Charging speed

Ok, I read that L3 DC charging slows way down when the battery is more full. Sure. Ok.

I guess it's harder to move those last few Lithium ions in after 80% SOC. But my question is: Does this also happen with L2 charging? I haven't seen anyone mention that.

Is there something different about AC charging that makes it easier to get the battery to100%? It's been a while since I took physics.

The slow down in charging speed does not happen with L2.

L2 charging speed does drop but not until the battery is almost full, somewhere in the high 90's % SOC.

Since the OP got 2 different answers, I'll break the tie: charging speed does drop on L2, and may do so as soon as it hits ~90% SoC.

Yes, as the battery nears full (90-95%) charging slows. Then, at the end, the car will start equalizing cells, and you will see what seems like a never-ending, very slow charge, for up to an hour after that. You can unplug it to avoid the EQ phase, but try to let it EQ the cells at least occasionally.

Wow, how interesting. Anyone know how this works?

I guess there must be some kind of rectifier to change the AC into DC, and we get some kind of power output that is already smaller than L3 charging. And since it is already smaller, it doesn't need to be throttled until much later?


Anyways, I was mostly curious whether estimating charge-time depends on how full or depleted your battery is.

You have the idea. Using L1 or L2 the charger on the car converts the AC input into the DC required to charge the battery. It monitors the state of the battery (via CAN bus I assume) to control the charging rate. That's why the charger is on the car and the box that has the cable to connect to the car is an EVSE (not a charger). With L3, the charger is outside the car and it performs the same functions as the charger on the car. It supplies the DC voltage directly to the battery and communicates with the car (via pins in the cable) to control the charging current.

BAsrican said:

Ok, I read that L3 DC charging slows way down when the battery is more full. Sure. Ok.

I guess it's harder to move those last few Lithium ions in after 80% SOC. But my question is: Does this also happen with L2 charging? I haven't seen anyone mention that.

Is there something different about AC charging that makes it easier to get the battery to100%? It's been a while since I took physics.

Click to expand...

The easiest way to visualize I suppose is trying to fill up a large barrel of water with a fire hose. When things start out, you can aim all that flow into the barrel easily but as it gets towards the top, would you really be able to top it off without spilling any water if it was going full blast the entire time? It's no different than filling up a gas vehicle, imagine if the gas pump was going full blast all the way to the top of the tank without stopping. I'm sure you've seen people with pumps that don't have a working auto-stop function and gas starts shooting out everywhere.
https://youtu.be/f1bvnMW2lGI?t=26

Same concept with any battery that recharges, you can fast charge it easily when empty, but when it gets towards full, it starts to build a physical pressure on the battery as heat and unless you slow down the charging near the end, you could easily over-charge the battery causing damage or worse such as so much heat build up that the battery explodes or burns. All EV have large packs with multiple batteries that all charge at slightly different rates. So you could fast charge them all at once and hope for the best, but you would end up damaging the batteries all the time or worse. To avoid EV fires all the time while charging (not going to make any Bolt jokes, I promise), the only way to do this safely is to slow down charging towards the top end so you can give time for all the cells to equalize together.

knightmb said:

All EV have large packs with multiple batteries that all charge at slightly different rates.

Click to expand...

All EVs have large packs with multiple cells that all charge at different rates.

Or perhaps:

All EVs have large batteries with multiple cells that all charge at different rates.


A battery is a collection of cells. Or much older, a battery is a collection of cannons. And a few other meanings, such a paired pitcher and catcher in baseball.

https://www.merriam-webster.com/dictionary/battery

I know, darn grammar police and all that.

BAsrican said:

Wow, how interesting. Anyone know how this works?

I guess there must be some kind of rectifier to change the AC into DC, and we get some kind of power output that is already smaller than L3 charging. And since it is already smaller, it doesn't need to be throttled until much later?


Anyways, I was mostly curious whether estimating charge-time depends on how full or depleted your battery is.

Click to expand...

Slightly more detail from an electrical engineer.

AC -> DC is done with a rectifier. This generates a DC voltage that is what's needed to charge the battery, and will vary depend on where the car is plugged in, the time of day and more.

Next is a DC -> DC converter that produces a current controlled voltage that matches what the battery needs. Why current controlled? At low charge levels, a constant current is used. The DC -> DC converter is producing a constant current and monitoring the voltage. Only when the battery is almost full is a constant voltage used, with declining current and thus power.

From a semiconductor company:



https://www.richtek.com/en/Design%20Support/Technical%20Document/AN024

(Edit, typo correction!!!)

(Edit, typo correction!!!)

Thanks to 91040

Pickle Rick: Jack over at evtv used to compare charging to a call for free beer at the bar; folks come rushing in at first when there is plenty of room, but as it fills up there aren't as many seats, and things start heating up as the place fills and there are no more seats; if you keep packing em in, then fights break out.

The OBC actually has 2 rectifier stages; the AC mains runs thru the first one and that DC gets filtered by a Power Factor Correction (PFC) Boost stage. Then a Pulse Width Modulation (PWM) stage chops the boosted voltage thru an H-bridge transistor section to create a high frequency AC. This AC then is fed thru a transformer, and the AC output runs into the second rectifier stage. This final DC is then filtered and sent to the pack as the current regulated charging voltage.

What the what?

Why do they do all that?

BAsrican said:

What the what?

Why do they do all that?

Click to expand...

The voltage being feed (120V-240V) in is less than the pack voltage (360V), so you have to use AC to boost the voltage above the pack voltage first before converting to DC.

knightmb said:

BAsrican said:

What the what?

Why do they do all that?

Click to expand...

The voltage being feed (120V-240V) in is less than the pack voltage (360V), so you have to use AC to boost the voltage above the pack voltage first before converting to DC.

Click to expand...

Even if it wasn't, the rectified input voltage isn't the voltage is needed to charge the pack.