AC/DC: Battery Power Conversion for AC Speakers (Part 1)June 08, 2016 by Seth Schaffer
This project details the process by which AC-powered speaker sets can typically be converted to battery power. I'll detail my own projects and provide advice for converting almost any set of speakers.
This article details the process by which AC-powered speaker sets can typically be converted to battery power. I'll detail my own projects and provide advice for converting almost any set of speakers.
This project is intended to be very easy to complete and requires little technical skill, though soldering in tight spaces can require some practice and may be necessary. It can also be easily expanded upon with any additional hardware you wish to add.
I will detail the process I went through for two specific cases: a prototype utilizing a very old 25W RMS Altech Lansing 2.1 speaker set, and a brand new Logitech Z333 2.1 speaker system, both of which use the same RC hobbyist NiMH battery packs as a power source. I'll be providing a full walkthrough for my most recent version.
I will also provide general advice for doing this on other speaker sets and using alternative battery technologies (Lithium Ion or Lithium Polymer are used in nearly all portable speakers on the market, after all). I will provide links for materials needed for whatever way you choose to go about the project, and I encourage others to modify the project to suit their needs, especially in choosing a speaker set to use.
In part one, I will cover how to plan your build. Part two will be a step by step "how-to" style guide, and part three will discuss my outcome, experience, and any changes I'd recommend making.
When choosing a speaker set, anything with a big enough cabinet for an enclosure will do, and one can even make their own separate enclosure if need be. It should ideally be small enough to be portable. However, if you want to make a single unit, choose speakers with satellites (the smaller pair) that can be secured easily to the woofer cabinet, either above or on the sides. It also definitely helps if there's a separate 'puck' for I/O. Some sets even have Bluetooth streaming built in.
For example, these curved satellite drivers shown on the right with attached stands won't work at all to make a single unit. Plus, the front-firing woofer with a side vent makes the whole thing asymmetrical.
Designing a Battery System
The power source simply needs to be chosen based on what the power output of your system will be and the desired run time per charge. NiMH batteries are the easiest to work with— and for low current uses like this, they're also the least dangerous among lithium and lead acid. They aren't too heavy, are easy to find, and are cheap. Also, it's easy to find inexpensive 7.2V packs, so wiring these in-series as I have provides a nominal voltage of 14.4V, well over the 12V necessary.
Another major advantage is their ease of use in terms of charging: They cannot be overcharged easily and will drop to a voltage below 12V without any risk of damage to the cells (though this is also true for 4S li-ion packs). When going for a very large scale project, however, I found that lithium batteries scale far better. You pay less per Ah for larger capacity batteries up to over 16Ah, whereas NiMH jump up massively in price from 3Ah to 5Ah and more beyond 5Ah.
It's also important to keep in mind that the batteries will need to be regulated to 12V. The power system needs to supply current equal to the RMS wattage of the system divided by 12 volts, as this will be the regulated voltage/drained battery voltage for the battery system. The voltage regulator dropout cannot be very large (2V or greater is common and severely limits the battery life of the system.) This means voltage regulators must be chosen accordingly.
It may help to add a voltmeter display to monitor the battery voltage, especially when using lithium batteries, though if the system is configured properly it will simply shut off when the batteries drain too much (like off-the-shelf portable speakers). For example, in my system I'm using 14.4V NiMH (2 * 7.2V in series) and these regulators with a 0.37V typical dropout, so my system can operate until the batteries drain to well under 12.5V in most circumstances. If you used a 2V dropout regulator, you would be losing a large portion of the rated battery capacity with 14.4V NiMH as 14V would be the minimum operating voltage.
For my first attempt at a prototype, I found my old Altech speakers had a poor quality transformer outputting 15V instead of 12V, so I didn't use any regulator at all. I used 5000mAh NiMH packs assuming that I'd get over 20 hours of battery life at reasonable volume levels. I imagine for most newer products (those were at least 10 years old) that kind of tolerance is not to be expected. I also had no way of knowing when the batteries needed to be recharged, and the volume wasn't very stable either. There was a good deal of distortion induced, not to mention the fact that the volume would drop off as the batteries' voltage dropped.
