AGM Against LiFePO4 for an RV House Bank
Written by RV Systems Lead, RV Systems and Electrical Lead, OCRV Center. Reviewed by Fleet Account Manager
In shortLithium roughly doubles usable capacity at a third of the weight. AGM asks nothing of the coach around it. The comparison is really about the wiring, not the batteries.
Is AGM or lithium the better house battery choice for an RV?
Lithium gives roughly double the usable capacity per nameplate amp hour and far more cycles, but it requires a compatible charger, correct fusing and monitoring. AGM asks nothing of the coach. Our Yorba Linda systems bay checks the existing DC bus before quoting either for Rancho Santa Margarita owners.
- AGM gives about half its nameplate as usable capacity
- Lithium holds voltage flat, so panel gauges become useless
- Class T fusing is the item most often left out
- Cold weather charging needs a heated lithium pack
- Systems labor rate
- $260 per hourBattery, charger and DC bus work
- AGM swap labor
- 1 to 3 hoursLike for like, including terminal cleanup and a load check
- Lithium conversion labor
- 3 to 14 hoursSet by charger, cable, fusing and alternator findings
- Diagnostic rate
- $285 per hourOne hour minimum, credited in full against an authorized repair
- Brands in the bay
- Victron, Magnum, Xantrex, Progressive Dynamics
Last verified
Nearly every conversation about this pair starts in the wrong place, with amp hours and price per amp hour. Those numbers matter least. What decides whether a lithium bank is a two hour job or a two day job is the four things around the batteries: what charges them, what wire connects them, what fuse protects them and what tells you their state of charge.
The table puts the two chemistries side by side on the criteria that actually change a work order. Below it, the four rows that owners underestimate, and an honest account of when the older chemistry is still the right call.
Usable capacity is the number that matters
A pair of AGM batteries labelled 100 amp hours each does not give you 200 amp hours. Taking lead acid much below half discharged shortens its life sharply, so the practical figure is around half of nameplate. A LiFePO4 pack of the same label gives you very close to all of it, and it does not mind being taken there repeatedly. That is why owners describe a lithium swap as doubling their capacity even when the label numbers match.
Weight moves with it. Lithium delivers that usable capacity at roughly a third of the mass, which matters on a travel trailer where tongue weight is a real constraint and on a van conversion where every pound counts. It also shifts the balance of the unit slightly, because lead acid sits low and heavy, and on a light trailer that is worth thinking about rather than ignoring.
The flat discharge curve, and why your gauge stops working
Lead acid voltage falls steadily as capacity drains, which is the only reason a panel voltmeter ever told you anything. Lithium holds voltage nearly constant across most of its usable range and then drops quickly at the end. So a coach that keeps its factory panel after a conversion shows full, full, full, then nothing, which is less information than the owner had before the upgrade.
The answer is a shunt based monitor that counts current in and out at the negative bus. It is a small fraction of the cost of the bank and it turns the whole system into numbers you can act on. It also makes the next electrical fault findable, because a shunt reveals a parasitic draw that no voltmeter would ever show. We fit one on every conversion for that reason alone.
What lithium asks of the coach around it
Four things, and any one of them can turn a battery purchase into a systems job. The charger has to run a lithium profile rather than a lead acid one, or the bank will chronically sit undercharged. The main cable has to suit the new continuous discharge, because lithium does not sag and therefore does not self limit the way lead did. The main fuse has to be able to interrupt a lithium short circuit current, which is what Class T fusing exists for.
The fourth is alternator charging, and it is the one that does real damage. A relay feed from the alternator to the house bank worked because lead acid refused high current when nearly full. A lithium pack with a healthy BMS will take everything the alternator can make for as long as it can make it, and the alternator was never sized for that. A DC to DC charger between the two is the correct answer, not an option.
When AGM is still the right answer
Three cases, and they are not unusual. An owner who stays on shore power at a park and only needs the bank to hold lights and a slide motor overnight gains almost nothing from lithium. An owner selling the unit within a year or two will not recover the conversion cost. And an owner who takes the coach to real cold gets a bank that refuses charge below freezing unless the pack is heated, which adds cost and complexity.
