Can You Mix Battery Brands, Sizes, or Ages in an RV or Van?
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.