The 3 Types of Batteries for Power Systems: FLA, AGM, and LiFePO4
The three primary battery types for off-grid, solar, and backup power systems are Flooded Lead-Acid (FLA), Sealed Valve-Regulated Lead-Acid (AGM/Gel), and Lithium Iron Phosphate (LiFePO4), each defined by their internal electrolyte state and anode/cathode chemistry. In a real circuit or installation, the battery type you choose fundamentally changes your charge controller's absorption voltage, float setpoints, equalization requirements, and whether you need active ventilation or temperature compensation sensors.
When makers and DIYers ask 'what are the 3 types of batteries', they are often confusing consumer 'Lithium-ion' (NMC/NCA chemistries used in EVs and power tools, which carry thermal runaway risks) with 'Lithium Iron Phosphate' (LiFePO4, the incredibly stable, non-combustible standard for 12V/24V/48V stationary banks). While NMC offers higher energy density, LiFePO4 is the only lithium chemistry you should be wiring into a home solar shed or server rack without specialized fire suppression.
Worked Numeric Example: Sizing a 24V Bank for 2.4 kWh Usable
To understand how these three types change your physical build and budget, let's size a 24V battery bank that must deliver 2.4 kWh of usable energy per day (equivalent to 100Ah at 24V drawn completely). Because lead-acid batteries suffer severe lifespan degradation if discharged past 50%, while LiFePO4 safely handles 90% Depth of Discharge (DoD), the rated capacity you must purchase changes drastically.
1. Flooded Lead-Acid (FLA)
- DoD Limit: 50%
- Required Rated Capacity: 4.8 kWh (24V × 200Ah)
- Physical Build: Four 6V, 200Ah golf-cart batteries wired in series.
- Weight: ~240 lbs (requires reinforced floor joists).
- Estimated Cost: $800 ($200 per battery).
2. Absorbent Glass Mat (AGM)
- DoD Limit: 50% (to achieve >500 cycle life)
- Required Rated Capacity: 4.8 kWh (24V × 200Ah)
- Physical Build: Two 12V, 200Ah sealed AGM batteries wired in series.
- Weight: ~260 lbs.
- Estimated Cost: $1,100 ($550 per battery).
3. Lithium Iron Phosphate (LiFePO4)
- DoD Limit: 90%
- Required Rated Capacity: 2.66 kWh (24V × 110Ah)
- Physical Build: One single 24V 100Ah server-rack battery (or two 12V 100Ah in series).
- Weight: ~65 lbs.
- Estimated Cost: $650 (for a high-quality 24V unit with internal BMS).
The Takeaway: Even though LiFePO4 has a higher upfront cost per *rated* amp-hour, it is actually cheaper per *usable* amp-hour and weighs 75% less than the AGM equivalent.
Where You Meet This in Practice
You will encounter these three chemistries in distinct environments based on their failure modes and maintenance profiles:
- FLA (Flooded Lead-Acid): Found in budget off-grid cabins, golf carts, and legacy forklifts. Where it fails: If you forget to top off the cells with distilled water monthly, the exposed lead plates sulfate and the battery bricks permanently. They also off-gas hydrogen during the absorption charge phase, requiring a vented battery box.
- AGM (Sealed Lead-Acid): Found in UPS data center racks, marine house banks, and mobility scooters. Where it fails: AGM batteries are highly sensitive to overcharging. If your charge controller's absorption voltage is set even 0.5V too high, the internal electrolyte boils off (dries out) and vents through the one-way valve. Once an AGM dries out, it cannot be rehydrated.
- LiFePO4 (Lithium): Found in modern RV conversions, home solar sheds, and telecom backup. Where it fails: Charging below freezing (32°F / 0°C) causes lithium plating on the anode, which permanently degrades capacity and can cause internal shorts. You must use a Battery Management System (BMS) with Low-Temperature Cut-Off (LTCO) to physically block incoming charge current in cold weather.
Decision Tree: Which Battery Chemistry Should You Buy?
