Can you mix different battery brands in a van?

Technically possible but not recommended. LiFePO4 batteries from different brands have slightly different BMS thresholds, internal resistance, and capacity tolerances. Mismatched batteries in parallel will charge and discharge unevenly — the battery with the lower BMS trip voltage will disconnect before the other, forcing all remaining current through the other battery. Same-brand, same-model, same-age is ideal for parallel banks.

Can you mix a new LiFePO4 battery with an old one?

Not recommended. An older battery has higher internal resistance and lower actual capacity than a new one. In parallel, the new battery does most of the work — the old one is along for the ride and may cause the new battery to underperform. Better to replace all batteries at once or add a new bank wired separately from the old one.

Mixing RV Batteries: Brands, Ages and Capacities

Do not combine batteries unless their manufacturers approve the proposed arrangement; chemistry, voltage, condition and BMS behaviour all matter.

Mixing batteries is tempting — you have one old battery and want to add a second. Here's why it's more complicated than it seems and what to do instead.

What happens when you mix batteries

In a parallel bank, batteries share current based on their internal resistance. Lower internal resistance = more current flows through that battery.

Mismatched brand/age: Battery A has 5mΩ internal resistance; Battery B (older, different brand) has 12mΩ. Battery A carries 70% of the current, Battery B carries 30%. Battery A heats up more, cycles harder, degrades faster.

Mismatched capacity: A 100Ah battery in parallel with a 200Ah battery creates an imbalanced bank. The 200Ah battery stores more energy but the 100Ah BMS trips first during discharge, leaving the 200Ah battery disconnected in a fault scenario that's confusing to diagnose.

Mismatched BMS thresholds: Different brands have different low-voltage cutoff points. If Battery A cuts off at 10V and Battery B cuts off at 11V, Battery B disconnects during a deep discharge while Battery A keeps going — then Battery A is left carrying all the load.

What's actually safe to mix

Same brand, same model, different production dates: Generally fine. Cells from the same production line have very similar characteristics. Internal resistance may differ slightly as one battery has more cycles, but the difference is small if both batteries are relatively new.

Same brand, same capacity, slightly different age (under 200 cycles): Usually acceptable. Monitor the bank for uneven heating between batteries.

AGM + LiFePO4 in parallel: Never do this. AGM and LiFePO4 have completely different charge profiles, voltage ranges, and self-discharge rates. They will fight each other continuously.

Better alternatives to mixing

Replace both batteries at once: The cleanest solution. Sell or repurpose the old battery.

Add a separate second bank: Wire a new 100Ah LiFePO4 as a completely separate bank on its own bus bar, charged by its own MPPT output. Use a battery combiner or manual switch to select which bank to draw from. More complex but avoids all the parallel-mismatch problems.

Upgrade to a single higher-capacity battery: A single 200Ah battery (LiTime 200Ah ~$480) is simpler and more reliable than two 100Ah batteries of different ages in parallel. No balancing issues.

Mixing LiFePO4 with AGM (for legacy builds)

Some RV converters combine a factory AGM starting/house battery with an added LiFePO4 house battery. The only safe way to do this is with a DC-DC charger between them — the DC-DC charger isolates the two batteries and provides the correct charge profile for each. Never wire AGM and LiFePO4 directly in parallel.

Worked planning check

An old 100Ah battery with reduced capacity and a new 100Ah battery should not be assumed to form a healthy 200Ah bank. In a series bank, the weaker unit can reach its limit first; in parallel, current may share unevenly. Obtain a supported design before connecting them.

The figures or dimensions in this example are explicit planning assumptions, not a product test or an installation specification. Replace them with your equipment data and actual measurements.

Planning questions

How much energy is in a 100Ah lithium battery?

At nominal 12.8V, the arithmetic is 1,280Wh. Delivered energy is lower after your reserve, losses and operating limitations; it is not a measured capacity result.

Can I mix battery brands or capacities?

Only with explicit manufacturer approval for the complete arrangement. Chemistry, voltage, age, current sharing and BMS behaviour can make a seemingly simple combination unsuitable.

Can lithium batteries charge in freezing weather?

Follow the exact battery’s permitted temperature range and heating conditions. Do not bypass a protective cutoff or infer charge permission from air temperature alone.

Use the free electrical planner to organise your daily loads and charging assumptions.