series connection of battery cells with FETS

A

atlantic

New Member
Hi,

I would like to connect 3 seperate battery cells together using MOSFETS (p or N canal).
please, see attached figure. Is the connection correct? Please, make suggestions.


can anyone here give me contact information of a person (or website) who can verify schematics?

Thank you.

Jason
 

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Just one question: why?

Surely it's easier to just use one MOSFET to turn the power to the circuit on and off? You probably don't need three.
 
I will like to connect the FETS and the battery cells as shown in the drawing. The question is more about the grounding of the various FETS.
 
You still haven't asked the question: why do you want to do that?

Sorry but I think it's a silly idea.

The batteries should be left permanently connected in series.
 
I do not want to connect the batteries permanently together but have them connected as shown in the new diagram.
Is the connection correct. Will it work?

Thank you
 

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  • FET_series..pdf
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  • FETseries..jpg
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It won't work at all.
Even if you solved the gate-drive issues of Mosfets whose sources are all at different potentials, and used P-chan Mosfets instead the N-chan as shown, whenever you turned off ANY Mosfet the current flow would be completely cut-off.
Might as well have a single Mosfet turning off the complete battery string, because essentially that is the end result you'll get.
 
atlantic said:
I do not want to connect the batteries permanently together but have them connected as shown in the new diagram.
Click to expand...
Again, why do you want to do that?

You could use relays but as I said before, there's no need to disconnect all the batteries, you only need to disconnect either the positive or negative. Trying to disconnect them all is pointless.
 
gentle men / gentle ladies,

many have written 'it would not work' So what is the solution? Again, I would like to connect the cells as shown in the diagram. Using MOSFETS to connect them in series.
 
Mike, your FETs are backwards. (D/S wrong way).
Atlantic, do you intend to both charge and discharge your batteries in this configuration? If so, you'll need back to back FETs.
 
yes, agreed, the diodes will pass the current with the FET off. charging won't be a problem as a FET will conduct current in both directions...
 
Use either of the following solutions attached.
 

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Last edited:
Thanx Mike, Kchriste and Hero999 for the susgested solutions.

Kchriste: Yes, I would have to charge the cells at one time and discharge them at some other time. What ever the case, the Mosfets must be BETWEEN the cells as drawn.
I believe the issue is, the Gate Source control voltage and potential between the cells.

Kchriste: Can you post a sketch?

Mike: Can you, please post another diagram. The one you posted is not very clear (parts of the circuit not visible)

Thank you
 
Kchriste: yes I would have to charge the cells at one time and discharge them at some other time but not using same Mosfets. For charge and discharge, I will use seperate Mosfets.

For both charge and discharge, the fets and the cells will be connected in series.
 
atlantic said:
Kchriste: Can you post a sketch?
Click to expand...
As noted by others, there is probably a better way to accomplish what you want without using a "switch" or FET between each cell. You are probably needlessly complicating the circuit.
But if you really must isolate each cell, then something like what I have attached should work. You'd need one "between" each cell and the 470pF caps could be rated for a lower voltage if the batteries are just low voltage cells:
 

Attachments

Ive seen that circuit before and I was impressed.

You could use logic level MOSFETs and it can be powered from a 5V logic circuit.

If 400V MOSFETs Y2 rated capacitors are used, it can safely switch the mains, although I'd be tempted to add a snubber network, put a high-voltage high-value resistor (5kV, 10M) across one of the capacitors and a zener across the MOSFET gates to minimise potential damage from static electricity.

The capacitors and logic gates could also be replaced with a tiny transformer and a two transistor oscillator.
 
Kchriste: Thanx for the sketch. I understand that, the proposed circuit could be used for both cell charge and discharge. You wrote 'one between each cell' Do you mean, the 'OUT' of one cell connected to the 'IN' of the other?

For Charge Mode: 'control signal from Low to High'?
Discharge Mode: 'control signal from High to Low?

Why the series connection of the cells using the FETS? Answer: Assume there are 6 cells which i want to connect in series. If I solder them together, I would not be able to disconnect the individual cells (electronically) when I want to. I want to be able to connect just 1 cell, 2 , 3 etc in series when I decide to.

Does anyone know how to realize this without using switches between them?

Thank you
 
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