The pre-made LiFePO4 batteries (LFP hereafter) you seen on Amazon and elsewhere are designed and marketed as drop-in replacements: pull your lead battery and “drop in” the LFP. This approach works in many cases because the size, shape, voltage range, and charging requirements are in the same ballpark as traditional lead batteries. It's not a perfect scenario, but will meet the needs of the average user. People with special requirements or an interest in technical details can do the reading/testing required for an optimal setup.

There are benefits, challenges, and differences to be aware of before breaking out the credit card.

Maybe. Probably. Here is a cheat sheet to the main issues:

1. what the LiFePO4 battery needs¹⁾
   • moderate charging voltage²⁾
   • relatively low float voltages³⁾, or no float at all
   • moderate charge & discharge currents⁴⁾
   • charging only above freezing 32F/0C
2. what your existing charging system provides (or can be configured to provide).
   • alternator voltage and rating if a combiner or DC-DC is present
   • solar charging (profiles, configurability, output rating, protections if any)
   • shore power charging (voltage, output rating, stages if any)

If the existing system cannot be configured to meet LFP's needs within the alternator's comfort zone then you are looking at replacing some components. For more specific guidance please share details about your setup, use case, budget, and priorities.

Note: undercharging lithium is not an issue unless you are running out of power. This is a radical change from lead-battery charging.

Here we need to be careful about charging voltages. Solar charging does not usually have enough current for that to be a problem.

Probably not, assuming your existing controller has a profile that meet's the LFP's stated needs, or has configurable setpoints that you can adjust to meet needs.

Use a lithium profile or take your battery manufacturer's specs and apply them thusly

Note: advanced or geeky folks may want to take a different approach.

Some controllers do use temperature sensing to cut off lithium charging near the freezing point. Some only do this in the preconfigured Lithium profiles (Renogy, on supported models).

Note that having a temp probe left over from lead battery charging does not mean low-temp cutoff is supported in the controller. With lead banks temp sensing is used to adjust charging voltage to battery temperature⁵⁾ and are not intended to turn off charging in freezing weather.⁶⁾ Read your controller docs to see if the temp probe can be used to sense freezing temps with Li banks.

In addition, some batteries have a low temperature charging disconnect built into the BMS, or internal heating to skirt the issue. The owner can also add external warming.

Charging LFP by alternator is understood by few but confidently⁷⁾ discussed by many. Battle Born summarizes the subject like this:

…can I use the alternator to charge the batteries? The answer is yes, you can. Under most circumstances you don't even need to modify your system

Alternator charging of aux battery banks is a balancing act regardless of battery chemistry. We want enough current that we get adequate charging in the time allotted without drawing so much current that we stress the alternator or battery bank itself.

This balancing act is more critical when the alternator is relatively small,⁸⁾ is the sole charging source, or when driving time is limited. When solar is present or drive times longer we can charge at a more leisurely pace.

Excess charging current is rarely a problem with solar, but can be an issue with charging from the alternator. Discharge and charge current is expressed in C. A 100Ah LFP is 100A at 1C, 50A at 0.5C, etc. Common current specs:

• 0.2C - the rate at which battery manufacturers rate cycle life – this might be telling us something..
• 0.4C - rate that appears to balance charging speed and cell longevity
• 0.5C - often given as the max recommended charge rate
• 1.0C - typical limit enforced by BMS

From the battery's point of view, alternator charging rates should be ≤0.5C, with 0.4C likely being the sweet spot. So ≤80A for a 200Ah bank. From the alternator's point of view, current should be no greater than the alternator can comfortably provide under expected charging conditions.

As the Battle Born quote above suggests, for average LFP banks and average cargo van alternators these criteria will often be met with no reconfiguration. If the criteria cannot be met then we will need DC-DC charging or some other approach to manipulate voltage and/or current.⁹⁾

There are cases where you may want to disable charging from the alternator. For example:

• temps are below freezing
• ambient temps are high and the vehicle is not moving (stuck in traffic)
• battery hits your charging targets
• combined solar + alternator charging exceeds your current preferences

Probably, all other things being equal (wiring, bank capacity, etc).

You've been watching that Victron video again, haven't you?

If you are reading this section your alternator is already charging the AGM bank without going all glowy and smelly. All other things being equal, a similar-sized Li bank will typically make similar demands on the alternator. As with all projects, it is important to pay attention and take reasonable precautions.

If you have a 100Ah of lead that has been pulling 25A and you are planning on 300Ah of LFP you can ballpark the new current acceptance will be 3 x 25A = 75A. Actual examples here.

But you might not need that much LFP capacity. Since lithium is not damaged by partial state of charge and can be taken to lower states of charge, it only takes ~62% Lithium Ah to replace lead's Ah. So unless your daily power requirements will increase you can actually get by with less lithium Ah than Lead and still get the same usable capacity.

• 100Ah lead (50Ah usable) = 63Ah of LFP
• 150Ah of lead (75Ah usable) = 93Ah of LFP
• 200Ah lead (50Ah usable) = 126Ah of LFP
• 220Ah lead (50Ah usable) = 136Ah of LFP
• 300Ah lead (50Ah usable) = 186Ah of LFP

An anecdote from secessus:

I replaced 220Ah of FLA GC with 100Ah of LFP and have the same functionality. Bank demand on the alternator dropped from 42A to 30A, largely a function of smaller bank capacity.

Yes, unless it has some weird hardcoded setpoints that are incompatible with the LFP bank. Or is rated higher than the charging current your LFP should receive.

If it was already handling a similar-size AGM bank then it will probably be fine.

Caveat: charging LFP with DC-DC can be somewhat harder on the alternator than charging AGM with DC-DC. AGM current acceptance tapers in Absorption which means the DC-DC will not be demanding full powah from the alternator. Depending on the setup Lithium can make the “full pull” until the bank is near full charge. In practice this might only make a real difference with long drives and heavily discharged batteries.

An esoteric caveat: it's conceivable there are edge cases where a vandweller has an oversized DC-DC charger (60A?) that has not destroyed a small alternator (90A?) yet because the small lead bank (100Ah?) has only been pulling a fraction of the alternator's capabilities. Shoehorning in a 300Ah LFP bank would max the 60A charger and place serious demands on the undersized alternator.

Shore power charging is relatively rare for vandwellers and has many factors feeding into it.

IF other forms of shore power charging are available or shore power charging will last at least overnight the shore power charger can be a simple, inexpensive, single-stage charger set to a relatively low voltage (13.4v - 13.6v?) This will allow near full charge and load support without overcharging.

If shore power charging is the only source of charging or if time on the charger is limited then a more aggressive, expensive, higher-capacity multistage charger may be warranted.

If the van will be on shore power for an extended period (days, weeks, months) then a small single-stage converter charger set to quite low voltage (13.2v?) would hold the battery at middling states of charge. It only needs to be a bit bigger than the expected loads so gradual charging can occur.

Yes, discharging too but that happens at much lower temperatures than the 32F we use as a hard limit for charging LFP cells.

If you will be using the batteries in ≤freezing temperatures¹⁰⁾ you will need one or more of these protections:

• a BMS that shuts off charging in freezing temps
• chargers that shut off charging in frezing temps¹¹⁾
• a way to warm the battery or otherwise prevent it from getting too cold

[Note from secessus: there is some evidence that LiFePO4 fares better when cell temps are held to warmer temps than just above freezing. For this reason I warm my bank to 50F.]