Lithium Charging Load Resistance?

[John-BOOM]

TL;DR Summary

What resistance does a lithium cell present to the charger PS?

I'm trying to understand the load-resistance of an A123 Nanophosphate High Power Lithium Ion cell.

https://www.buya123products.com/uploads/vipcase/844c1bd8bdd1190ebb364d572bc1e6e7.pdf

My understanding is that resistance increases as the cell charges.

The datasheet says "Internal Impedance (1kHz AC typical, mΩ) 8". I don't understand what that means.

Here's my (probably incorrect) attempt to apply Ohm's Law, at the low-SOC and full-SOC of the cell:

Low SOC
charging at 4A
R = V/I
R = 2.5/4
R = 0.625

High SOC
charging at 0.00001A
R = 3.7/0.00001
R = 370K

 

[berkeman]

Welcome to PF.

Are you trying to design a Li-Ion battery charger? Is that why you chose the last part of your new username?

https://globalnews.ca/news/2453214/watch-hoverboard-bursts-into-flames-during-users-first-ride/

 

[Baluncore]

TL;DR Summary: What resistance does a lithium cell present to the charger PS?

The datasheet says "Internal Impedance (1kHz AC typical, mΩ) 8". I don't understand what that means.

8 mΩ = 0.008 Ω
During charge or discharge, the cell will appear to have a fixed voltage with an 8 mΩ series resistance.
Your charger will have to limit the current to below 10 amps, or below 3 amps, depending on the charge time.

 

[DaveE]

A good working approximation is 0. That 8mΩ is mostly the interconnect wires. Your wires are probably more than that.

Li-Ion chargers and/or battery packs aren't a great project for DIY trial and error. Study first so you don't start fires. @berkeman's photo wasn't a joke.

 

[Baluncore]

Li-Ion chargers and/or battery packs aren't a great project for DIY trial and error.

I don't think it would be too hard to design a Li cell charger, so long as you take the following issues into account. A lithium battery charger that is kind to the battery is a switching constant current source. It also will sense the temperature of the battery.

There are several situations when it will turn off.
1. After a fixed integral charge transfer. Q = amps * time in seconds.
Q = 3 amp * 45 min * 60 sec = 8100 coulombs.
Q = 10 amp * 15 min * 60 sec = 9000 coulombs.
2. If the charger gets too hot, or the cell is outside the range of maybe 0 °C to 45 °C.
3. Depending on cell temperature, when the open circuit cell voltage exceeds 3.1 to 3.7 volts.
4. When the open circuit cell voltage is below 1.5 volts, because it is short-circuited.

Once the cell has been charged with the constant current, it can be floated at the temperature dependent open circuit cell voltage, probably from a low-power supply.

Start by designing a voltage reference that mimics the cell temperature profile. That can be used to detect an overcharge, then later, to float the charged cell.

During the charging process, the charging current will need to be interrupted momentarily to measure the open circuit cell voltage.

 

[DaveE]

I don't think it would be too hard to design a Li cell charger

... for an analog EE.

In fact, it's simple enough that if you have to ask us instead of doing the required research online, and then applying normal design skills, then you aren't doing it right. There must be 1000 app notes describing these chargers. I haven't looked, but I wouldn't be surprised if TI, Maxim, or Analog will sell you a single chip solution.

Most DIY types can't do what you think is simple, primarily because they don't know to ask the right questions or how to find the answers. That's OK, nobody knows everything. But this is not a beginner's project. Reread the OP and his "ohm's law" model.

 

[DaveE]

Why not just buy one?
https://www.amazon.com/dp/B09PYBGSKT/?tag=pfamazon01-20

 

[Borek]

Geez, I am starting to be afraid of reading datasheets.

 

[Baluncore]

I don't think it would be too hard to design a Li cell charger, so long as you take the following issues into account.

... for an analog EE.

I made my statement conditional, in the hope it would give some idea of the complexity involved. After writing that, I would buy one myself.

 

[Baluncore]

Geez, I am starting to be afraid of reading datasheets.

I am afraid of trusting them, since every well written datasheet now reads like it was written by ChatGPT.

 

[DaveE]