Electronics > Beginners
Effect of switching current on Li-ion battery packs
VEGETA:
--- Quote from: ogden on February 08, 2020, 04:27:42 pm ---
--- Quote from: VEGETA on February 08, 2020, 03:53:01 pm ---I have attached the simulation so you can check yourself.
--- End quote ---
Yes indeed you have zero impedance battery, ideal (and small) capacitor after battery, no battery wires "modelled", L3 & L4 also are ideal. You would want to update circuit and run simulation again.
--- End quote ---
So what is your suggestion? i posted the spice simulation so you can modify and test your ideas to see if it fits. I am open to these modifications.
--- Quote ---That "not-orthodox type of grounding" is |O. What's the problem to make OUT- as simulation ground?
--- End quote ---
The circuit is indeed not so straightforward, if you are interested and have time to waste then check this thread were this circuit was developed from scratch.
The circuit makes the GND as the reference and as Vout+ itself. There is a DC-DC isolated module which acts as an isolated power source to power op-amps, MCU, LCD, and control circuit (DAC,ADC,etc..) which has its negative reference at the Vout+ itself... this is called the floating regulator. I learned this idea from Kleinstein here in this forum as he witnessed it in commercial supplies. That is all the trick in the circuit, the rest is normal.
The topology of this circuit is not really the most important here but rather the pulsing current, whether 2s2p pack of 18650 can handle it or not.
ogden:
--- Quote from: VEGETA on February 08, 2020, 04:37:57 pm ---So what is your suggestion? i posted the spice simulation so you can modify and test your ideas to see if it fits. I am open to these modifications.
--- End quote ---
It is up to you to find out & introduce impedance of your battery, wires, said capacitors & inductors. Don't ask me.
--- Quote ---The circuit makes the GND as the reference and as Vout+ itself.
There is a DC-DC isolated module which acts as an isolated power source to power op-amps, MCU, LCD, and control circuit (DAC,ADC,etc..) which has its negative reference at the Vout+ itself... this is called the floating regulator. I learned this idea from Kleinstein here in this forum as he witnessed it in commercial supplies. That is all the trick in the circuit, the rest is normal.
--- End quote ---
I am afraid that you confuse real world ground with simulator ground. Those can be two different things. If your simulation ground (triangle symbol) placement actually breaks simulation - you are doing it wrong. Just introduce something like "GND1" for MCU, LCD / whatever (name that floating ground "GND1") but connect simulator ground to OUT- where it shall be.
[edit] Whether 2s2p pack of 18650 can handle pulsing current you will ask *after* you correctly simulate that pulsing current.
VEGETA:
--- Quote ---It is up to you to find out & introduce impedance of your battery, wires, said capacitors & inductors. Don't ask me.
--- End quote ---
Ok, I will but I wouldn't mind a hint.
--- Quote ---I am afraid that you confuse real world ground with simulator ground. Those can be two different things.
--- End quote ---
Real world ground (triangle symbol) is exactly where I want it to be. The ground of the design is the output positive itself.
--- Quote ---If your simulation ground (triangle symbol) placement actually breaks simulation - you are doing it wrong.
--- End quote ---
Simulation works and it is not broken. As I told you, the design is exactly as we wanted it to be.
--- Quote ---Just introduce something like "GND1" for MCU, LCD / whatever (name that floating ground "GND1") but connect simulator ground to OUT- where it shall be.
--- End quote ---
Hmm what should be the benefit of changing the terminology if the final result in real world is the same and simulation works?
--- Quote ---[edit] Whether 2s2p pack of 18650 can handle pulsing current you will ask *after* you correctly simulate that pulsing current.
--- End quote ---
I fail to understand why is reversing the direction of current or changing the probing method will affect this? the current pulsing is in the design itself as the LT3757A explains and we picked the 0.003R shunt resistor to allow such high current peaks in order to be able to output enough final output current.
ogden:
--- Quote from: VEGETA on February 08, 2020, 05:32:05 pm ---Real world ground (triangle symbol) is exactly where I want it to be.
--- End quote ---
:palm:
Triangle symbol is not real world ground in LTspice. It is simulation ground. There is no real world ground as such in LTspice. Decision of which wire is real world ground is up-to you.
--- Quote ---I fail to understand why is reversing the direction of current or changing the probing method will affect this? the current pulsing is in the design itself as the LT3757A explains and we picked the 0.003R shunt resistor to allow such high current peaks in order to be able to output enough final output current.
--- End quote ---
There are two problems in your simulation: #1 use of ideal, non real components #2 wrong inconvenient for simulation connection of LTspice ground. Problem #2 has nothing to do with current pulsing.
--- Quote ---Hmm what should be the benefit of changing the terminology if the final result in real world is the same and simulation works?
--- End quote ---
Changing simulator ground changes ground reference of simulation voltages. Illogical simulation ground connection discouraged me to even press "run" button because I knew "measure" tool have wrong reference ground.
fcb:
Running your simulation (as supplied), if you look at the current draw on the power source (battery) at say 4.85ms, you get this:
You'll see that the current is being drawn by your converter in pulses. Any inductance in the supply to this (e.g. battery chemistry, construction, leads, etc..) will have an effect on this ability to draw current and likely affect your stability/function of your converter.
If I put in a small ESR for the battery (say 0.02ohms) and just look at the ripple current that C13 (which says 4.7uF x2 - but is actually set to 4.7uF not 9.6uF?):
Peak currents of around 3.6A in C13 - that needs to be a pretty special 4.7uF capacitor to take that ripple. I would increase it and model is with at least it's ESR included.
Here's what happens if you change:
C13 to C1210C106K3RAC 10uF from KEMET (Mouser stocked part)
L7 (wire), 100 mm of 4mm CSA wire (https://www.eeweb.com/tools/wire-inductance)
You'll notice that the ripple current experienced by the battery is much lower. Perhaps adding a bit more inductance (a filter basically) will reduce this further (lower EMI, lower heating due to AC component in battery). I'd probably add a delibrate PI filter, or perhaps add a capacitor across the battery directly to create this filter with any wiring.
You'll have to do some calculations with C13 to see if it will take the ripple (clue: Dissipation Factor!)
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