Updated: Mar 21, 2021
Part One: Researching the options. How big? How much? Lithium or AGM?
Old battery system
Our Cub Escape camper trailer came fitted with a 100Ah AGM deep cycle battery, a Projecta 240V AC charger, and an Anderson plug to connect to the vehicle alternator. Supplemented by our 200W solar blanket, this system has worked well for us for the five years we've owned it.
But the AGM battery is starting to show signs of age, going flat in under a day unless augmented with solar and not holding a charge very well in the workshop. Rather than just replace it one-for-one with another AGM battery I took the opportunity to re-assess our electrical needs for sustained off-grid use.
Working out our electrical needs
The first step to any change to your electrical system ought to be a thorough assessment of what you actually need. There's no point in dropping huge dollars on a system that vastly exceeds your requirements, or saving money on one that doesn't.
Before you buy anything or even look at options, you need to consider:
what you need your electrical system to do
how much power (battery capacity) you need
the space and weight you have available for the battery
how you plan to charge the battery
What do I need it to do?
I want my battery system to be able to do two main jobs: camp for 48 hours off the grid (a typical Friday night to Sunday arvo camping trip), and support our long range expeditions which are typically driving from camp to camp every day - perhaps six hours of total driving (with breaks), and sixteen hours stopped (say, from 4pm to 8am).
While I will use solar to supplement the system, I don't want to rely on it, because on a long trip there are plenty of cloudy days and shady campsites to deal with.
My system needs to run a fridge, power the water pump, run or recharge our camp lights, recharge two phones, run two small 12V fans on hot nights, and recharge various other device batteries (cameras, drone, laptop, iPad, GPS etc) as required. It also needs to charge effectively from my vehicle, solar, or 240V AC mains power.
The worst case scenario would camping for 48 hours, then driving for a day, then camping for another two nights. In that situation the battery needs to completely recharge in one day of driving.
How much capacity?
Having defined my mission, now I need some specifics. How much power will I use in 48 hours?
To gather this information, I wrote down everything I could imagine having to power or recharge while camping. Next, I used the specifications in the manual or on the label, or derived through my own previous measurements, to record power consumption for each device in Watts.
Note: In most cases, the maximum power figure in the device specifications will overestimate your power consumption - for example my fridge claims it will draw 70W, but that might only be for a few minutes in any 24 hour period. On average, like most 12V fridges, it only draws about 15W. Bear that in mind when considering intermittent loads like fridges, water pumps, or those suddenly-popular 12V ovens.
Next I worked out how many hours per day my devices would run. For a fridge, it's obviously 24. But my 5W phone chargers? They typically only take two hours per phone. Times two phones, that's four hours per day. Multiply Watts by running hours to calculate Watt-hours - those 5W phone chargers running for four hours therefore use 20Wh (5Wx4h). I did this for all of my electrical devices and came up with a total of 1,400W per 24 hours.
1400Wh is about 110Ah, which explains why my old 100Ah battery is beginning to fail: lead acid batteries don't like being discharged below about 50% capacity, and I'm demanding up to 110%! How is that possible? Well, I don't always run or charge literally every device I have every day, so in fact my true average usage will be closer to half that 110Ah figure. This still draws more out of the AGM battery than it likes in a perfect world, but it's OK.
So, my battery bank capacity for 48 hours needs to be 220Ah, or 2800Wh.
Space and weight constraints
My Cub trailer stores the battery under the bed, directly over the axle. This is a great spot for it as, provided the battery is less than 325mm tall once installed, it means I have virtually unlimited physical space for it. Not all installations are so lucky - in many cases you'll be dealing with a storage box or locker with quite tight dimensions.
However, the Cub's cargo payload isn't great, and the huge storage volume means it's tempting to overload it. I haven't weighed the existing battery, but it's likely to weigh between 20-25kg depending how ruggedly it's been made. I'd like to match that weight as closely as possible to avoid overloading the trailer.
This is where it starts to get interesting. A lot of people stop their planning when they've worked out how much battery they need and what space they have available to fit it. Just buy the biggest DC-DC charger they can find and call it done. But will that actually work?