Use this decision matrix to select the exact chemistry and a proven, reliable part number for your build. Do not mix chemistries in the same bank.
| Your Scenario & Constraints | Recommended Chemistry | Concrete Pick (Part / Model) |
|---|---|---|
| Budget is under $200/kWh, you have a vented outdoor shed, and you don't mind monthly watering and periodic equalization charges. | FLA (6V Golf Cart) | Trojan T-105 RE (6V, 200Ah) |
| Indoor installation (closet/bedroom), zero maintenance allowed, primarily for standby UPS use (rarely cycled below 80% SoC). | AGM (Sealed 12V) | Fullriver FT200-12 AGM (12V, 200Ah) |
| Daily deep-cycling (solar/RV), want a 10-year lifespan, limited space/weight capacity, and budget allows $400-$800 per 100Ah block. | LiFePO4 (Server Rack) | EG4 48V 100Ah Server Rack (5.12kWh) |
| Marine environment, extreme vibration, budget is secondary to safety and spill-proof operation, but daily cycling is required. | LiFePO4 (Marine Drop-in) | Relion RB12100 (12V, 100Ah) |
Common Confusions and Charging Profile Mistakes
The most common way DIYers destroy a new battery bank is by applying the wrong charging profile. Your solar charge controller (MPPT) or inverter/charger must be configured for the specific chemistry.
- The 'Lithium Float' Mistake: Lead-acid batteries require a 'float' stage (typically 13.5V for 12V systems) to maintain 100% SoC without overcharging. LiFePO4 batteries do not need and should not be held at a float voltage indefinitely. Set your LiFePO4 float to 13.5V-13.6V (just enough to keep the BMS awake) or disable float entirely if your inverter allows it, relying on absorption up to 14.4V.
- The Equalization Hazard: FLA batteries require a periodic 'equalization' charge (pushing voltage up to 15.5V+ to intentionally boil the electrolyte and mix the stratified acid). If you accidentally run an equalization profile on an AGM or LiFePO4 battery, you will permanently vent the AGM or trigger the LiFePO4 BMS over-voltage disconnect, potentially damaging connected electronics with voltage spikes.
- Alternator Burnout: LiFePO4 batteries have incredibly low internal resistance. If you connect a 12V LiFePO4 bank directly to a vehicle's standard alternator to charge while driving, the battery will pull maximum current continuously, overheating and melting the alternator diodes. You must use a DC-to-DC charger (like a Victron Orion-Tr Smart) to limit the current draw.
FAQ: Battery Bank Edge Cases
Can I wire an old AGM battery in parallel with a new LiFePO4 battery to save money?
No. The resting voltage curves are entirely different. A fully charged AGM rests around 12.8V, while a fully charged LiFePO4 rests at 13.4V. If wired in parallel, the LiFePO4 will endlessly dump current into the AGM trying to equalize the voltage, draining itself and potentially triggering its BMS low-voltage disconnect. Always keep chemistries on separate busbars with separate charge controllers.
Do I need a special multimeter to test LiFePO4?
Standard multimeters measure voltage fine, but voltage is a poor indicator of State of Charge (SoC) for lithium. A LiFePO4 cell will read between 13.1V and 13.3V for 80% of its discharge cycle. To know your actual capacity, you need a shunt-based battery monitor (like a Victron SmartShunt) that uses Coulomb counting to track exact amp-hours in and out of the bank.
What happens if my LiFePO4 BMS shuts down due to low voltage?
If you drain the battery past the BMS low-voltage cutoff (usually ~10.5V for a 12V system), the BMS opens the internal MOSFETs to protect the cells from copper dissolution. Most standard solar charge controllers cannot 'wake up' a dead lithium battery because they require a minimum battery voltage to turn on. You will need a dedicated lithium battery charger with a '0V recovery' or 'wake-up' function to force a small current into the cells and raise the voltage above the BMS threshold.
For deeper technical specifications on battery degradation curves and thermal limits, refer to the testing data published by Battery University and the installation guidelines from the U.S. Department of Energy. Always verify your specific inverter's firmware supports custom lithium charging profiles before wiring up your bank.
