Lithium Conversion…the nitty gritty

Thought I would share some of the analysis that supported our lithium conversion mod. It’s a long read…really long, but if you tough it out to the end, I think you will step away with a pretty good understanding of the lithium battery environment.

When our Trojan battery gave up the ghost, research into lithium batteries and supporting components went into high gear. There is a ton of great information out there, and I did eventually narrow my research down to a handful of sources which I decided seemed legit, and generally had their shit together. I’ll list these at the end.

One thing to keep in mind is that this is MY take on the whack of information out there that I used to form the scope of the conversion, and the charging parameters. You may well form a different view, and that’s cool…the key is knowledge, and putting it to use in your own mod.

Here is the link to the first post about the component installation mod:   https://routealto80.blogspot.com/2023/05/lithium-conversion.html

Capacity Needs

Before making a battery purchase, you should to take a good hard look at how you camp. Are you mostly boondocking or only occasionally? Are you running a 12v fridge, as that guy definitely eats up the amps. Do you run an inverter to power that coffee maker, or do you simply enjoy your morning joe from a simple V60 pour over? Are you working remotely, enjoy evening movies, or have a CBAP machine? All this factors into the capacity of the battery you may need. In the end though, it kinda boils down to a 100, 150 or 200 amp hour battery, or multiples thereof. If you really like to plug stuff in, get as large a capacity battery as your wallet will allow. If you are somewhat mindful of power consumption, then a 100ah or the new to the market 150ah “drop-in” batteries will be plenty.

If you do go with a single large capacity battery, or two 100Ah ones, you will also need to figure out where and how to house them. Once you start reconfiguring the tongue area, or a location inside, your mod will become more complex. Hence the beauty of a “drop-in”, if that will suit your needs.

The Battery

In terms of features, an internal battery management system (BMS) that provides low temperature cutoff is critical. Lithium cells do not like to be charged at 0C(32F) or below, so it is important that the BMS used by the manufacturer cuts off the charging amps at these temps. Early SC lithium battery installs did not appear to have this low temp protection, which led to the addition of that somewhat confusing big red battery cutoff that lurks near the electrical cubby. With the evolution of battery choices, and of SC’s lithium knowledge, I think the need for that cutoff handle has gone the way of the dodo. Any good quality battery manufacturer will utilize a robust BMS that will protect your investment from a number of conditions that could potentially damage the lithium cells inside. Bluetooth connectivity to the BMS is really a nice to have, as a good shunt based monitor will give you a lot of useful information.

Some batteries have built in heating pads to warm the cells enough to allow charging to re-start post low temp cutoff. Most of these configurations require shore power to run the heaters. If you have shore power though, you have 12 volts from the converter to the distribution panel, to run the Alto, and the BMS will prevent the battery from receiving the juice. Some batteries will self power the heating pads, but I find that concept at odds with itself. The battery self depletes to warm up enough to enable charging, only to self deplete again once the cell temperature drops. Now it may not be that simple…but perhaps it is. In any case, my view is that heating ability is a nice feature to have, especially if you are connected to shore power to run those pads. 

No real need to dive too deep into the type of lithium cells inside that battery case, unless you are building your own, but here are some basics. LiFePO4 cells, be it cylindrical, prismatic, or pouch style, have a working voltage of 2.5 to 3.65 volts per cell. That’s just the way this chemistry works. So for example, a battery marketed as a 12v 100Ah, will have series and parallel topologies of cells to achieve the desired specs. Simple electrical theory states that a series circuit adds voltages, while a parallel circuit will add the amps. Four cells of whatever style will be wired up in series to produce the required 12 volts (actually around 13.6v), and then groups of these four cells are wired up in parallel to achieve the specified capacity. The number of parallel cell groups required to achieve the specs depends on the Ah capacity of a single cell. So you could have one group of four 100Ah cells, or perhaps two groups of 50Ah cells. BattleBorn uses cylindrical cells, so tucked inside one of their batteries will be a whole bunch of groups all wired up in series/parallel to achieve that 100ah capacity.

Charging That Battery

Understanding how a lithium battery charges will help you get the most out of your investment. Tailoring the phase voltages to achieve a happy balance between charging efficiency and cycle longevity is a worthwhile analysis. The lithium chemistry accepts a charge much better than a flooded lead acid (FLA). A simple analogy is that it takes firehose force to push electrons into an FLA battery, where a lithium will fill itself nicely with just a garden hose. A subtle but big benefit for the recovery abilities of lithium.

The charge/discharge curve of a lithium battery is very flat, unlike an FLA, which is very much like a ski hill. Under charge, a lithium battery rises to a flat plateau quickly, then travels along that plateau with tiny changes to cell voltage, all while packing in those amps. This is why using voltage alone to estimate state of charge is even more difficult with lithium than with an FLA. More on that later.

