Battery Questions

[Urahara]

Hi

This might be a basic/silly question, but since I have really no foundation in electronics, I guess I have to tap on the experts here.

Suppose I have a circuit (pic chip + lcd/oled character module + keypad) that requires 5V to operate. For batteries, I could use either be a 1x9V or 4x1.5V AA (total 6V) battery. Since either types of battery > 5V, I would need a voltage regulator to drop the input battery voltages down to 5V, correct? if so, I would need a low drop out regulator?

If I would like to maximize the usage time, which type of battery would be a better choice?

The way I look at it, the 9V would be better since it has a longer way to go before dropping to below 5V. But AA batteries are easier to find.

Thks!

 

[kchriste]

4 AAs, or even 4 AAAs, would last longer than a 9V battery because they have more mAh. You would need a low drop out regulator if you used the AAs or AAAs, but not if you used a 9V. The preceding assumes you are using alkaline batteries. NiMH batteries put out a nominal 1.2V per cell but are typically at 1.5V when fresh off the charger so you'd need 5 of these to get the full capacity out of them.

 

[parts-man73]

9V is the worst choice IMHO, It has the lowest mA rating. If your goal is to maximize running time, the more mA, the longer it will last. The 9V also has a lower voltage drop at the regulator, the regulator dissipates this as heat and the efficiency goes down.

The lower voltage would be better, as less voltage is dropped at the regulator, efficiency is higher, less heat produced, less energy wasted. Higher mA ..... all this equals longer usage time before the batteries go dead.

A "standard" 7805 regulator needs a volt to a volt and a half above 5 volts to maintain a constant output. A LDO regulator can operate with much lower overhead.

Also, look over the data sheet for the regulator you end up choosing and use the proper capacitors on the input and output side of the regualtor.

Good luck!
Brian

 

[Urahara]

Thks for the prompt response!

Another question : if I use 2xAA batteries instead of 4, the voltage would be 3V. What can I use to bring 3V up to 5V to power the circuit? Also, I suppose 2xAA would still have more juice than a single 9V battery?

 

[ArtemisGoldfish]

If you can, get either a DC-DC converter (significantly better efficiency) or a buck-boost converter (Will still output a certain voltage even when the supply voltage goes lower than the desired output)

 

[Hero999]

What's the minimum voltage it will operate down to?

What's the maximum voltage?

It might be able to take 6.5V for a short length of time in which case it can be run directly from 4 AA cells.

If it can work down to 4V then you'll get the maximum battery life.

If it draws under 50mA but has an absolute maximum voltage of say 5.5V then a LM2936-5.0 is ideal.

If the cirecuit requires a minimum of 4.5V to operate, then you really need to use 5 AA cells with the LM2936 to get decent battery life.

 

[Urahara]

Have not worked out the min, max voltage required (that's another one I will have to figure out later ). But what I do know is the following :

1) 18F4620 uC is used
2) 4x4 keypad
3) 20x2 LCD or OLED character module
4) maybe drive one or two leds
5) DS18B20 digital temp sensor is used occasionally. if used, to take a reading say, every 30 sec for maybe 20 mins
6) Take a light reading from a Cds photocell, also occasionally
7) Battery operated, and also from mains using one of those AC-DC wall adapter. If latter is used, battery will be "disconnected"

You reckon I am ok with just plain 4xAA batteries, or I will still need those LDO regulators?

Would be grateful if you could advise how to estimate the voltage required too

Thks!

 

[Hero999]

Have a look at the datasheet for your :mu:C.

If it can't withstand 6.4V then you need a LDO regulator.

 

[Urahara]

Datasheet says "Absolute Max Rating" is 7.5V. So I guess I can get away w/o the LDO.

Thks!

 

[kchriste]

I would use the LDO regulator. It'll make the circuit design easier if you have a steady 5V available, especially for the CDS cell reading. There are also boost regulator ICs available designed specifically to generate 5V from 1.5-3V. The LTC3525, MAX1674 or MAX1675 are examples of this.
If you don't use a regulator, then you'll need to have a voltage reference IC for the ADC so your CDS cell readings stay consistent as the supply voltage drops. Without a regulator you could add a diode in series with the 4 AA cells to drop the voltage by 0.7V. The diode could also be part of the mains/battery switchover circuit.

 

[Willbe]

You can also use a forward biased 1N4001 to get a ~1 V drop at low series impedance, kind of like a 1 V Zener. With diodes and resistors you might get close enough to your design voltage.

 

[Urahara]

Thks!

KChristie's input on the voltage ref for the CDS is a valid point. Certainly lost sight of that one.

Will look into both designs and see what works for me.

 

[Oznog]

Datasheet says "Absolute Max Rating" is 7.5V. So I guess I can get away w/o the LDO.

Thks!

Oh no. That's a bad idea. Spec sheets are always vague about the "less than absolute max but more than Electrical Specification". The controller's spec'ed for 5v. Anything more may cause problems and will put the part at risk of early death. You need the LDO for sure with 4x alkaline batts.

On the other side you want the part to not get screwy when Vin gets low, because the batts may drop to 0.9v/ea before you wanna call it "dead". The boost reg is good for that. BTW, look at the freq vs Vin graph, the part is often unable to operate at top freq when Vin<5v. Anyhow, a boost reg is simpler than a "buck AND boost" reg. So 1 or 2 or 3 alkaline batts could boost to 5v without need for a buck capability AND could retain the 5v power until the batts get "really dead" not just "somewhat dead".

Look up Mouser or Digikey for these, you won't find them at Radio Shack. There are hundreds. Most require an inductor as an external component so you'll need to do the selection math in the spec sheet. There are also less efficient charge pumps to double the voltage followed by a linear LDO reg to drop it back down to 5v, this will also require external components.

 

[Hero999]

Oh no. That's a bad idea. Spec sheets are always vague about the "less than absolute max but more than Electrical Specification". The controller's spec'ed for 5v. Anything more may cause problems and will put the part at risk of early death. You need the LDO for sure with 4x alkaline batts.

But he isn't pushing the IC to its absolute maximum rating, the nominal voltage is just 6V and the absolute maximum voltage he can expect is 6.4V for a fully charged set of cells, way short of the absolute maximum of 7.5V.

Most MCUs are based on HC(T) logic which is stable between 2V and 6V.
https://www.onsemi.com/PowerSolutions/taxonomy.do?id=612&type=Family

If anything, a simple diode is all that's required for reverse polarity protection.

So long as he's not pushing the MCU to its maximum speed, then operating down to 0.8V per cell shouldn't be a problem, even with a 0.7V diode drop.

 

[philba]

I'd be tempted to run this off of 3 AAs. Same battery life. That pic will run all the way down to 2V according to the data sheet.

 

[Nigel Goodwin]

I'd be tempted to run this off of 3 AAs. Same battery life. That pic will run all the way down to 2V according to the data sheet.

They usually work perfectly well off three AA's - and saves needing a regulator.

 

[philba]

yup, the most efficient power supply is a passive wire.

 

[Hero999]

I'd be tempted to run this off of 3 AAs. Same battery life. That pic will run all the way down to 2V according to the data sheet.

It'll probably work at lower voltage than 2V so you might get away with using two AA cells and still have reasonable battery life.

 

[philba]

probably but I think he's going to need some references so a bit more head room would help. I suspect his LCD doesn't like lower voltages.

 

[Urahara]

Yup, LCD spec sheet says btw 3-5V. So 3 batteries seem like a safe choice.