I still remember the first AGM bank I babied through a Michigan winter, watching the voltage sag to 11.9 under load and doing math in my head about how much longer the furnace fan would run before the battery gave up. That rig eventually got a LiFePO4 upgrade, and the difference wasn’t subtle. It was like swapping a bicycle for an e-bike.
If you’re standing in the battery aisle trying to decide between another AGM and finally making the lithium jump, this comparison is for you. No hype, just the numbers that actually change how your rig behaves.
The quick version
- AGM gives you roughly 50% usable capacity before damage; LiFePO4 gives you 80-100%, so a 100Ah lithium battery often replaces 200Ah of AGM.
- LiFePO4 weighs about a third of an equivalent AGM bank, a real factor for payload-limited vans and tow-behind trailers.
- LiFePO4 lasts 3000-6000 cycles versus 300-500 for AGM, which usually makes it cheaper per year even though the sticker price is higher.
- AGM tolerates a wider charging profile; LiFePO4 needs the right voltage settings or a battery management system will cut the connection.
- Cold weather favors neither by default: standard LiFePO4 will not charge below freezing without protection, while AGM charges fine but delivers less power in the cold.
What Each Chemistry Actually Is
AGM stands for absorbed glass mat, a type of sealed lead-acid battery where the electrolyte is held in a fiberglass mat instead of sloshing around as liquid. It is a mature, well understood chemistry that has powered RVs and boats for decades.
LiFePO4, or lithium iron phosphate, is a different chemistry entirely. It uses lithium ions moving between electrodes instead of a lead-acid reaction, and it includes an onboard battery management system, or BMS, that protects the cells from overcharge, over-discharge, and short circuits.
That BMS is the single biggest reason these two batteries behave so differently under real use. AGM has no brain. LiFePO4 does, and that brain will shut things down if you feed it power the wrong way.
Usable Capacity: The Number That Actually Matters
Here is where most first-time buyers get surprised. A 200Ah AGM battery is not really a 200Ah battery in practice.
Pulling AGM below 50% state of charge repeatedly shortens its life dramatically, so most manufacturers and installers treat 50% as the practical floor. That means your real usable capacity is closer to 100Ah.
LiFePO4 can be discharged to 20% or even 10% state of charge on most drop-in batteries without meaningful damage, and some BMS units allow discharge to near 0% safely. A 100Ah LiFePO4 battery therefore delivers 80 to 100Ah of real, usable power, which is roughly what you’d get from a 200Ah AGM bank.
| Spec | AGM (100Ah rated) | LiFePO4 (100Ah rated) |
|---|---|---|
| Usable capacity | ~50Ah | 80-100Ah |
| Weight | 60-65 lbs | 22-28 lbs |
| Cycle life (to 80%) | 300-500 cycles | 3000-6000 cycles |
| Charge voltage (14.x) | 14.4-14.7V absorption | 14.2-14.6V, shorter absorption |
| Cold charging | Fine to about 20F, reduced output | Most BMS block charging below 32F |
| Approx price (100Ah) | $200-$280 | $700-$1000 |
Weight: Where Lithium Wins Without an Argument
A group 31 AGM battery weighs around 60 to 65 pounds. A comparable 100Ah LiFePO4 battery from Battle Born, Li Time, or Ampere Time weighs 22 to 28 pounds.
For a van with a 3500 to 4000 pound payload rating, that difference across a 400Ah bank is over 130 pounds saved. That’s a full-size generator’s worth of weight back in your budget for water, gear, or passengers.
Boats feel this too, though less dramatically since payload is rarely as tight a constraint as it is on a Class B van build.
Charging Behavior: Where People Actually Get Burned
A mistake I see constantly is someone swapping AGM for LiFePO4 and leaving their old converter in place, assuming a battery is a battery. It isn’t.
Standard AGM converters like older Progressive Dynamics or WFCO units without a lithium mode hold absorption voltage for hours and taper current slowly. That profile undercharges LiFePO4 and can cause the BMS to think something’s wrong, cycling the battery offline mid-charge.
Warning: Never assume your existing lead-acid charger, converter, or alternator setup is fine for lithium just because the voltage numbers look similar on paper. Check the actual charge profile, and consult our guide on how a LiFePO4 battery actually works to understand why the BMS reacts the way it does.
LiFePO4 wants a bulk and absorption voltage in the 14.2 to 14.6V range for a 12V system, with a short absorption stage, then a drop to float around 13.6V or lower. AGM wants similar bulk voltage but a longer absorption hold and a higher float, often 13.2 to 13.8V, to keep the plates topped off.
Lifespan and Real Cost Over Time
AGM batteries typically deliver 300 to 500 full cycles before capacity drops meaningfully, translating to roughly 3 to 6 years of moderate RV use. LiFePO4 batteries are commonly rated for 3000 to 6000 cycles, which stretches into 10 or more years for most weekend and part-time users.
Run the math over a decade. Two or three AGM replacements at $250 each easily approaches the price of one LiFePO4 battery that’s still going strong. This is the calculation I walk customers through when the sticker shock hits, and it’s covered in more detail in our breakdown of what a lithium battery bank really costs.
There’s also a data point people underweight: how long the battery holds up depends heavily on depth of discharge and temperature, topics we cover fully in our deep dive on LiFePO4 lifespan.
Cold Weather: Neither Chemistry Gets a Free Pass
This is where the comparison gets nuanced. AGM will accept a charge in freezing temperatures, though it delivers noticeably less power output as the mercury drops, sometimes 20% less at 32F than at 80F.
Standard LiFePO4 batteries, on the other hand, have a BMS that blocks charging below 32F entirely to protect the cells from lithium plating, a real degradation mechanism. Discharging still works fine in the cold, just not charging.
If you camp in freezing temperatures regularly, look at self-heating lithium batteries, which run 100 to 150 dollars more but include internal heating pads that activate automatically before accepting a charge.
A Real Scenario: Weekend Warrior vs Full-Time Boondocker
A couple I helped last spring ran a travel trailer three or four weekends a month, mostly at campgrounds with shore power. For them, I actually recommended sticking with AGM a little longer, since their usage pattern didn’t stress the battery’s weak points.
Contrast that with a full-time boondocker running a fridge, inverter, and Starlink off-grid for weeks at a stretch. That person burns through AGM’s usable capacity daily and needs the deeper discharge and faster recharge that LiFePO4 provides.
Your use case matters more than any spec sheet. Match the battery to how you actually camp, not how you imagine you might camp someday.
Making the Switch Without Regret
If you decide lithium is right for you, budget for more than just the battery. Plan on a compatible charger or converter, proper fusing per ABYC standards, and possibly new cables if you’re stepping up to higher amperage.
- Confirm your converter or charger has a lithium charge profile, or budget for one that does
- Check battery box dimensions against your chosen lithium battery’s exact measurements
- Verify fuse and wire gauge sizing for the higher continuous discharge lithium allows
- Decide if you need a self-heating or low-temperature-protected battery for your climate
- Set a realistic budget knowing lithium costs 2.5 to 3.5 times more per battery upfront
Whichever way you go, do the comparison with your actual camping habits in front of you, not a spreadsheet built for someone else’s rig. AGM still has a place for light, seasonal use, but for anyone who dry camps regularly or hates hauling dead weight, LiFePO4 earns its keep faster than the price tag suggests.
