Solar MPPT Add-On (24 V Pack)
Harvest sun to recharge your 2kW station. This section shows controller sizing, safe fusing, and simple string options.
Controller & Array Sizing
- Battery: 24 V LiFePO₄ (8s), absorb ≈ 28.4–28.8 V.
- MPPT Rating: 60 A at 24 V is a sweet spot (up to ~1.6 kW PV at STC, derated in practice).
- PV Input Voltage: Choose MPPT with 100–150 V max PV Voc. Keep your string Voc (cold) below this.
Rule-of-thumb harvest:
Daily Wh ≈ PV_Watts × SunHours × 0.75.
Example: 800 W × 4 h × 0.75 ≈ 2.4 kWh/day.
String Options (Examples)
- 2× “200 W 24 V” panels in series: Vmp ≈ 36 V × 2 = 72 V, Voc ≈ 44 V × 2 = 88 V → great for 100–150 V MPPT.
- 2S2P (four panels): ~800 W; Isc sum doubles on the parallel combiner.
- Cold-day Voc check: Voc_cold ≈ Voc_STC × (1 + |tempCoeff| × ΔT). Example: 44 V × (1 + 0.003×35) ≈ 44 V × 1.105 = 48.6 V per panel; two in series ≈ 97.2 V.
Use your panel datasheet for exact Voc, Vmp, Isc, and temp coefficient.
Wiring & Protection
[ PV String A ]--MC4--.
+--( PV Combiner )-- 15 A string fuses* -- PV Disconnect --> MPPT PV+
[ PV String B ]--MC4--'
MPPT PV- <-- return
MPPT BAT+ -- 80 A ANL/Class-T fuse --> Bus+ (size ≥1.25 × I_charge)
MPPT BAT- -------------------------------------> Bus-
Fans/12V Buck (optional) <-- from Bus via 5–15 A inline fuse
* Fuse each string when ≥2 strings in parallel (protects backfeed).
- PV side: 10 AWG PV cable for strings; 15 A MC4 fuses per string when paralleling.
- PV disconnect: A small DC isolator (≥150 V DC) between combiner and MPPT.
- Battery side: Fuse MPPT output near the battery/bus. For a 60 A controller, an 80 A ANL is typical.
- Wire gauge: Size for ≤3% drop. For ~40–60 A short runs, 6–4 AWG copper is common; check a voltage-drop calculator.
Connect sequence matters: Battery to MPPT first, then connect PV.
Disconnect PV first when shutting down, then battery. This protects the controller.
Charge Current & Limits
- Max charge current: keep ≤ 0.5 C unless your battery spec allows more. For 24 V 100 Ah → ≤50 A.
- MPPT setting: limit charge current to your battery’s allowed value if PV array can exceed it.
- Absorb/float: follow your battery maker (typical absorb ~28.4–28.8 V; float ~27.0–27.2 V or disabled).
Array Planning Quick Math
- Controller ceiling: I_charge ≈ PV_Watts / (Charge_Voltage × η). Example: 1,000 W / (28 V × 0.98) ≈ 36.5 A.
- Runtime from solar: If you harvest 2.4 kWh/day and average load is 200 W, you can cover ~12 h of use daily.
- Shading kills strings: Prefer fewer, longer strings in full sun rather than many shaded parallels.