12 V Lead‑Acid (AGM/SLA) Solar Charger

Build a 12 V Lead‑Acid (AGM/SLA) Solar Charger. Robust 12 V system for lights/routers. Two paths: (A) off‑the‑shelf PWM controller (recommended), or (B) DIY PWM core for learning.

Chosen specs

Panel: 50 W, 12 V nominal, Vmp ≈ 18 V, Voc ≈ 22 V, Isc ≈ 2.8–3.0 A
Battery: 12 V 18 Ah AGM (sealed lead‑acid)
Charge targets (25 °C): Absorption 14.4 V, Float 13.7 V; temp‑comp −3.6 mV/°C/cell
Load: Up to ~3–4 A continuous if panel/battery sized accordingly

Path A — Using a ready PWM controller (best reliability)

Wiring.

Panel + ─ Fuse(5A) ──> Controller PV+

Panel − ─────────────> Controller PV−

Battery + ─ Fuse(10A, near battery) ─> Controller BATT+

Battery − ────────────────────────────> Controller BATT−

Load +/− (optional) ──────────────────> Controller LOAD +/−

Controller settings

Battery type: Sealed/AGM
Absorb/Bulk: 14.4 V (25 °C)
Float: 13.7 V
Low‑voltage disconnect (LVD): 11.0–11.4 V
Temp sensor: connect to battery side if available

BoM

PWM Controller: 10 A class (common generic or branded)
Panel: 50 W, 12 V nominal
Battery: 12 V 18 Ah AGM
Fuses: 5 A panel+, 10 A battery+
Wiring: 14 AWG panel→controller (≤3 m), 12 AWG controller↔battery if higher load
TVS: SMBJ24A across PV input (optional)

Path B — DIY PWM core (educational)

Concept: MOSFET chops panel current; comparator supervises battery voltage with hysteresis; TL431 provides reference; NTC adds temp‑comp. (Use only if you’re comfortable; a small mistake can over/under‑charge.)

Target values (example)

Sense divider: R1 = 120 kΩ, R2 = 10 kΩ → 14.4 V maps to ~1.2 V ref (if using TLV431 1.24 V ref)
Hysteresis: Rfb ≈ 330 kΩ → ~200 mV; so resumes at ≈14.2 V
Float mode: Add second threshold ~13.7 V (second comparator) or reduce duty cycle after tAbsorb ~2 h (MCU optional)

BoM (DIY core)

IRLZ44N (logic‑level MOSFET, 47 A, low Rds(on)) + heatsink
TLV431 (1.24 V ref) or TL431 (2.495 V) and adjust divider accordingly
LM393 dual comparator or rail‑to‑rail op‑amp
NTC 10 kΩ epoxy bead + 22 kΩ series for temp‑comp curve (tune)
Snubber parts: 100 nF film, 100 Ω 0.5 W
Diodes: SS56 blocking diode
Misc resistors/caps, perfboard, terminals

Testing (both paths)

Verify controller off‑line thresholds with bench supply first (11–15 V sweep).
In sun, measure: panel V ≈ 17–19 V open; battery climbs to 14.4 V then holds; current tapers.
Confirm float 13.6–13.8 V after absorption.

Universal Protections & Build Quality

Fuses close to sources: panel+, battery+; size at 1.25× expected max current.
Reverse polarity: series Schottky or PFET ideal diode for battery outputs.
TVS diodes on long leads and where loads are inductive.
Wire gauge: keep voltage drop <3% at max current; use 18 AWG for ≤2 A, 14 AWG for 3–10 A short runs.
Thermals: ensure buck/boost heatsinks or mount to aluminum plate.
Enclosures: IP65+ with cable glands; include drip loops. For Li‑ion, don’t fully seal—allow venting.

Quick Bring‑Up Checklists

5 V no‑battery: 5.00 V set, load 1 A holds ≥4.9 V; panel Voc ~22 V; buck input >7 V.

Li‑ion: TP4056 P+ = 5.0 V; charge LED on; battery rises from ~3.6 → 4.2 V; boost gives 5 V under 0.5–1.0 A.

12 V SLA: Bulk to 14.4 V; after 1–3 h at 14.4 V (sun permitting), float at 13.7 V; LVD ~11.2 V.

Optional Upgrades

MPPT controller for 12 V system improves harvest with larger panels.
Power‑path management IC (e.g., CN3065/CN3791 or LTC4054 family) for better Li‑ion load sharing.
Ideal diode ORing (LM5050/LM74800) for robust reverse protection and minimal drop.
Instrumentation: tiny wattmeter or INA219 for current/voltage logging.

Simple Sourcing Guidance

All modules listed (LM2596/MP1584 bucks, TP4056 with protection, XL6009/MT3608 boost, 10 A PWM controllers) are widely available from general electronics marketplaces and local vendors. Choose branded cells (Samsung/Panasonic/LG/Sony) with genuine protection boards.