# Getting a precise (16.5V) voltage (how to use a step down..) from 18650 battery?

#### Caros M

Joined Aug 29, 2016
12
Hello,

I'm looking to built a Li-On battery to deliver 16.5V at 3A, based on 18650B cells.
For this purpose, I will create two pack of 6 cells in series and join the packs in parallel to have more current.

Now, i would like to find how to cut off the cell pack before it goes under 16.8V, while the included PCB off the cells would theoretically permit the pack to go down until 15V, and how to precisely transform the tension which will be between 16.8V (2.8V*6=16.8V) and 25.2V (4.2V*6=25.2V, at full charge each cell have 4.2V), to a constant 16.5V with safety.

Two work around :

First one :
The cut off from the PCB of the cell is 2.5V, i think put 7 cells in series (6*2.5V=15V less than the 16.5V needed) wouldn't be the best idea, because it save the cell to cut off before 2.5V (2.8V would give 2.8V*6=16.8V minimum, should be perfect including the margin error), and at full charge with 7 cells i would have 4.2V*7=29.4V, which make a lot to step down.
i'm looking to cut off the cells before it goes under 2.8V, in the pack it makes under 16.8V for 6 cells in series.

So, which module could regulate theses two pack to cut off before the tension drops to 16.8V ?

Second one :
-To transform the tension (16.8V-25.2V) to 16.5V precisely, I found this step down on ebay :

Should it be really precise during all the discharge, are I need to add an other component ?

Adding capacitor would be a great idea? Which capacitor I should buy, or how to calculate the capacity of the capacitor I need from my voltage and current input / output? (arround 3A / 16.8/25.2V input and 3A / 16.5V output)

About safety, which kind of module can protect the step down itself, to cut off if the voltage ascent to 16.7V or drop under 16V?

#### dannyf

Joined Sep 13, 2015
2,197

#### Caros M

Joined Aug 29, 2016
12
I don't know how precise it should be. the purpose is to recharge laptops, so i guess something like +-0.5V (for output voltage) would do the job.

#### dannyf

Joined Sep 13, 2015
2,197
The simplest one would be a DC DC converter activated by a comparator or a MCU with ADC.

#### Caros M

Joined Aug 29, 2016
12
The simplest one would be a DC DC converter activated by a comparator or a MCU with ADC.
I'm new to built circuit, it would be difficult for a neophyte, isn't it?
I'm looking for a DIY circuit with notice, theses kind of stuffs on ebay have, howeover, if you are ready to help me, it should be a good solution.

#### tcmtech

Joined Nov 4, 2013
2,867
I don't know how precise it should be. the purpose is to recharge laptops, so i guess something like +-0.5V (for output voltage) would do the job.
Unlikely you even need any voltage regulation for that application being the charging and power systems inside the laptop takes care of all of that on it's own. Pretty good chance the laptop will take anything you throw at it from a 5 cell (15 - 21) volt battery based power source.

#### Caros M

Joined Aug 29, 2016
12
The simplest one would be a DC DC converter activated by a comparator or a MCU with ADC.
How to write on a MCU/ADC? is it difficult? which pieces I need? Is this be sufficient to be sure the voltage is always correct? (not need to add capacitors or something else..?)

#### Caros M

Joined Aug 29, 2016
12
Unlikely you even need any voltage regulation for that application being the charging and power systems inside the laptop takes care of all of that on it's own. Pretty good chance the laptop will take anything you throw at it from a 5 cell (15 - 21) volt battery based power source.
Yeah I guess.. but i would prefer to deliver exact voltage, I don't want to take the risk, and i'm eager to learn! Although I don't know if it's difficult or not. If it is, any ressources would be helpful

#### tcmtech

Joined Nov 4, 2013
2,867
The general rule I have always used when running mismatched power packs on device is that anything that is less than 1/3 more than the nominal rating is not going to be a problem which in your case a 16.5 VDC nominal input will be fine up to at least 22 volts.

Also even though your battery pack may be reading 21 volts at full charge as soon as you put the several amp load on it that a laptop will draw while charging its own internal battery pack it's going to lose a few volts as soon as it's connected pulling it down to a more realistic 18 - 19 volts.

If it was me I would put together a battery pack and put a load on it similar to your laptops power pack amp rating and see how far it drops from full charge voltage at that load rate and compare that to the actual power packs no load open circuit voltage. Odds are they are only going to be close enough to not matter to the laptops actual internal power control systems!

#### Caros M

Joined Aug 29, 2016
12
The general rule I have always used when running mismatched power packs on device is that anything that is less than 1/3 more than the nominal rating is not going to be a problem which in your case a 16.5 VDC nominal input will be fine up to at least 22 volts.

Also even though your battery pack may be reading 21 volts at full charge as soon as you put the several amp load on it that a laptop will draw while charging its own internal battery pack it's going to lose a few volts as soon as it's connected pulling it down to a more realistic 18 - 19 volts.

If it was me I would put together a battery pack and put a load on it similar to your laptops power pack amp rating and see how far it drops from full charge voltage at that load rate and compare that to the actual power packs no load open circuit voltage. Odds are they are only going to be close enough to not matter to the laptops actual internal power control systems!

Now on a laptop that retails for $<400 new I wouldn't expect that to have any degree of protection. Thread Starter #### Caros M Joined Aug 29, 2016 12 I have two Panasonic Toughbooks that retailed for$2300 new at one point in their life and I never worry aobut that sort of stuff (...) of protection.
(thanks for info)
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So the best seems to be a DC DC converter activated by a comparator or a MCU with ADC.

Some links for comparator and how to program them and made them activate the DC DC step down?
MCU with ADC, what it is exactly? How again program / make them work for my desired voltage?

#### tcmtech

Joined Nov 4, 2013
2,867
I would do the actual battery at load voltage VS power pack open circuit voltage testing first to see what level of voltage drop you are dealing with.

Without that information no one here can recomend what method of voltage reduction to use.

#### Caros M

Joined Aug 29, 2016
12
I would do the actual battery at load voltage VS power pack open circuit voltage testing first to see what level of voltage drop you are dealing with.

Without that information no one here can recomend what method of voltage reduction to use.
I will base on 18650B panasonic cells (datasheet).

I will use two x 7 in series, so it will start at 29.4V to end at 17.5V. The voltage drop is 11.9V.

#### dannyf

Joined Sep 13, 2015
2,197
1) find a dc/dc converter with the shutdown or enable pin; see if it is active high or low;
2) design a comparator (or a mcu) that will go to the right output when the voltage drops below your threshold.
3) sit back and relax.

#### tcmtech

Joined Nov 4, 2013
2,867
I will base on 18650B panasonic cells (datasheet).

I will use two x 7 in series, so it will start at 29.4V to end at 17.5V. The voltage drop is 11.9V.
Just can't bring yourself to do that actual power pack open circuit voltage test and battery pack under load test huh?