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I am building a system with 4x AA batteries. (3V output) I like to understand the battery level for the system. I cannot control which battery people use (li-ion vs alkaline) or quality of the batteries (name brands such as energizer vs. noname)

I have a processor with ADC and I can put additional hardware if necessary. Also, I have a booster where I generate 3V, on the paper 3V is constant between 1.8V -3V input so, presumably I have a decent 3V ADC reference. My current plan is to observe voltage drop and turn the battery indicator red at a predetermined level.

Does this make sense? I know the drop is not linear but I cannot think of a better way.

UPDATE I realized I didn't think this problem throughly.. Russel informed me about several key points. So let me expand and ask questions:

My input AA battery, current thinking is to use 4 of them increase total available power, not because I have to make them serial and produce 4-6V. I want product to work for a long time.

I need internally 5V and 3.3V.

I need a battery indicator, at least I need to understand when things are getting low.

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  • \$\begingroup\$ It does not make much sense to me. 4 x AA batteries produce 3V output? Or do you use them as input to a circuit that produces 3V output? Lateron you talk about 1.8V - 3V input? \$\endgroup\$ – Wouter van Ooijen Sep 11 '11 at 16:01
  • \$\begingroup\$ @wouter, sorry it wasn't clear. We are trying to get 3V and 5V in our system using AA batteries. 4 battery comes from the total available energy requirement for the lifetime of the product \$\endgroup\$ – Frank Sep 12 '11 at 3:46
  • \$\begingroup\$ When you say 3V do you mean 3.0V or 3.3V? \$\endgroup\$ – Matt B. Sep 12 '11 at 5:22
  • \$\begingroup\$ @matt 3.3V but I guess everything in there would work with 3V wihtout a hitch. \$\endgroup\$ – Frank Sep 12 '11 at 7:53
  • \$\begingroup\$ What exactly is your question? Is it about a design for powering your system, or for monitoring battery life? If it is about powering: How much current is consumed at 5V and at 3.3V? If it is about battery life: would seem a pretty hopeless job to me, if you allow the user to change batteries and battery types. \$\endgroup\$ – Wouter van Ooijen Sep 12 '11 at 14:37
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Assumption: 3V minimum supply voltage wanted by system.

Best solution:

  • Linear LDO regulator with 3 cells

  • or Buck Converter with 4 cells.

The 4 cell solution gets slightly more of the total available energy from the battery pack and has 4/3 of the battery volume so 4 cells would run for about 1.4 x as long as 3 cells.


Batteries people use will PROBABLY have a per cell voltage of from about 0.9 Volt minimum to about 1.6 Volt maximum.

You are very unlikely to have people use LiIon (3V to 4.2V/cell). AA (= 14500) cells are available in LiIon but are extremely rare on the consumer market.People who have them will usually be educated enough to not use them in this application.

It is possible that people could have primary Lithium AA (about 3V) or other very rarely available cells, but this is extremely unlikely. Mercury, Zinc Air, LiFePO4.

Voltages below are the reasonably extreme limits Most likely cells are -

  • NiCd - 0.9 - 1.3 V
  • NimH - 0.9 - 1.4V
  • Alkaline - 0.8 - 1.6V
  • "Zinc Chloride / LeClanche - 0.8 - 1.5V
  • LiIon - 3 - 4.2V

ZnCl is standard cheap torch or radio battery.

Alkaline batteries are nominally 1.5V but consistently measure 1.55V or slightly more when fully charged. NimH are nominally 1.2V but can be 1.35V soon after charging.

So, excluding LiIon te voltage range is about 0.8 - 1.6V/cell.
If you want rechargeables to be ttreated well then 1.0V is sensible lower limit at low currents so 1.0 - 1.6V

SO:

3 cells: 3 x 1.0 - 1.6 V = 3V- 4.8V
4 cells: 4 x 1.0 - 1.6V = 4V - 6.4V

(The above is for new and exhausted cells. For "sensible" voltages of 1.0 - 1.5V th range is 3-4.5V and 4 - 6V for 3 and 4 cells)

In the most unlikely case of LiIon being used then 3 cells gives 9 - 12.6V and 4 cells gives 12V - 16.8V. Unusual but needs to be designed for if damage is to be avoided.

