How long does it take to charge an RV battery?
A charge has two phases, and the second is why amp-hours divided by charger amps comes up short, by an amount that depends on the chemistry. In bulk the charger pushes constant current and the battery takes all of it; in absorption the charger holds a constant voltage and the current tapers as the battery fills. Lithium stays in bulk to about 95%, so its tail is minutes; lead-acid hands over at 80%, and the last fifth takes about as long as the first four. Not every amp-hour you push in is stored, either: lithium is 99% efficient, AGM about 85%, flooded about 80%. The same 200 Ah bank, half flat, on the 45 A converter most trailers come with:
| 200 Ah bank from 50% | Bulk | Absorption | To 100% | Stop at 80% | Ah ÷ amps says |
|---|---|---|---|---|---|
| Lithium (LiFePO4) | 2 h 1 min | 45 min | 2 h 46 min | 1 h 21 min | 2 h 13 min |
| AGM | 1 h 34 min | 3 h 29 min | 5 h 3 min | 1 h 34 min | 2 h 13 min |
| Flooded | 2 h 53 min | 6 h 25 min | 9 h 18 min | 2 h 53 min | 2 h 13 min |
2 × 100 Ah at 12 V on a 45 A converter; bulk to 95% (lithium) or 80% (lead-acid), absorption averaged at 30% of the bulk current, charge efficiency by chemistry. The last column is the plain amp-hours ÷ amps estimate, with no tail and no losses.
On the AGM bank the tail is 3 h 29 min of a 5 h 3 min charge, 69% of the time for the last 20% of the amp-hours, and the simple division comes up short by 2 h 50 min. That gap is why a generator run "never seems to finish".
Will a bigger converter charge my batteries faster?
Charging runs at the lower of two ceilings: what your chargers supply, and what the battery accepts. A bigger converter helps only when the bank will take the extra current, which is worth checking before buying one. Battle Born's BB10012 data sheet gives 50 A as both the recommended and the maximum charge current for its 100 Ah battery (0.5C); past that the converter is idling. One 100 Ah lithium from 20%, then two of them, by converter size:
| Converter | 1 × 100 Ah lithium: flows | Time to full | 2 × 100 Ah: flows | Time to full |
|---|---|---|---|---|
| 30 A | 30 A | 3 h 5 min | 30 A | 6 h 10 min |
| 45 A | 45 A | 2 h 3 min | 45 A | 4 h 7 min |
| 55 A | 50 A (bank caps it) | 1 h 51 min | 55 A | 3 h 22 min |
| 60 A | 50 A (bank caps it) | 1 h 51 min | 60 A | 3 h 5 min |
| 80 A | 50 A (bank caps it) | 1 h 51 min | 80 A | 2 h 19 min |
20% → 100%, lithium accepting 0.5C (50 A per 100 Ah). Where the bank caps the current, every larger converter gives the identical time.
On the single battery, going from 45 A to 80 A saves 12 min, and 55 A already gets all of it. On the pair, the same upgrade saves 1 h 48 min (4 h 7 min → 2 h 19 min) because a 200 Ah bank accepts 100 A. Same two products, opposite answers, decided by the bank. That's why the result panel names the binding ceiling first, and when the bank is the ceiling it recommends nothing.
A three-stage converter starts in bulk, full rated current until the battery reaches the absorption voltage. It then holds that voltage (absorption) and the battery draws less and less as it fills; this is where the hours go on lead-acid. When the current falls off, it drops to float and just holds the bank. The voltages are the makers'. WFCO's WF-8900 in lead-acid mode goes to 14.4 V while it's delivering full current, drops back to 13.6 V when the current falls to about 5 A or four hours have passed, and floats at 13.2 V (operator's manual); its lithium mode holds 14.6 V instead. Progressive Dynamics' Charge Wizard uses 14.4 V boost, 13.6 V normal and 13.2 V storage (Battery Management 101). Battle Born recommends 14.4 V bulk and absorption from a charger set between 14.2 and 14.6 V, the same window its data sheet gives for cell balancing. A converter that sits at 13.6 or 13.2 V never reaches the 14.2–14.6 V the battery makers specify for a full, balanced charge, so a lithium bank needs a lithium profile more than it needs more amps. The lithium vs AGM calculator puts that converter on the upgrade's bill.
How long to charge RV batteries with a generator?
The generator only makes 120 V for the converter, and the converter's output amps are what reach the battery, so the charge takes exactly as long as the converter takes: a "3,500 W generator" charging through a 45 A converter charges at 45 A. What the generator has to carry is the converter's input: at 14.4 V out and 85% efficiency, a 55 A converter draws about 932 W, which any inverter generator carries with the fridge and lights on top. The tool checks it against the generator saved in My Rig; with the air conditioner running too, the generator size page does the whole sum.
| Converter | Draws from the generator | Spare on a 2,200 W inverter generator |
|---|---|---|
| 30 A | 508 W | 1,692 W |
| 45 A | 762 W | 1,438 W |
| 55 A | 932 W | 1,268 W |
| 60 A | 1,016 W | 1,184 W |
| 80 A | 1,355 W | 845 W |
Amps × 14.4 V ÷ 0.85. A generator's own 12 V "DC charging" outlet is a trickle: Honda's EU2200i lists 100 W, 8.3 A (spec sheet), which would take 11 h 7 min to bring one 100 Ah lithium up from 20%. Charge through the converter.
The fuel-saving generator strategy follows from the first table: run it through bulk, when every amp the converter makes goes into the bank, and stop when absorption begins. On lead-acid that is 1 h 34 min instead of 5 h 3 min for the 200 Ah AGM above; let solar or the next campground finish the tail. Doing that every day sulfates a lead-acid bank, so give it a full charge every few days. Lithium doesn't mind a partial charge, and its tail is 45 min anyway.