For my more recent, modern attempt, I used the same batteries. But since the speakers I'm using had a whole separate power supply board (as opposed to just a transformer and diodes) I knew the original voltage would be stable near 12V to the amp, and I needed to regulate my batteries' output voltage. I also wanted to devise a better solution for charging using a standard barrel jack to allow a smart charger to hook up to the system. Below is a circuit diagram of what I constructed for use with my system, and it should work with any proper combination of batteries, charger, and regulators. Keep in mind, it would be helpful (though not strictly necessary) to add tantalum capacitors to reduce the output noise at both the input and output of the regulator circuit. The exact values will be shown on the datasheet for your chosen regulators, here 10uF.
Author's note: Previously, this diagram did not include filter capacitors and featured two regulators used in parallel. However, this arrangement is problematic due to the inexact voltage outputs of the regulators, and would almost certainly cause nearly all of the current to flow through only one regulator. There is no simple, effective way to combat this issue. I was unaware of this fact when first assembling my circuit, but you should definitely use a single regulator with a high enough current capacity for the entire load.
The main switch prevents the batteries from leaking through the regulator circuit even with the amp control switch open, so the batteries should hold their charge for a very long time. I found many large 'on-off' SPST toggle switches fit well in the hole where the power cord used to enter the woofer, which is also rather convenient.
Planning the Internal Layout: Time for Teardown
While my prototype went smoothly due to a spacious cabinet, I wasn't nearly as lucky on my second attempt. The Altech speakers had everything except a large transformer mounted to a single board while my new Logitech set had a separate power supply board, which was inaccessible without removing the amp + I/O board. Because of how everything was squeezed into the cab, however, I needed to cut the AC power cord to even see, let alone access, the power supply board. And the power supply was glued down so it would be irreplaceable after removal.
At least Logitech was nice enough to use pin headers instead of soldered connections. Not that it helped initially.
After managing to remove the board and disconnect all the pin connectors, I was able to remove the power supply by prying with a screwdriver (very professional, I know) since it was screwed onto a piece of wood which was in turn glued onto the bottom of the cabinet. And then more epoxy had been placed over the screws in case you could somehow operate a screwdriver inside the cabinet through that slot.
I wish I was kidding. So after finally getting that board out, I could finally lay my batteries flat on the bottom of the cabinet. Now is when the layout of everything can be properly planned out. This is really dependent on your specific speaker set, but to give a better idea of what to expect, I'll detail the process I went through in my specific case.
Wiring: The Hard Part
This article is getting long and I also want to go into a lot of detail in my build log. I will use step-by-step details with images of my work progress in my next article to properly explain everything I did. While the circuit itself is deceptively simple, making so many safe electrical connections in such tight spaces was a hassle to say the least. Plus, if you choose to add anything to the internal electronics (such as an internal Bluetooth receiver or BMS/charging circuitry), it can complicate matters. You'll probably have to make some decisions on the fly in terms of fining places to mount various bits of hardware, but when in doubt, using excessively long wires usually won't hinder the performance of the system in the end, so plan ahead accordingly. Keep in mind you should always be conservative with the wire gage used and you'll be fine.
|2x 7.2V NiMH Batteries (3Ah)||$32.99||Link|
|NiMH Charger w/ USB (<= 3Ah)||$19.38||Link|
|Voltage Regulators||$0.50 to $5.00||Based on your power requirements|
|Total:||About $65 for Essentials (not including speakers)||May be cheaper with LiPo|
|Bluetooth Receiver w/ NFC||$19.99||Link|
|4S LiPo (1Ah)||$8 + shipping||Link|
|4S LiPo (3Ah)||$18 + shipping||Link|
|4S LiPo (5Ah)||$28 + shipping||Link|
|NiMH Charger (<= 5Ah)||??? suddenly unavailable||Link|
|12V to USB Converter||$8.00||Link|
|LiPo Warning Monitor||$6.00||Link|
|Li-Ion/LiPo 14.8V 5A BMS||$9 + shipping||Link|
|LiPo/LiFe Balance Charger||$19.99||Link|
|Voltage LED Display||$6 (may be less on Ebay)||Link|