Effect of switching current on Li-ion battery packs

[VEGETA]

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.

 

Seriously ? Especially using those "firey" brand 18650 cells like SureFire, MustFire and etc. 

we don't have these brands that you mentioned but rather other brands that performs good. They are cheap and good, not good as panasonic that is.

I am working on implementing protection and charging circuitry to make sure even the unprotected cells can work without problems. Now the performance is another story since it depends on the batteries but my point is that the device should work with any battery. It is the user's fault for getting a cheap battery that lasts for a short time.

[ogden]

So the question is: should I use 2S2P or 4P? since they are using same current per cell (~7A) -> at maximum ever state.

This part was already answered:

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.

For DC-DC converter also it is better to minimize input current because you minimize losses in the switches, rectifiers and inductors.

[VEGETA]

This part was already answered:

I get it but it is as this:

2S2P: max current from battery is ~15 amps but per cell battery is about 7A.
4S1P: max current from battery is ~7 amps which is the same as per cell.

What you mean is 15 amps of total current is worse since more current means more heat... thus per cell current is not the lone factor?

For DC-DC converter also it is better to minimize input current because you minimize losses in the switches, rectifiers and inductors.

this makes sense. I wonder if adding bigger caps will enhance the system... but without hindering the speed. I remember once I tried to add an inductor after the battery (not the 50nH wire one) but circuit didn't work.

Concluding from all this that 4S is the correct choice here with making sure enough capacitors directly after the battery. Correct?

[ogden]

I wonder if adding bigger caps will enhance the system... but without hindering the speed.

Sooner or later you get diminishing returns. You balance caps for ripple_voltage vs cost/size.

[VEGETA]

I wonder if adding bigger caps will enhance the system... but without hindering the speed.

Sooner or later you get diminishing returns. You balance caps for ripple_voltage vs cost/size.

I've modified the simulation a bit to consolidate some parts and so on... I also made ESR for each passive part as much as I could. Kindly see it.

Is this ripple good enough?

Also, see the very first +40A overshoot thing... I couldn't fix it even by putting 4.7u inductor.

[ogden]

Is this ripple good enough?

That is current ripple of battery, right? Where is voltage ripple on input capacitors? You have to look at both.

Also, see the very first +40A overshoot thing... I couldn't fix it even by putting 4.7u inductor.

Ignore it. It is inrush current of input capacitor charge.

[VEGETA]

That is current ripple of battery, right?

Yes.

Where is voltage ripple on input capacitors? You have to look at both.

I have attached photos for the voltage ripple, the 2nd photo is when output is stable. voltage ripple is about 0.15V, so is it good?

Ignore it. It is inrush current of input capacitor charge.

I would ignore it but isn't it harmful to the batteries and supplies in general? It was oscillating and doing some overshoot then stabilize before adding caps though.

[ogden]

I have attached photos for the voltage ripple, the 2nd photo is when output is stable. voltage ripple is about 0.15A, so is it good?

You mean 0.15V? - Yes, looks good. To me seems like input caps size is right. Why you are refusing to reconnect simulation ground to "OUT-" so you don't have to use math like in this case?

Ignore it. It is inrush current of input capacitor charge.

I would ignore it but isn't it harmful to the batteries and supplies in general?

That "short circuit" is very short. In real world some spark will fly but nothing bad will happen. Obviously do not overdo capacity of input capacitors. The bigger they are, the louder bang will be when you connect battery

[VEGETA]

You mean 0.15V? - Yes, looks good.

yes I modified it to be "v". I guess this is the best it gets.

To me seems like input caps size is right.

Yes since increasing them will increase the inrush current plus will make it slower.

Why you are refusing to reconnect simulation ground to "OUT-" so you don't have to use math like in this case?

what math?

This simulation is exactly as the one it should be in real-life, that is how this design is.

If I connect ground to OUT-, then I will have to modify a lot of stuff like the isolated DC-DC ground and the ground at the output... since all referenced to it. It looks like negative current and so on but in real-life it is typical behavior. Check the thread where this thing got developed in for more info.

Why do you differentiate between real-world ground and simulation ground?

You can do this modification as a gift to you brother if you want to xD.

[fcb]

From your earlier description I’m assuming you are permenatly connecting the battery to the converter? So no switch?

Back in the 90’s we built high power dc boost converters (250W+), these were connected to large NiMH packs as and when needed using expensive Fischer connectors. As the topology was basic ‘boost’ we needed to (1) protect against high in-inrush current when connected (2) disconnect under low voltage battery conditions and (3) disconnect under output short conditions.

We used a big MOSFET in series with the battery and drove it from the microcontroller. It had no measurable impact on converter efficiency and allowed us to gently fill the front end filter capacitors and bring up the converter. We could detect shorted outputs, etc.. easily and protect battery and converter.

[VEGETA]

From your earlier description I’m assuming you are permenatly connecting the battery to the converter? So no switch?

Back in the 90’s we built high power dc boost converters (250W+), these were connected to large NiMH packs as and when needed using expensive Fischer connectors. As the topology was basic ‘boost’ we needed to (1) protect against high in-inrush current when connected (2) disconnect under low voltage battery conditions and (3) disconnect under output short conditions.

We used a big MOSFET in series with the battery and drove it from the microcontroller. It had no measurable impact on converter efficiency and allowed us to gently fill the front end filter capacitors and bring up the converter. We could detect shorted outputs, etc.. easily and protect battery and converter.

Yes, batteries are always connected. However, I could add a hardware switch to turn the entire device on or off. What is your recommendation?

I assume you mean put the hardware switch after the input capacitors so that the caps are always charged so no inrush current will occur, right?

