Electronics > Beginners
Effect of switching current on Li-ion battery packs
VEGETA:
Hello,
I've nearly completed the design of a linear lab PSU with SEPIC switching pre-regulator which can deliver 30v 3 amps from 2S2P battery pack.
The pack should be around 8.4v maximum, say 8v typical... simulation in LTSpice is good.
However, the pack is about 5000 mAH or say 4500 mAH but the switching maximum current according to attached image is around 20-30 amps and zero amps.
So I wonder if this could be done? looking forward to your answers.
Note: green is final output current at 30v... and blue is the ripple switched current from the battery pack (nevermind the negative since it is probed that way).
fcb:
It will depend on the inductance of your battery pack and switching frequency.
But I would expect significant heating from your pack - can you try reducing the ripple with capacitors on the input of your converter?
Rigolon:
Batteries are really complex if you go deep in how they work, and i'm not an expert but here is my thoughts on the subject.
In a 2S2P battery pack you have 4 batteries. 2 pairs connected in series, each pair consists in 2 batteries connected in parallel.
So to have 5000mAh 2S2P battery pack, each battery has the capacity of 2500mAh. This is important, so keep that number in mind.
The capacity of the battery represents the value that would fully discharge a battery if draw constantly in 1 hour. (There are more variables to this but I will not get into that). Let's say a 2500mAh means you can supply 2.5A for an hour and then the battery dies, if you supply double (5A) the battery dies in half an hour.
But here is the catch.
The thing is that the capacity of the battery it's not a real value for every case. Usually lower consumption means even more time that can be used and higher means less time. You would expect that in a 2500mAh battery that supplies only 250mA you would get 10 hours, but in reality you can get more, lets say 11~12hours. The same applies for higher consumption. If you go 10 times the capacity current (25A) you'd believe that you can supply current for 1/10 of the time (6 minutes). But in reality will supply less, perhaps 3 to 4 minutes or so. All these values are only illustrative as this characteristics changes with the type of batteries and a lot more specs.
Although when using high frequency the batteries will show behavior of capacitors that would allow higher current output, I wouldn't think it would be able to output 10 times higher the value specified in the capacity for much longer than in constant DC.
Also there is a max current output that over this value will damage the battery, this are all specs to care for.
I'm not saying that you wouldn't be able to do what you want. But as I'm not sure that you will, I'm sharing what I know about the subject so you can check a few things before risking blowing up your battery pack.
EDIT: my knowledge is more related to lead-acid batteries and they are really different than Li-Ion batteries. As I checked a few information it seems they don't lose much capacity even when discharging with 10C rate (10x 2500mA). And since you are using pulsed I guess you won't have much problems.
But on important thing to remember is that when using pulsed discharging the battery allows less cycles (life-time) than rather using constant low DC consumption.
VEGETA:
--- Quote from: fcb on February 07, 2020, 04:29:46 pm ---It will depend on the inductance of your battery pack and switching frequency.
But I would expect significant heating from your pack - can you try reducing the ripple with capacitors on the input of your converter?
--- End quote ---
Hmmm for the absolute state which is 30v 3a total output power would be 90 watts. The design is optimized to reduce heating by using the pre-regulator at 1v difference from linear one which is done using ltspice and all loops are compensated for stability.
for the pack it would get 20 amps of pulses but also 0 amps within the same pulse... shouldn't this count? we know that in order to get overheating you need prolonged overload current but this is not the case here.
Notice that this is the maximum case, normal uses are way below this. However, I wanted to make sure this works.
--- Quote --- if you supply double (5A) the battery dies in half an hour.
--- End quote ---
Yes, but that is the user's responsibility right? i mean he knows he will drain his batteries fast. However, my responsibility is to make it safe and working perfectly without errors.
--- Quote ---Although when using high frequency the batteries will show behavior of capacitors that would allow higher current output, I wouldn't think it would be able to output 10 times higher the value specified in the capacity for much longer than in constant DC.
--- End quote ---
I put 2 in series and 2 in parallel to make it 5000 mah to allow more capability. Now each pair should get 10 amps in pulses.
fcb:
--- Quote from: VEGETA on February 07, 2020, 05:49:10 pm ---
--- Quote from: fcb on February 07, 2020, 04:29:46 pm ---It will depend on the inductance of your battery pack and switching frequency.
But I would expect significant heating from your pack - can you try reducing the ripple with capacitors on the input of your converter?
--- End quote ---
Hmmm for the absolute state which is 30v 3a total output power would be 90 watts. The design is optimized to reduce heating by using the pre-regulator at 1v difference from linear one which is done using ltspice and all loops are compensated for stability.
for the pack it would get 20 amps of pulses but also 0 amps within the same pulse... shouldn't this count? we know that in order to get overheating you need prolonged overload current but this is not the case here.
Notice that this is the maximum case, normal uses are way below this. However, I wanted to make sure this works.
--- Quote --- if you supply double (5A) the battery dies in half an hour.
--- End quote ---
Yes, but that is the user's responsibility right? i mean he knows he will drain his batteries fast. However, my responsibility is to make it safe and working perfectly without errors.
--- Quote ---Although when using high frequency the batteries will show behavior of capacitors that would allow higher current output, I wouldn't think it would be able to output 10 times higher the value specified in the capacity for much longer than in constant DC.
--- End quote ---
I put 2 in series and 2 in parallel to make it 5000 mah to allow more capability. Now each pair should get 10 amps in pulses.
--- End quote ---
OK. I think you are missing the point of what I'm trying to say.
If you look at your graph, the blue trace is solid - this represents a dynamic signal (what frequency?) - basically you are drawing an AC current from your battery. So what matters is how your battery is able to cope with an AC current! And this will depend on cell construction, chemistry, state of charge etc..
If your battery has any inductance at all, then there will be an effect and you can't treat your load totally within the DC domain.
With sufficient capacitance (and the right type of capacitors) between the battery and the converter you will 'smooth out' the AC ripple - reducing the AC component you are presenting to the battery and then you are treating the battery more as DC device.
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