3 voltages from one battery

[AGCB]

I'm building a project that will be powered by 1-4 18650 batteries. It will need 3.3, 5 and 12 volt circuits. I'm planning on using a step-up converter for the 12 volt and 5 volt but how shall I get the 3.3? There is a good chance that at some time the per battery volts will drop to ~3 volts which will be too low for a 3.3 volt regulator Would it be OK to step up to 12 volt 1st and then down to the others. I would have to use a step-up with enough current to run everything. Or should I split the batteries or just put multiples in series and start out with say 7.8 volts? Just wondering if there would be any preferred method>
Thanks
Aaron

 

[gophert]

I'm building a project that will be powered by 1-4 18650 batteries. It will need 3.3, 5 and 12 volt circuits. I'm planning on using a step-up converter for the 12 volt and 5 volt but how shall I get the 3.3? There is a good chance that at some time the per battery volts will drop to ~3 volts which will be too low for a 3.3 volt regulator Would it be OK to step up to 12 volt 1st and then down to the others. I would have to use a step-up with enough current to run everything. Or should I split the batteries or just put multiples in series and start out with say 7.8 volts? Just wondering if there would be any preferred method>
Thanks
Aaron

I would use an 11.1V battery and step up to 12 and down to everything else. Step down from the battery voltage, otherwise, you will have losses going up plus losses going down.

If you use 7.8v, you may run into problems maintaining 5V with a step-down converter at the end of the 7.8v battery's charge - we need to know the "dropoutvoltage" of any regulator you plan to use. Assume you need 2v above any stepped down voltage - linear or smps regulator will likely need some voltage.

 

[dr pepper]

A buck/boost converter might do the job, but its not a good idea to run a lipo that low.

 

[Nigel Goodwin]

A buck/boost converter might do the job, bit its not a good idea to run a lipo that low.

You should include battery protection on the battery pack, this will prevent it been over-discharged (and give other desirable features as well).

I would also suggest NOT using a single 18650, but either place four in series (and step down for all voltages - using switch-mode regulators), or four in series parallel, and step up for 12V, and down from 12V for the rest.

 

[BobW]

It would help to know the current draw for each voltage.

 

[AnalogKid]

What is the load current for each voltage? A linear regulator to take the 5 V down to 3.3 V makes sense at 0.1 A, but not at 10 A.

ak

 

[AGCB]

This will be a TFT monitor with uC on the back and a variety of circuits which may include RTC, PING distance module, temperature sensor, lightning sensor, EMIC text to speech module and others. None of these are high current devices.

You should include battery protection on the battery pack

I was planning on having a low battery alarm but after your post and some research, may incorporate a protection circuit as well or instead.

Thanks.
Aaron

 

[Nigel Goodwin]

I was planning on having a low battery alarm but after your post and some research, may incorporate a protection circuit as well or instead.

They are so cheap it makes no sense not to add one.

It's also a good idea to prevent your battery pack setting on fire

 

[AGCB]

Nigel
Would you suggest a part number or web site please. Thanks

 

[Nigel Goodwin]

Nigel
Would you suggest a part number or web site please. Thanks

It depends on how many batteries you are placing in series - variouis types are available at Banggood and Aliexpress.

This is an example for three batteries in series (3S):

https://www.banggood.com/3S-20A-Li-...-p-1118043.html?rmmds=search&cur_warehouse=CN

 

[BobW]

It would help to know the current draw for each voltage.

 

[Mike - K8LH]

I purchased a couple of these single 18650 modules last year (see below) which include charger and BMS circuitry as well as 3.3v and 5.0v outputs. The outputs operate fine while charging, too. I'm so pleased with these modules that I just ordered a dual 18650 module to try out.

As for supplying 12 volts, I've been very happy with the little boost modules shown below.

Search for these items on AliExpress or Ebay....

Good luck. Cheerful regards, Mike