1) Key terms in 60 seconds
DoD Depth of Discharge — how much capacity you use. Cycle One full charge+discharge. C-rate Charge/discharge power relative to capacity. BMS Battery Management System — protects lithium cells.
2) Usable energy & lifespan
| Battery type | Usable DoD (typ.) | Cycle life (typ.) | Notes |
|---|---|---|---|
| LiFePO₄ (lithium) | 90–100% | 2 500–6 000+ | Delivers most of nameplate energy with minimal degradation. |
| AGM / Gel (sealed lead-acid) | 50% (better: 30–50%) | 300–800 | Deep cycling shortens life quickly; keep DoD shallow. |
| Flooded lead-acid | 50% (better: 20–50%) | 300–600 | Maintenance required; ventilation needed. |
Example: A “5 kWh” lithium gives ~4.8–5 kWh usable; a “5 kWh” lead-acid bank safely offers ~2.5 kWh (50% DoD). You’ll often need 2×–3× the lead-acid capacity to match lithium’s usable energy.
3) Charge/discharge power (C-rate)
- Lithium (LiFePO₄): commonly 0.5–1C continuous, short peaks higher (check BMS/inverter limits).
- Lead-acid: recommended 0.1–0.2C charging; higher rates cause heat, gas, and shorter life.
Practically: lithium can absorb solar faster on a good day and deliver higher peak loads without big voltage sag.
4) Temperature, safety & maintenance
- Lithium (LiFePO₄): very stable chemistry; BMS protects against over/under-voltage & temp. Avoid charging < 0 °C unless battery is heated.
- Lead-acid: tolerant to cold but heavy, gasses if overcharged (ventilation). Flooded cells need water top-ups.
5) Total cost of ownership (TCO)
Upfront, AGM/Gel is cheaper. Over 5–10 years of regular load-shedding cycles, lithium almost always wins on R/kWh delivered because it lasts longer and you buy less capacity for the same usable energy.
| Scenario | Lead-acid bank | Lithium bank | Comment |
|---|---|---|---|
| Essential loads ≈ 3 kWh/day | ~6 kWh nameplate (50% DoD usable ≈ 3 kWh) — often replaced in 2–3 years | ~3.6–4 kWh nameplate (usable ≈ 3.2–3.8 kWh) — lasts 5–10 years | Lithium uses smaller bank and endures deep cycles better. |
| High peak appliances | Voltage sag; large bank required for surge | Handles surges better (check BMS limits) | Lithium maintains voltage under load. |
6) Charging profiles
- Lithium: simpler CC/CV; charge to ~56.0–56.8 V for 48 V packs (brand-specific), no equalize/float required. Follow battery vendor profile.
- Lead-acid: bulk/absorb/float; periodic equalize for flooded; temperature-compensated charging recommended.
7) When to choose which?
- Cycle daily due to load-shedding or self-consumption.
- Need compact, high-power, long-life storage.
- Value lower R/kWh over the system’s life.
- Have very low cycle frequency (occasional backup only).
- Have tight upfront budget and accept reduced lifespan.
- Can allocate space/weight for a larger bank.
Quick sizing example
Goal: ~3 kWh usable per day.
- Lithium: 3.6–4.0 kWh nameplate (90–100% DoD) ≈ 3.2–3.8 kWh usable.
- Lead-acid: 6 kWh nameplate (50% DoD) ≈ 3 kWh usable.
Result: Lithium requires roughly half the nominal capacity for the same usable energy.
FAQ
Is LiFePO₄ safe?
Yes — LiFePO₄ is among the safest lithium chemistries. Use reputable brands and follow installation guidelines.
Can I mix old and new batteries?
Avoid mixing ages/capacities. In lithium banks, follow vendor rules for paralleling. In lead-acid, mixing causes imbalance and premature failure.
Do I need a BMS?
Lithium batteries include a BMS. Ensure CAN/RS485 comms with your inverter where supported for best performance and warranty compliance.
This guide is informational. Always follow manufacturer specifications and local electrical codes; use a qualified installer for compliance and CoC.