Has it been a year already?
When we bought our bus last summer it had no house battery bank or inverter installed – critical features of utmost importance for two Technomads who depend on ample electricity for both our work and social lives.
We knew this was an area we needed to immediately address if we wanted any sort of autonomy on the road other than strictly living pole-to-pole. And with a week in the desert without any access to hookups camped at Burning Man looming ahead of us, we knew we had to figure out our electrical plans quickly.
The safe, simple, and sane option would have been buying a moderately sized bank of AGM batteries and a basic run-of-the-mill Inverter/Charger.
But, what’s the fun in that?
Instead – we jumped into the deep end of battery geekery, and decided to go lithium.
(Specifically – we went Lithium Iron Phosphate aka LFP aka LiFePO4)
But in the RV market (as opposed to the more common electrical vehicle – EV – applications), lithium remains a fairly new experimental playground. Relatively few other pioneers have begun playing with this technology for building house battery banks.
And the upfront costs of going lithium are substantial – even building our own battery bank from raw cells would cost more than going with quality well-proven AGM batteries.
But yet over time, with a little faith in the potential, the cost analysis of a lithium battery bank actually starts to look pretty darn good.
And after all, if faced with a choice between cool next-generation unproven technology or a well known traditional choice, what else should self respecting Technomads pick???
So we decided to go for it, and after a mad dash all-night installation push we managed to get our new batteries and inverter installed just in time for Burning Man last year, with only minutes to spare.
And, amazingly, everything worked.
Not only worked for our bus, but for powering a friend’s bus with a failing generator at Burning Man too!
And with only a few hiccups along the way, things are still purring along nicely a year later.
But, a year later, do we still think that it was really worth it to spend nearly as much on an electrical system as we did on buying our bus in the first place?
How has our first year on lithium been? Would we do it again?
Have there been challenges? What have we learned along the way?
With a year’s experience under our belt, here’s some of what we’ve learned…
Getting Charged Up
A 12V lithium battery is made up of four cells in series, with a nominal cell voltage of 3.2+ volts/cell. This is in contrast to a common lead acid battery, which is crafted from six 2.1 volt cells. The end result of these cells in series is that a fully charged lithium battery has a higher resting voltage (13.4V) than a lead acid battery (12.6V), but the two battery chemistries are close enough in operating voltage to be mostly compatible.
One of the challenging differences is that lithium batteries ideally prefer a slightly different charging profile than commonly found in chargers designed for lead acid batteries – and only a couple inverters or chargers have a pre-programmed “Lithium” setting yet.
Instead, there are usually just the traditional presets for Flooded, AGM, and Gel.
You can probably get away with using one of these presets (and some lithium battery makers sell batteries intended to be “drop in” replacements for regular AGM’s), but one of the primary reasons we went with our fancy Victron inverter and control panel is that it let us manually set a charging profile for our batteries – disabling temperature compensation, setting a target charge voltage, and letting us play around to find an ideal float voltage. It even let us set very low charge currents for our balancing experiments. (See below for more on balancing.)
But even the Victron proved slightly less than ideal when it came to charging. LFP batteries can be bulk charged to nearly 100% capacity and do not need an extended absorption phase, but even the Victron would not let us set an absorption time to anything less than 1 hour.
Still that was close enough to ideal that we’ve been able to come up with a charge profile that has been working reliably for the past year.
Our current charge settings are bulk / absorption charge to 14.2V, absorb for 1 hour (we can’t lower this), and then float at 13.6V. We have temperature compensation, equalization, and battery “storage mode” features disabled as well.
The Victron charger is rated at 120A charge current, but we’ve never managed more than 111A out of it. With that we can usually fully charge mostly depleted batteries in just 3 or 4 hours of generator time, and there is no need for extended run-time waiting for a slow absorption phase to finish topping off the final 20% of the batteries.
When we do add a solar controller to the mix, we will also be sure to seek out a fully programmable one so that we can make sure that it matches our charge parameters well.
One other note – I’ve noticed that the batteries do not get roasting hot when they are charging at this rate, thanks to LFP’s low internal resistance.
Overall – charging hasn’t proven to be much of a problem at all.
Four Cells? Or Twenty? If you look into our battery bay, you will see twenty blue boxes (roughly the size of cereal boxes) – all strapped and bolted together into a single large (but light 140lbs!) slab. Each of these boxes is a 3.2V 100Ah LFP battery cell from Elite Power Solutions.
These cells initially came bolted together as a 4-pack, thus forming a single 100Ah 12.8V battery.
To build our desired 500Ah 12.8V battery bank, we disassembled five of these batteries, and reassembled four banks of five modules wired in parallel to build several larger 500Ah 3.2V cells.
