How to convert 14.8V @ 10A Lipo battery output to 12V@10A ? - Page 1
Forum section: electronics / projects

author: muthukural001
author_rank: Regular Contributor
timestamp: July 20, 2019, 11:56:37 am
content: 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

author: ogden
author_rank: Super Contributor
timestamp: July 20, 2019, 01:15:05 pm
content: Quote from: muthukural001 on July 20, 2019, 11:56:37 amRight. 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.

author: muthukural001
author_rank: Regular Contributor
timestamp: July 20, 2019, 01:31:56 pm
content: Hi ogden,Thanks...I am looking for the buck converter to do the job...Do u know any Buck converter for this?.

author: MagicSmoker
author_rank: Super Contributor
timestamp: July 20, 2019, 01:41:42 pm
content: 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).

author: IconicPCB
author_rank: Super Contributor
timestamp: July 20, 2019, 01:49:18 pm
content: What will the converter drive?Can the load tolerate the noise and ripple of a switcher?Is a linear regulator necessary / essential?

author: muthukural001
author_rank: Regular Contributor
timestamp: July 20, 2019, 02:28:10 pm
content: 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...

author: MagicSmoker
author_rank: Super Contributor
timestamp: July 20, 2019, 02:40:00 pm
content: Quote from: muthukural001 on July 20, 2019, 02:28:10 pmThe load is a motor controller driving a DC motor? More than likely it will work just fine on 14.8V.

author: ogden
author_rank: Super Contributor
timestamp: July 20, 2019, 03:25:40 pm
content: Quote from: MagicSmoker on July 20, 2019, 02:40:00 pmI do not see any need for regulator for DC motor as well. Problem solved

author: tunk
author_rank: Super Contributor
timestamp: July 20, 2019, 05:44:38 pm
content: I guess that battery fully charged is around 17V.If you go for one of the chinese DC-DC converters, you should know that theirspecifications 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 alwaysdraw 10A.

author: patrick1
author_rank: Contributor
timestamp: July 20, 2019, 05:53:07 pm
content: i know its abit knasty, but depending on the usage, could just use a resistor divider too the base of a well heatsinked transistor

author: mariush
author_rank: Super Contributor
timestamp: July 20, 2019, 06:28:36 pm
content: 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/5481911The 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 inhttps://www.analog.com/media/en/technical-documentation/data-sheets/108345fh.pdfbut 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 13.5vAlso, 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 rectifierAlso... 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 voltageA 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 1vFor example see this 1.25$ 20A bridge rectifier :https://www.digikey.com/product-detail/en/smc-diode-solutions/GBJ2006TB/1655-1863-ND/7244891Could 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 1-1.1vA 12v relay would work with 10v..15v, no worries.

author: james_s
author_rank: Super Contributor
timestamp: July 20, 2019, 06:38:10 pm
content: 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.

author: Simon
author_rank: Unknown
timestamp: July 20, 2019, 08:09:25 pm
content: 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.

author: IDEngineer
author_rank: Super Contributor
timestamp: July 20, 2019, 08:29:25 pm
content: Quote from: james_s on July 20, 2019, 06:38:10 pmI 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.

author: ogden
author_rank: Super Contributor
timestamp: July 20, 2019, 09:16:46 pm
content: Quote from: Simon on July 20, 2019, 08:09:25 pmI bet it is 4-cell 3.7V LiPo.

author: Simon
author_rank: Unknown
timestamp: July 21, 2019, 09:17:11 am
content: do LiPo's discharge to 3.2V? i thought lithium stuff was 4.2 to 3.7V

author: sleemanj
author_rank: Super Contributor
timestamp: July 21, 2019, 10:35:46 am
content: Quote from: Simon on July 21, 2019, 09:17:11 am3.7 is the nominal voltage, not the minimum voltage.3.2v would be a fairly modest discharge level, 2.7v is fairly deep.

author: ogden
author_rank: Super Contributor
timestamp: July 21, 2019, 01:33:13 pm
content: Quote from: sleemanj on July 21, 2019, 10:35:46 amI 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

author: Siwastaja
author_rank: Super Contributor
timestamp: July 23, 2019, 06:10:28 am
content: The motor controller does the conversion for you, you don't need to regulate the input. The motor controller actuallyisthe 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.Quote from: ogden on July 21, 2019, 01:33:13 pmWouldn'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%.