Author Topic: how many batteries in series for a 12V system?  (Read 1927 times)

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Offline ZeroResistance

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how many batteries in series for a 12V system?
« on: May 24, 2017, 09:49:24 pm »
I am in a dilemma regarding the number or series Li-ion cells for a 12V system.
My loads are BLDC motors running on the 12V power and other circuits that would run on 5V / 3.3V.

Should I use 3 cells or 4 cells in series for Li-ion.
The charging voltage of the cells is 4.2V.
so for
3 Cells = 4.2 x 3 = 12.6V.
4 Cells = 4.2 x 4 = 16.8V.

If I take the nominal voltage of the cell as 3.7V
3 Cells = 3.7 x 3 =  11.1V.
4 Cells = 3.7 x 4 =  14.8V.

So my first question is should I consider the voltage as 4.2V or 3.7V?

Currently I'm planning to use a 19V charger because I had thought of using 4 cells.

The other questions are can I directly provide this voltage to the motors which would probably result in overvoltage in a 4 cell system and under voltage in a 3 cell system.

Or should I put in a 12V SMPS regulator after the batteries?

TIA
 

Online retrolefty

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Re: how many batteries in series for a 12V system?
« Reply #1 on: May 24, 2017, 10:03:35 pm »
Quote
Currently I'm planning to use a 19V charger because I had thought of using 4 cells.

 A proper balanced Li charger will adjust it's voltage and current automatically to suit the number of cells in use. One does not just provide a constant voltage to charge these kinds of battery.

Quote
The other questions are can I directly provide this voltage to the motors which would probably result in overvoltage in a 4 cell system and under voltage in a 3 cell system.
I would go with a 3 cell Li to power a nominal 12 volt load or system.

Quote
Or should I put in a 12V SMPS regulator after the batteries?

 One normally doesn't use regulators for powering higher current motor loads.

 
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Offline ZeroResistance

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Re: how many batteries in series for a 12V system?
« Reply #2 on: May 24, 2017, 11:09:43 pm »

 A proper balanced Li charger will adjust it's voltage and current automatically to suit the number of cells in use. One does not just provide a constant voltage to charge these kinds of battery.


I meant to say that I am currently planning to use a 19V adapter.
For the charging system I was planning to use the BQ24600 from TI it can charge upto 6 cells in series however I don't see that has any cell balancing features.

So you say 3 series cells that would give a voltage range from 9 to 12.6V if I consider 3V as the min voltage and 4.2V as the max voltage of the cell. Is this OK?

Ok! agreed on the regulator part. You see that I have to power the bldc motor controller which is 3rd party and I don't know if it will operate from 9 to 12.6V but I can check on that
 

Offline NANDBlog

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Re: how many batteries in series for a 12V system?
« Reply #3 on: May 24, 2017, 11:35:31 pm »
I usually shortcut these problems by using LiFePO4 cells, with a nominal voltage of 3.2V. Unless you have a specific reqirement, or price is an issue they are better anyway.
 
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Offline ZeroResistance

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Re: how many batteries in series for a 12V system?
« Reply #4 on: May 25, 2017, 12:35:49 am »
LiFePO4, that chemistry is highly safe right? So 3.2 nominal you say? but charge voltage would probably be 3.6
so
3.2 x 4 = 12.8V. (For nominal) and
3.6 x 4 = 14.4V. (for max)

but the min discharge voltage would probably be 2.8V
2.8 x 4 = 11.2V. (for min)

So this gives you a range of 11.2 to 14.4, which is quite good, it seems to be centered around that 12V mark.

How do you manage to charge these do you use a  charge controller and what about cell balancing?
Know any reputed source for these?
 

Offline Audioguru

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Re: how many batteries in series for a 12V system?
« Reply #5 on: May 25, 2017, 12:41:39 am »
Why don't you use brushless motors and balanced chargers designed for Lithium batteries? My electric radio controlled model airplanes do.
 

Offline NANDBlog

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Re: how many batteries in series for a 12V system?
« Reply #6 on: May 25, 2017, 12:49:38 am »
You buy a cell balance charger, or build in a board dedicated for that. I dont know the application, since you havent told us, nor capacity.
 

