Electronics > Beginners
Effect of switching current on Li-ion battery packs
GeorgeOfTheJungle:
--- Quote from: VEGETA on February 08, 2020, 04:37:57 pm ---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.
--- End quote ---
Many 18650s can handle 15..20 A max constant current nowadays (e.g. LG INR18650HG2), but you've got to look that carefully in the cells' specs: Intended Use (power tools) and Maximum Continuous Current.
In general the rule (for 18650s) has always been more current (max A) => less capacity (Ah/Wh), and in this sense 4S is better than 2S2P, and 2S2P better than 4P, because 4S lets you get the same watts with less amps.
At those currents you won't get anywhere close to the nominal capacity (Ah/Wh) because the nominal capacity is usually calculated at less than 1C. For example, that INR18650HG2 has 3Ah @0.6A, but I2R losses at 20A are at least a THOUSAND times more than at 0.6A... ((20/0.6)2)
At those currents life expectancy is a disaster.
VEGETA:
Kindly see newer modified version in attachment, now total battery peak currents are about 1 amps of ripple (14.5A - 13.5A) instead of pulses of 20 amps and 0. Nice improvement right?
what I did is adding around 50nH of inductance with 0.005R series resistance which is the PCB trace since I will surface mount the regulator very closely to the surface mounted batteries... so it would be even better than this.
Combine that with now 3 of 10uF capacitor that you choose to get this result with about 3 amps of these capacitors ripple current current (assuming I could get cheaper ones from LCSC in a big reel with same specs or close). I guess this is what you meant previously right? but your capacitor ripple is lower.
What I have mistaken hugely is the switching current at Isense and Rsense resistor of the LT3757A which is required... I assumed that the current drawn from the battery should be similar... but you guided me that it shouldn't.
--- Quote ---At those currents life expectancy is a disaster.
--- End quote ---
Now maximum battery pack current is about 15 amps continuous which means each battery gives 7.5 amps, this is much better. There is a very short (~3 ms) of more current at first but this is not continuous.
--- Quote ---and in this sense 4S is better than 2S2P, and 2S2P better than 4P, because 4S lets you get the same watts with less amps.
--- End quote ---
I thought about this solution and maybe it is better overall? luckily the design allows this flexibility, any configuration works since it is a wide input SEPIC design... However I will need to adjust the charging and protection circuit for the complete circuit according to the final decision... which has nothing to affect this main circuit.
Do you think I could pull maximum of 30 volts 3 amps out of say 4S1P cells of ~ 2000-2500 mah? the design allows this assuming the source is capable.
ogden:
As you see - capacitors indeed reduce power supply ripple :) Yes, high current Li-Ion cells can sustain currents and discharge rate above 1C. There are even 10C cells/packs. Problem with such - they are more expensive than 1C rate cells, they degrade faster. For example 18650 cell at 4C discharge (2500mAh @ 10A) at the end will be around 50..60oC degree hot. Frequently overheated cells do not live long. (picture from internet search)
VEGETA:
--- Quote from: ogden on February 08, 2020, 07:28:27 pm ---As you see - capacitors indeed reduce power supply ripple :) Yes, high current Li-Ion cells can sustain currents and discharge rate above 1C. There are even 10C cells/packs. Problem with such - they are more expensive than 1C rate cells, they degrade faster. For example 18650 cell at 4C discharge (2500mAh @ 10A) at the end will be around 50..60oC degree hot. Frequently overheated cells do not live long. (picture from internet search)
--- End quote ---
Well, at 15A max we got 7.5A per cell which is doable even for Chinese 18650 batteries.. which is my aim. To be able to use this thing with any battery.
I made another simulation with 16v input (4 batteries in series) and the total current from battery was around 7 amps with peaks between 6 amps and 7 amps total.
So the question is: should I use 2S2P or 4P? since they are using same current per cell (~7A) -> at maximum ever state.
BravoV:
--- Quote from: VEGETA on February 08, 2020, 07:49:14 pm ---Well, at 15A max we got 7.5A per cell which is doable even for Chinese 18650 batteries.. which is my aim. To be able to use this thing with any battery.
--- End quote ---
:palm:
Seriously ? Especially using those "firey" brand 18650 cells like SureFire, MustFire and etc. :scared:
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