Please note, this is a running document, as long as I'm able to update it, and still need to, so I don't expect to type out the entire thing in one sitting. Buckle up and enjoy the ride! Part 1:
History
I've wanted to build a grab-n-go portable battery box for many years, and I'm going to jump into that pool with both feet, explaining my choices of why I make them, etc... I invite anyone who's curious to join in, and follow along.
Ever since I saw Jehu Garcia's YouTube video 7+ years ago, although I was very critical of that original box in the video, I'd wanted to make my own & do better. Been a fan of his work since long before then, and he'd done better battery work than that one. For all of it's shortcomings, it was one of the pioneers in showing the world that they could do that sort of thing. It was inspirational and lots of folks have posted videos showing their own twist in building such since!
His inspiration for building it was from the Inergy Kodiac, (which has since been discontinued).
The Kodiac specs were loosely what the PEP BB for building a
"Charge and Carry" lithium battery power box are based on, and I say "loosely", because the BB requirements are both missing some specs and have at least one spec wrong. They say:
To complete this BB, the minimum requirements are:
- 1100 watt hour capacity
- 1500 watt inverter
- reasonably water resistant case
- One
220VAC 30 amp RV outlet -
There is NO such thing!*
- Four 110VAC 15 amp outlets
- Four USB plugs
- Two 12V sockets
- Power meter with display
- Solar input plugs
- Plug to charge the unit from 110V or 220V
What they don't say, is that the solar charge controller in the Kodiac is a 600 watt charge controller, which could become significant if one wants to use this box more than 1 day in a row. Jehu's box only had a 300 watt charge controller (built-in to the inverter), so that was even worse.
*The 30-amp RV outlet is a 120 VAC outlet, NOT a 240 VAC outlet. The 50-amp RV outlet is 240 VAC, but the NEMA TT-30R is the outlet in question (TT stands for travel trailer), and it bears a resemblance to the obsolete NEMA 10-30R outlet, which WAS a 240 VAC outlet, and sometimes folks might wire them wrong because of the confusion about this.
This is all well and good, but maybe you're not trying to build this exact thing, or maybe your needs/wants/desires are different? How to build the system of YOUR preference starts in the design phase.
Design phase
Where to start? Start with the end in mind! Figure out what you want to do, so you can tell when you've accomplished it. If you have a specific goal or set of goals, figure out what it will take to power those goals, and work backwards from there.
Going the other way is prone to wasting money, resources and time, only to find that what you built won't do what you really want it to have done in the first place. If your goal is just to meet the BB for this, your end result will look VERY different than it would if your goal was to be able to power your refrigerator for an indefinite time period, or be able to run a circular saw building furniture at a cabin, etc...
Along the way, there will be a few major design choices that will need to be made, because they will either restrict your options going forward but save money immediately, or allow for other choices, but likely cost more for that luxury.
One of the early choices is what system voltage to run at. This is not necessarily the same as the output voltages, for example, one can have a 25.6 V system voltage, and still produce 12 volts for 12 V outlets. The Kodiak actually had a system voltage of 11.1 volts... Common options are normally "12 Volt", "24 Volt", or "48 Volt", with pros and cons for each of them, plus if being able to generate 120 Vac (or 230 Vac) is a goal, the system voltage will lock you into which inverters you can use, and thereafter, it can't be changed without replacing the inverter and probably wasting the money spent on the first inverter...
You don't have to make the choice first, just be aware that the choice is coming, and it will lock the build into that path once you do. Keep this in mind when shopping for inverters, so it can help you evaluate your choices.
Choosing an inverter (or so many options, how to choose?)
As alluded to before, inverters will have an input voltage (range) that ties them to the system voltage, but they also have other choices of options. Modified sine wave or pure sine wave? Pure sine wave output more closely resembles what utility power looks like, while modified sine wave will look closer to a square wave pattern, if you look at them on an oscilloscope. More importantly, the modified sine wave is electrically noisy, and can cause problems with sensitive electronics, or in some cases, won't even run them at all. But it's cheaper... Stick to the pure sine wave whenever there's a choice and the budget will allow it, as there are lots of folks who end up wishing they'd bought the pure sine wave versions, or end up switching them out for such.
