3 or 4 18650 batteries to get precise 12V ?

[wwwill]

Hello everybody,
This is my first message here, I hope I do everything right. Tell me otherwise.

My question may sound silly to you, but I can't really figure.
I want to create an autonomous device and one of the boards onboard needs a 12V 4A power supply (I am pretty sure the 4A is for another configuration, 2A is probably more than enough).
I plan to reuse some 18650 batteries found in old laptop batteries.
But the question is "how to get precisely (enough) 12V no matter how charged the batteries are ?"

The batteries are each 3.7V as regular voltage.
So, 3 in series would be 11.1V when charged.
And, 4 in series would be 14.8V when charged.

So the logic would be 4 in series with a DC step-down to 12V.
But does a 3-pack with a DC step-up to 12V would not waste less energy ?

I cannot really figure...
Thanks a lot everybody !
Guillaume

[danadak]

You can use a buck/boost regulator to handle a battery over its discharge
cycle.

Several vendors offer them, TI, NSC, Micrel, ON.....


Regards, Dana.

[wwwill]

You can use a buck/boost regulator to handle a battery over its discharge
cycle.

So for you I should use 3 batteries in series and a boost regulator.
Thanks.

[danadak]

Depends on design requirements. For same batteries as you raise
the # of batteries you can get longer service out of a charge,
but sacrifice volume/size constraints.

Also can be an impact on regulator efficiency, consult data
sheet.


Regards, Dana.

[Rick Law]

You can never get precise voltage with the lithium 18650 since the battery's voltage changes as it discharge.

Fresh charge, it would be around 4.2V, but as it discharges, it will drop to 2.6V where most  battery protection circuit cuts in and disallow further discharge.  So, with 4x18650, you are over 12V with fresh batteries (4x4.2V=16.8V) and near end of discharge at 2.6V each, you have 10.4V.  Your voltage depends on the state-of-charge.

If you do want to keep it at around 12V, boost/buck is your best bet.

[Audioguru]

The battery protection circuit is not in each cell, it is in the battery pack that you do not have. If a cell is discharged lower than about 3.2V then it is ruined or destroyed. You MUST have a circuit to measure each cell's voltage and disconnect the load when the voltage gets below about 3.2V per cell.

How will you charge the battery so each cell receives a balanced charge and so the battery does not explode or catch on fire?

[ebclr]

This little baby will do 12V with any battery combination you want, ( need do adjust trimpot )



https://pt.aliexpress.com/item/LM2587-DC-DC-Boost-Converter-3-30V-Step-up-to-4-35V-Power-Supply-Module-MAX/32379182065.html?spm=2114.02010208.3.11.NvAp8a&ws_ab_test=searchweb201556_0,searchweb201602_3_10057_10065_10056_10068_10037_10055_10054_10069_301_10059_10033_10058_10032_418_10073_10017_10070_10060_10061_10052_10062_10053_10050_10051,searchweb201603_2&btsid=3a7a9150-cc51-4ddc-9dcf-c6dc4a1e5843

[eventhorizon]

I have almost the same one as shown above as a buck converter and if I connect it to 12.19v supply the best I can get out if about 11.53 volts so loss in the circuit on this design will limit the use of the batteries as they get close to the 12v output on the batteries.  I think at least with this buck converter you may only get 50% give or take (no numbers written down or calculators used to get that number) use from your batteries before the buck cant keep you at 12v so I would say with my experience at least with this and limited amount of playing with these things that a buck wont get you the best results.

[suicidaleggroll]

You need a buck/boost.

Boost only with 3 cells will not work when they're fully charged.  Buck only with 4 cells will not work when they're nearing empty.  A buck/boost will work whether you have 3 or 4 cells regardless of their state of charge, provided you stay within its operational voltage range.

You will also need a cell balancing and over/under voltage protection circuit so you don't destroy the cells.  Lithium Ion is very touchy when it comes to under/over voltage or current, and if something goes wrong they can catch on fire or explode.  DO NOT skimp on the balancing and protection circuitry.

[Rick Law]

...

So the logic would be 4 in series with a DC step-down to 12V.
But does a 3-pack with a DC step-up to 12V would not waste less energy ?

I cannot really figure...
Thanks a lot everybody !
Guillaume

It depend a lot on the board you choose.  Some are more efficient than others.  General rule I've found is, the higher the input voltage, the better the efficiency -- which is kind of basic physics really.

