LiFePO4 (Lithium) Battery Build (part 1)

Our camper (travel trailer) has a typical 12V lead acid battery to run most of the devices. Solar does a decent job of keeping the battery topped up, but sometimes we end up in a campsite with a lot of shade and have been finding it increasingly hard to keep the battery sufficiently charged for 4-5 nights. This is particularly pronounced when we get up in higher elevations and will often run the heater at night to keep the temperature tolerable.

We’ve tried AGM batteries in the past but they’re roughly 2x the cost of a regular “wet” cell and don’t really provide much benefit. Honestly just replacing the $99 “deep cycle” battery every couple years has been more cost effective, though it rarely provides any major energy storage benefit. (Except the one time I killed a battery in Big Bend by running too many fans and draining it down to 10.5V overnight…)

I’ve been debating a lithium-based option like the Renogy or Battleborn drop-in LiFePO4 batteries for a while, but at $900 the price is steep for only a bit more capacity. Recently though a friend of mine turned me on to the DIY Solar Forum, where a bunch of people were building their own high capacity lithium batteries for a fraction of the cost. I did the math and decided to make the plunge, which ultimately led me down a rabbit hole in an effort learn everything I could as quickly as possible so I could build and test one before our next long summer trip.

This post has a number of acronyms or terms, which may be used interchangeably at times, even when the meaning isn’t 100% identical. Terms like:

  • SLA = Lead Acid battery (even for wet cell batteries which aren’t sealed). This is probably what’s in your car.
  • AGM = Absorbent Glass Mat battery (it’s like an SLA but actually sealed)
  • LiFePO4 is one of many lithium battery chemistries
  • Lithium Ion = LI = Li-ION or LION, which is a specific battery chemistry, probably in your cell phone
  • Lithium in this post refers to LiFePO4, even though it’s a generic term for all Lithium-based batteries
  • V = Volts
  • Ah = Amp hours. In a 12V battery, 10Ah means you can power a 12V device that draws 10A of current for 1 hour, or 1A of current for 10 hours. I will ignore battery efficient at low/high outputs

If you’re a purist, get over it.

Why Go Lithium?

Cheap lead acid batteries are available at Sam’s Club or Walmart for about $100. My last two Duracell 105Ah batteries came from Sam’s Club. They work, but unless you’re regularly on shore power or run a generator in my experience it’s really hard to get enough capacity from them. This is due to a few reasons.

First, the cheap “deep cycle marine” batteries in my experience are really inferior to high quality deep cycle batteries. I’ve read this is due to thinner plates, the use of recycled materials, etc. I can say from experience though that after two seasons, even with good maintenance practices, these tend to only have 70-80% capacity remaining. High quality AGM deep cycle batteries seem to hold up better over time (except when you completely drain them… d’oh!), but are roughly double the cost at $200-250. Lithium chemistries, on the other hand, are known for lasting thousands of cycles – the typical LiFePO4 battery will still have 80% of its capacity after 2000-3000 cycles, or roughly 7-10 years.

Second, you can use much more of your lithium battery capacity safely. Lead acid batteries should really not be discharged below 50% (though I will go as low as 30% sometimes), whereas lithium can be fully charged and discharged (though the manufacturers recommend keeping and using them in the 10%-90% range. Thus a 100Ah lead acid battery only has maybe 50-70Ah usable, whereas a 100Ah lithium battery is designed for 80% utilization but can be cycled up to 100% if desired.

Third, lithium chemistry batteries weight a LOT less than lead acid. Don’t believe me? Go check your periodic table. That $99 100Ah Duracell is 59lbs, while a 280Ah LiFePO4 battery pack (without the case) is only 50lbs.

So in short I expect given roughly the same space (Group 31 battery size) and weight (60 lbs) to normally have about 4x the usable capacity (230Ah, assuming 10-90% state of charge) vs the old lead acid battery (roughly 60Ah usable). [Note: If I fully cycle the LiFePO4 battery that’s 280Ah, and if I compare that to deep cycling the SLA battery that’s 70 or maybe 80Ah, so I feel the ~4x ratio holds].

Why Not Go Lithium?

There are really only a couple reasons not to do this. The first and most obvious is cost. While the actual cost to build a lithium battery with 4x capacity is on par with purchasing 4 high quality AGM batteries, in practice the latter doesn’t really make sense for me to do because of the weight and space required. (Also because I really don’t *need* more than ~150Ah of lead acid capacity).

The other reason is battery management. LiFePO4 batteries in particular don’t like being charged below freezing or discharged significantly below. In fact, charging your LiFePO4 battery when the cell temperature is below freezing is known to destroy them quickly. To compensate, the battery will need to be relocated inside the trailer and will need a battery management system. More on this in part 2…

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