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LT3652 MPPT threshold set to 6.5 V — too high for 6 V nominal solar panel

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Valutazione

Difficoltà
4/5
Tempo stimato
3-5 giorni
Idoneità per principianti
48/100
Tipo di issue
Bug
Chiarezza
Abbastanza chiara
Stato di attività
Tranquilla
Ambito
embedded-iot

Direzione di ricerca

No file or test is named. Start by locating the schematic entry for U4 and the R22/R69 VIN_REG divider, then confirm the specific panel’s V_mp or I-V curve with its vendor. Done means selecting and documenting a resistor value whose calculated threshold matches the panel specification.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

bug

Summary

The LT3652 MPPT (input voltage regulation) threshold is set by the R22/R69 resistor divider on the VIN_REG pin. With R22 = 143 kΩ (top) and R69 = 100 kΩ (bottom) and the LT3652 internal threshold of 2.7 V:

V_MPPT = 2.7 × (1 + R22/R69) = 2.7 × (1 + 143/100) = 2.7 × 2.43 = 6.56 V

The solar panel is specified as 6 V nominal. A typical 6 V panel has:

  • V_oc (open circuit, full sun) ≈ 7.0–7.5 V
  • V_mp (maximum power point) ≈ 5.6–6.1 V (varies by manufacturer)

The MPPT threshold (6.56 V) is above the panel's maximum power point and close to its open-circuit voltage. This means:

  1. The MPPT circuit will only activate when VIN approaches the near-open-circuit condition, i.e., when the panel is barely loaded at all.
  2. Under normal charging (meaningful load), VIN will be at or below 6 V — the charger will operate in full-power mode (no MPPT limiting), drawing whatever current it wants regardless of where that leaves the panel operating point.
  3. In weak-light conditions, the panel may not sustain 6.56 V at all, so the MPPT circuit never intervenes.

Result: MPPT is effectively non-functional for the specified panel.

Recommended fix

Set V_MPPT to approximately 80% of V_oc per standard practice, or target V_mp directly if V_mp is known. For a 6 V nominal panel:

  • Target V_MPPT ≈ 5.8–6.1 V

With R22 = 143 kΩ fixed, solve for R69:

R69 = R22 / (V_MPPT / 2.7 - 1)
    = 143k / (6.0 / 2.7 - 1)
    = 143k / 1.222
    ≈ 117 kΩ  →  use 120 kΩ (standard E24 value)

Suggested change: R69 from 100 kΩ → 120 kΩ (gives V_MPPT = 5.97 V ≈ 6.0 V)

Or for a more aggressive MPPT point (80% of 7.2 V V_oc):

  • R69 = 143k / (5.76/2.7 - 1) = 143k / 1.133 ≈ 126 kΩ → use 130 kΩ (gives V_MPPT = 5.74 V)

Note: The correct V_mp of the specific panel model should be confirmed before changing this value. Ask the panel vendor for the I-V curve or V_mp spec.

Affected components

  • U4: LT3652 solar charger
  • R22: 143 kΩ (top of divider, VIN_REG to GND)
  • R69: 100 kΩ (bottom of divider) — this is the value to change
  • Net: MPPT_VIN_REG

🤖 Generated with Claude Code

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