The other main choice one will have with inverters, is the power output rating. This is where the end goal comes home to roost, because choosing too small here means you can't run whatever device you were hoping to run! Keep in mind, more power = more expensive, and also more power output = more power input, so as that scales upwards, the system voltages will need to go up too. Buying a really huge inverter will force you to also buy huge battery power to run it, so exercise some caution and restraint, as needed
When I sat down with my wife and chatted about "What would we ever want to use something like this for?", in light of our planned trip to WL, she first said "Lights!", which can be done pretty easily, and on a low power budget. The next thing, was refrigeration, since a week of camping would go better for us, if we had a way to keep food cold. Fine - there are several manufacturers of electric refrigerator coolers. Looking at the specs of the BougeRV 30 L one, it's reported to draw an average of 55 watts, although when cooling down initially, can go upwards of 100 watts. Charging phones & tablets made the list, as well as a truly luxury cooking item - a sous vide cooker (1000 watts?). Don't know how close I'll get to that, but lets clarify that and reduce it to hard numbers.
I estimate about 50 watts for LED lighting (aim high), and 8 hrs of use, that's 400 watt-hrs for lighting.
Charging electronics? Both of our phones can likely stay charged for 50 watt-hrs /day, but the tablets will be much greater power draw. I'm going to guess 500 watt-hrs/day to start, and then check it with a Kill-o-watt meter later.
Let's start with 550 watt-hrs for electronics (for now)
Refrigeration - If the 30 L refrigerator used 100 watts all the time, that would add up to 2400 watt-hrs/day. It won't be that high, but aim high for calculations, just in case.
That brings the total up to 3 Kwh/day, without the fancy cooking gear for total power usage! I can surely build/bring a box that can handle that amount of power for a day, but it's going to be tough to be able to bring that amount of solar to recharge 3 Kwh/day, minus inefficiency losses (maybe 20%?)...
That brings us to a sidebar interruption to our thought process - charging capacity! If we were to bring a 200 watt solar panel, and we got the functional equivalent of 4 hrs of sun each day, that would only allow us to recharge 800 watt-hrs of power, and using more than 3.6 Kwh/day isn't sustainable. We'd need to have at least 5 of those panels, or have more than 4 hrs of functional daylight, just to keep up, and that still doesn't include the fancy cooking gear.
About that fancy cooking gear... Not sure if we'll end up pulling it off, but it does highlight another inverter (and battery) criteria that explains why it matters. Peak power demand matters. If that were used at the same time as everything else, the peak power requirement from our inverter would be likely 1000 watts, but the peak power demand from the battery system will be 1100 watts, for the inverter, 100 watts for the refrigerator, 50 watts for lighting, 50 watts for phone charging, and 100 watts for tablet charging, totaling 1400 watts...
Not only does the inverter have to be able to supply 1000 watts (normally pretty easy to get that), the battery system will need to provide nearly half again as much, but at a much lower voltage, so for a 12.8 Vdc LiFePO4 battery, that comes out to about 110 Amps. The system battery has to have enough current capacity to source that, hopefully without exceeding the 1C rate, to keep the batteries healthy longer. For this example, a 100 Ah battery bank will be too small!
That brings us back around to the system voltage, because if this were a 24 Vdc system, the numbers would be much more favorable, because at that same power output, the demand from the battery would be only about 55 amps. As an added bonus, the 24 Vdc system can use smaller wires for battery power, and lose less heat from resistance losses in the wires (which also eats up efficiency, so I'd stay with as large a wire size as will fit). For a LiFePO4 battery, that means cells in groups of 8, which is likely to increase the size and weight of such a system, but hopefully not so much as to make it impractical, but that depends on the choice of battery.
A 48 Vdc system will only need to supply about 28 amps, although the need to carry batteries with cells in groups of 16 starts to off-set the portability of it all...
If we add the fancy cooking gear, with a budget of using it for only 2 hrs/day, our daily power needs go up to 5 Kwh/day, plus efficiency losses. or 6 Kwh, but that also drives up the solar requirement to 8 panels, to be able to charge enough to sustain it.
In any case, that shows how there's a careful dance with capacity vs. weight/size. I don't actually know which choice I'm going to make right now, as I should probably chat with my wife to find out how badly she'll want to have the fancy cooking gear in use - while Harbor Freight still has the portable 200 watt panels on sale for the next 2 days...
I'll return to discuss choice of batteries in the next part.
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