# Most efficient power management when splitting battery voltage

#### Binary Buddha

Joined Sep 24, 2016
40
So if I wanted to reduce the voltage of a 9V battery down to 3.7ish voltage... what would be the best way? Using resisters to split the voltage or doing the voltage regulator dance? Is there any other method to step down voltage?

Obviously, I'm trying to keep the result circuit size to a minimum whilst trying to maintain optimal power efficiency.

#### AlbertHall

Joined Jun 4, 2014
12,324
How much current do you want to draw from the 3.7V?

#### Binary Buddha

Joined Sep 24, 2016
40
How much current do you want to draw from the 3.7V?

Various... Where would you draw the line? Let's say 3000mA (10W) for one thing at the moment.

There's also the idea of making a 9V battery supply 5V 2.1A and what not.

In short, I've heard that a voltage regulator is the best solution for low-steping voltage if the power supply in continuous and consistent. But, I have heard anything about if you use a battery. And I know know battery power is quite the opposite. The battery voltage continuously drops when it's being used. Which would throw off the design specs of any circuit.

#### crutschow

Joined Mar 14, 2008
33,965
A standard 9V "transistor" battery will never deliver 10W for any significant length of time (beyond a few seconds).
You will need a much larger battery for that.
How long do you want the battery to last at that load?

#### Binary Buddha

Joined Sep 24, 2016
40
A standard 9V "transistor" battery will never deliver 10W for any significant length of time (beyond a few seconds).
You will need a much larger battery for that.
How long do you want the battery to last at that load?

Okay... Then let's say we chained a bunch or C or D batteries then.

#### crutschow

Joined Mar 14, 2008
33,965
Alkalines will give you 1.5V per cell.
So what exact voltage and current do you need, and for how low before the battery is dead?

#### Binary Buddha

Joined Sep 24, 2016
40
Alkalines will give you 1.5V per cell.
So what exact voltage and current do you need, and for how low before the battery is dead?

I'm actually looking for the theory of what would be the best to do. Not to be spoon fed a solution.

Would you be able to explain to me what would be best under certain circumstances? I know it's not "simple". Hence why I'm asking for guidance.

From your responses, (thank you btw) I'm assuming that if I need high current it's best to use resistors to split the voltage. Which would lead to the question of what should I do with with the second Vout from the split? Can or should or just ignore it? I know that most of the voltage regulators can go up to 5 amps out max. So, question would lead to if I need less than 5 amps would it be better to use a voltage regulator over resistor voltage splitting?

#### AlbertHall

Joined Jun 4, 2014
12,324
You would use regulators for high current.
For very low output currents you can use resistors and the current through the divider can be much greater than the output current and thus keep the voltage reasonably constant.

#### crutschow

Joined Mar 14, 2008
33,965
I'm assuming that if I need high current it's best to use resistors to split the voltage.
Just the opposite.
Resistors are only good for very low current.

So you need a regulator as AH stated.

A linear regulator is simple but is not efficient as it wastes energy (Vin-Vout) * Iout as heat.
So if you need higher efficiency you would use a more complex switching regulator, which generally has efficiencies in the 85-90% range.

#### DickCappels

Joined Aug 21, 2008
10,117
Large voltage ratio...Medium to high currents...High Efficiency...This sounds like a job for a Buck Converter!

#### Binary Buddha

Joined Sep 24, 2016
40
You would use regulators for high current.
For very low output currents you can use resistors and the current through the divider can be much greater than the output current and thus keep the voltage reasonably constant.
So, if I were to use something like an LM317 to drop 12V to 5V USB would I be able to hit 4+ amps? What would the max amps be on a LM317? Trying to build a usb fast charger into a dashboard thing I'm making.

Large voltage ratio...Medium to high currents...High Efficiency...This sounds like a job for a Buck Converter!

View attachment 130629
lol... I'm too noob to start playing around with buck converters yet. I'm still learning how to use my LM317s.

#### MrChips

Joined Oct 2, 2009
30,416
So, if I were to use something like an LM317 to drop 12V to 5V USB would I be able to hit 4+ amps? What would the max amps be on a LM317? Trying to build a usb fast charger into a dashboard thing I'm making.

lol... I'm too noob to start playing around with buck converters yet. I'm still learning how to use my LM317s.
You need to rethink your strategy.

12V @ 4A = 48W
You have to get rid of 7V @ 4A = 28W

You have to waste 28W in a resistive device such as LM317. The LM317 is going to get very hot.

A more efficient solution is a buck converter.

#### OBW0549

Joined Mar 2, 2015
3,566
So, if I were to use something like an LM317 to drop 12V to 5V USB would I be able to hit 4+ amps? What would the max amps be on a LM317?

#### recklessrog

Joined May 23, 2013
985
I'm with DickCappels, A buck converter is the way to go.

#### Dodgydave

Joined Jun 22, 2012
11,233
Lm2678 best solution....