The battery is the most expensive single component in an off-grid system, and the one most likely to fail first. Choosing the right chemistry determines how long your system lasts and what each stored kWh actually costs.
Lead-acid is the oldest rechargeable battery chemistry, dating to 1859. Two lead plates sit in a sulfuric acid electrolyte. During discharge, lead sulfate forms on both plates; during charge, the reaction reverses. Flooded lead-acid (FLA) batteries require periodic topping up with distilled water as electrolysis consumes the water during charging. Valve-Regulated Lead-Acid (VRLA) batteries — sold as AGM (Absorbent Glass Mat) or gel — are sealed and maintenance-free, but cost 20–40% more than flooded equivalents.
Lead-acid batteries have a flat discharge curve around 12.0–12.8 V (for a 12 V battery), dropping sharply below 50% state of charge. Discharging below 50% dramatically accelerates plate sulfation — the irreversible crystallization of lead sulfate that permanently reduces capacity. This is why lead-acid batteries are rated for only 50% depth of discharge (DoD) in practice.
LFP uses an iron phosphate cathode and a graphite anode in a lithium-salt electrolyte. The iron phosphate crystal structure is exceptionally stable — the phosphate-oxygen bonds don't break down under heat the way cobalt or nickel oxides do, which makes LFP inherently resistant to thermal runaway. Cell voltage is 3.2 V nominal (vs 3.7 V for NMC lithium), so a 4-cell series pack gives 12.8 V — a direct drop-in replacement for 12 V lead-acid.
LFP's discharge curve is remarkably flat: the voltage stays between 3.1 and 3.3 V from 10% to 90% state of charge. This flat curve means your appliances receive consistent power regardless of how full the battery is — unlike lead-acid, where voltage sags noticeably below 50%.
NMC lithium batteries offer higher energy density than LFP (about 150–220 Wh/kg vs 90–160 Wh/kg for LFP), packing more kWh into less weight and volume. Tesla Powerwall uses NMC. However, NMC has lower thermal stability than LFP and requires more sophisticated battery management systems (BMS) to prevent thermal runaway. Cycle life is typically 2,000–4,000 cycles — less than LFP. NMC also uses cobalt, a conflict mineral with supply chain concerns.
| Parameter | Lead-Acid (FLA) | Lead-Acid (AGM) | LFP | NMC |
|---|---|---|---|---|
| Cycle life (80% DoD) | 300–800 | 500–1,200 | 4,000–6,000 | 2,000–4,000 |
| Usable DoD | 50% | 50% | 80–95% | 80–90% |
| Round-trip efficiency | 75–80% | 78–82% | 92–96% | 90–95% |
| Self-discharge (/month) | 3–5% | 2–3% | 1–2% | 1–3% |
| Weight (per kWh) | 25–30 kg | 25–30 kg | 6–10 kg | 5–8 kg |
| Upfront cost (₹/kWh) | ₹6,000–9,000 | ₹10,000–15,000 | ₹18,000–30,000 | ₹22,000–35,000 |
| Cost per usable kWh-cycle | ₹1.50–3.60 | ₹1.70–3.00 | ₹0.35–0.80 | ₹0.55–1.10 |
| Operating temperature | −20 to 50°C | −20 to 50°C | −20 to 60°C | −20 to 55°C |
| Maintenance | Monthly water topping | None | None | None |
| Thermal runaway risk | Low (hydrogen gas is a risk) | Low | Very low | Moderate |
Upfront price is misleading. A ₹9,000/kWh lead-acid battery that delivers 50% DoD over 600 cycles gives you 300 usable kWh-cycles from each kWh of nameplate capacity. That's ₹30 per usable kWh (₹9,000 ÷ 300). A ₹24,000/kWh LFP battery at 90% DoD over 5,000 cycles delivers 4,500 usable kWh-cycles — ₹5.33 per usable kWh (₹24,000 ÷ 4,500).
The LFP battery costs nearly 6× less per unit of energy actually delivered over its lifetime. Factor in the round-trip efficiency difference (95% vs 78%), and the gap widens further: for every 100 Wh of solar energy you put into a lead-acid battery, you get 78 Wh back. With LFP, you get 95 Wh back. Over 5,000 cycles, that efficiency gap alone represents 850 kWh of wasted solar energy per kWh of battery capacity.