SO:

With 3 cells you JUST get down to 3V at 1.0V/cell. Below that with eg ZnCl run very flat. Below 1.0V all cells have very little capacity remaining so a 3V LDO regulator would be "good enough" This could be linear with almost 100% efficiency at 3V in and 3/4.5 = 66% efficiency at 4.5V. A buck converter may run in the 80% - 90% range. The relatively low Vin and range of voltages means the 90-95% achieveable with buck in some cases would not easily apply here. So, a linear LDO regulator would work well enough here.

With 4 cells you JUST get down to 4V at 1.0V/cell. (Or down to 4 x 0.8V = 3.2V with eg ZnCl run very flat. So a 3V LDO regulator would easily provide 3v out. This could be linear with around 75% max efficiency at very low battery voltages and as little as 50% efficiency at 6V in. A buck converter may run in the 85% - 90% range - slightly better than with 3 cells. SO a buck converter would make a lot of sense with 4 cells.

If you want to protect against people using LiIon cells you need to protect against 3 x 4.2 = 12.6V or 4 x 4.2 = 18.8V Vin.
You could either shut down the system with a warning at these voltages (especially if a linear regulator was used) or accept the lower overall efficiency with a buck converter) as a system optimised for half the voltage of LiIon cells would probably be less than optimum when LiIon was used.

At the bottom end I'd cut off operation at 1V/cell to protect rechargeable cells - unless you wanted every last 'drop' of energy. (With 3 cells you need just over 1V / cell with an LDO linear).

Buck-boost with 3 cells allows you to get slightly more from the batteries with 3 cells but has less efficiency overall so probably makes no sense compared with a linear LDO.

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  • \$\begingroup\$ I think he means lithium primary AA cells such as Energizer Ultimate Lithium (formerly known as e^2), which are LiFeS2 (lithium iron disulfide), NOT lithium-ion. They're commonly available at retail (Walgreens carries them), while Li-ion bare cells are not. They're more expensive than alkaline on a $/W-hr basis but they do last longer and weigh less so they're a premium product for people who care more about that than a few bucks. Great for cameras, walkie-talkies, camping. The voltage is a little higher than alkaline, so it's smart to make sure your system won't fry if somebody uses them. \$\endgroup\$ – Matt B. Sep 12 '11 at 2:58
  • \$\begingroup\$ Also it used to be that the upper voltage limit quoted by manufacturers was 2.0V per cell for NiCd and 1.8V for NiMH (during rapid charging). Maybe it has changed in more recent formulations. You're not likely to see that in practice, but it may be a good idea to consider 2.0V/cell as the upper limit of AA and make sure your circuit doesn't fry with that as the input. Energizer Ultimate Lithium can start at 1.8V upon first use, so 2.0V isn't an unreasonable safety margin. \$\endgroup\$ – Matt B. Sep 12 '11 at 3:09
  • \$\begingroup\$ @Matt B. is right, I was considering this Lithium batteries in AA form, I don't think they provide 3V. This is for a retail product. \$\endgroup\$ – Frank Sep 12 '11 at 3:45
  • \$\begingroup\$ @Frank - you ssid LiIomn - which are the <= 4.2V seconmdsry (ie rechargeable ) cells - clarification re meaning primary Lithium noted. FWOIW I have some 14500 = AA LiIom cells which would deop into a normal AA battery position. Vanishingly few people will hsve these. \$\endgroup\$ – Russell McMahon Sep 13 '11 at 5:15
  • \$\begingroup\$ @Matt.B - the maximum vpltage you are liable see is the maximuj charger terminal voltage during charging. With NimH most g neagachargers will have en endpoint voltage of 1.45 V. Some may be slightly lower and a relative few may be slightly higher. This voltage is usually a secondary effect of battery chemistry when charging and will be seen even when negative delta V endpoint detection is used. \$\endgroup\$ – Russell McMahon Sep 13 '11 at 5:21
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To clarify the original question: Frank is designing a system that takes 4 x AA cells, which will have an ADC and regulated 3.3VDC and 5.0VDC rails (which he offers could be used as a reference for the ADC). He wants to use the ADC to measure the remaining battery capacity, or at least turn on a red LED when the battery is low and needs replacing. The AA cells can be of any chemistry, capacity, age, etc. It sounds like the system may be low-power but the power use isn't stated.