Because I'm installing the battery in a trailer, it's a looooong way from the vehicle alternator. The total return wire run is around 15m. It could be more. And while I can control the size and capacity of the cables on the vehicle side (because I haven't installed them on the 200 yet!), I have to take it on faith that the wiring in the Cub is reasonable. It's mostly hidden inside the chassis members and concealed behind bodywork, but it looks to be 8 gauge to me. And it joins to the vehicle through a 50A Anderson plug.
A quick look at a 12V wiring chart shows that any hope of running a 50A DC-DC charger back there is out of the question. The most popular 50A DC charger on the market draws up to 80A, and even if the wiring allowed that (it doesn't), the Anderson plug won't. Best case, the 50A charger will sense the voltage drop and reduce its output, in which case I could have just bought a smaller, cheaper charger. Worst case, it will try and suck down 80A until either a fuse blows or there's an electrical fire.
So, the most powerful DC charger I think is sensible to use is 25-30A.
A good lead-acid battery has a charging efficiency around 70% - that is, for every 100Wh applied by the charger, the battery will restore 70Wh of capacity. Many lithium battery suppliers are a bit coy about their charging efficiency figures, but they are better than lead-acid. Those that are willing to quote numbers usually nominate low-to-mid 90%, so I will assume 90% for planning. It's actually a bit more complex than this and will depend on where in the charging cycle you are - in both types of battery, charging up to nearly full is much faster than the final little bit. But it's a reasonable number to plan around.
If I drive for six hours per day, a 25A charger should therefore deliver somewhere around 105Ah to a lead-acid battery and 135Ah to a LFP battery. Probably less, since in real life I'm going to stop the engine and interrupt the charge at least once during the day even if I do six hours of total running time.
You can already see there is a problem here - I wanted to put 220Ah back into the battery after 48 hours, and I'm only doing half that. If I use a lead-acid battery, I'm not even covering 24 hours of usage, let alone 48 hours! This is where many people go wrong in their system design.
So... what now?
I have several options to deal with this charging shortfall. Firstly, we could use less power - don't underestimate that as an option! Secondly, we could turn this moderate job into a major one and substantially upgrade the wiring to the trailer, including use of high-current Anderson plugs. That would allow a higher capacity DC charger. Thirdly, we could use solar charging to supplement the DC charger.
I don't want to completely re-wire my trailer and I don't want to rely on solar, as I've described above. So that leaves using less power. But how?!
I started by being a bit more realistic on my power demands. Am I really going to charge my laptop twice in one weekend? And the drone? Probably not. And how many of the things I do intend to charge, actually have to be charged from the trailer, and not the second battery pack I intend to install in the car? The phones definitely, because we keep those by our bedsides, but the WiFi dongle? Can some of this charging wait two days until we're in the car and can charge from the alternator?
I also made the decision that, with the exception of the built-in interior light inside the trailer, all of our other camp lighting would be battery powered. Modern battery lanterns are incredibly bright, incredibly efficient, and can last for days (or even weeks) before a quick charge from USB. The main advantage is that battery lanterns effectively separate the time you use the light from the time it demands electrical power. By using them at night and then recharging them from the car later while driving, the electrical power for your camp lighting is basically free. I've bought a few different lanterns to see how they work, and will post a review later.
With these changes, I could bring our usage down to about 70Ah per day, or 140Ah over 48 hours. This is still a bit short of the 105-135Ah we're able to recharge in a day, so we need to be mindful that we can't do two 48-hour camps in a row unless we deploy the solar to supplement. But this is as good as I can make it.
Happy campers using one of our new USB rechargeable lanterns
My budget for this project is limited to $4,000, and I really hope to find some room in that for some new solar panels. I currently have a perfectly good AC charger, and a spare DC charger left over from my previous vehicle, but these are not suited to lithium batteries. So if I go lithium, I need to factor in the cost of replacing both chargers.
My budget is adequate, but not generous, for getting this project done with lithium and it's very generous for doing it with lead-acid.
