AGM Bank: Alternator Sense Lead and Charger Temperature Compensation
The house bank is four 100 Ah AGM batteries (400 Ah total). A 60 A alternator with an external regulator charges the bank through a 6 AWG cable with a 15 ft one-way run. The regulator's voltage-sense lead is connected at the alternator output, not at the battery. At 60 A output the alternator terminal reads 14.6 V and the battery post reads 13.9 V. The AGM maker specifies a maximum charge rate of 0.2C, absorption at 14.4 V, float at 13.5 V, and temperature compensation of -30 mV/°C above 25 °C. Engine-room temperature is 45 °C. A 40 A shore charger is set to 14.4/13.5 V and has no battery temperature sensor. The owner says the bank never gets above about 70% state of charge after a 5-hour motor and that the shore charger takes 14 hours.
1. Calculate the voltage drop in the alternator charge cable and explain the charging symptom. What is the first correction?
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Vdrop = 2KIL/CM = 2 x 10.7 x 60 x 15 / 26,240 = 19,260 / 26,240 = 0.73 V. That matches the observed 0.7 V difference, so the cable is the cause. The regulator is sensing at the alternator, so it holds 14.6 V there; the battery only receives 13.9 V. For an AGM bank needing 14.4 V absorption, 13.9 V is below the absorption setpoint, so the bank stays in bulk or partial charge and never reaches full. Move the voltage-sense lead to the battery positive post (with a small fuse), or use a remote-sense regulator, and then confirm the battery sees 14.4 V at 25 °C. Do not simply raise the regulator setpoint; that would overcharge once the drop is fixed. Also check the negative return and all terminations.
2. Evaluate whether the 40 A shore charger is adequate for the 400 Ah bank, including the effect of 15 A of DC loads and the missing temperature sensor.
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40 A is 0.10C for a 400 Ah bank, below the 0.20C maximum, so it is not dangerous. But with 15 A of DC loads, net charge is 25 A, or 0.0625C. Replacing 120 Ah (50% to 80%) takes about 4.8 hours at 25 A before the absorption tail, so 14 hours is plausible if the bank started low. A 60 A charger would give about 45 A net and is still only 0.15C; it would shorten recharge time. Temperature is the bigger issue: at 45 °C the compensation is (45 - 25) x -30 mV = -0.6 V, so absorption should be about 13.8 V and float about 12.9 V. With no sensor and fixed 14.4/13.5 V, the charger overcharges the AGM bank at 45 °C. Add a battery temperature sensor or reduce the setpoints and verify against the battery maker's data.
3. Is the 60 A alternator itself inadequate? Explain what measurements and decisions you would make before recommending a larger alternator.
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The 60 A alternator is 0.15C, within the 0.20C maximum, so it is adequate in principle for a 400 Ah bank; it is not the first thing to replace. In a 45 °C engine room, alternator output may derate, and the 60 A rating may not be available continuously. First correct the sense lead and add battery temperature compensation; then measure actual charge current and battery voltage at the battery posts. If the owner wants faster recharge with heavy DC loads, a larger alternator or a second charge source may be justified, but the charge cable and fusing must be sized for the new output. Do not assume the alternator's hot rating equals its cold rating.