High Amp DC-DC boost board

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61

Hi Andrew

That should work. You could still have it “no break” if you wanted by moving the diodes to the 12V point. That way you could connect the smaller “transit” battery before removing the large battery and vice versa.
You will get there I am sure.
Cheers Bob

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62

Hi Andrew
Add on to above
Moving the ORing diodes from the 5V point to the 12V point has another advantage. There would be no potential problem with the diode voltage drop and this effect on RPi which seems to be a worry with these devices.
Cheers Bob

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63

Just to finally round out the question, the easiest and cheapest way to acheive exactly what I was aiming for, was to go for the 3s BMS and 3 x 18650s.

12v input to the BMS will both charge the batteries and run my Pi and all other hardware without issue.
Plugging in or unplugging the 12v main feed has zero effect on the Pi or my other hardware.

So.

  • Single 12v input
  • backup battery power that runs everything, not just the Pi
  • the ability to plug and unplug at will with no side effects
  • no complicated rules to follow in order to not reboot or under power anything.

All in all, that’s a big win as far as I am concerned.
Thanks to everyone that provided me with input, you all helped me to get to the good solution in the end.

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64

Hi Andrew

No it won’t.
Charging: The 3s LiPo will require 12.6V to charge. The lead acid battery could possibly be 13.6 or 13.8V fully charged which could possible damage the LiPos. There have been reports of exploding cells and fire due to incorrectly charging these.
Other hardware: The mAhr capacity of a 3s arrangement will be approximately the capacity of 1 cell. In series the mAhr capacity does not add up, the voltage adds. The back up should only run the RPi.

If both batteries are connected for extended periods Diodes to isolate will be required to prevent one attempting to charge the other. Then the one with the highest voltage will pick up more of the load. If this is the “back up” battery it will be depleted to the same voltage as the “main” battery so when you need the “back up” it will probably be flat. This can be overcome with circuitry to disconnect the “back up” battery when the main supply is present as with some solar controller cum charger cum battery booster devices but then you are faced with the change over delay you have with your present system.

I would still be going for the simple system of connecting your “back up” battery only while in transit and separately charging it, but that is me, simple.

I don’t think you will achieve any of these points. Your 3s arrangement will probably not run everything.

By the way, what is wrong with the RPi rebooting or won’t it do this reliably. If that is the case I would be looking for something that will.
Cheers Bob

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I think that you’ve missed a few points, and there’s a couple of things that I will agree on.

I didnt explicity say it, and I rounded the input voltage in what I said to be brief, however I AM supplying the BMS with 12.6v (I did say 12v)
Secondly, the BMS will not over volt the cells, that is litterally it’s job.

Yep, totally understand that, no question.

Not sure what you mean by this.
If you mean only the Pi, not the screen or other things, then that is absolutely incorrect.
It is sitting on my bench now, happily running the Pi 4, HDMI screen, 3 x DC encoder motors and DSLR camera, only from the 3 x 18650s connected to the BMS, then the 12v output converted down to the 5v for the Pi, 6v for the motors and 9v for the camera.

You are totally correct on this front and I missed that, easy to fix though by putting diodes on the supply line.

I get it, you dont understand what I am trying to do.
Your solution doesnt meet my requirements, no matter how many times it is raised.
I am not saying that it wont work, it will, but I need to solve all my requirements, not just one.

Yes it totally does.

I’ll go back, again, to my requirement list.
The Pi rebooting in ‘normal’ circumstances is not the end of the world, however it is absolutely not an ideal situation for a couple of reasons.

  1. hard rebooting a Pi will eventually corrupt the SD card, just a fact of life. Avoiding that is obviously ideal.
  2. For the short term application of my hardware I have a hard requirement that the Pi isnt rebooted, it needs to stay connected and do so for config, transport, moving to specific location and then final setup before it can be repowered by the much more permanent power supply.
    It is specifically not ideal to do this with batteries that are external to my setup as multiple boxes joined by a wire tend to get knocked, bumped and eventually disconnected.
    This is why I require the backup power to be internal to my device.
    It’s not ideal for that supply to have to be cabled externally to my device to be plugged and unplugged with the external battery.
    This is due to the fact that all of this is for a camera slider dolly.
    It ALL moves and anything that poses a snag risk is not acceptable.

Yes, I could mount all that internally to the device and have a switch, but past experience tells me that switches get knocked and in the dark (which is where this all works) it’s really easy to switch the wrong switch or understand what position the switch needs to be in for what mode.

This is entirely the reason why I need the solution to be capable of running by itself or when powered from a single external cable.

As I said, i do appreciate the help Bob.

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66

Hi Andrew
Yes I appreciate where you are coming from and realise why you are trying to make this absolutely foolproof but I really don’t know of too much off the top of my head which will do all that you need. Doing your back up etc at the 12V level will get around the diode voltage drop at 5V which would remove one worry.
But if both batteries remain connected This will still happen.

and the only way to overcome that

You really need a way to disconnect the back up battery when the main battery is disconnected but still charge the back up with the main battery while leaving it in the box with everything else. Also have a “no break” during connection and disconnection to prevent any rebooting etc. Off the top of my head I don’t know of any “maker” type product to do this but that does not say one will not exist. Will do a bit of head scratching.

I have seen “no break” systems but on a very much larger scale. One that comes to mind was essentially Diesel back up. A DC motor rotated a flywheel of some 3 or 4 tons which was coupled to the diesel via a magnetic clutch. In the event of power failure the energy in the flywheel was enough to keep the motor spinning as a generator and start the diesel. Very abruptly I might add. Brute force but it worked and very reliable.Used on unattended aircraft nav aid stations.

I am not absolutely conversant with the BMS usually used but I understood they equalised cell charging by regulating the charging current in individual cells. They do this by bypassing some of the current around any cells that are charging quicker than others. The maximum charging voltage (12.6V for 3s connection) organised externally. Maybe some do this internally, I don’t know for sure.

Keep going. This is a great project and I have always admired some of the results obtained with time lapse photography techniques.
Cheers Bob

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67

Thanks Bob!

There are Battery Protection BMS boards, and there are Charger BMS boards.
I have a combination of a dozen different varieties, usually from 3S build your own kits, or removed from individual 3S LiPo battery packs.
All of these are made for discharge and charge, and work with a dumb 12v supply.

I’m still testing, but will include the diodes for reverse protection.

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