Building a Festival Ready 12V Rechargeable Power System Part 1: Introduction
Planning to take the Red Tin and Outdoor Sound System off-grid and starting by working out what batteries and solar charging panels would fit the bill.
Over the past few months, I have been developing my DJ system in a red tool box and a sound system to go with it. Both are powered by 12 volts and are designed to be used at festivals and pop-up venues. What I need now is a portable, rechargeable power system to complement them.
The plan is that this would involve some kind of battery and a solar panel for charging. As well as being portable it would need to be water and mud proof. It also occurred to me that if it could charge a mobile ‘phone or two as well as its primary function, this would be a bonus.
Research – what is already out there
I started researching what would fit the bill and discovered there is an awful lot of information out there about domestic solar power installations, mostly from people who want to sell you an array of solar panels to put on your roof. There was also some information aimed at live-in vehicles and boats and, although that fitted the 12-volt remit, none of the ones I found were anything like portable. So I was going to have to bite the bullet and try and work it out for myself.
How much power?
I needed to work out how much power The Red Tin and the Sound System would use over two or three hours and what kind of charge I could expect from a solar panel in a typical British summer.
I have the data sheet from the UDOO x86 — the single board mini computer that powers the Red Tin — that includes figures on power consumption. The same goes for the USB Sound card that takes a separate 12V input, rather than being powered via USB. The screen I fitted suggests it needs a minimum 2 Amp power supply but provides no other information. Also in the tin are some LED lights, the NuTube pre-amp and a fan, all of which should consume minimal power.
Then there is the sound system. Again there are data sheets for the amps so I could have a stab at estimating their power consumption.
Having said all that I thought it would be wise to get some proper measurements, so I know what I am dealing with and do not end up with something that will provide power for 30 minutes and then die just as the party is getting started.
The Red Tin was connected to a 12-volt power supply and a bench digital multimeter to get accurate readings. I started up the computer then started the operating system. The Red Tin is dual boot Ubuntu and Windows 10 and I decided to boot into the latter as I thought, if anything, it would be more power hungry and I wanted to get a “worst case scenario” of the power consumption. I could immediately see a spike in the power use when I started the OS and again when I launched the DJ software. I put the system through its paces loading and playing multiple tracks of everything from Country to Punk and Dance Music. The power consumption averaged between 1.5 Amps and peaking at about 2 Amp.
I repeated the process with the sound system connected and this time measured the power consumption for the amps. This gave an average reading of about 500 mA, peaking at 1 Amp for bass-heavy dance music played at approaching full volume.
Given the above observations, I reckoned if I used 2.5 Amps in my calculations this would give a good conservative figure. This meant that a battery with a 12 Amp Hours (Ah) rating should give me just under 5 hours of playing time.
Choosing a solar panel
I also needed to be sure that whatever solar panel I selected would be capable of charging the batteries during an average summer day. The consensus seems to vary between 4 and 6 hours of sunshine for an average British Summer’s day, so it seems sensible to opt for 5 as a midpoint estimate when doing my calculations.
I learned that all panel ratings are performed under test conditions of 1000W/m sq and 25C. Although all 10W panels will give 10W per hour under these test conditions, they will not all perform the same under other conditions. If I was going to use 5 hours of sunshine a day as my guide, a 10W panel should produce about 50 watt-hours a day in admittedly favourable conditions.
Once I started looking at solar panels I realised I was also going to need a solar charge controller or regulator as they are sometimes called.
A controller has a number of functions: it prevents the batteries overcharging if the panels generate too much voltage, which would seriously damage the batteries, or even cause them to explode; the controller also prevents current from flowing back to the panel from the battery once it gets too dark for the panel to generate power.
Having researched all the options and calculated as best I could, given the unpredictability of the British weather and the kind of power source I was going to need, I found a folding solar panel that was designed to be portable and had the advantage of having a built-in controller. (888-3695). This certainly looked like a good option.
In the next post, I will be looking at putting the system together as a rugged, portable unit. I may be too late for this summer’s music festivals, but we recently had the first planning meeting for the festival stage I am helping to put on next year, so my head is in festival planning mode already.
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Well the simplest way off the top of my little brain is if the use any battery powered power tools that use 18v packs just use as them in parallel to get you capacity and regulate the voltage to 12v. Many are 4 Amp hr packs and recharge quickly. Can always rebuild the packs with better 18650 cells. Or build your own custom pack and use a TI, Maximum or other multi cell battery charging / monitor IC hooked up to solar panel or other power supply. Quick and easy with few parts. Just a thought
Well said Fluffy! - I was going to suggest exactly the same thing!
I would also recommend considering a LiFePO4 battery pack.
Lithium batteries generally weigh between 1/3 and 1/2 for the same usable capacity and can last up to 10 times as long when used as a deep-cycle battery. If you compare cost vs benefit for both chemistries - LiFePO4 is the winner by far!
As Fluffy 1 reported - a lead-acid battery should never be discharged beyond 50% depth of discharge, but LiFePO4 cells can be happily discarged to 80% so if you aim for a 10 Ah pack then you need at least a 20 Ah lead-acid or a 12 Ah Lithium!
If I may be so bold. You need to look at your power calculations again.
Lead Acid batteries cant be called upon to produce their full AH capacity. In other words you cant use 100% of the available energy.
Only 20% of the energy is the norm, pushing it to 50% occasionally. I suggest you double the size of the battery and the Solar Panel.
Also instant current readings are notoriously unreliable, especially for something like an audio system. Meter response and specifically the meters internal A/D sampling rate can miss a significant, if not nearly all the current spikes. A better method would be to measure total consumption over a long period like an hour, even better 5 hours if that is what your target it.