A truck bed camper or a canopy-covered bed doesn’t need a big power system to run a 12V fridge, lights and phone charging for a long weekend. A single 100Ah lithium battery, one or two 100W flexible panels, and a charger that also uses the truck’s alternator will cover most trips. LiTime’s 12V 40A DC-DC charger with MPPT combines the alternator and solar charging in a 5.8 × 1.9 × 7.4-inch box, according to the company’s spec sheet. Renogy lists its 100W flexible panel at 5.3 lb and 0.1 inch thick, so you can glue it to a fiberglass cap without drilling the roof. This guide covers sizing, parts and wiring order, based on the makers’ datasheets and manuals.
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The system at a glance
- Collect: one or two Renogy 100W flexible panels on the cap or camper roof.
- Charge: a LiTime 12V 40A DC-DC charger with built-in MPPT. It takes solar input and alternator input from the truck’s starter battery.
- Store: a LiTime 12V 100Ah Xtra-Mini LiFePO4 battery, which is small enough to fit beside a wheel well.
- Use: a fused 12V distribution block feeding the fridge, lights, fan and USB outlets.
That’s four main parts plus cable, fuses and connectors. There’s no inverter in this build. Skipping AC keeps the system cheap and efficient. If you later want to run a laptop brick or a coffee maker, add an inverter sized to the load.
Step 1: Add up what you’ll run
Start with your daily load in amp-hours (Ah) at 12V. The figures below are an example for a weekend camper. The fridge figure comes from Dometic’s listed 0.92 Ah per hour for its CFX3 25 at a 90°F ambient. The other rows are typical estimates, so swap in the numbers from your own device labels.
| Load | Draw | Hours/day | Ah/day |
|---|---|---|---|
| Compact 12V compressor fridge (hot weather) | 0.92 Ah/h average | 24 | ~22 |
| LED strip lights (10W total, estimate) | ~0.8A | 4 | ~3.3 |
| Phone and headlamp charging (estimate) | ~30 Wh/day | n/a | ~2.5 |
| Small 12V vent fan on low (estimate) | ~1.5A | 8 | ~12 |
| Total | ~40 Ah/day |
The fridge is the biggest single load. For help choosing one that fits a tight bed space, see our new guide to the best 12V fridge for a car, which compares 25–36 L models.
Step 2: Pick a battery that fits the space
At about 40 Ah a day, a 100Ah LiFePO4 battery gives about two and a half days with no charging at all (estimate). Space is usually the real limit in a truck bed, so a compact case helps.
| LiTime 12V 100Ah Xtra-Mini (LiTime spec) | Value |
|---|---|
| Size | 9.02 × 5.43 × 8.19 in (Group 22NF) |
| Weight | 19.13 lb |
| Energy | 1,280 Wh (12.8V, 100Ah) |
| BMS / continuous current | 100A charge and discharge, 500A for 1 second |
| Charge voltage / recommended current | 14.4V ± 0.2V / 20A |
| Charging temperature | 32°F to 122°F |
| Low-temperature protection | Charging stops below 32°F and resumes at 41°F or above |
| Extras | Bluetooth, IP65, M8 terminals, 4P4S expansion |
| Cycle life | 4,000 at 100% DOD; 6,000 at 80%; 15,000 at 60% |
| Warranty | 5 years |
The low-temperature cutoff matters in a truck bed, because the battery sits at outside temperature. If a frosty morning stops it from charging, the battery will start accepting charge again once it warms to 41°F. LiTime says not to use these batteries as starter batteries. This one stays on the house side only.
See the LiTime 12V 100Ah Xtra-Mini on Amazon or on LiTime’s store ($375.99 on Oct. 17, 2026).
Step 3: Choose panels that match the charger
This is the step people most often get wrong. Here are Renogy’s datasheet figures for the RNG-100DB-H flexible panel (current G3 version):
| Renogy 100W flexible | Value |
|---|---|
| Vmp / Imp | 20.7V / 4.84A |
| Voc / Isc | 24.4V / 5.16A |
| Size / weight | 43.0 × 22.9 × 0.1 in / 5.3 lb |
| Temperature coefficient of Voc | -0.29%/°C |
| Output cables | 12 AWG, 27.6 in |
| Price (Renogy store, Oct. 17, 2026) | $149.99 |
LiTime’s product page for the DC-DC charger gives a maximum PV input of 26V in its feature list and “600W/30V” in its spec table. The safe approach is to plan around the lower 26V figure, so wire two panels in parallel, not series. Two in series would reach about 48.8V open-circuit and exceed both ratings. Two in parallel stay at 24.4V and deliver about 9.7A at maximum power.
Cold weather pushes panel voltage up. Using Renogy’s -0.29%/°C coefficient, a 24.4V panel reads about 26.2V at 32°F and about 26.9V at 14°F (my estimates). Both are inside the 30V spec-table figure but slightly over the 26V figure. If you camp in freezing weather, confirm the true limit in LiTime’s manual or with LiTime support before you buy panels.
If you’d rather use higher-voltage panels, such as the 200W flexible models with a 36.4V Voc, run them through a standalone MPPT controller like the one in our SunSwift 60A MPPT review, and use the DC-DC charger for alternator input only.
See the Renogy 100W flexible panel on Amazon or on Renogy’s store.
