Lithium iron phosphate (LiFePO4) batteries — what's in modern Mars caravans — have one important quirk: they should not be charged below 0°C. Discharge is fine. Use is fine. Charging cold is the problem. Here's why, what your BMS does about it, and what it means for your trip.

The Short Version

Charging a lithium battery below 0°C causes lithium plating — metallic lithium deposits inside the cell that don't go away. Over time this permanently damages capacity and creates internal short-circuit risk. To prevent this, every quality lithium battery has a Battery Management System (BMS) that blocks charging when the battery is too cold.

What This Means in the Real World

You wake up in the high country. The van's been sitting at -2°C all night. The sun comes up, your solar panels start producing — but your battery doesn't accept the charge.

What's happening: the BMS has detected the battery temperature is below its low-temperature charging cutoff (often 0°C or +5°C depending on manufacturer). It's protecting the battery by refusing the incoming current. Your solar regulator might still show production, but it's going nowhere — or the regulator is also seeing the rejection and stopping.

This is not a fault. It is the battery protecting itself.

When You'll Run Into This

• High country / alpine areas — Vic high country, Tasmanian highlands, Snowy Mountains in winter or shoulder season
• Inland desert mornings — clear nights at altitude can drop well below zero even when daytime hits 25°C
• Southern winter generally — anywhere south of about Sydney can see overnight sub-zero
• Storage — vans stored unheated in cold areas can have batteries at 0°C or below for extended periods

What Your BMS Does (and Doesn't Do)

Modern Mars vans use lithium batteries with built-in BMS. Standard protection includes:

• Low temperature charging cutoff (charging only)
• High temperature cutoff
• Over-charge protection
• Over-discharge protection
• Cell balancing

The low-temperature cutoff temperature varies by battery brand — commonly 0°C or +5°C. Some premium batteries have internal heating elements that warm the cells before allowing charge (these are rarer but increasingly common).

What it doesn't do: warm the battery for you (unless it has internal heating). The BMS waits for the cells to warm naturally.

Discharge Is Fine — Use the Van Normally

Below freezing, you can still use the battery — lights, water pump, fridge, heater fan, USB charging, even the inverter. Discharge is unaffected by low temperatures (down to roughly -20°C, well below anything you'd reasonably camp in).

So the practical impact is: at sub-zero, you can use power but you can't replace what you're using. Until the battery warms enough for charging to resume.

How to Manage It

If Camping in Sub-Zero Temperatures

1. Plan for limited recharging on cold mornings. Solar might not start contributing until late morning when the battery has warmed.
2. Run your diesel heater overnight if you have one. Apart from keeping you warm, the heater fan circulates warm air through the van, which slowly warms the battery compartment (depending on where the battery is located).
3. Park where the morning sun hits the side of the van that has the battery compartment — speeds up warming.
4. Reduce overnight loads. Switch the inverter off. Set the fridge sensibly. Limit USB device charging overnight to one or two essentials.
5. If you have a 240V hookup or generator, charging from those is fine — same low-temp cutoff applies though, so the same warming is needed before charge accepts.

If Storing the Van in Cold Conditions

1. Disconnect or switch off the battery isolator before storage. Stops it slowly self-discharging.
2. Charge to 50–70% SOC before storing (lithium prefers being stored part-charged rather than full).
3. Check periodically — every 1–2 months.
4. If you're in a freezing area for the whole winter and the van isn't being used, this is fine. Lithium handles cold storage well — it just won't accept charge while cold.

What Not to Do

• Don't try to "force" charge by overriding the BMS. Some monitoring systems let you see BMS status. None of them allow you to override the low-temp cutoff for good reason.
• Don't apply external heat directly to the battery (heat gun, hair dryer, electric blanket). Uneven heating can damage cells.
• Don't panic. A battery that's refusing charge because it's too cold is fine — it's working as designed.

How to Check Battery Temperature

Higher-end battery monitors and some BMS apps show internal battery temperature. If yours does, check it during cold mornings. If the BMS is rejecting charge, the temperature reading will explain why.

If your monitor doesn't show battery temperature, infer it from ambient — battery compartment is usually within a few degrees of outside air, slightly slower to change. Allow a few hours after dawn before expecting normal charging behaviour.

Premium Option — Heated Battery Banks

Some lithium batteries come with built-in heating elements. They sense the cell temperature and warm the battery (using a small amount of its own stored power) before accepting charge. Eliminates the cold-morning recharging problem entirely.

If you regularly camp in alpine conditions or winter inland, ask your Mars branch about heated battery options — they're available as an upgrade on some builds.

For Your Mental Model

Think of lithium like an athlete. They perform fine in heat or cold. They train (charge) fine in normal conditions. But ask them to train hard from cold-stiff muscles and you'll cause injury.

Your BMS is the coach saying "warm up first." Trust it.

Quick Summary

• Discharge below 0°C: fine.
• Charge below 0°C: blocked by BMS to protect the battery.
• This is a feature, not a fault.
• Battery warms naturally as ambient warms — charging resumes.
• Plan cold-weather trips with limited morning recharge time in mind.
• Heated batteries are available if it's a recurring issue.

Related: How Your Mars Solar System Works · Battery Basics: Monitor, Protect & Extend · Cold Weather Camping