Personal experience (aka-not expert opinion but what I discovered by messing around) with a dozen or so boost (LM2577/MC34063..) buck (LM2596/XL1509...) at about 300mA-700mA 12V out:
- With buck, you get 90-95% efficiency.  When input gets too near output voltage, regulation fails and you have 100% efficiency lost.
- With boost, you get a very wide range of efficiency.  The lower the input voltage, and/or, the higher the current draw, the worst it gets.  For my use (1x18650 to 12V at 300mA), I get Watt-out at about 60% of Watt-in.

That said, I am chicken (ie: don't want risk), so I stick with 1x(protected)18650 boost.  There is no need to balance cell.  There is no need to worry about a good cell feeding the destruction of a bad cell should a failure occurs.  Awful efficiency, but absolutely no concern of multi-cell issues.

If efficiency is your primary concert, use 6x(protected)18650 and buck it.  But you better make sure your cells are balanced and in good shape.  That many 18650 is practically a bomb if you abuse it.  In my view, even if you have a current-limit with the board, a hardware-fuse on the cell-side is an absolute must.  If there is any excess current, the hardware fuse ensures that the serialization of the 6x18650 is cut even if there is a protection board failure.

Oh, beside protection board failure... there are those cheap cells advertised to have protection board but shown to be without the protection board.  The hardware fuse give you some level of comfort at least.

[mariush]

Most cheap buck or boost regulators you find on ebay using lm2577  or lm2596 or xl15** chips are generally only good for about 2-3A .  In addition to that, the people that make them put inductors and diodes on them that would make them work with wide range of input and output voltages, but they're not the best choice. The datasheets recommend specific inductor values if your input and output voltages are within some ranges ... if you don't use the recommended uH value, the chip may overheat or the efficiency suffers.

In general, buck regulators are more efficient than boos regulators or SEPIC (buck-boost) regulators, so it would make sense to buy something that regulates the voltage down. Buck regulators can achieve 95%+ efficiency, while some boost regulators may only reach up to 80-85%.

Also, you could just go for a ready made pack designed for higher voltage than 12v, instead of using cheap possibly crap 18650 cells.  You should check out Hobbyking , they have a huge selection of batterry pack. Pick something with 12v+ and capable of 2A+ output and you're set. They also have proper chargers that can balance the individual cells inside such battery packs.

[Rick Law]

Most cheap buck or boost regulators you find on ebay using lm2577  or lm2596 or xl15** chips are generally only good for about 2-3A .
... ...

Besides the spot-on guide line above from Mariush...  Also bare in mind that many cheap boards list the max power under ideal situation and a small-print "with heat sink".  Many of the cheap boards comes without heat sink at all.  A heat-sink-less boost/buck chip at 2-3A could be hot enough cook itself.

I use an LM2596 with small heat sink for my "bench supply".  I have a mini-fan (1 inch) which I turn on at > 1A and I don't fell comfortable using it over 2.5A for anything over a minute.

[ez24]

... instead of using cheap possibly crap 18650 cells.

The OP is using batteries from laptops, so they are likely good quality like mine are (Dell)

[Cerebus]

Before plunging into the question of what regulator topology works best there is a more important question to be asked first. What exactly are you powering? Perhaps it will be quite happy with the range available between a fully charged stack and a discharged one - many circuits supply voltage requirements are nominal and they'll happily work across a range of voltages. Only once that question is out of the way can you really decide the way forward.

[Red Squirrel]

Honestly I would just run it directly off 4 batteries (add fuse), but check datasheet of the devices to make sure they will handle the voltage, but they probably will.  If the device is low power draw (ex: micro controller) a simple linear reg might work too. Not as efficient though.

[KL27x]

+1 if the board is small and the IC's can be indentified, it might be possible to run it directly from batteries. It might have only a few components getting 12V, and the rest distributed through regulators that drop that voltage anyway.

But if 12 V is what you need, I don't see why you need buck/boost. 4 cells and a buck could work. You don't lose too much capacity stopping at 3V under load. The end is near, anyway. And board will most likely work slightly below 12V, if you want to push your batteries.

Common cutout voltages go from 2.7 to 2.9, judging from the most available/common voltage detectors on the market, and from my personal experience you don't get much from the difference there, nor between that and 3.0.

[Zero999]

How precise does it need to be?

What's the specified operating voltage range of the board?

It's possible it already has a regulator built-in and will work over a fairly wide range of voltages. Check the datasheet/schematic.