battery pack - multiple voltages

[Roberts_Curiosity]

Short form question: Can I get different voltages by tapping into the middle of a battery pack?

I need 3.3vdc and 5vdc rails for a small toy project. 
It's basically blinky leds, a single board computer and a bluetooth module.
I'm thinking of using 4 rechargeable 1.2vdc 1000mAh lion battery cells in series.
That will give me 4.8vdc, which is probably close enough to the 5vdc.
The max current draw will be about 100mA at startup for 10 seconds, and 50mA after that.

I'm thinking of getting 3.6vdc from between the 3rd and 4th cells.
But, I've never seen anything that does this.
And I've never seen any hint of this in electronics texts.
So, there must be something bad about this idea.
But, what is it? 

Thanks in advance for your helpful thoughts and comments!

[jeroen79]

You will be discharging the cells unequally.
Cells 1-3 will provide current for the 3,3V and the 5V circuits and will be discharged deeper than cell 4 which will only power the 5V circuit.

If you don't take this into account when declaring 'low battery' you risk overdischarging cell 1-3.
And if you don't take this into account when recharging you risk overcharging cell 4.

[Roberts_Curiosity]

You will be discharging the cells unequally.
Cells 1-3 will provide current for the 3,3V and the 5V circuits and will be discharged deeper than cell 4 which will only power the 5V circuit.

If you don't take this into account when declaring 'low battery' you risk overdischarging cell 1-3.
And if you don't take this into account when recharging you risk overcharging cell 4.

Can I deal with the overcharging risk by always charging all 4 cells?
Oh, wait... If I charge them as a pack, then cell 4 might have a higher state of charge than cells 1-3.
Hmm...
So, charging gets very complex and expensive -- for a toy.

I think I'm starting to get the picture.  Is this the right logic path?  Or, did I miss something?

Thanks!   

[Jwillis]

1.2V Li-ion Battery? NiMh maybe? Never heard of a 1.2V Li-ion battery . Can you show the brand ?

[madires]

The crucial question is how much current is drawn on each rail.

[Jwillis]

Regardless of the current draw I would use a buck converters to get different voltages from any type of battery pack.

[Kean]

I've seen many devices that tap a lower voltage off batteries in series.  In general it is only a good idea for very low power consumption and with non-rechargeable batteries.  As pretty much any type of battery does not have a consistent voltage throughout their usage, a regulator (linear, buck, etc) is almost always a better solution to power modern electronics, and also ensure even discharge across all cells.

1.2V Li-ion Battery? NiMh maybe? Never heard of a 1.2V Li-ion battery . Can you show the brand ?

While I also suspect the OP may have meant NiMh, there are AA & AAA sized LiIon cells which contain tiny buck converters to output somewhere in the range of 1.2V to 1.5V.  They will then have either a microUSB connector for charging, or in the case of some (AAA is too narrow for microUSB) a third terminal for charging and thus require a special charging adapter.

[tooki]

I need 3.3vdc and 5vdc rails for a small toy project. 
It's basically blinky leds, a single board computer and a bluetooth module.
I'm thinking of using 4 rechargeable 1.2vdc 1000mAh lion battery cells in series.
That will give me 4.8vdc, which is probably close enough to the 5vdc.

Most Bluetooth modules are designed for 3.3V, and many actually contain a single-cell LiIon charger.

Instead of sticking to 5V, consider using a 3.3V microcontroller. For example, an Arduino Uno uses 5V normally, but it’ll actually run down to 2.7V.

If you take 3V as a sensible cutoff point for a single LiIon or 3 series NiMH cells, you’ll still be well within the range for that microcontroller.

If you go with a nominal 3.3V microcontroller like an ESP32, I’d recommend using a 3.3V buck-boost converter. The freshly-charged voltage of 3 NiMH cells can easily be around 4.5V (which would fry a 3.3V MCU), but you also need it to work on lower voltages, hence buck-boost.

But the easiest is an MCU with a wide voltage range like the ATmega328 in the Arduino Uno. I’ve run them off of 3 series NiMH before and it works great.

[Roberts_Curiosity]

Indeed they are NiMH - sorry for the confusion!

But, I'm sure learning stuff from the conversation.

[m3vuv]

You will be discharging the cells unequally.
Cells 1-3 will provide current for the 3,3V and the 5V circuits and will be discharged deeper than cell 4 which will only power the 5V circuit.

If you don't take this into account when declaring 'low battery' you risk overdischarging cell 1-3.
And if you don't take this into account when recharging you risk overcharging cell 4.

a 1.2v lion cell,am i going nuts but thought they were all 3.7v nominal  and 4.2 charged,am i missing something here??

[m3vuv]

sorry about my last post,i posted before reading the last thread,ive done what the op is asking with ebike batterys for power take offs for various bits mostly 5v for sat navs and phone chargers,and 12v for lights,worked ok with a decent bms,the bms is the way to go but doubt in a toy its cost efective or practical.

[tunk]

Also remember that a fully charged NiMh is 1.4-1.5V, and depending
on the application, fully discharged down to 0.9V (or even lower).

[Roberts_Curiosity]

I also suspect the OP may have meant NiMh

Indeed they are NiMH - sorry for the confusion!

I've seen many devices that tap a lower voltage off batteries in series. 

I have never seen one, and would like to.
Can you point me to one that might be available to buy in the US?

[Kean]

I've seen many devices that tap a lower voltage off batteries in series. 

I have never seen one, and would like to.
Can you point me to one that might be available to buy in the US?

I've seen it used in older RC toys, some older multimeters and other low power handheld devices with liquid crystal displays, and most recently in a batch of battery powered handheld barcode scanners I was recently tasked to repair.  Off the top of my head I can't think of anything I could directly point you to buy, but obvious candidates would be cheap motorized toys with some "smarts" (e.g voice chip or remote control) that take 4 or more AA/AAA cells.  It is becoming less comon in modern devices due to the use of rechargable lithium batteries and lower cost voltage conversion chips.

A quick Google search did find someone querying a robot arm kit that did it: https://electronics.stackexchange.com/questions/23527/does-wiring-a-multi-cell-battery-pack-to-provide-two-different-voltages-adversel

The Battery University website has basically the same comments as above on this topic, about halfway down the page: https://batteryuniversity.com/article/bu-302-series-and-parallel-battery-configurations

[madires]

This seems to be another case of the classic XY problem.

[mariush]

AA and AAA rechargeable Ni-MH batteries will be nearly 1.35v when fully charged, so you'd have 4 x 1.35v = 5.5v

BUT, you should also keep in mind that someone may insert non-rechargeable batteries in your toys, and then you may have up to 1.65v for a very short time right after one pulls batteries out of a package... normally there's gonna be 1.5v or less. So anyway, with 4 batteries, 6v or more is quite likely.

So if you have something that's quite sensitive to maximum input voltage, maybe you should consider using a LDO with very low drop (like 50-100mV) as a sort of protection against over voltage.
For 3.3v, I'd just use a dc-dc converter chip that runs at high-ish frequency, so that you'd only have to use a small surface mount inductor and maybe a diode and a capacitor to complete the circuit.