Step 4: Let the truck do half the work
A truck camper has something a cabin doesn’t: an alternator that runs every time you drive. LiTime rates the charger at 40A output, up to 600W from the alternator side, and up to 93% charging efficiency. It works with conventional and smart alternators. For smart alternators, LiTime says to connect the supplied IGN signal wire to the vehicle ignition port, so the charger knows when the engine is running.
LiTime lists a few other useful features. A built-in voltage-sensitive relay keeps the starter and house batteries isolated, and solar can trickle-charge the starter battery once the house battery is full. A temperature sensor adjusts charging to the ambient temperature. An emergency reverse-charging mode lets the house battery top up a weak starter battery.
| LiTime 12V 40A DC-DC with MPPT (LiTime spec) | Value |
|---|---|
| Charge current | 40A |
| Battery types | SLA, AGM, GEL, Calcium, Lithium |
| Min. solar input voltage | 10V |
| Size / weight | 5.83 × 1.89 × 7.44 in / 2.43 lb |
| Operating temperature | 14°F to 113°F |
| Waterproof | No (LiTime warns it isn’t for marine use) |
| Price (LiTime store, Oct. 17, 2026) | $209.99 |
Because the charger isn’t waterproof, mount it inside the camper or cap, not out in the open bed.
See the LiTime 40A DC-DC charger with MPPT on Amazon or on LiTime’s store.
How the daily energy budget balances
| Source | Assumption | Ah/day (estimate) |
|---|---|---|
| One 100W panel | 4.84A × 5 sun hours × 75% real-world factor | ~18 |
| Two 100W panels (parallel) | Same assumptions | ~36 |
| Alternator via DC-DC | 1 hour of driving at up to 40A | up to ~40 |
| Daily use (from Step 1) | Hot-weather example | ~40 |
Two panels nearly cover the example load on a sunny day while you’re parked, and any driving adds a cushion. In cooler weather, the fridge draws less and the budget gets easier. In shade or winter, expect less solar and lean more on driving. To understand where the 5-sun-hour assumption comes from and why rated watts rarely show up in practice, read solar panel specs explained.
Step 5: Wire it in the right order
- Mount the battery and charger first. Keep the DC-DC charger close to the house battery to keep its output cable short, and secure the battery so it can’t shift on rough roads.
- Run the alternator feed. Route a cable from the starter battery positive to the charger’s input, with a fuse near the starter battery. Size the cable for 40A plus the run length; our 12V wire gauge chart and voltage drop guide walk through it. Truck-to-camper runs are often long, so check voltage drop rather than guessing.
- Connect the IGN wire if your truck has a smart alternator, as LiTime instructs.
- Connect the charger output to the house battery, with a fuse close to the battery positive.
- Connect the panels last. Mount them (see below), join them in parallel with a branch connector, and bring the cable in through a sealed roof gland. Cover the panels or wait until dark before making the final connection.
- Add the load side. A fused distribution block from the house battery feeds each 12V device on its own fuse.
- Set the battery type on the charger to lithium and confirm it charges in the Bluetooth app.
Mounting on a cap or camper roof
A Renogy 100W panel is 43 × 22.9 inches. Two side by side need about 46 inches of width, or two end to end need about 86 inches of length, before you leave room for roof racks, vents and lights. Measure your cap before ordering. On a short bed, one panel plus more alternator charging may be the realistic layout.
Renogy’s flexible-panel manual calls for mounting flat or with less than a 30° bend and recommends structural adhesive instead of drilling through the panel. For full surface preparation and bonding steps, see our flexible solar panel install guide. The same adhesive method works on a fiberglass cap as on an RV roof, but confirm with your cap maker that the roof skin can carry a bonded panel.
Parts list and cost (estimate)
| Part | Qty | Maker price, Oct. 17, 2026 |
|---|---|---|
| Renogy 100W flexible panel | 2 | $299.98 |
| LiTime 40A DC-DC with MPPT | 1 | $209.99 |
| LiTime 12V 100Ah Xtra-Mini | 1 | $375.99 |
| Cable, fuses, branch connector, roof gland, fuse block | 1 set | varies by run length |
| Core parts total | ~$886 before wiring |
FAQ
How much solar do I need for a truck bed camper?
For a fridge, lights, a fan and phone charging, 100–200W is a common range. Using the estimates above, two 100W panels produce about 36 Ah on a sunny day, close to a 40 Ah hot-weather load.
Can I charge the camper battery while I drive?
Yes. A DC-DC charger like LiTime’s 40A model takes alternator power and charges the house battery at up to 40A. It also works with smart alternators when you connect the IGN wire.
Should I wire two 100W panels in series or parallel?
With this charger, parallel. LiTime lists a 26V maximum PV input in one place and 30V in another. Two Renogy panels in series would be about 48.8V open-circuit.
Will a lithium battery charge in a cold truck bed?
Not below freezing. LiTime’s Xtra-Mini stops charging under 32°F and resumes at 41°F or above. Discharging is rated down to -4°F.
Do I need an inverter?
Not for 12V fridges, lights, fans and USB charging. Add one only if you have AC appliances, and size it to their surge and running watts.
Can I use the starter battery instead of a second battery?
It isn’t a good idea for overnight loads. A separate house battery charged by a DC-DC charger keeps the starter battery ready to start the truck.
Bottom line
For a truck bed camper, a small system is easiest to live with: one 100Ah LiFePO4 battery, one or two 100W flexible panels in parallel, and a DC-DC charger with MPPT so driving and sunshine both refill the battery. Match the panel voltage to the charger’s input limit, keep the charger dry inside the cap, and size the cable for the long run back to the starter battery.