Lithium (LiFePO4)  Safe to discharge to roughly 20% remaining, needs a BMS to protect against over-discharge/over-temp/cell imbalance, and does not need a maintenance/float charge the way lead-acid does
AGM  Sealed, no watering needed, but should not be regularly discharged below 50% without shortening its lifespan
Flooded lead-acid  Needs periodic watering and equalization charging, and is the most tolerant of the three of overcharging but the least tolerant of deep discharge
Never mix chemistries  In the same bank — different internal resistance and charge acceptance rates cause one battery to be chronically over- or under-charged relative to the others

— On lithium: a BMS protection trip from a deep discharge, over-temperature, or cell imbalance is far more common than a truly dead battery
— On lead-acid/AGM: sulfation from repeated deep discharge or long storage without a maintenance charge
— A parasitic draw pulling the bank down faster than expected — check for anything left switched on, and inverter standby draw
— Charge controller or converter not configured for the correct battery chemistry — see the chemistry section above

— Charge the battery fully and let it rest at least an hour with no load before reading voltage
— A battery monitor with a shunt (like a Victron SmartShunt) tracking actual amp-hours in and out is far more reliable than voltage alone, especially on lithium where the voltage curve is nearly flat across most of the charge range
— A load test that shows voltage collapsing quickly under a moderate load, even though resting voltage looked fine, points to reduced internal capacity
— Compare against the battery’s rated amp-hour capacity — a battery holding less than roughly 80% of its rated capacity is generally considered end-of-life

— Lithium batteries store best around 50–60% state of charge, not full — storing at 100% for months accelerates capacity loss
— Lead-acid/AGM batteries should be stored fully charged and kept on a maintenance/float charger — letting them sit discharged causes sulfation within weeks
— Disconnect or use a battery disconnect switch for long storage to eliminate parasitic draw entirely, rather than relying on a maintainer alone
— Extreme cold reduces usable lithium capacity and most BMS units block charging below freezing — check your BMS’s low-temperature charge cutoff before winter storage or cold-weather charging

— Partial panel shading (a vent, AC unit, or antenna shadow) can drop output far more than the shaded area alone would suggest, especially on non-optimized arrays
— Charge controller in the wrong mode or with the wrong battery profile loaded
— Loose or corroded connections at panel junction boxes or the controller — a common cause of intermittent output
— Battery already at full charge and the controller has stepped down to float — not a fault, just normal behavior

— Low-voltage shutdown: battery voltage sagged under load, often from undersized cabling or a battery bank that’s smaller than the load demands
— Overload shutdown: total connected load exceeded the inverter’s rated capacity, common with AC units, microwaves, and coffee makers on the same circuit
— Overtemperature shutdown: inverter mounted in a poorly ventilated space, or ambient temps at the mounting location are too high
— A brand-specific fault code narrows this down immediately — see the Victron or PD/WFCO/Xantrex fault code guides linked above before assuming

Repeated BMS trips, a battery that won’t accept a charge at all, inverter faults that recur after a reset, or any system with mixed brands and no clear documentation are all worth a full diagnostic. Networked systems (Victron, and similar) in particular benefit from certified diagnosis — log data reveals the real cause in minutes versus hours of guesswork.