2AA Batteries and one supercap cell

Hi to all, let's start simple: what should I do if I wanted to play with a couple of AA batteries 1,5v and 1 or 2 supercap cell (2.7v each) to power up a small LED?

I'd like to build a small demo board where, given a programmable device to implement some logic (example like a mid range PIC like 16F690, 16F684) I can set the % or the balance of power flowing to the LED from these 2 sources (or a motor if that suit better as example) - I'd like to balance the power absorption from these two reservoir. Example, if I would attach a trimmer to the PIC, I could set the flow like:

_ 100% from battery
_ 20% from battery, 80% from supercap
_ 100% from supercap
.. and so on

As sidenote, I can stabilize the working DC level to 5v it via a simple switching circuit like max756, so 2AA and 1 supercap cell would be stepped up to 5v independently (or down, depending on power sources voltage, or even a SEPIC..but..). But then I'm missing the building step to implement the power flow control

I feel I really miss some building blocks in electronics, one in particular being the capability to actually ""control"" power flows (double brackets necessary..).

I've posted this simple example, not to be solved by some cheap tricks (diodes, limiting resistors, relays..), rather to really get the point: how do you treat paraller power sources? adding more, at which level do you stabilize, regulate and control power sources?

I was thinking of having just one switching regulator, then I could feed it with the supercap cell and 2AA.. but the problem is still the same, I've only shifted the place where to balance the flow of power.. what's the keypoint?

Hope I explained the problem clearly, have you got any ideas?
Tnx.

Have some experience with juggling power from multiple generators as a single power system.
All the generators are connected to their own regulator to put them at the same voltage level before combining them to provide stable power to the main bus.
Then there is a whole mess of protection and monitoring that can disconnect a generator if there is a problem.


But yeah... I think I know what your getting at.

You could use a trimmer on each source, as long as the PIC was constantly monitoring the output of each source and adjusting the trimmer accordingly. This is how voltage regulators work, also why they heat up and waste a great deal of power.

I was thinking of having just one switching regulator, then I could feed it with the supercap cell and 2AA

Click to expand...

AA batteries are in parallel with the super cap? The battery would power the system and charge the cap till it is full. The cap then could provide additional current when the system pulls more current than the batteries can provide.

Not really a control of current flow, but many portable system that use super caps do this.

First, thanks for the answer.

What I wanted to demonstrate is to ""find"" (well, absolutely not invent..) some techniques on how to regulate the flow of currents - I wanted to do it by assembling a small demo system, 2AA and 1 supercap element, just to play it safe.

What I would like to achive is to show how to control currents by some mean of automated way (a simple PIC would do all the thinking) - what I'm missing is the building block, the first step, the basic mechanism on how to control current flow, without just parallelizing power sources.

I could also change the problem on how to power balance 2 AA batteries powering a load in a controlled way; I can easisly take the supercap out of the project, it's not the subject of the experiment

If you were to light up a LED from two AA batteries, but you want full control on currents flowing outside the batteries, and you want control on them, like a knob where you set the absorption of the two.. I'm totally missing this techniques.. nd I can't find more on book - any suggestion?

Tnx.

I too, do not know much about this topic, but am happy to help you search.

Here are some keywords you can search on.
Cell balancing
Battery management system
State of charge monitor
Coulomb counting

Thanks for the keywords, here some more consideration:

A naive idea would be that given a stabilized output voltage, eg 5v, each parallel unit would just cheat their outputs by few millivolts less/more individually, just enough so that having them in parallel would force them to pump less/more current (not interested in assorbing it back..), while keeping the bus voltage steady - this is just an idea, the problem is the wrong feedback from the output terminal that my units would receive back.

From what I can see, even a simple switching regulator has always (by design) a simple resistor network on the output, so if I parallelize multiple unit they would simple go crazy, becaus their feedback signal would be poisoned by the output of the other converter; what I'm feeling right now is to investigate in the building block of a more distributed system where each power source is outputting some current and it must cooperate (and know the existance of) other power modules.

It certainly sounds esotic and complicated.. that's why I started asking taking as base example a simple system..

There are many types of regulators. I know that method works with transformer / rectifiers from generators.

I can't say if a normal regulator that uses a resister network would "go crazy", you would think that it would continue to adjust. If it was me, I would slap one together and take some measurements.