Switched-mode power supplies for the Raspberry Pi, the ultimate way to get your Raspberry Pi Zero to run off of alkaline AA batteries, right? No, actually it isn’t. The ultimate way is to connect Nickel Metal-Hydride (NiMH) AA batteries almost directly to your Raspberry Pi.
So, what exactly is going on here? The primary problem at stake is the limits imposed by the power consumption of the Raspberry Pi Zero and the current supply ability of your batteries. Despite the Raspberry Pi Zero being considered a “low-powered computer,” it is still a relatively high power consuming device on an absolute scale. Really, 250 milliamps at 5 volts? That’s 1.25 watts of power. Sure, that’s not much power compared to traditional desktop PCs and traditional incandescent light bulbs in wall and ceiling mounted lighting fixtures, but that’s an awful lot of power to draw from AA batteries. According to Wikipedia (and other sites not referenced), a single alkaline AA battery can supply up to 0.5 amps of current before its charge supplying ability starts to sag significantly below its rated charge capacity in milliamp-hours (1400 mAh) or kilo-Coulombs (5 kC).
20200329/https://en.wikipedia.org/wiki/AA_battery
So, what does this mean if you’re using 3 alkaline AA batteries to
power a Raspberry Pi Zero? The Raspberry Pi without a camera
connected will draw 1.25 watts of power at idle, so that’s 1.25 W /
4.5 V = 278 mA of current. With a camera, you’ve got double trouble,
so that’s 556 mA of current. Yes, that’s literally just over the 0.5
A mark! Your alkaline AA batteries will not last for long if they are
powering a complete Raspberry Pi Zero system, and you will only be
able to get a fraction of the rated charge capacity before your
alkaline AA batteries have voltage sagged too low to be usable any
longer.
Now, this is exactly the problem. What does a switched-mode power supply do when the voltage sags from the batteries? It boosts the current draw. With alkaline AA batteries, this is going to further exacerbate the problem you already have and the boost switched-mode power supply on alkaline AA batteries will end up buying you almost nothing. So, point in hand… if you really want to power your Raspberry Pi project using alkaline AA batteries, only use a buck switched-mode power supply. Or, limit the minimum current that your boost switched-mode power supply operates on to 3 V to keep the current boost effect under control. If you want to get longer battery life with alkaline AA batteries, prefer to wire more batteries in series to provide a higher supply voltage at a lower current, the alkaline AA batteries will thank you with many times increased energy delivery in return. So, let’s answer the question: If you’re consuming 5 watts of energy, how many AA batteries should you use to limit the current draw from them to 100 mA?
P = V * I
P = 5 W
I = 0.1 A
V = P / I
V = 5 W / 0.1 A = 50 V
50 V / 1.5 V per AA battery = 33.33 AA batteries
Whoa… now that’s a lot of AA batteries. If we make compromises… limit power draw to 2.5 watts and tolerate drawing 200 mA of current, then we can get much more reasonable numbers.
2.5 W / 0.2 A = 12.5 V
12.5 V / 1.5 V per AA battery = 8.3 AA batteries
But still, it’s not very impressive. Think of the size and the weight of all those AA batteries? That hardly makes for a good portable project, much less a “mobile device.”
So, if at the very least, if you want to be more satisfied with getting power from a battery power source that is not too far detached from the fundamentals of electronics, you’re better off using Nickel Metal-Hydride AA batteries, AAA work well too for shorter-term uses. These batteries work much better with high current draw applications, their rated charge capacity will not sag nearly as much under high current load as alkaline batteries do. Also, with a nominal cell voltage of 1.2 V, you can connect 4 of these batteries in series directly to a Raspberry Pi Zero and get 4.8 V of power, well within the 2.9 V to 5.2 V operating range of Raspberry Pi Zero. By contrast, 4 alkaline AA batteries would push you over the limit that the Raspberry Pi can handle.
But, how much is too much for NiMH batteries? Looks like the “too much” is well outside the range you may work with for Raspberry Pi. NiMH batteries can definitely do well supplying 1.5 A of current, and apparently NiMH batteries can also do well supplying 10+ A of current. However, please note that before we start talking about such high current draws, you’ve got to make sure that the wires and connectors (if any) from your battery to your switched-mode power supply are rated to handle that much current.
20200329/https://en.wikipedia.org/wiki/Nickel%E2%80%93metal_hydride_battery
20200329/DuckDuckGo maximum current draw nimh battery
20200329/https://www.eevblog.com/forum/projects/maximum-aa-battery-current-draw/
So, here is the crux of the point. Only if you are using NiMH batteries is it safe to connect them to a boost switched-mode power supply that will even boost when the voltage runs down to 2 V. However, please note that at this point, chances are your wires and connectors (if any) will be the limit on your maximum current draw, so plan accordingly! If unsure, set the minimum battery voltage so that battery current is limited to 1 A.