Then we chained four of these “cells” in series to make our battery bank.
We did it this way to give the EMS one battery with four cells to monitor. If we hadn’t constructed this single large slab, we would have needed a separate EMS and set of sense boards for each of the five separate batteries.
This is how we can simultaneously have both twenty and just four cells in our system.
EMS Goodness & Glitches
Though it is not strictly required, it is generally considered a good idea to pair a lithium battery bank with some sort of an energy management system (EMS) to protect the expensive battery bank from accidentally being over-charged or over-discharged, both of which can cause permanent and maybe even unrecoverable damage to LFP batteries.
The EMS measures the voltage across the entire battery pack, as well as across each of the cells. It also measures the individual cell temperatures, the overall battery state of charge (as a percentage), and the charge/discharge current. All of this information can be displayed via a composite TV-out cable that we either hook up to our backup camera monitor, or display on our computer monitor via a video capture card.
The EMS has two alarm wires that can be triggered – the high voltage alarm triggers if any individual cell reads over 3.7V, and the low voltage alarm triggers if any cell drops below 2.8V. And both alarms trigger if there is some sort of runaway high temperature situation.
These alarm wires were initially connected to two Tyco EV200 Contactors that could completely disconnect the battery in the event of an alarm situation, saving the battery from harm. I’ve now modified the system so that one contactor works as a battery disconnect wired up to the alarms, and the other can be triggered to bridge in current from the bus’s engine alternator to charge our batteries and/or run a roof AC while we are underway.
The EMS has done its job admirably protecting our batteries (especially the one time we left our bus for several days to attend to a family medical situation, and the shore power breaker tripped when our driveway surfing host plugged in a space heater inside on the same circuit!), but the EMS hasn’t been without issues.
For one – all the settings are programmable, but only by sending the EMS back to Elite Power Solutions. I wish there was a way that a user could adjust the alarm triggers or installed battery capacity more easily. As you will see in the section on balancing below, this resulted in a bit of a headache for us.
I also discovered that overall EMS pack voltage readout always reads roughly 0.36V higher than it should – a small issue that in cooperation with Elite we discovered was because the EMS had initially been calibrated for a much higher voltage battery backs (96V or higher is common in electric vehicles) than the 12.8V target nominal voltage in our system.
The EMS UI also has a few display bugs – all of which have reportedly been fixed now.
At some point we plan to send our EMS back in to Elite for reprogramming and upgrading the internal software, hopefully solving all of these issues.
One areas where the EMS became a problem was in initially balancing our battery cells.
In a battery – ideally all the cells should be balanced, charged with exactly the same voltage and having an identical capacity. Usually when a battery “dies” it is because a single cell has gone bad and is no longer a match for the others.
Theoretically, when we purchased our cells they were all supposed to be matched and pre-balanced, but we did not have time to use Elite’s test equipment to confirm this when we first worked with them to build our battery bank.
When I finally did have time to do some testing of my own, I discovered that we were getting substantially less capacity than I expected as a result of the cells being a bit out of balance. As a result, our 500Ah battery was only able to deliver 420Ah before triggering the low voltage cutoff in the EMS system, shutting the batteries down.
The EMS sense boards on each cell have a built in balancing shunt circuit that was supposed to make balancing our cells easy. The shunt engages when the cell reaches 3.7V, and the green light on top of the sense board turns red as a 1/2A balancing load is placed upon the cell.
Assuming you are charging slowly enough so as not to overwhelm the shunts (and this is another place the Victron’s programability comes in handy) you can very slowly and carefully charge your battery bank to 14.8V (3.7V x 4) and the shunts will hold down the out of balance high cells until all the cells are balanced and equal.
This is how it was supposed to work.
But every time I tried, the EMS would end up triggering a high-voltage alert and disconnect the battery after 30 seconds. Often it seemed as if the EMS was disconnecting the batteries before the shunt even began to do any work.
I eventually traced down the problem — the EMS was programmed to disconnect the battery 30 seconds after it detected a 3.7V reading on any cell. But the voltage metering of the cells was off slightly – the EMS would register a cell as being 0.03V – 0.05V higher than they actually were. A trivial amount, but it caused the EMS to disconnect the battery when a cell reached 3.65V, before the balancing shunt ever had a chance to engage at 3.7V.
Elite recognized the problem, and they have since redesigned the shunts to trigger at 3.55V, and for applications like ours they now recommend changing the EMS over-voltage alarm to trigger at 3.8V. This should give ample enough headroom for balancing to occur without the EMS disconnecting the entire bank.
With the problem identified (and not wanting to deal with sending the EMS back to Elite), I set about to manually balance our cells by using alligator clips and an incandescent 12V bulb. I used the bulb to put a load on the high cells, and over the course of several days of trial and error I was eventually able to manually bring the cells into perfect balance.
