Author Topic: How to convert 14.8V @ 10A Lipo battery output to 12V@10A ?  (Read 6956 times)

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Offline muthukural001Topic starter

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Hello All,

I have been searching for a DC-DC converter to down convert my 14.8V,10000mAh battery output to 12V 10A current output. Since the cutoff voltage for 14.8V battery is 12.8V, the dc-dc converter is expected to regulate 12V even when the input voltage is 12.8V.But, I am not getting any dc-dc converter with above spec...Is there a dc-dc to do the above ?...If yes, please suggest...

I don't think that putting diodes that has 0.7v drop in series is a good idea for my design...Because, at 14.8V, I will get 12V but when the input voltage goes to 12.8V, the output after the diodes will be ~10V...Using a zener diode will not work for 10A load as there is no zener with that much power rating
 

Offline ogden

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Re: How to convert 14.8V @ 10A Lipo battery output to 12V@10A ?
« Reply #1 on: July 20, 2019, 01:15:05 pm »
there is no zener with that much power rating

Right. ANY linear regulator or series diodes have to dissipate lot of heat, worst case 28W ((14.8V-12V)*10A). You would want to use powerful enough buck (synchronous rectification) converter.
 

Offline muthukural001Topic starter

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Re: How to convert 14.8V @ 10A Lipo battery output to 12V@10A ?
« Reply #2 on: July 20, 2019, 01:31:56 pm »
Hi ogden,

Thanks...I am looking for the buck converter to do the job...Do u know any Buck converter for this?.
 

Offline MagicSmoker

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Re: How to convert 14.8V @ 10A Lipo battery output to 12V@10A ?
« Reply #3 on: July 20, 2019, 01:41:42 pm »
Synchronous rectification is of questionable value when duty cycle is >70%.

That said, there are plenty of cheap Chinese buck converters on Amazon, ebay, etc., rated for 12A or more. I make no claim as to their quality, etc., and I obviously don't know what might be easiest for the OP to obtain in his country.

One caveat: delivering 12V out at 12.8V in requires a duty cycle very close to 100% and many buck converters can't go above 90% or so because they use a charge pump (aka bootstrapped) gate driver for the buck MOSFET (which requires turning the MOSFET off every so often to recharge the bootstrap capacitor).

 

Offline IconicPCB

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Re: How to convert 14.8V @ 10A Lipo battery output to 12V@10A ?
« Reply #4 on: July 20, 2019, 01:49:18 pm »
What will the converter drive?
Can the load tolerate the noise and ripple of a switcher?
Is a linear regulator necessary / essential?
 

Offline muthukural001Topic starter

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Re: How to convert 14.8V @ 10A Lipo battery output to 12V@10A ?
« Reply #5 on: July 20, 2019, 02:28:10 pm »
A1: The converter will drive the motor driver which drives the 12V@10A DC motor...
A2: The motor driver datasheet does not say about the acceptable ripple voltage...But, in the reference design, I can see the bulk capacitor 680 uF 63V to meet acceptable voltage ripple levels.
A3: Since I am going to use Battery source, using linear regulator (LDO) would waste the power.For example, if the motor takes 12V 10A,I will loose  8 to 28W for 12.8 to 14.8V  input voltage respectively...
 

Offline MagicSmoker

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Re: How to convert 14.8V @ 10A Lipo battery output to 12V@10A ?
« Reply #6 on: July 20, 2019, 02:40:00 pm »
A1: The converter will drive the motor driver which drives the 12V@10A DC motor...

The load is a motor controller driving a DC motor? More than likely it will work just fine on 14.8V.
 

Offline ogden

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Re: How to convert 14.8V @ 10A Lipo battery output to 12V@10A ?
« Reply #7 on: July 20, 2019, 03:25:40 pm »
The load is a motor controller driving a DC motor? More than likely it will work just fine on 14.8V.

I do not see any need for regulator for DC motor as well. Problem solved :)
 

Offline tunk

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Re: How to convert 14.8V @ 10A Lipo battery output to 12V@10A ?
« Reply #8 on: July 20, 2019, 05:44:38 pm »
I guess that battery fully charged is around 17V.
If you go for one of the chinese DC-DC converters, you should know that their
specifications are a bit optimistic, so you may look for a 15-20A converter.
Also, the current draw of the motor depends on the load, so it may not always
draw 10A.
 

Offline patrick1

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Re: How to convert 14.8V @ 10A Lipo battery output to 12V@10A ?
« Reply #9 on: July 20, 2019, 05:53:07 pm »
i know its abit knasty, but depending on the usage, could just use a resistor divider too the base of a well heatsinked transistor
 

Offline mariush

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Re: How to convert 14.8V @ 10A Lipo battery output to 12V@10A ?
« Reply #10 on: July 20, 2019, 06:28:36 pm »
Digikey has a category of products for this, dc-dc converters:

For example, this would work but it costs around 30$ : https://www.digikey.com/product-detail/en/tdk-lambda-americas-inc/I6A24014A033V-001-R/285-2434-ND/5481911

The datasheet only shows 14v in -> 12v out in graphs, but it will probably work with as little as 13v.

If you want super cheap and don't care about efficiency, you could just grab two or three  LM1084  (linear regulators) and parallel them using a 0.1 ohm resistor for each one as balancing resistors.  See page 15 in https://www.analog.com/media/en/technical-documentation/data-sheets/108345fh.pdf  but replace "2 feet of AWG18 wire" with a 0.05 ohm 1w resistor or something like that (AWG18 has around 0.021 ohm per meter/3ft of resistance)

Each 1084 has a peak current of 5A, so if you use 3, each regulator would have around 3.3A of current going through it.