There is also a simple case for AGM: it drops in. Same tray, same cable, same charger, same fuse, one to three hours of labor and a load check. On a coach that is otherwise healthy and an owner who wants the problem to go away, that is a perfectly respectable decision and we will not talk anyone out of it.
How the cost shape differs, without inventing a total
AGM cost is almost entirely parts, with one to three hours of labor at $260 per hour. Lithium cost is parts plus a variable amount of systems work: a shunt and Class T fusing on the easy end, and a charger, a DC to DC unit, new cable and new fusing on the harder end. That is why a lithium quote can vary by a factor of three between two coaches with the same batteries, and why anyone quoting lithium without inspecting the DC bus is guessing.
The inspection is short. Reading a converter label, measuring cable, checking the main fuse and identifying whether an alternator feed exists usually fits inside an hour of diagnostic time at $285 per hour, and that hour comes off the invoice if the work proceeds. Photographs of the battery compartment and the converter label often get most of the answer before the unit arrives.
AGM lead acid bank compared with LiFePO4 lithium bank
| Criterion | AGM lead acid | LiFePO4 lithium |
|---|---|---|
| Usable capacity | Roughly half of nameplate before cycle life suffers | Close to the full nameplate figure |
| Voltage under load | Sags progressively, so a panel voltmeter estimates charge usefully | Nearly flat until nearly empty, so a voltmeter tells you almost nothing |
| Charge acceptance | Tapers as it fills, which naturally limits charging current | Accepts high current until nearly full, which is what stresses chargers and alternators |
| Charger requirement | Any standard multi stage lead acid profile | A lithium profile, either selectable on the unit or programmed |
| Alternator charging | A relay or solenoid feed is tolerable | Needs a DC to DC charger sized to what the alternator can sustain |
| Main fusing | ANL or MEGA class is generally acceptable | Class T on the battery positive, sized to the bank and the inverter |
| Monitoring | Panel voltmeter is adequate for daily use | A shunt based amp hour counter is effectively required |
| Weight per usable amp hour | High, and it sits low in the unit which helps balance | Roughly a third of AGM, which changes tongue and axle loading |
| Charging below freezing | Accepts charge, with reduced efficiency | Refuses charge to protect cells unless the pack is heated |
| Cycle life | Several hundred cycles at moderate depth of discharge | Several thousand cycles at deep discharge |
| Install labor | 1 to 3 mechanical hours for a like for like replacement | 3 to 14 mechanical hours depending on how much of the DC bus needs work |
| Failure behavior | Gradual capacity loss you can watch coming | An abrupt BMS shutdown with very little warning |
AGM wins for an owner who mostly stays on shore power, wants the cheapest possible replacement, and does not want to touch the wiring: it drops in, it works with whatever charger the coach came with, and it tolerates cold. Lithium wins for anyone who actually camps off power, tows weight sensitive, or runs an inverter hard, because usable capacity roughly doubles for a third of the weight. The catch is that lithium is only a drop in when the charger, the cable, the fusing and the alternator arrangement all happen to suit it, and on most coaches at least one of those does not.
Appendix: questions and answers
- Why is only half of an AGM bank usable?
- Because depth of discharge and cycle life trade against each other in lead chemistry. Taking an AGM battery to eighty percent discharged repeatedly will cost you most of its expected cycles. Staying around fifty percent is the accepted compromise, which is why the industry figure for usable capacity on lead acid is roughly half of what the label says.
- Does a generator charge lithium any differently?
- The generator itself does not care, because it feeds the converter or the inverter charger rather than the bank directly. What matters is whether that charger has a lithium profile. A generator run through an old lead acid converter will charge a lithium bank badly and slowly, which owners often blame on the generator when the profile is at fault.
- Is lithium worth it on a solar only build with no inverter?
- Often yes, because solar harvest is limited by daylight and lithium accepts charge much faster in the hours you have. A lead bank tapers early in the afternoon and wastes available sun. The requirement is an MPPT controller with a lithium setting, which most modern controllers have and most older PWM units do not.
- Can I keep my AGM bank as a backup after adding lithium?
- Not in the same circuit. Two chemistries in parallel fight each other because their voltage curves differ, so one ends up doing all the work. If you want a separate reserve, it has to be genuinely separate, with its own charging path and its own switch, and that is a wiring project rather than an afterthought.
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