How long does it take to charge RV batteries while driving?
Through the 7-pin plug alone, most of a day. The charge line is long, thin and shared with the running lights, and by the time it reaches the trailer the voltage is barely above the battery's, so it delivers "around 5 amps of maintenance current" (Mike Sokol, RVelectricity). A DC-DC charger (a Redarc, Victron Orion or Renogy unit at the trailer end, on its own heavy cable) boosts the alternator's voltage and turns the drive into a useful charge:
| Source while driving | 1 × 100 Ah lithium from 20% | 2 × 100 Ah from 20% |
|---|---|---|
| 7-pin only (~5 A) | 18 h 31 min | 37 h 2 min |
| 20 A DC-DC charger | 4 h 38 min | 9 h 16 min |
| 30 A DC-DC charger | 3 h 5 min | 6 h 10 min |
| 50 A DC-DC charger | 1 h 51 min | 3 h 42 min |
Hours of driving, engine running the whole time, nothing else drawing from the bank. Sokol's "about 4 hours" for a 25 A charger on 100 Ah lands between the 20 A and 30 A rows.
A DC-DC charger needs the alternator to spare its rating (a 50 A unit is fine on a modern truck, marginal on a small alternator with the AC on), and needs cable sized for the amps over the run. The 7-pin's wire won't do it: a 30 A charger over 20 ft needs 2 AWG at 3%. Tick "DC-DC" and "converter" together in the tool and the sources add, which is what happens when you drive to a campground and plug in: the total is then capped by the bank.
How long does solar take to recharge RV batteries?
Panels don't charge at night, and a flat-mounted array delivers about 70% of its rating even in the peak-sun hours, so a solar refill is counted in days. 400 W is 20.6 A into a 12 V bank in full sun (400 × 0.7 ÷ 13.6 V), a useful addition to a converter and a slow charger on its own. The tool adds solar as amps in full sun when you tick it with another source, and answers solar alone in days of average sun:
| Array | Amps in full sun | 100 Ah lithium from 20% | 200 Ah lithium from 20% | 200 Ah AGM from 50% |
|---|---|---|---|---|
| 200 W | 10.3 A | 1.6 days | 3.3 days | 2.2 days |
| 400 W | 20.6 A | 0.8 days | 1.6 days | 1.1 days |
| 600 W | 30.9 A | 0.5 days | 1.1 days | 0.7 days |
| 800 W | 41.2 A | 0.4 days | 0.8 days | 0.6 days |
Days of average sun at 4.5 peak-sun hours (contiguous-US annual mean) × 0.7 flat-roof derate, charge efficiency by chemistry, nothing else drawing. The solar page uses your state and season.
Four 6 V golf-cart batteries make a 440 Ah bank at 12 V. Half flat, on a 55 A converter, bulk puts back 132 Ah in 3 h. Then absorption: the remaining 88 Ah take 6 h 40 min, at an average of about 17 A, while the generator burns fuel at nearly the same rate as before. That is 69% of the run for the last fifth of the charge, which is the case for stopping a lead-acid generator charge at the end of bulk on most days.
Why is my battery stuck at 80% (or 13.6 V)?
Three usual causes, in order of likelihood. The converter is a single-stage unit fixed at 13.6 V (the output Progressive Dynamics calls typical for RV converters), or a lead-acid profile that has timed out of its 14.4 V boost; either sits below the 14.2–14.6 V a LiFePO4 battery needs to finish and balance, and the fix is a converter with a lithium profile rather than a bigger one. The bank is cold: the BMS blocks charging below a temperature that varies by maker, 25 °F for Battle Born's BB10012 (manual), 41 °F (+5 °C) for Victron's Lithium Battery Smart, and below 32 °F, resuming above 41 °F, for LiTime's 12V 100Ah TM. A winter morning's charge can be zero amps at any converter size, and the tool doesn't model temperature, so it says so. Or it is simply lead-acid in absorption, doing what the first table shows (3 h 29 min for the last 20%), and nothing is wrong. A shunt monitor answers which one at a glance; voltage alone can't tell 80% from 100% on lithium, whose curve is flat across the whole middle of its charge (the battery voltage chart prints three published LiFePO4 charts side by side, and they disagree).
Sources
- 100 Amp Hour 12 Volt LiFePO4 Deep Cycle Battery, Model BB10012 (data sheet), Battle Born Batteries: 50 A maximum charge (0.5C); absorption and cell balancing at 14.2–14.6 V; charge temperature 25–135 °F; 100 A continuous, 200 A for 30 s; 28 W heater on the BB10012H
- The Basics of Charging LiFePO4 Batteries, Battle Born Batteries: 14.4 V bulk and absorption recommended; chargers set to 14.2–14.6 V
- BB10012 Manual and Installation Guide, Battle Born Batteries, hosted by Solacity: BMS low-temperature charge protection below 25 °F
- Operator’s Manual, WF-8900-AD-MBA Series, WFCO (Arterra Distribution), hosted by Forest River: Lead-acid mode: 14.4 V bulk until the current falls to about 5 A or 4 hours pass, then 13.6 V, then 13.2 V float; lithium mode 14.6 V
- Battery Management 101, Progressive Dynamics: Charge Wizard: 14.4 V boost, 13.6 V normal, 13.2 V storage
- 12,8 & 25,6 Volt Lithium-Iron-Phosphate Batteries Smart, Victron Energy: Charge temperature +5 °C to +50 °C (41–122 °F)
- LiTime 12V 100Ah Lithium Trolling Motor Battery, LiTime: Low-temperature charge protection: stops below 32 °F, resumes above 41 °F
- Honda EU2200i Super Quiet Series Generator Technical Specifications, American Honda Motor Co.: DC output 12 V, 100 W (8.3 A)