C generally does not give any problems either. Above that will produce heat which will gas the electrolyte which in foil packs and prismatics will swell the cell.

So first decide on your charge current. Next the chemistry determines the voltage to set the charger at . 3.65 is a max for LFP and 4.2 a max for LiIon.

So for LFP set the current to .2C ,set the voltage to 3.65v and wait until the current starts to drop. Watch the voltage and switch it all off when the cell terminals reach 3.5v. Going higher produces heat and plates lithium metal on to the SEI layer so don't go there but once on first charge.