How to Troubleshoot Sanctuary Batteries
The basics for how the Sanctuary battery's management system (BMS) works, what to watch for with lithium cells, and easy steps for fixing common issues.
BMS Basics
The BMS is a required component of any lithium battery. Lithium batteries may be damaged if overcharged or over-discharged. The BMS will monitor battery temperature, the voltage of each cell, and the current of the pack. It will disconnect the battery if subjected to potentially damaging conditions.
One common problem when connecting the battery to the inverter is that when the inverter’s battery is at 0V, the capacitors draw so much current from the battery that the battery thinks it’s a short circuit and will disable discharging. A single battery can charge one inverter up to about 32V before disabling discharge. The inverter then settles down to about 11V. The battery needs a voltage greater than its own to re-enable discharging.
The “battery awaken” feature on the inverter will raise the voltage on the battery terminals. Depending on the firmware version, it can be 51V or more. Also, depending on the firmware version, it may be done automatically.
Sometimes the battery will disable both charging and discharging. Usually disconnecting one power cable for 10 seconds will re-enable discharging. But when everything else is within proper ranges and it still won’t re-enable, it seems to have a bug in the BMS logic.
The battery has a circuit breaker that can be commanded off electronically. If the BMS senses that current is still flowing after the MOSFET switch has been turned off, it will command the circuit breaker to trip. The BMS can also turn off the breaker if it needs to turn off to prevent over-discharge.
Lithium Iron Phosphate Cell Basics
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Cell voltage does not vary much between 20% to 80% SOC.
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When fully charged, the difference between the highest and lowest cell may be 300 mV or more.
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When fully discharged, the difference between the highest and lowest cell may be 700 mV or more.
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The BMS will disable charging above 3650 mV max cell voltage.
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The BMS will disable discharging when the minimum cell voltage drops below 2400 mV.
When a cell voltage drops too low, the cell may start a chemical reaction similar to rusting and be permanently damaged. These cells may be dangerous to charge. The BMS will disallow charging if any cell is below 1400 mV or the cell min-to-max difference is greater than 700 mV.
Troubleshooting
Use the following information to assist in battery troubleshooting: See also the attached balancing cells sheet as needed.
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Check if charging and discharging are enabled. Either check the data from the inverter or read it from the BMS via RS-485.
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If charging and discharging are enabled, and the battery voltage when disconnected is still 0V, remove the front cover and check the circuit breaker.
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If the voltage on the inverter’s battery terminals is low, around 11V, use the battery awaken function to increase the voltage to above 51V.
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For single battery systems, if discharge is still disabled, disconnect the cable at the battery power terminal for 10 seconds and then reconnect.
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For multiple battery systems, if discharge is still disabled, disconnect one battery cable from each battery. Set the system charging current to 100A or less. We need to check each battery’s voltage before reconnecting. Otherwise, high current can flow from the high SOC battery to the low SOC battery. Starting at the lowest battery, reconnect its cable first and allow the inverters to charge it. Then as the voltage increases to within 0.5V of the next highest battery voltage, connect that one next. Continue until all batteries are reconnected. Then, when all batteries are connected and charging is enabled on all batteries, set the system charging current to what it was before.
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If a cell voltage is too low, it can be charged with a DC power supply. Set the power supply’s open circuit voltage to no more than 3.65V. Do not exceed 100A charging this way. Most of our power supplies can supply 5A, which is sufficient to get the voltage up in a reasonable amount of time. If one cell is charged too much, it can reduce the Amp-hours of the pack due to that cell now being the highest SOC and preventing the rest of the cells from charging when it hits 3.65V. Connect the power supply positive wire to the positive terminal of the cell and the power supply negative to the negative terminal of the cell. Once the cell voltage is the same as the rest of the pack, see if the BMS wakes up when the battery is charged from the battery terminals on the top of the case.
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If everything is in spec and the BMS won’t communicate:
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Try a different RS-485 connector from the plug on the BMS circuit board.
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Do a hard reset by unplugging the main power cables from the battery and turning off the breaker. Then unplug the 16-pin connector for the cell voltage taps, and plug it back in. Turn the breaker back on and reconnect the battery. Apply charging voltage to wake up the BMS.
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Replace the BMS or RMA the battery.
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If the battery breaker trips repeatedly, test if there’s a short in the charge/discharge MOSFETs.
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If the SOC is less than 80% and the maximum cell voltage is less than 3350 mV, set the inverter to charge the battery. Using BMS software, disable charging. Reconnect the BMS to the inverter. Once the inverter starts charging, if the breaker trips or the battery still accepts current when charging is disabled, then the BMS is defective. Be sure to check the inverter’s data on the battery state before making conclusions.
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If the SOC is greater than 30% and the minimum cell voltage is greater than 3000 mV, set the inverter to discharge the battery. Using BMS software, disable discharging. Reconnect the BMS to the inverter. Once the inverter starts discharging, if the breaker trips or the battery still discharges current when discharging is disabled, then the BMS is defective. Be sure to check the inverter’s data on the battery state before making conclusions.
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Voltage at battery terminals (on top of battery)
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Above 50 Volts = normal
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Between 44 and 50 V = low state of charge
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Less than 20 Volts, no load connected = This is the typical voltage when the BMS has disabled discharging.
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0 Volts = BMS breaker is probably tripped (needs to be reset, may have a cell drifted)
Voltage of individual cells
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Within 0.5 Volts of each other = good.
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Above 2.4 volts but outside of 0.5 volt variance = needs balancing.
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Any Cell lower than 1.5 Volts = bad cell.
Note: If all cells appear to be at 3.2 (or balanced), but the total cell voltage (tested at the battery lugs just above the cells) is less than 51.2 volts (3.2 volts x 16 cells), then a cell connection may have broken. Test the stacking voltage, adding one cell at a time to confirm.