Solar batteries: complete AGM and lithium guide

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Solar batteries: complete AGM and lithium guide

Ah, kWh, cycles, BMS, AGM, GEL and LiFePO₄: the concepts that matter before buying or sizing a solar battery.

Energy — How long the battery can supply loads.
Power — How much instantaneous demand it can support.
Depth of discharge — How much nominal capacity is usable.
Compatibility — Inverter, controller and BMS must match electrically.

Ah, Wh and kWh

Amp-hours describe electric charge, not complete energy. Use watt-hours to compare batteries at different voltages. A 100 Ah 12 V battery and a 100 Ah 48 V battery are not equivalent.

Nominal energy (Wh) = voltage (V) × capacity (Ah)
Example: 12 V × 100 Ah = 1,200 Wh nominal. An AGM planned at 50% DoD offers about 600 Wh before losses. A 12.8 V 100 Ah LiFePO₄ planned at 90% offers about 1,152 Wh before losses, when permitted by its maker.

AGM, GEL and LiFePO₄ compared

There is no universal winner: initial cost, weight, peak current, temperature and expected life pull in different directions.

On a phone, swipe the table horizontally to see every column.

Criterion AGM GEL LiFePO₄
Initial cost Low/medium Medium Higher
Planning DoD Often ~50% Often ~50% Often 80–90% if permitted
Weight High High Much lower per usable kWh
High current Good, model-dependent Often less suited to peaks Good when cells and BMS permit
Cold charging Within stated limits Within stated limits Often prohibited below 0 °C
Management Correct charging required Precise thresholds matter BMS essential
Lifetime Sensitive to heat and deep discharge Good in correct cyclic use Often thousands of cycles, design-dependent

Indicative planning values, not universal specifications. The selected product datasheet prevails.

Nominal capacity is not usable capacity

Depth of discharge, efficiency, temperature and current reduce available energy. Lead-acid also exhibits the Peukert effect: effective capacity falls at high current. Lithium voltage is relatively flat, so a voltmeter alone is a poor state-of-charge gauge.

Deeper discharge gives more runtime today but may shorten tomorrow's life. Good sizing balances autonomy, DoD and reserve.

What a BMS really does

A Battery Management System monitors cell voltages, current and temperature and may disconnect the pack. It does not replace a fuse, suitable cable, charger or engineering. A 100 A BMS label does not automatically guarantee 100 A continuous battery capability.

On a phone, swipe the table horizontally to see every column.

Protection Monitors Does not fix
Over-voltage Cells charged too high Repeated wrong charger profile
Under-voltage Cells discharged too far Undersized battery
Over-current Current above threshold Thin cable or missing fuse
Temperature Charge/discharge limits Poor installation environment
Balancing Cell differences Damaged cells or weak pack design

12, 24 or 48 V?

For the same power, higher voltage reduces current, voltage drop and cable burden. Every component must still support the nominal and maximum voltage.

On a phone, swipe the table horizontally to see every column.

Load power Ideal current at 12 V at 24 V at 48 V
600 W 50 A 25 A 12.5 A
1,500 W 125 A 62.5 A 31.3 A
3,000 W 250 A 125 A 62.5 A

What shortens battery life

  • sustained high temperature;
  • leaving lead-acid discharged;
  • charging lithium below its minimum temperature;
  • wrong charge thresholds or lead-style equalisation on lithium;
  • excess current, loose connections and imbalance;
  • long storage at extreme state of charge, depending on chemistry.
Size for the coldest winter, the hottest equipment room and the battery's internal temperature—not a pleasant spring afternoon.

Choose by answering seven questions

  1. Daily energy use in Wh?
  2. Required sunless autonomy?
  3. Inverter peak power?
  4. Allowed DoD for expected life?
  5. Installation temperature range?
  6. Chemistry and communication supported by charger/inverter?
  7. Adequate space, protection, ventilation and service?

Calculate the requirement before choosing chemistry

First define energy and current; then compare lead and lithium for the real duty.

AGM and GEL batteriesLithium batteries

Frequently asked questions

How many kWh is 100 Ah?

It depends on voltage: about 1.2 kWh nominal at 12 V and 4.8 kWh at 48 V.

Can I replace AGM with lithium and keep everything?

Not automatically. Check charger, controller, inverter, BMS, temperature, fuses and cable.

Do two batteries in parallel simply double capacity?

Theoretically yes, but they need compatibility, symmetric wiring and branch protection.

Does voltage show state of charge?

Only roughly at rest; LiFePO₄ voltage is particularly insensitive across its middle range.

Which lasts longer?

Chemistry matters, but temperature, DoD, current and system quality often dominate.

Related guides

Sizing and troubleshooting · MPPT controller guide

Technical sources

Percentages are planning examples; always use the selected battery's specified limits.