# Parallel to Series Switching Batteries

1. ## Parallel to Series Switching Batteries

Hello!

In this occasion, i would like to implement a system which can let me switch automatically parallel to series mode of two batteries represented on the image as b1and b2.

The idea is to control it using a micro which is continuously measuring the voltage of the Batteries (3.6V) and the idea is that once the batteries be 1.8V, inmediately changes to series (1.8 +1.8 = 3.6) and the system can continue working.

My problem is that i dont know which kind of switching devices must i use because i will drive pulse currents until of 300mA.

I hope you can help me with this.

Thanks!

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2. ## Re: Parallel to Series Switching Batteries

If the battery voltage drops to half of when it is new then it is already dead and connecting two in series will cause their total voltage to drop extremely quickly, you gain almost no extra time.

Are the battery cells Lithium rechargeable? They are destroyed if their voltage drops below 3.0V and attempting to recharge them might cause a fire.

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3. ## Re: Parallel to Series Switching Batteries

Originally Posted by Audioguru
If the battery voltage drops to half of when it is new then it is already dead and connecting two in series will cause their total voltage to drop extremely quickly, you gain almost no extra time.

Are the battery cells Lithium rechargeable? They are destroyed if their voltage drops below 3.0V and attempting to recharge them might cause a fire.
Thanks for your great explanation. Now, according you told me about the point where the batteries get half charge; the idea is that at least let the board working for about one day and can notify to a server in order to change it inmediately.

PD: The system has a GPRS module.

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4. ## Re: Parallel to Series Switching Batteries

It would be helpful if you show two schematics, one for charging (before switchover), and one after switchover.

Questions:

Do you wish to use one or more relays?

What is the source charging voltage?

Do you need for the negative end of the battery to be 0V ground?

5. ## Re: Parallel to Series Switching Batteries

Hi,

Audioguru is correct.

When the voltage is below 3V, then don´t draw any current from the battery. Not in prallel, nor in series.

In this diagram you see, that from 3.4V down to 3.0V the charge goes from 1920mAh to 2000mAh.
This is only 4% of the charge.
http://www.richtek.com/battery-manag...-discharge.png
* This is less than the tolerance from one battery to the other.
* Or a couple degree of temperature difference
In my eyes it is not worth any effort.

In case you use a switching regulator --> Maybe use a SEPIC one to get higher output voltage. Take care of no_load_input current. Consider to use burst mode.
In case you use a linear regulator --> use a switching regulator.

Klaus

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6. ## Re: Parallel to Series Switching Batteries

It would be helpful if you show two schematics, one for charging (before switchover), and one after switchover.

Questions:

Do you wish to use one or more relays?
I dont want to use mechanical relays because my application must be ultra low consumption.

What is the source charging voltage?
There is no charging mode, the batteries are of 9Ah and i will use until 4 2*2 parallel by pair. These are the Batteries:
https://www.amazon.com/Ultralife-ER2...FY6P6K8QQHEE21

According with my calculation, i dont need to charge the Batteries.

The Load is Attached after the D7 Diode... Look at the image attached VCC.

Do you need for the negative end of the battery to be 0V ground?
Yes i do, thats what i thought to implement the application that i need. A system which changes automatically parallel to series the batteries.

7. ## Re: Parallel to Series Switching Batteries

The datasheet for the battery shows that when it is near the end of its life then the voltage drops extremely quickly.

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