As mentioned earlier, the cells inside the battery range in voltage from 2.5 to 3.65, from low to fully charged. When charging, the BMS takes the input voltage, and distributes it accordingly, based on an individual cells requirements. It also monitors the current and amps moving into the cells, and performs a process to balance the cell voltages. The BMS prevents the cells from being overcharged, or overdrawn, ensuring they operate within a variety preset limits to avoid any damage. All these BMS functions result in a stable battery that should provide years of reliable service.

Where the charge process gets interesting is the interpretation of just how to set these voltages to achieve a stress free charge. You are probably thinking…why are we now talking about individual cell voltages when I have a 12 volt system? In short, because this is how the BMS is looking at the cells, keeping them charged and balanced. Bear with me here. 

During my research I found a You Tube channel called Off Grid Garage. Andy did a bunch of charge/discharge testing with a single 100Ah prismatic cell, and I soon found it much easier to understand what was going on during the process when relating it all to a single raw cell. Converting it back to the 12v RV way of thinking is just a matter of multiplying a cell voltage by four. The next little bit will be about a single cell, so try to wrap your head around it.

Say we have a single 100Ah cell that we want to charge. How fast that cell charges, and how many amps are accepted is a function of both the voltage and current applied. The lithium cell will happily accept the 3.65 max cell voltage, and a high rate of current, but what sort of long term degradation is occurring if we do this all the time. In reality though, charging at a very high current rate is really only a test bench scenario, as we would never achieve that sort of current with the average solar set up in the Alto. We could though, get perhaps 40 amps of current out of the Intellipower converter while on shore power, but certainly less charge current from the solar while boondocking, unless you have some crazy huge solar panel installation.

A stress reduced, moderate approach to charging is best for long term cell health, and realistically, this is what will occur when using only solar to charge the battery. The solar amps produced will generally be lower, and fluctuating with the day’s cloud cover. While on shore power, the 12v converter will put out higher and more consistent amps, so the charging will be faster. Although the amps will be higher, it really is not so extreme as to cause much stress on the cells, or long term degradation.

The Flat Plateau

In the sample graphs below, let’s establish a bulk set point of 3.65 volts, charging at a constant 5 amps. This probably represents a somewhat averaged example of charging with solar on a sunny day with cloudy periods. This particular test takes the cell from a depleted state voltage of 2.5v, to a maximum voltage of 3.65. As you can see, the voltage rises quickly, to 3.23v after an hour, with 7.04 amps stored. The main plateau is reached after 5 1/2 hours of charging. At this point, around 30 amps have been stored by the cell, at a voltage of 3.32v. After approx eight more hours, the amps stored by the cell has more than doubled, to 70, yet the voltage is only 3.37, a mere 5/100’s of a volt more. This is why it is so difficult to determine the state of charge of a lithium battery by voltage alone. Insanely small voltages changes along the plateau, yet the amps stored or remaining at the same voltage reading can be significantly different. The flat plateau continues like this, eventually reaching rated capacity of 100 amps at 3.378 volts. This particular charge cycle took around 22 hours before reaching the bulk charge cutoff at 3.65 volts, with 109 amps packed into the cell.

Towards the end of the plateau, the voltage started to climb a little quicker, by this point the cell is already over the rated amp capacity, and once it reaches the 3.65v set point, the controller moves into absorption phase, where the voltage becomes constant, and the current begins to drop off. This is called the tail current, and once it drops to the amp cutoff set point, it will flip into the float phase. The set point voltage for float is typically around 3.4 volts. Once the charging has stopped, the cell will settle into a “resting” voltage, which will be a tad higher than the float set point voltage. 

The cell is now fully charged, and generally, it has packed away more amps than its rated capacity, another subtle benefit of a lithium cell. The cell now supplies amps as needed to support the load. This continues until the voltage drops to the re-bulk offset voltage. The re-bulk offset is the value below the float set point voltage when the controller will start up the bulk charge cycle again. So in this example, if float is at 3.4v, and the re-bulk offset is say .05v (.2v when converted to 12v values), then the bulk charging cycle will not start until the cell drops to 3.35 volts. The offset allows for cell voltage fluctuations caused by load spikes, so that controller does not unnecessarily place the charge process back into bulk phase. So that in a nutshell is an overview of how a lithium cell charges.

That was a long recharge, from depleted to full at 5 amps of current took 22 hours, and the math bears this out when filling up a 100 amp hour cell. We would not normally be trying to recover the full 100 amps, but regardless, a low current means a slower recovery. Now let’s look at the second graph, the 30 amp charge curve.

Using the same voltage reference points of the 5 amp chart, unsurprisingly, we see a much faster time to recover,  The entire charge cycle only took a little over 3 1/2 hours when you have gobs of current on the job. In real world usage, perhaps we are only needing to recover 20-30 amps, and on a sunny day with good panel exposure, even at 5 amps current, that recovery happens in a very reasonable amount of time.