The 1084 has a typical voltage drop of around 1.3A at 5A of current, but keep in mind they must be cooled... the total amount of heat they can dissipate is around 15 watts.

So LM1084/LT1084/*1084 could be used to output 12v with as little as 13.5v

Also, you should think about the maximum voltage the motor supports.
When the voltage drops below some threshold, it may be easier and cheaper for you to just use a mechanical relay to switch a dc-dc regulator and direct / through a bridge rectifier

Also... if you know the voltage is not gonna be above 14-15v and your motor isn't super picky about voltage then in theory you could use a bridge rectifier and a mechanical relay to drop some voltage

A bridge rectifier has 4 diodes inside, and depending on how you connect the wires, you could have two diodes in series to drop around 2v or you could have one diode and drop only 1v

For example see this 1.25$ 20A bridge rectifier : https://www.digikey.com/product-detail/en/smc-diode-solutions/GBJ2006TB/1655-1863-ND/7244891



Could have a voltage comparator or microcontroller or something enable the relay only when voltage goes above 13v... in default state relay is connected so that only 1v is dropped, if voltage is above 13v relay turns on and switching changes so you drop 2 x 1-1.1v
A 12v relay would work with 10v..15v, no worries.
 

 

Offline james_s

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Re: How to convert 14.8V @ 10A Lipo battery output to 12V@10A ?
« Reply #11 on: July 20, 2019, 06:38:10 pm »
If you're just running a DC motor you don't need a regulator at all. Most DC motors will happily run at twice their rated supply voltage and usually down to 1/10th or less. If maintaining a specific RPM is critical then you should use a motor with a tachometer output and PWM control of the speed, otherwise just run it directly, it will be fine.
 

Online Simon

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Re: How to convert 14.8V @ 10A Lipo battery output to 12V@10A ?
« Reply #12 on: July 20, 2019, 08:09:25 pm »
12.8V is a fully charged lead acid battery at rest (more in fact), do you have a lead acid battery? if you cut off at 12.8V it may never work. If you are powering a "nominally" 12V motor 14.8V is fine.
 

Offline IDEngineer

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Re: How to convert 14.8V @ 10A Lipo battery output to 12V@10A ?
« Reply #13 on: July 20, 2019, 08:29:25 pm »
If you're just running a DC motor you don't need a regulator at all. Most DC motors will happily run at twice their rated supply voltage and usually down to 1/10th or less. If maintaining a specific RPM is critical then you should use a motor with a tachometer output and PWM control of the speed, otherwise just run it directly, it will be fine.
I agree and was going to post something similar. If he doesn't need to control speed closely and just wants to protect the motor from "excessive" voltage, he could just do a raw PWM circuit and run it at high duty cycle. A MOSFET won't dissipate too much heat, no problems with gate drivers that suffer from high duty cycles, and he can ratchet the effective applied voltage down to whatever he likes.
 

Offline ogden

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Re: How to convert 14.8V @ 10A Lipo battery output to 12V@10A ?
« Reply #14 on: July 20, 2019, 09:16:46 pm »
12.8V is a fully charged lead acid battery at rest (more in fact), do you have a lead acid battery?

I bet it is 4-cell 3.7V LiPo.
 

Online Simon

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Re: How to convert 14.8V @ 10A Lipo battery output to 12V@10A ?
« Reply #15 on: July 21, 2019, 09:17:11 am »
do LiPo's discharge to 3.2V? i thought lithium stuff was 4.2 to 3.7V
 

Offline sleemanj

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Re: How to convert 14.8V @ 10A Lipo battery output to 12V@10A ?
« Reply #16 on: July 21, 2019, 10:35:46 am »
do LiPo's discharge to 3.2V? i thought lithium stuff was 4.2 to 3.7V

3.7 is the nominal voltage, not the minimum voltage.

3.2v would be a fairly modest discharge level, 2.7v is fairly deep.
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Offline ogden

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Re: How to convert 14.8V @ 10A Lipo battery output to 12V@10A ?
« Reply #17 on: July 21, 2019, 01:33:13 pm »
3.7 is the nominal voltage, not the minimum voltage.
3.2v would be a fairly modest discharge level, 2.7v is fairly deep.

I would say too low. "3.7V nominal" LiPo range for (0% - 100%) charge level is 3.0V - 4.2V / cell.

BTW best battery-related info resource I know: https://batteryuniversity.com
 

Online Siwastaja

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Re: How to convert 14.8V @ 10A Lipo battery output to 12V@10A ?
« Reply #18 on: July 23, 2019, 06:10:28 am »
The motor controller does the conversion for you, you don't need to regulate the input. The motor controller actually is the buck converter you are looking for, just using the motor as the inductor.

Make sure the controller can handle the maximum battery voltage. If not, pick/design one that can.

BTW best battery-related info resource I know: https://batteryuniversity.com

Wouldn't trust it. Although it's been through massive cleanup and rewrite process, they are responsible for originally creating many urban myths still hanging around. I don't have time to evaluate it now, but in the past their bullshit rate was almost 100%.
« Last Edit: July 23, 2019, 06:13:24 am by Siwastaja »
 


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