Offline grifftech

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Re: how many batteries in series for a 12V system?
« Reply #7 on: May 25, 2017, 01:06:53 am »
My "12V" lithium cordless drill battery is just over 11 volts open circuit and has 3 cells
 
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Offline Mechatrommer

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Re: how many batteries in series for a 12V system?
« Reply #8 on: May 25, 2017, 01:57:20 am »
I am in a dilemma regarding the number or series Li-ion cells for a 12V system.
use 3 cells, its considered as 12V battery.

So my first question is should I consider the voltage as 4.2V or 3.7V?
(4.2 + 3.7) / 2 = 3.95 ;D

Code: [Select]
3 Cells = 4.2 x 3 = 12.6V.  ----> 0.6V overvoltage
3 Cells = 3.7 x 3 =  11.1V. ----> 0.8V undervoltage

4 Cells = 4.2 x 4 = 16.8V.  ----> 4.8V overvoltage
4 Cells = 3.7 x 4 =  14.8V. ----> 2.8V overvoltage

The other questions are can I directly provide this voltage to the motors which would probably result in overvoltage in a 4 cell system and under voltage in a 3 cell system.
general answer is yes. undervoltage, motor runs a little bit slower and cooler. overvoltage, motor runs a little bit faster and hotter. thats all.

Or should I put in a 12V SMPS regulator after the batteries?
from my experience... no.
if something can select, how cant it be intelligent? if something is intelligent, how cant it exist?
 
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Offline ZeroResistance

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Re: how many batteries in series for a 12V system?
« Reply #9 on: May 25, 2017, 05:33:35 am »
Excellent answer Mechatrommer!!
Thanks also to Audioguru, Griggtech and Nandblog!!

So the general consensus is to use 3 cells of 3.7V (nominal) and 4.2V (max) voltage. This approximates a 12V system. Although Nandblog also suggested to use LiFePO4 batteries.

One point that I missed out is that the system is normally being powered by the mains and when the mains fails it goes to battery backup mode.
So if we take 3 cells of 4.2 volts that gives us 12.6V charging voltage so out input voltage from the would have to be higher than this so lets say it would need to be at least 2 volt higher so 14.6V or approximate it to 15V.

So in mains operation the motors will be operating at 15V from the mains adapter and when on battery it would move to 12.6V. Would this higher volatge be ok for the motors especially considering the fact that the system will be in mains mode for a larger percentage of time.
 

Offline ZeroResistance

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Re: how many batteries in series for a 12V system?
« Reply #10 on: May 25, 2017, 03:30:25 pm »
Quote from: Mechatrommer
it should be ok..
15V to the motors should'nt be a problem right? it will only run hotter and faster?
 

Offline Mechatrommer

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Re: how many batteries in series for a 12V system?
« Reply #11 on: May 25, 2017, 05:20:56 pm »
15V to the motors should'nt be a problem right? it will only run hotter and faster?
12V motor supplied with 15V should'nt be a problem. it will only run hotter and faster... and shorter life. how short is anyone's guess. i run a 12V motor on 24V supply for few months and then went dead.
if something can select, how cant it be intelligent? if something is intelligent, how cant it exist?
 

Offline ZeroResistance

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Re: how many batteries in series for a 12V system?
« Reply #12 on: May 25, 2017, 08:15:22 pm »
Thanks mechtrommer , I would want the motors to have as long life as they could possibly have without overstressing them.
How about going for a 12V mains adapter and then boosting it to 15V and  feeding that to the battery charging circuit.
While the loads are  fed directly through the adapter.
 

Offline Nusa

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Re: how many batteries in series for a 12V system?
« Reply #13 on: May 25, 2017, 09:27:42 pm »
You're asking for validation without giving information about the motor or application. We don't know, so we can only cite generalities.

I'd start by finding the spec sheet for the particular motor you have in mind. It'll likely specify an operating voltage RANGE as well as a nominal voltage. It'll also likely cite a noload rpm and current, an nominal rpm and current, and a stall current. I'd also expect torque info and maybe thermal characateristics.

And then there's the nature of the application. For instance, a power screwdriver is a potentially high current heavy load application that may even stall, but it typically only runs for a few seconds and then rests for a while. Contrast that with a cooling fan that runs continously at a constant load.
 


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