Real-world example: A farmhouse running a 5 kWh daily load needs approximately 10 kWh of lead-acid batteries (50% DoD) or 5.5 kWh of LFP (90% DoD) for one day of autonomy. The lead-acid bank costs ~₹90,000 upfront and lasts 2–3 years in daily cycling. The LFP bank costs ~₹1,32,000 upfront and lasts 12–15 years. Over 15 years, the lead-acid path costs ₹4,50,000–₹5,40,000 (5–6 replacements) versus ₹1,32,000 for one LFP bank.
Lead-acid batteries require a three-stage charging profile: bulk (constant current until ~80% SoC), absorption (constant voltage, tapering current, for 2–4 hours), and float (lower constant voltage to maintain full charge). The absorption phase is critical — cutting it short causes sulfation. A lead-acid battery takes 6–10 hours to fully charge from 50% SoC, even with ample solar power available.
LFP batteries accept charge at a constant rate (1C or higher) until nearly full, then taper briefly. A 100 Ah LFP battery can charge from 10% to 100% in under 2 hours at 1C. There is no absorption phase to wait through. This means an LFP bank captures more of the available solar window — particularly important during India's monsoon, when sunshine may be limited to 3–4 hours of broken cloud.
Lead-acid batteries hate partial cycling. If a lead-acid battery is regularly cycled between 50% and 80% SoC without periodic full charges, sulfation builds up on the plates, permanently reducing capacity. This is the single most common cause of premature lead-acid failure in solar systems — the panels don't produce enough during monsoon to complete the absorption stage, and the battery slowly dies.
LFP has no memory effect and no sulfation. It can cycle between any state-of-charge levels indefinitely with no penalty. This makes LFP fundamentally better suited to solar, where the battery's charge state depends on weather and loads, not a controlled charging schedule.
Lead-acid capacity drops significantly in heat. At 40°C (common in Indian summers), lead-acid self-discharge doubles and cycle life is reduced by 30–50%. At 50°C, grid corrosion accelerates and the battery may swell or vent gas. Flooded batteries in hot environments need water topping every 2–4 weeks.
LFP performs well up to 45°C with minimal degradation and can handle peaks to 60°C. Its flat discharge curve remains flat across the temperature range. For Indian climates — where battery enclosures on rooftops routinely reach 45–55°C in summer — LFP's thermal tolerance is a practical necessity, not a luxury.
Lead-acid batteries produce hydrogen gas during charging (especially overcharging). Hydrogen is explosive at concentrations above 4% in air. Battery rooms for flooded lead-acid banks require ventilation, spark-free fixtures, and acid-resistant flooring. Sulfuric acid spills from cracked cases are corrosive.
LFP batteries are sealed and produce no gases during normal operation. The iron phosphate cathode is thermally stable up to 270°C (versus 150°C for NMC). LFP cells don't go into thermal runaway under nail penetration, crush, or overcharge tests — they may swell and vent, but they don't ignite. This makes LFP safe to install indoors, in enclosed spaces, and in residential settings without special ventilation.
Alloy Charge PowerVault: The PowerVault uses automotive-grade LFP cells with an integrated Battery Management System that monitors each cell's voltage, temperature, and current. The BMS provides cell-level balancing, over-charge/over-discharge protection, short-circuit protection, and thermal management. Rated for 6,000+ cycles at 90% DoD in ambient temperatures up to 50°C.
Lead-acid remains viable for infrequently cycled applications: emergency backup systems that sit on float charge 99% of the time and cycle only during occasional grid outages (a few times per month). In this use pattern, cycle life is less important than upfront cost, and lead-acid's lower price per kWh nameplate capacity is an advantage. Lead-acid is also appropriate where the system will be replaced or decommissioned within 2–3 years — too short for LFP's longevity advantage to pay off.
For daily-cycling solar systems — which is most solar installations — LFP wins on every metric that matters over the system's lifetime.
We'll analyse your load profile, backup requirements, and solar generation to recommend the right LFP capacity for your property.
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