Monitoring Battery Level, without wasting too much energy

barkly

Hi,

I have a solar panel currently charging two 6V batteries after being sent through a solar regulator and was hoping to either monitor the battery levels or the voltage/current being output from the regulator. Was hoping to monitor this through a IOIO microprocessor that has analog inputs 0-3.3V or digital ins that register Hi for V>3.3V.

Any ideas? keeping in mind I want to limit the power consumption from this monitoring circuit.

Cheers

berkeman

I've used a micropower comparator before to monitor the battery voltage in a small RF node. Let me look for the part number...

berkeman

MAX919 and MAX921: http://www.maximintegrated.com/gloss...arator/gpk/989

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barkly

Wont I need a positive and negative rail to ensure the op amps operate though?
Did you just power from the microprocessor?

berkeman

Are the two batteries separate or in series to make 12V?

You would power the comparator probably from the same 3.3V rail as the uC, so the output voltage can be compatible. No need for any negative rails here unless I'm missing something. Those comparators are rail-to-rail comparators (double check me on this by looking at their datasheets and application notes at the link I provided).

barkly

In series to make 12V.

Yer I just read the data sheet, your correct in them being rail-to-rail so can just tie negative input to GND.

barkly

can see this is going to be a problem with this part as the max in voltage is 5V. Does anyone have any experience with IC current sensors? What is the power usage on these components like?

barkly

should mention that the batterys are sealed lead

berkeman

Do they show any applications with high-impedance voltage dividers? You don't need any bandwith in a battery level monitor...

barkly

not that i can tell, starting to think voltage divider will be the best solution for now

sophiecentaur

It strikes me that you only need to monitor at very long intervals (30minutes, perhaps). A very low power timer circuit can be used to turn on your main measuring and analysing circuits for a few ms every 30 minutes - the power drain would hardly even overtax a watch battery.
The processor will drain more current than a basic voltage divider on its input, surely.

barkly

Theres an external battery, running into a IOIO which is connected to a android phone. I have a solar panel that runs into a solar regulater charging the external battery. Basically I am just wanting to get an idea on either the output of the solar regulator power or the external battery, without wasting to much energy as I already know the android phones battery status.

sophiecentaur

In this case, a simple milliammeter in series would give you a very good idea and it would consume very little power.

barkly

Was hoping to input it into the microprocessor so I can utilize the information in a program..

pantaz

If you really want to get that technical with it, there are a variety of battery/power monitoring ICs available with stand-by currents in the micro-amp range (I believe I've seen some with pico-amp stand-by ratings.)

MATLABdude

You already have an analog to digital sampling port on your micro, so I'd suggest making use of it (to reduce power consumption, you can turn off your A to D module and tri-state the input). I'd build a 2 or 2.5 or 3 to 1 voltage divider (say, with two 4k7, or a 3k6 and a 2k4, or a 3k6 and a 1k8) so you can get the measured voltage within range of your analog to digital input.

If you're concerned about the constant drain through the voltage divider (in spite of choosing values in the kOhm range), you could put in an N-channel MOSFET at the tail of the voltage divider. The MOSFET should be off until you drive the gate to 3.3V (do it through a small resistor, 1k or less) when it'll hopefully put the MOSFET in the linear region. By doing this, you'll introduce a slight resistance (see the MOSFET's datasheet for the Rds(on) parameter), but if you choose high enough values for your voltage divider, this should have fairly negligible impact.