5 amp charge curve

30 amp charge curve

Playing With The Numbers

If we take a closer look at how that cell charged, and you can really only do that with test bench data, you start to see some interesting tidbits come into view. For example, if the cell is over the rated capacity at 3.475v, why stress it by charging it to 3.65v, just to squeeze in another few amp hours? Current level being applied has the greatest impact on the speed of recovery. Interesting to note as well, a lower current charge cycle will packed in more amps, at the same voltage reading, than a higher level of current is able to do. However, that takes a far longer time to achieve. So what are some reasonable voltage settings for the charge phases? Slower, steady charging, will easily achieve an over rated capacity charge, and ensure the longest cycle life possible. 

Now that we understand how an individual cell charges, and the voltages involved, let’s flip back to our 12v RV world so we can easily understand what sort of settings to use in the controller. So we have seen that 3.4v (13.6v) will achieve a full charge, and 3.5v (14.0v) achieves an over capacity charge, perhaps a value of 3.475v (13.9v) is a good balance between charge speed (current dependent) and battery life cycle longevity.

So right now, everyone is thinking, that is not what the manufacturer is recommending for charge voltages. Most state 14.2v - 14.6v for bulk charging. If testing shows over capacity can be achieved at lower voltages, what’s the deal here? My view is that there is a bit of marketing going on. Generally the hype is around how fast and full a lithium recharges, which in fairness is a valuable feature, but a fast recovery needs some solid current behind it, as it is not really voltage driven.

My Numbers

I use 13.9v as my bulk voltage, or as Victron likes to call it, Absorption Voltage. Next to determine is the float voltage set point.

A key difference between lithium and FLA battery charging is that lithium does not like the trickle current float phase. This is because of the incredibly low self discharge rate of the lithium chemistry. An FLA on the other hand, starts to discharge as soon as current is removed. The float phase algorithm of an FLA charger keeps a small trickle current flowing to the battery, to stave off the inherent self discharge. Not required for a lithium, and certainly detrimental to the cell’s lithium chemistry over the long term.

So where to set the float voltage? Not only do we want to eliminate that trickle current, we also have to factor in the lithium battery “resting voltage”. What to hell is that?

It would be natural to expect that once the battery is fully charged at 13.9 volts (or your setting), the monitor voltage reading would show that value until amps are pulled out under load, and the voltage begins to drop. Not so. Once in the float phase, and no longer charging, a lithium battery will settle down to a resting voltage of around 13.6 volts. All those charging amps are still packed into the battery, now the lithium chemistry settles down to this resting voltage when off charge and not under load.

Given this, if one were to set a controller float voltage higher than the resting voltage, you could end up in a continual charging loop, initiated when that battery settles down to its resting voltage, and that is certainly not good. This is why you see recommended float voltages of around 13.4 volts. As the battery is now fully charged, is resting at 13.6v, and the float voltage is set to 13.4v, the controller has no need to supply any sort of charging current to the battery…which is a good thing.

So then some loads are applied, the battery supplies those amps, and if the loads continue, the voltage will eventually drop to the re-bulk value, and the charge cycle will start all over again. With a well set up charging profile, the battery will be nicely exercised, will be again charged at a moderate pace, then will settle down and rest until the cycle is required to start over again.

Is your head about to explode?

Is all this detailed analysis moot, because of the inherent excellent performance and longevity of a lithium battery. Perhaps…but the upshot is that we now have a better idea of how a lithium battery works, and how it could be charged. I think a key point is to know how your particular charging profiles are actually set up, and review them with some new understanding, to ensure you will get the most out of your lithium battery over the long term. Absolutely nothing wrong with following a battery manufacturer’s recommendations, and that would be the comfort zone for many of us. But if like me, you are keen to research, analyze, and then tinker…just go for it.

Interesting information resources

Lots of great info and ideas to be found at these sources. Time well spent.

Victron Energy: https://www.victronenergy.com/

Off Grid Garage: https://off-grid-garage.com/        https://www.youtube.com/@OffGridGarageAustralia

The Explorist: https://explorist.life/                        https://www.youtube.com/@EXPLORISTlife

Far Out Ride: https://faroutride.com/                      https://www.youtube.com/@FarOutRide/

Will Prowse: https://www.mobile-solarpower.com/        https://www.youtube.com/@WillProwse

So after a summer of camping, I’m super keen on our lithium conversion, perhaps even a tad fanatical. The lithium battery certainly recovers quicker, and the availabilty of a larger amount useable amps provides a nice comfort zone while off grid. The programmability of the Victron components, and the detailed state of charge information available through the app is incredibly useful, and unsurprisingly addictive. 

Make the move to lithium, coupled with some solid monitoring...it is so very worth it!