Otherwise, if using alkaline, to limit the current draw from the batteries, you should cut-off at 3 V. But that’s already what the Raspberry Pi Zero does on its own due to the tolerance limits of the internal buck switched-mode power supply… it has a minimum fixed buck voltage of about 1.1 volts, and since the core voltage must be 1.8 volts, the supply voltage must be at least 2.9 volts. So, if these limits are already in place, and you plan on connecting no more than 3 batteries to your Raspberry Pi Zero, and you will either use alkaline AA batteries or NiMH AA batteries but don’t know in advance, then there is absolutely no reason to use an additional switched-mode power supply in front of the Raspberry Pi.
Otherwise, if you really want to use 4 or more AA batteries in front of the Raspberry Pi but don’t know their chemistry in advance, then it makes sense to use a predominantly buck switched-mode power supply. And actually, it wouldn’t be a bad idea either to use an exclusively buck switched-mode power supply either, given the constraints that I’ve mentioned. As you can see, if you want to build a project that can take any random AA battery as a battery-powered voltage supply, the chain is only as strong as the weakest link.
This is why a lot of computer-grade electronics don’t run off of standard AA battery cell sizes or the like. There are just too many dooming assumptions that must be made. Yeah, you can design your device to run off of AA batteries, but surely the user will not be happy. The battery won’t run long enough on a single charge… the user will want rechargeable batteries for convenience, the user will want a light battery pack, the user wants to continue using the computer while the batteries are charging, the user doesn’t want to have to remove batteries for a charge or carry around two battery packs, and the user would rather recharge frequently than carry around a giant battery pack that doesn’t need to be charged or changed very frequently. With all of these assumptions in place, the only thing that matters for standardization is a standard way to plug in to recharge the batteries, and that amounts to an AC wall adapter that sends power to the computer, and the computer does all the rest with charging the batteries.
Sure, those are pretty restrictive assumptions, but can’t you at least have some degree of serviceability and standardization on those laptop battery packs? Why can’t you like open up those laptop battery packs and get a series of standard AA cells that can be swapped out with the right chemistry? Well, first of all, if you let the user open up the battery pack, they might replace with the wrong chemistry, and then you’d have to handle that possible case with more complex logic inside your smart battery pack. Second, lithium-ion cells are dangerous. Third, modern lithium-ion battery packs don’t use standard can-shaped cells anymore. Each additional restriction and complication gets added simply because of the rawness of sheer consumer demand and their lack of interest to have an electronic that a novice can open up and service to even the most basic extent. Finally, the most dooming assumption that has been made with modern smartphones is that the user will never bother replacing their own battery, heck they’re not going to want to own the same smartphone for that long. So, the smartphones are now sealed up with no modular design for replacing the battery.
For makers, it’s a sad story, I know. But ‘tis the course that all modern mass market electronics must follow. It already happened long ago with the migration from through-hole components to surface-mount devices in computer-grade electronics. It was only a matter of time that likewise would happen with the battery power supplies of computer-grade electronics.
So, yeah… in conclusion, I dropped you some great guiding facts, assumptions, and principles for battery-powered Raspberry Pi projects. One thing I didn’t cover was the idea of using D cell batteries, but presumably you wouldn’t want to use those anyways due to their sheer size and weight. In general, Raspberry Pi consumes too much power to be easily usable with basic AA battery types. Alkaline AA battery power is more of the domain of lower-power microcontrollers like Arduino.
Oh, and one last point I left open about switched-mode power supplies and Raspberry Pi. Indeed, you must first select your guiding assumptions on what kind of battery and load you’re using before switched-mode power supply decisions become relevant. Second, once you do, there are a few more guiding tips to completing your journey if you do decide to use a switched-mode power supply.
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Low current draw, of course, is king. If your power consumption is below 5 volts at 500 milliamps, you have a whole host of options available to you, particularly some very nice maker-friendly boards that put a collection of high quality surface mount components together with 1/10 inch through-hole pads for the essential connections.
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If you must use a high-current draw, your choice of switched-mode power supplies is suddenly more narrow. You might find you need to build your own, but then you’d opt to use through-hole components and find yourself with a much smaller market of opportunities due to your omission of all the nice surface-mount devices there are out there. So then, another factor at steak becomes more crucial. Remember this key: if you need high current draw, chances are you’ll be picking a switching controller instead of a switching regulator. This is a PMIC (Power Management Integrated Controller) that controls an external MOSFET of your own choosing for the main power switching, and this allows you to size it so that it can handle much higher current loads than a builtin switching MOSFET can handle on the inside of a switching regulator.