[ogden]

Why do you differentiate between real-world ground and simulation ground?

Because measure tool is referenced against simulation ground. Seems like you don't even try to comprehend what I am telling. So I give up.

You can do this modification as a gift to you brother if you want to xD.

Thanx, no. If you can't delete few triangles, name those busses as "VGND" or similar and put single triangle where it shall be - at battery "-" terminal, I am not going to.

[fcb]

From your earlier description I’m assuming you are permenatly connecting the battery to the converter? So no switch?

Back in the 90’s we built high power dc boost converters (250W+), these were connected to large NiMH packs as and when needed using expensive Fischer connectors. As the topology was basic ‘boost’ we needed to (1) protect against high in-inrush current when connected (2) disconnect under low voltage battery conditions and (3) disconnect under output short conditions.

We used a big MOSFET in series with the battery and drove it from the microcontroller. It had no measurable impact on converter efficiency and allowed us to gently fill the front end filter capacitors and bring up the converter. We could detect shorted outputs, etc.. easily and protect battery and converter.

Yes, batteries are always connected. However, I could add a hardware switch to turn the entire device on or off. What is your recommendation?

I assume you mean put the hardware switch after the input capacitors so that the caps are always charged so no inrush current will occur, right?

No - you need to keep the path between the input capacitors and the SEPIC as short as possible.

In your circumstance I would put a lowRDSon N-channel MOSFET in the negative of the battery and then control the gate with a switch.  Probably also have some sort of soft-start (slug the rise time of the gate with a capacitor) - you won't have to slug it much to reduce the in-rush current massively.  Probably make sure your controller IC is setup to come on after your input MOSFET is hard on.

Your input MOSFET will cost alot less than a physical switch that can cope repeatedly with the in-rush current.

[VEGETA]

No - you need to keep the path between the input capacitors and the SEPIC as short as possible.


In your circumstance I would put a lowRDSon N-channel MOSFET in the negative of the battery and then control the gate with a switch.  Probably also have some sort of soft-start (slug the rise time of the gate with a capacitor) - you won't have to slug it much to reduce the in-rush current massively.  Probably make sure your controller IC is setup to come on after your input MOSFET is hard on.

Your input MOSFET will cost alot less than a physical switch that can cope repeatedly with the in-rush current.

But won't this make the sepic converter always on? I mean if I put a mosfet at the negative side of the battery, how can it be powered with the microcontroller? in this case, maybe you meant powering the mosfet with a physical switch to its gate connecting it to the battery?

The LT3757A has a soft-start pin, going beyond the 10nF capacitor will hinder its working as I tested it in simulation. So if we go with this solution, we might put a 10nf at the gate of the mosfet to make it a bit slower. I guess this is better than using an analog switch.

I tried playing around with soft-start pin of the sepic ic but couldn't do much thus i kept it as is (10nF).

Let's say we went with this inrush current as is, is it gonna be a problem assuming physical or mosfet switch?

Now with 4S pack the voltage is 16v, thus I need to lower it a bit to feed it into the isolated DC-DC converter module. Previously I used a boost to make it from 6-8.4v to 12v but now it should be from say 12-16.8v to 12v using a buck converter assuming 3v is the lowest voltage each cell can reach. If we go to say 2.5-2.7v, pack voltage is gonna be 10-10.8v so it is less than 12v! in this case the buck will output about 9v or even less. I need to solve this the cheapest and simplest way possible. 

[fcb]

Just put your on/off switch in line with R1.

[VEGETA]

Just put your on/off switch in line with R1.

(Attachment Link)

Thanks for this circuit, I will try to modify it to consolidate parts and make it suitable for 16v input.

The rest of my questions needs more investigation to be solved.

regards

[fcb]

Just put your on/off switch in line with R1.

(Attachment Link)

Thanks for this circuit, I will try to modify it to consolidate parts and make it suitable for 16v input.

The rest of my questions needs more investigation to be solved.

regards

That circuit will work fine at Vbattery=16V. If you are concerned about exceeding V_GSmax (+/-20V) stick a 10V zener across the gate and source.

[VEGETA]

Just put your on/off switch in line with R1.

(Attachment Link)

Thanks for this circuit, I will try to modify it to consolidate parts and make it suitable for 16v input.

The rest of my questions needs more investigation to be solved.

regards

That circuit will work fine at Vbattery=16V. If you are concerned about exceeding V_GSmax (+/-20V) stick a 10V zener across the gate and source.

I intend to have a 13v zener diode elsewhere (input of DC-DC module), so I will put another one for this circuit. Also, probably getting consolidated parts like 10k or 100k resistors only. putting a small cap on the on-off switch will help in bouncing effect if it happens with a traditional switch (not tactile button).

[fcb]

Just put your on/off switch in line with R1.

(Attachment Link)

Thanks for this circuit, I will try to modify it to consolidate parts and make it suitable for 16v input.

The rest of my questions needs more investigation to be solved.

regards

That circuit will work fine at Vbattery=16V. If you are concerned about exceeding V_GSmax (+/-20V) stick a 10V zener across the gate and source.

I intend to have a 13v zener diode elsewhere (input of DC-DC module), so I will put another one for this circuit. Also, probably getting consolidated parts like 10k or 100k resistors only. putting a small cap on the on-off switch will help in bouncing effect if it happens with a traditional switch (not tactile button).

You don't need to debounce the switch - the capacitor across the gate-source will do that for you.

[VEGETA]

Nothing new here but re-arranged GND stuff to be referenced in a better way, still the circuit is the same.

Now GND is the battery negative and I assigned "float GND" to the floating supply circuit.