Answer: When you're using bare AA cells (instead of an assembled battery pack), the user can put in anything at any time, including a mix of alkaline and NiMH (or any chemistry that fits), mix of dead and fresh cells, cells in backwards, you name it. Trying to guess the remaining capacity simply from the voltage is pretty much impossible.

However, if you only want to turn on a red LED when the battery is very low, you may be able to simply detect when the battery voltage goes below some low threshold. Alkaline, NiMH, lithium primary, etc. all have their particular range of voltage but in all cases the voltage will nosedive when they're dying. NiMH in particular should not be discharged below something like 0.9 or 1.0V or the cells may suffer permanent damage. Disposable cells such as alkaline can be run down below that but won't offer more than a tiny amount of extra runtime. So you can set a low-voltage detector at something like 1.05V per cell or 4.20V for your 4 x AA and turn on the red LED when the battery is below that threshold. You might experiment over a range of 1.0V to 1.1V to see what works best to give an appropriate amount of warning time for your system.

This method works best when the load on the battery is smallish and fairly constant. If the battery is loaded with heavy pulses, the voltage will dip during the pulse and recover, causing the low-voltage detector to trigger on those dips instead of when the battery is "really" dead. If using the ADC rather than a dedicated voltage detector, it's easy to undersample badly, or even cause dips by measuring if the processor takes much power. The more often you measure, the faster you run down the battery.

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  • \$\begingroup\$ great answer thanks.. What is the best way of doing this, assuming user can put in anything? \$\endgroup\$ – Frank Sep 14 '11 at 1:20
  • \$\begingroup\$ One way is to put a low-power standalone voltage detector with built-in reference (MAX836 or similar) on the battery and feed the output to a pin on your processor. For a pulsed load you may need to run its input through an RC filter or something so that it doesn't trigger until the voltage has been low longer than a typical pulse. Once it triggers, you may want the red light to stay on until the battery goes back above some "good" threshold so that the red light isn't blinking on & off unpredictably. Could need a second detector or some form of hysteresis, or reset on a change of batteries. \$\endgroup\$ – Matt B. Sep 14 '11 at 3:41
  • \$\begingroup\$ If your load is very small, like a microcontroller, maybe you can just sample with the ADC once every 10 minutes or whatever seems reasonable depending on your estimated battery life. The point of having a hardware voltage detector is the power used is very low compared to cranking up a big processor to handle a voltage measurement, but if the processor is very low power it may not help. Don't forget that the LED is only information for the user. If the product may have NiMH in it then you also need a hardware cutoff to make sure the product doesn't over-discharge rechargeable cells. \$\endgroup\$ – Matt B. Sep 14 '11 at 3:45
  • \$\begingroup\$ In general a low-battery light of this type isn't worth that much because it doesn't help a whole lot. By the time the red light comes on, there's only enough energy left for a little bit more usage. It only keeps the user from confusing a dead battery with a broken product. Maybe gives them some time to, say, use a flashlight to find more batteries for the flashlight in the dark. The product should try not to hurt the user by losing data when suddenly turning itself off. \$\endgroup\$ – Matt B. Sep 14 '11 at 3:55

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