power

[Roger_tech]

If I wanted to connect two 4S Lipo batteries, with unequal charges in parallel, and get a constant output voltage, with minimal heat dissipation, how would I do this?

 

[Martaine2005]

#1 you shouldn’t parallel batteries with unequal voltages.
#2 batteries don’t give constant voltage, the voltage will change as the battery depletes.

What do you want to do?

 

[Roger_tech]

This isn't for a practical project, it was a task given to me, and I don't need constant voltage output, for a long period of time, I just need it to not fluctuate so much in a short period of time, where changes due to battery discharge can be ignored.

 

[Martaine2005]

You can use a buck/boost converter and adjust it to your desired voltage or use a voltage regulator.

 

[Delta Prime]

This is how I read your question .
Connecting 4 series LiPo cells in parallel with an additional 4 series LiPo cells is how I read your question? That is a 4S2P battery pack with each cell having different voltage levels. is this correct? If so, you’re in big trouble.
See here.

 

[Roger_tech]

You can use a buck/boost converter and adjust it to your desired voltage or use a voltage regulator.

Can I connect two batteries with different charges at the same time to Buch/boost converter?

 

[Roger_tech]

This is how I read your question .
Connecting 4 series LiPo cells in parallel with an additional 4 series LiPo cells is how I read your question? That is a 4S2P battery pack with each cell having different voltage levels. is this correct? If so, you’re in big trouble.
See here.

Two battery packs with 4 cells in series, each pack has a different voltage, I need to connect the two of them in parallel and without the batteries blowing up, or heating too much, I need a, output voltage.

 

[Alec_t]

You could connect each 4S pack via a respective diode to a common load. The load would then be powered by whichever pack had the higher voltage. Or you could connect each pack via a respective resistor to the common load. The load would then be powered by a weighted average of the two pack voltages.

 

[Roger_tech]

At first, I tried to balance the batteries, using a safe amount of current through resistors, but that idea was rejected, because I was generating too much heat, so no resistors, so I'm thinking of choosing your idea of diodes. `thanks

 

[Harald Kapp]

As I understand the op's question this is not about powering a load but about balancing the two battery stacks such that the unequal charges become equal. So the diodes as suggested in post #8 will not help.

The first try by the op to use a resistor to balance the charges is viable, but obviously dissipates to much power.
A less power intensive way could be a switch mode battery charger which takes the battery stack with the higher charge as input and charges the battery stack with the lower charge on the output side. Using switch mode technology reduces power dissipation as much as possible (my humble opinion). One will need a supervisor circuit and a suitable algorithm (ADC + controller) to estimate the state of charge of both battery sztacks. Once they have reached (almost) equal state of charge, the switch mode charger can be removed and replaced by a direct connection. Not a simple setup, I admit.

Much easier: First charge both battery stacks to full, then connect them in parallel.

 

[Roger_tech]

As I understand the op's question this is not about powering a load but about balancing the two battery stacks such that the unequal charges become equal. So the diodes as suggested in post #8 will not help.

The first try by the op to use a resistor to balance the charges is viable, but obviously dissipates to much power.
A less power intensive way could be a switch mode battery charger which takes the battery stack with the higher charge as input and charges the battery stack with the lower charge on the output side. Using switch mode technology reduces power dissipation as much as possible (my humble opinion). One will need a supervisor circuit and a suitable algorithm (ADC + controller) to estimate the state of charge of both battery sztacks. Once they have reached (almost) equal state of charge, the switch mode charger can be removed and replaced by a direct connection. Not a simple setup, I admit.

Much easier: First charge both battery stacks to full, then connect them in parallel.

The objective was not balancing the packs, it was simply about getting a voltage output (constant, ignoring the decrease over time due depleting battery charge). Balancing the packs was an idea that I thought of. Anyhow thanks for your opinion

 

[Martaine2005]

The objective was not balancing the packs

They have to be balanced.
Without a diode, the weaker cells will cause the Good cells to deplete quicker and generate heat.
I still don’t understand what you’re trying to achieve.

 

[Alec_t]

I still don’t understand what you’re trying to achieve.

Join the club.

 

[Roger_tech]

Join the club.

I know I suck at explaining, this is a test, it was given to me during an interview, I was asked to design a PCB for this and submit it.

 

[Roger_tech]

Two 4s Lipo batteries at 16.8V and 14.8V, connect them in parallel, minimal heat dissipation. This is what they asked me to do.

 

[crutschow]

Two 4s Lipo batteries at 16.8V and 14.8V, connect them in parallel, minimal heat dissipation. This is what they asked me to do.

Did they state what the purpose of this was?
Without that, it's not reasonable to do a design, and it's a poorly asked problem.

 

[Roger_tech]

Did they state what the purpose of this was?
Without that, it's not reasonable to do a design, and it's a poorly asked problem.

No, they didn't talk about an actual use for this design, and I don't think they meant to, I think they just want to see how I approach a problem that isn't very common. I mean, no one simply connects batteries with different charges in parallel, right?

 

[Martaine2005]

I mean, no one simply connects batteries with different charges in parallel, right?

Right!
Maybe that should be your answer.

 

[Roger_tech]

Right!
Maybe that should be your answer.

I don't think that's it, they explicitly asked me to design a PCB, and send them the Gerber files, I interviewed for the PCB designer position.

 

[Martaine2005]

I don’t get it. Paralleled cells of different voltages is a no go. Well, of course you can but not advised.
Can you add circuitry to each battery before they are connected in parallel?