Electronics > Beginners

Effect of switching current on Li-ion battery packs

<< < (8/9) > >>

VEGETA:

--- Quote from: fcb on February 10, 2020, 08:55:36 am ---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.

--- End quote ---

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:

--- Quote from: VEGETA on February 10, 2020, 08:15:59 am ---Why do you differentiate between real-world ground and simulation ground?

--- End quote ---
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.


--- Quote ---You can do this modification as a gift to you brother if you want to xD. :-+

--- End quote ---
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:

--- Quote from: VEGETA on February 10, 2020, 09:08:21 am ---
--- Quote from: fcb on February 10, 2020, 08:55:36 am ---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.

--- End quote ---

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?

--- End quote ---
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:

--- Quote ---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.

--- End quote ---

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.  |O

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

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod