Hi there....long time reader first time poster

I have a project I'm working on which drives a 12v Pump using 4 Lithium batteries in series regulated with an Texas TL2575. When, and for how long this comes on it determined by an ATMEGA328P and DS3231. This setup has to run for 3 months so every little bit of power i can save is valuable. I've found a few buck converters which have pretty low Iq that will provide the 3.3v i need BUT they are expensive.

So here is my maybe stupid question....Can i connect the batteries in series and still draw 3-4.2v from one of the cells, or even all 4? If i draw it from one will the other cells even out the draw? If i could do it this way i can drop the 3.3v regulation and just power the Atmega straight from the battery and save myself some power.

Sorry if this is a stupid question but i cannot find an answer anywhere else.

 

Running the Atmel controller from just one battery in a series string will unbalance the charge and you will end up with that battery flat while the others are charged. This will most probably result in the battery being reverse charger when the motor runs and that will kill the battery and most probably the controller.
The TL2575 may not be the best reg to use. A low quiescent power linear reg is often better, and put the controller to sleep as much s you can.
It could be better to have a separate battery for the controller, and if you can, a small solar panel to keep the charges up.
Measure the current the controller needs, and then the current the TL2575 uses. You may find the switcher consumes more current than the controller so if that is the case, a linear reg will be better for light loads.

 

Thanks dendad! I'd been mulling it over this eve and reading similar questions on here and had started thinking a seperate cell for the uP might be a better idea as you say. A backup solar cell might work as well. The controller is asleep almost all the time. It just wakes up, checks the RTC, if it's not time to pump it goes back to sleep.

Interesting point about the TL2575. I'll check out some LDO's. I didn't want to over complicate my first post with too much info....the pump draws about 600mA when in use but is only on for 5-10s a few times an hour. When it is off the TL2575 is in shutdown so uses very little current. Not sure if this makes a difference?

Thanks a lot for your help!!

 

Is the TL2575 just to supply the controller or the pump too?
If you have the TL2575 to supply just the controller, how do you keep the 5V (or 3.33V) to the controller going when the TL2575 is shutdown?
Does the controller have a separate reg and use the TL2575 ON/OFF pin to control the pump?

 

Sorry for not being clear.
Yeah the TL2575 is just to power the pump. It is turned OFF via the EN pin the rest of the time.
In my current prototype I use an additional little LDO to get from Vbat to 3.3v for the ATMEGA. I wanted to switch to something more efficient as my understanding was going from 12v-16.8v Vbat down to 3.3v using an LDO was wasting a lot of power.
The suitable switching regulators for this all seem to have silly high Iq (5mA!?!) or cost a lot.
I think your suggestion of a seperate battery is the way to go...if I can find a solar cell which will keep that topped up that'll be even better!!