It was annoying – but also educational.
And now a year later, I’ve just run through a series of load tests and manually tested the cell balance with a multimeter at various states of charge, and it looks like our cells haven’t drifted out of balance at all in the past 11 months.
Capacity: You Can Never Have Too Much Juice
An RV power system requires balancing three variables – storage capacity, power consumption, and power generation.
In our past home-on-wheels, we achieved an elegant balance having just enough solar and storage capacity combined with an extreme focus on energy efficiency such that we barely ever even needed to think about running a generator unless we absolutely needed some air conditioning.
We aim to eventually reach a similar balance point with our bus, but at the moment we still have a long way to go before we reach anything close to an elegant energy balance – we can only tackle so many bus projects at a time!
On the generation side, we still plan to install 500W – 1000W of solar on the roof, ideally as much as we can manage while preserving Zephyr’s aesthetics and elegant curvy lines. This much generation capacity should keep us from needing to run a noisy and unpleasant generator very often. We’re still deep in research on what our ideal choice for parts are, and will of course share that project as it progresses.
We also aim to work on enhancing (or even replacing) our generator to make it much less noisy and unpleasant, for the times when we do need to use it. No amount of solar will ever be able to keep up with air conditioning demands.
On the conservation side – we intend to soon replace our interior lighting with LED’s, eliminating one of our most substantial loads. We are also researching more power efficient air conditioning units (look for a future article on this topic), and we have replaced our propane traditional RV fridge (horribly inefficient when run on electricity) with an energy efficient marine-style Vitrifrigo 12V fridge.
As for capacity…
When we set out building the system last year the goal was to have enough battery capacity to run a roof AC (and all our other onboard systems) for “almost 3 hours on a charge”, allowing us to run errands on hot summer days while leaving the cat basking in cool comfort inside.
Yes, it is always all about the cat.
And the system has admirably lived up to calculated expectations – just this week I did a capacity test and managed 2hrs 42min of runtime with the AC on max-cool with high fans, and with all our other usual computers and lights on.
With the fans lower and especially with the thermostat turned down to allow the compressor to cycle, we would get substantially more runtime.
I have the inverter programmed to cut off before the EMS low-voltage alarm triggers to ensure battery longevity (even lithiums don’t like going too far below 80% drained too often) and because I have the EMS set up to require a manual reset if the low-voltage alarm is triggered (to make sure that if there is a problem, we know about it). In this test, the battery was at 16% capacity and 424Ah had been consumed when the inverter shut down. The overall bank voltage was still at 12.52V, well above the EMS warning line.
Ultimately, we know that we want more than 500Ah of battery on board – we just didn’t want to invest that much upfront last year until we had some chance to test out the technology further.
But now that we are sufficiently happy with how lithium has performed, we are ready to invest in more.
With a 1000 – 1200Ah battery bank (particularly coupled with a more energy efficient AC unit) we should have enough battery capacity to run the air conditioning over night when we are traveling and boondocking places where a generator would be frowned upon. This would also give us more days of autonomy with non AC loads, because even once we have solar there will still be days with minimal sun.
To upgrade our system – we would either expand our current battery bank to double the capacity, or if we find the right home for our current bank we would be open to selling it and starting from scratch. Though LFP batteries are theoretically OK with mixing older and new cells (better than lead acid, anyway), it would still be ideal to try and keep all the cells identically aged.
So… If you are interested in a totally awesome, well tested and fully functioning 500Ah lithium battery bank sometime in the next year, let us know. *grin*
Boosting Awesomeness & Inverter Oddities
We’ve only had an opportunity to live fully off-grid on a handful of occasions in the past year, so our batteries haven’t had too many deep cycles to really stress test them.
But where we have gotten plenty of use out of our electrical system has been using the boosting power of our inverter to make driveway surfing easy, allowing us to without worry run heavy loads like air conditioning or microwaves on dinky 15A circuits. Using our inverter’s power boosting feature to combine battery capacity with shore power allows us to stay places comfortably that would otherwise be near impossible.
Being able to limit shore current demands and boost when necessary are such incredibly useful features that I would never want to own an inverter that lacks these features.
Unfortunately, there aren’t very many inverters that can do this. Our Victron MultiPlus 3000VA is one of the only options out there.
After a year of heavy use, we remain mostly impressed with it – the overall most annoying issues we’ve discovered (discussed in this other post) is that the shore current limit can’t be set below 15A.
I’ve also noticed that the inverter is incapable of giving “just a little boost”, and this wastes energy and needlessly drains battery capacity.
