LRDT Long Range Desert ToolsLRDT
Build
Build / Electrical

Electrical and auxiliary power

How much power does the truck use in a day, how long do the batteries last, and does charging keep up? List what draws power, the batteries it draws from and what charges them. The fridge works from the maker’s own test points, every figure says where it came from, and anything nobody has confirmed stays unknown instead of turning into a runtime.

How to use this 4 steps
  1. Pick your truckIts wheelbase fills in for the weight figures, and its factory battery and alternator show for the model year, for reference. Nothing about its wiring is assumed.
  2. List what draws power and what stores itThe fridge, lights and anything else, each with the hours it runs and the battery it draws from. Then the starter battery, any auxiliary battery and any power station.
  3. Add what charges themSolar panels and their controller, the alternator, a charger between batteries, a combiner. Then a month and place for the sun, and how long you drive.
  4. Read the day, the runtime and the balanceEnergy for a parked day, a driving day or one night, how many days each battery lasts with nothing charging, and whether charging keeps up.

Nothing here is saved. The page reads your saved rig to pick the truck and never changes it. Copy the link to keep your list.

Your truck

Pick your truck, then list what draws power.

Profile
Show

Daily energy

1What draws powerThe fridge, lights and the restAdd

Each load has one or more modes (a dual-color light bar might be white, amber or both), each with its input power, the hours it’s on and how much of that time it actually runs. Blank means unknown, not zero: put 0 only for a mode that’s really off.

2Batteries and power stationsWhat stores the energyAdd

Each battery and power station is its own store. They only pool if you say they’re joined. Usable energy is nominal × depth of discharge × availability. The depth of discharge defaults are LRDT planning figures (lead acid and AGM 50%, NMC and LiFePO4 80%); your battery maker’s limit replaces them.

3Charging and other partsSolar, chargers, the winchAdd

Everything that charges a battery is a charger: a solar controller (its panels say which controller they feed), the alternator or a DC-DC charger, a charger that moves energy from one battery to another, or shore power, which is recorded but never counted off grid. A combiner joins two batteries while they charge. Panels, the winch and anything else count in the weight. A winch from the catalog gives its current at the line pull you enter, from the maker’s own table.

4Weight and positionWhere each part sitsAdd
in

Where these numbers come from Sources, formulas, status labels

Energy figures come from published ratings, the hours you enter and the maker’s test conditions, not a meter on your truck. A fridge in the sun, warm contents or an open lid uses more. Use at your own risk.

Battery runtime

No charging

Idealized: no charging, no delivery losses unless you entered an inverter, no temperature or age effects unless you entered availability, and a low-voltage cutoff (the fridge’s own included) can stop a load before this energy is used. Use at your own risk.

Charging and recovery

Where, when and how long you drive

A daily planning balance from typical-year sun, the figures you enter and the makers’ ratings, not a meter on your truck. Clouds, shade, heat, a battery held near a charger’s switch voltage and charge taper all put back less. Use at your own risk.

What it weighs

Static, level ground

Static weights on level ground. Nothing here approves a mount, a battery box or a location. Use at your own risk.

Not worked out yet

Later phases

Wire sizing and the wiring diagram come next. Until then the page works out neither, and shows nothing as zero in their place.

    How the numbers work

    How is a load’s daily energy worked out?

    Input power times the hours it’s on times the share of that time it actually runs: E = P × t × D. A light that’s on is on, so D is 1. When the maker publishes an average over time, as Dometic does for its fridges, the cycling is already in the figure and no duty factor is applied on top.

    Why not use the fridge’s rated current?

    The 7.5 A Dometic prints for the CFX3 35 is the most it’s rated to draw, for sizing the wire. The compressor cycles, so its average is far lower. Dometic publishes that average as Ah per hour at set ambient and interior temperatures: 0.47 at 20 °C and 0.98 at 32 °C, both with a 4 °C interior. At the 12 V reference that’s 135.36 and 282.24 Wh a day.

    What about other temperatures?

    Published mode answers only at Dometic’s own test points. Planning estimate mode draws a straight line between them, or past them, and labels every figure estimated. Measured mode takes your own 24-hour reading from the fridge’s app or a battery monitor, which beats both. Sun on the box, warm contents, opening the lid and freezer use all add draw that no published point covers.

    Why does the starter battery need a number from me?

    How much a starter battery can give before the truck may not start depends on the battery, its age, the cold and the engine, so there’s no planning default for it. Until you say how much of it you’re willing to use, its runtime stays unknown. Auxiliary batteries and power stations get the LRDT planning depth of discharge for their chemistry unless you enter the maker’s limit.

    Why is my runtime unknown when most of it is filled in?

    Because one load with no power rating, or no battery it draws from, could be the biggest one. The page shows the known part and a runtime from only the known loads, labeled incomplete, so you can see the arithmetic without reading it as your truck’s runtime. Delete a load you don’t have, or put 0 hours on a mode you don’t use, and it stops counting as unknown.

    How is solar worked out?

    The panels’ nameplate watts times the month’s peak sun hours at your place (from NREL’s PVWatts, flat or tilted) times a system factor for heat, a flat mount, dust, shade and wiring losses. The controller then caps the power at its output current times the battery voltage: a Victron 75/15 gives 15 A, about 180 W at 12 V. Two 100 W panels at 4 sun hours and a 65% factor make 520 Wh, under that cap. The 65% is an estimate until you measure yours; a day’s yield from the controller’s app is better still.

    Why does a car charger to a power station only count while charging?

    DJI’s 1 kW car charger charges the station only while the car battery is above a voltage you set, 13.5 V by default. A resting battery sits below that, so in practice it runs only while the engine or the sun is charging, and it takes its share of that charge before the batteries it draws from do. The page counts it then, and never as a load on a resting battery.

    What does a battery do to the axles?

    A part of mass m sitting x behind the front axle on a wheelbase L puts m × x ÷ L on the rear axle and the rest on the front. Behind the rear axle the front share goes negative: it lifts weight off the front. A replacement battery counts as its mass minus the stock battery’s. Without a position the page counts the weight and leaves the axle shares unknown.

    Use at your own risk. These calculators are here to help you plan and to inform your decisions, not to sign off on a build. Many numbers are estimates worked out from factory figures, and some factory figures are not yet confirmed. Check every number against your own truck, its manuals and a qualified shop before you buy parts, cut, weld, lift, regear or drive on it. Long Range Desert Tools is not liable for anything you do based on these calculators or their results. The full terms are in our policies.