Lithium vs AGM
It's tremendously popular right now to run off to the nearest 4WD store waving fists of dollar bills and shouting "lithium! lithium! lithium!" with no real idea why. Paid advertorials on your favourite 4WD TV show may have convinced you that lithium batteries are smaller, lighter, more powerful, and come with a free handjob. But lithium batteries are not really some magical unicorn product that will better your life in every way.
Lithium batteries - in this case, lithium iron phosphate, or LFP - are just 12V DC batteries that perform exactly the same role in your electrical system as the century-old lead-acid battery technology you're used to. They simply use a different set of chemicals inside the plastic case to generate that 12V, and like any technology, they come with their own sets of pros and cons.
I need 140Ah of capacity. Deep cycle lead-acid batteries don't like being drained below 50%, and LFP batteries don't like going below 20%. So I'd need a 280Ah lead-acid battery pack or a 175Ah LFP battery.
Size and weight
A good, 300Ah lead-acid battery (as close as I could get to 280Ah) is about twice the size of my current lead-acid battery, and over three times the weight (>70kg). A 200Ah LFP battery (as close as I could get to 175Ah) is about the same size as the 300Ah lead-acid, but exactly the same weight as my current 100Ah lead-acid (<25kg).
I have the room for the lead acid, but 70kg-plus is too heavy.
In my calculations above, I worked out that after a 48 hour camp, it will be touch-and-go recharging a lithium battery, and a lead-acid will fall short quite substantially thanks to its lower charging efficiency. That's already a win for lithium, but that same efficiency also means lithium responds better to imperfect solar conditions than does lead-acid.
The lithium battery is really the only suitable option when you consider the charging requirements I have.
A 200Ah LFP battery is worth $1,800-$2,500. A 300Ah lead-acid battery is worth $600-$900. Both will easily last 5 years. The LFP requires replacement AC and DC chargers, at a combined cost of about $1,000. The lead-acid does not require either charger.
The economics favour lead-acid, but can't overcome the weight penalty.
I had a good look around the lithium battery market, and in my opinion it's not yet a consumer-friendly, mature market with established quality and price standards. If you don't need a lithium battery, my suggestion is don't get one. If you can make lead-acid work for you, then I think the current state of the market means that you should. There's a real danger of buying a bad lithium battery, or getting ripped off, or both.
The problem is that few - possibly none - of the lithium battery brands on the market are actually battery manufacturers. A small number assemble their batteries from components to a bespoke design. Most seem to buy pre-assembled cells, but add their own battery management system and case. Quite a few buy in the whole thing drop-shipped from Alibaba, and all they had a hand in was the sticker on the box. None have yet established a dominant market position and none have yet earned a reputation as being substantially better quality than anything else.
I settled on four main brands I was comfortable with - Revolution Power, Deep Cycle Systems, Enerdrive, and Amptron. These are a mix of prismatic cell types and cylindrical cell types, which I once thought was important but now I suspect probably isn't. As long as they're proper LFP cells and not dozens of phone batteries taped together.
There are probably other brands that should be in this list, but these four are stocked by a reputable retailer I've dealt with before, so I'm happy that they would not carry them if they were beset with endless warranty claims.
Having narrowed it down to these four, I immediately eliminated one brand because their battery was a slimline design that didn't suit my installation space. Of the remaining three, I picked Enerdrive because they offered a package with matching Enerdrive AC and DC chargers, they have a reputation for good technical support, and their battery uniquely (in the ones I considered) had a reset button.
(Lithium battery management systems are designed to shut down when completely discharged, to protect the battery cells. When shut down, they will not accept a charge unless re-started by applying voltage across the terminals to 'wake' the BMS. The Enerdrive has a button to push instead.)
The whole pack, including the chargers and a tray, cost $3,200. I could have saved a couple of hundred by buying the battery, DC charger, and AC charger separately. But the Enerdrive units seem to be reasonably well-regarded, and the fact that the chargers and battery are the same brand means there is no chance I will find one component doesn't suit the others.
As I write this, the battery has arrived and the rest of the parts can't be far behind. I will post a second article about installing them, and then maybe a third after I give the system a test. Stay tuned.