For example, often we only need an extra amp or two of AC power (commonly needing a total draw of 17A with a 15A shore limit), but when the Victron’s Power Boost engages it drops the shore power load to 10A and makes up the entire difference off of the batteries, drawing 50A of DC and draining more capacity than would otherwise be needed.
One lithium-related area that is most annoying is that our Victron constantly thinks that it is suffering from a “low battery” condition when used with our lithium bank. This low battery warning is triggered when the inverter senses a battery voltage 1V above the programmed low voltage cutoff, but since lithium sags so little as it is being drained the inverter essentially always thinks that it is low when it is in use – even though the Victron battery monitor shows 90%+ capacity remaining.
The Victron battery monitor also has its share of lithium-unfriendly features. For example – it handily calculates a “time remaining” display to let you know how much longer you can keep running the current load — before the battery reaches 50% capacity. This is a fine target cutoff point for lead-acid, but I wish I could program this calculation to target our preferred 80% capacity cutoff point.
Alas, this is not programable.
Another battery monitoring issue I am trying to understand is that the battery monitor seems to incorrectly calculate its percentage remaining – overestimating how much capacity remains. The monitor knows we have a 500Ah battery, but as an example – during my last capacity drain test the Victron battery monitor indicated at one point that -280.6Ah had been consumed, but that a battery percentage remaining was 52%. I’m not sure how they can determine that a battery that has been 58% drained has 52% remaining – I am still hoping to get an answer from Victron support explaining this.
One final issue that we’ve discovered with the Victron – when it overloads (for example – we accidentally try and start two AC units at one time) the inverter is supposed to auto-reset when the error condition goes away, and sometimes it does. But other times, it fails to auto-reset and must manually be switched on/off in the battery bay to get it to restart.
Annoying, but fortunately a rarely encountered problem.
One upgrade to our electrical system I completed earlier this year (just before we set off from Florida to spend the summer roaming) was setting up a bridge between our bus alternator and the battery / inverter – primarily so that we can run the roof AC while underway without constantly needing to run our annoying generator.
We probably wouldn’t have survived driving in the summer without this bridge in place.
Unfortunately though – the bus alternator at our typical driving RPM’s runs roughly just a tad below break even with the demands of the inverter and roof air conditioner while underway, meaning that if we run the AC full blast we can reach our destination with a partially depleted battery after a day of driving.
If on the other hand we can make due without full AC, the alternator is plenty hefty enough to fully charge our battery after just a couple hours on the road. This comes in quite handy if we’re planning to dry camp overnight, we arrive with a fully charged battery and know it will be refilled quickly when we hit the road the next day. This has been fitting nicely with our goal of only driving a couple hours a day.
Ultimately I’d like to increase our engine power generation capability and/or switch to a more energy efficient roof AC unit so that we always run at a net positive.
Any Other Annoyances?
Setting up a battery system like ours may sound like a lot of intimidating work, but it really hasn’t been too hard at all.
And the best part – once it is built and configured (and balanced!) it for the most part has “just worked” and not been an issue at all.
It is nice having an electrical system that we feel as if we can rely on.
Lately, the biggest chore involving the battery system has been the need to constantly hoist my toolbox and parts bins out of the way to show our system off… *grin*
Speaking of which, we recently gave a talk at a bus rally about our Lithium battery bank and streamed it live. You can catch a recording of that recording by clicking here if you want to go over the basics covered in our previous articles.
Worth It? So far – yes!
The real payoff from a lithium battery bank comes over time, and ideally this system is designed to last for years (decades even) of trouble-free service. Over the past year lead battery costs have gone up (by about 10% it seems) but the price of LFP batteries have remained constant, making the math even more appealing if we were starting out now.
So we’d certainly still go lithium if we were starting again from scratch.
Ask us again next year. And the year after.
But, I somehow doubt we’d change our mind.
Not unless a battery technology even cooler comes along that is!
By the way, we should note – we are not trying to motivate anyone to follow us on this path. We are not selling these batteries, we are not affiliates with any battery dealer, we paid for all our components and nor do we have any financial stake in the technology beyond our own systems. We are simply full time RVing technomads who are designing our own cutting edge home & office on wheels, and are sharing our research & project. Of course we’d love to have more folks out there pioneering and helping us take the arrows in our backs. Right now, we do not consider this technology ready for the mainstream, and those contemplating this technology need to be a bit savvy with electrical and battery technology.
Other Posts In this Series:
Promise of Lithium #1: Lead Acid Battery Downsides
Promise of Lithium #2: Lithium Ion Battery Advantages
Promise of Lithium #3: Lithium RV Battery System Cost Analysis
Boosted Electrons = Better Views
All our Lithium Ion Battery Posts