AGM vs Lithium for the Sleeper House Bank

Every sleeper electrical build eventually arrives at the same fork: fill the battery tray with AGM, or spend up for lithium. The forum answer is "lithium, obviously" — and for a van-lifer parked in Arizona, it usually is. For a carrier running Highway 11 in January, the answer has more moving parts: LiFePO4 chemistry refuses to accept charge below freezing, your alternator was never designed to feed it, and the weight savings that sell lithium matter less in a Class 8 tray than they do in a camper. This is the honest version of the tradeoff — usable capacity, weight, cold behaviour, cycle life, and charging — written for trucks that see real winter.

What does "100Ah" actually mean in each chemistry?

The spec-sheet number is the same; the usable number is not. Lead-acid chemistry — AGM included — ages fast when you routinely discharge below about 50% state of charge. A 100Ah AGM is therefore a 50–60Ah battery in honest overnight terms, and its voltage sags under heavy load, which is what trips inverters early on a "half full" bank. LiFePO4 delivers nearly its full rating: a 100Ah lithium battery gives you 90–95Ah of genuinely usable capacity, holds voltage almost flat until it is nearly empty, and does not resent deep discharge. Practical translation: two lithium batteries in the tray replace three to four AGMs in delivered amp-hours. When you size a bank against a real overnight load — fridge, CPAP, inverter overhead — usable amp-hours is the only column that matters, and it is the column lithium wins by the widest margin.

How much does the weight difference matter in a Class 8?

A Group 31 AGM weighs in around the high-60s to low-70s in pounds; a Group 31 LiFePO4 runs roughly half that. Replace four AGMs with two lithiums delivering the same usable capacity and you shed something like 200 lb. On a camper van that is transformative; on an 80,000 lb GVW combination it is a rounding error on the scale ticket — real, occasionally useful if you run heavy on the steer-axle side, but rarely the deciding factor. Where the weight does matter is your back: wrestling a 70 lb battery in and out of a tray at the frame rail in a February parking lot is a genuinely different job than lifting a 35 lb one. Drivers who service their own banks feel the difference every swap; drivers who pay a shop never notice.

What happens to each chemistry at minus 20?

This is the section that decides the question for Canadian lanes. Discharging in deep cold: both chemistries lose capacity as temperature drops, AGM more steeply — a cold-soaked AGM bank might deliver 70–80% of its rated capacity, and its already-thin usable half shrinks with it. Lithium discharges in the cold reasonably well. Charging is where lithium has a hard line: LiFePO4 cells must not accept charge below 0°C — plating damage is permanent — so every reputable lithium battery's BMS simply refuses charge below freezing. A cold-soaked lithium bank after a minus-25 night will run your loads and then sit there declining your alternator's help until the cells warm up. The fixes are real but not free: heated variants that draw power to warm themselves before charging, insulated boxes, or interior mounting. AGM, by contrast, charges in the cold happily — slowly, but without a lockout. If your winters look like the ones in our winter driving guide, a non-heated lithium bank in the frame-rail tray is a spec-sheet purchase that will disappoint you in January.

Why does the alternator care which one you buy?

AGM was designed for the world your truck already lives in: drop the batteries in the tray, let the alternator charge them, done. Lithium is not a drop-in on the charging side. Its flat voltage curve and low internal resistance let it pull heavy current for as long as the alternator will give it — which can run a stock alternator hot for a long stretch — and the alternator's charge profile never brings LiFePO4 properly to full. The correct lithium install puts a DC-DC charger between the start batteries and the house bank: it caps the current the alternator sees, delivers the right voltage profile, and isolates the start bank so a hungry house bank can never drag your morning start down with it. That is an extra component, extra wiring, and ideally professional installation — budget line items that belong in the same honest spreadsheet as everything in our owner-operator startup cost breakdown.

How do cycle life and lifespan compare?

Cycled to 50% daily, an AGM bank is typically a 2–4 year component; a quality LiFePO4 bank is rated for thousands of full cycles — call it 8–10 years of nightly use before meaningful fade. Over a decade, one lithium purchase replaces two to three full AGM sets, plus the labour of each swap. That is the honest economic case for lithium, and it is a per-year argument, not a purchase-day argument: the chemistry with the higher entry price is routinely the lower-cost bank per year of service for a full-time truck. The catch is that the payoff assumes the bank survives — which loops back to cold charging. A lithium bank that spends winters being deep-cycled and never properly recharged will not see its rated cycle life, while AGM tolerates that abuse pattern with less drama and simply wears out on schedule.

So which bank should you actually build?

Build AGM if: your budget is tight now, your bank lives in the exposed frame-rail tray, you run northern lanes all winter, and you want a system with no extra charging hardware to install or fail. Accept the 50% rule, size the bank double your overnight load, and plan on replacement as routine maintenance.

Build lithium if: you are in the truck full-time for years, you can mount the bank somewhere it stays warmer or buy heated cells, and you will do the install properly with a DC-DC charger. You get double the usable capacity per tray slot, a bank that holds voltage under the microwave, and a per-year cost that undercuts AGM over the life of the truck.

Either way, know your actual state of charge rather than guessing from a voltage number — particularly with lithium, whose flat curve makes voltage nearly useless as a fuel gauge. A shunt-based monitor is the cheap insurance on either chemistry, and the difference between managing a bank and quietly murdering one.