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__Tango
24 September 2012, 0102
So a normal CCCV charges to 80% at a constant current and then switches to constant voltage and finally shuts off.

My question is, how does the charger circuitry "know" when the pack is at 80% or whatever to switch modes or to turn off completely?

Hugues
24 September 2012, 0449
i'm also thinking about this lately too,
my charger goes like this:
- i want to charge at say 10 amps until 3.65 V
- so charger starts to ramp up from 0 to 10 amps in a few seconds,
- then voltage slowly goes up, that's the CC phase
- when it reaches 3.65 V, charger slowly decreases amps to keep voltage of 3.65 +/-, that's the CV phase
- then after a certain time at 3.65 V(15 min in my case), the charger stops.

don't know if this helps ?

Nuts & Volts
24 September 2012, 0519
You set the maximum total voltage of the charger, say 87V, and the maximum current, say 10A. When you plug the charger into the pack it outputs a voltage above pack voltage so that according to ohms law the current is 10A. Eventually the voltage of the battery increases and so must the charger to maintain 10A output. Once the charger output voltage reaches the set point it it stops increasing voltage and just lets the current drop until it reaches a preset minimum current.

__Tango
24 September 2012, 0936
Hughes, I did know that part, I'm more looking for information about how the charger knows when to change from "CC" to "CV" and then when to finish.

Kyle, I'm not quite sure i understand. you say "according to ohms law". What is R in this case? So during the "CC" phase, in order for the voltage to change to make ohms law make the current 10A, does that mean that the resistance changes?

Thanks!

Nuts & Volts
24 September 2012, 1018
The resistance is that of the battery pack+wiring. This also is your secondary voltage.

So I_charger = (V_charger - V_bat) / R_bat. All the charger really does is increase V_charger to a preset max.

liveforphysics
24 September 2012, 1100
It doesn't "know" anything and nothing actually is changing.


If you set any lab power supply to your packs desired maximum voltage and connect it to your battery, your battery will be charged at whatever the current the supply is capable of delivering until the power supply reaches the maximum voltage you set it for (which is the maximum voltage you want your pack to be). When it reaches this point, current naturally must taper off as the voltage of the pack comes closer to the voltage of the power supply (only the resistance of the wiring and Ri of the cells creating the voltage difference). This is because current can only flow when you have a difference in voltage.

Saying it happens at 80% or whatever is kinda legacy from old very high resistance laptop batteries etc. If you have beefy charger leads and a pack made of good modern cells, you can easily go >95% at CC before current tapers a bit. My nano-tech pack on deathbike has such low resistance it goes to 99% before any current tapering.

As far as when a charger terminates, if it's something like a lab power supply set to the desired maximum voltage, it never would need to terminate anything, it's impossible to overcharge if your output voltage isn't exceeding the desired pack voltage, current naturally approaches the limit zero. For example, if you've got a 20s LiFePO4 based pack sitting at 72v, and you connect it to a power supply capable of delivering a million amps for eternity, and it's output voltage is set to not exceed 72v, you can connect that fully charged battery and forget about it, it's never going to have any overcharge etc, and it wouldn't matter if it was connected to a charger capable of 100mA or 100kA, when the voltages are the same, current won't be flowing.

Most people don't charge from lab supplies though, because it's costly and requires some basic knowledge of what voltage you want to set etc, and they wouldn't know how to tell when it's done (just look at current). So, they put some general compromise between getting a fully charged battery and how long a user waits for charge time, typical is to wait until current drops to 1/5th C (so if it's a 10Ah pack, they wait until current naturally tapers as pack voltage approaches charger peak voltage output until only 2amps of charge current is flowing, then they click it off and give you a signal that you're fully charged). The point you pick is a compromise between waiting longer before it tells you it's charged and getting the last 1-2% charge into the batteries.

podolefsky
24 September 2012, 1156
Hughes, I did know that part, I'm more looking for information about how the charger knows when to change from "CC" to "CV" and then when to finish.

Kyle, I'm not quite sure i understand. you say "according to ohms law". What is R in this case? So during the "CC" phase, in order for the voltage to change to make ohms law make the current 10A, does that mean that the resistance changes?

Thanks!

Like Kyle said, R is the internal resistance of the battery (plus wires which is negligible).

Any charger or regulated power supply has two limiters - a voltage limiter and a current limiter. When the current limiter is on (CC mode), the charger adjusts V_charger until I = (V_charger - V_batt) / R gives you the current limit. As the cell charges, it increases V_charger to maintain the set current.

When V_charger hits the voltage set limit (like 3.65V), the voltage limiter switches on (CV mode). "Switches" is kind of misleading. It just keeps V_charger at the set voltage and as V_batt approaches V_charger, current naturally decreases.

In that sense, the charger "knows" when to switch because at some point V_charger hits the set voltage and it stops increasing V_charger.

DaveAK
24 September 2012, 1207
I was going to hazzard a guess it was ESP.