Deep Discharge Handling

Activation / Wakeup Function

When a battery is deeply discharged, the BMS shuts off its MOSFETs and goes silent. The charger must wake the battery before normal charging can begin. This page covers exactly how that works and the critical safety risks involved.

Safety Warning: During Activation Mode, the charger produces a forced output with no reverse polarity protection and no short circuit protection. Always verify connector polarity before plugging into a deeply discharged battery.

Why Is Activation Needed?

Under normal conditions, the BMS keeps its charge and discharge MOSFETs open when the vehicle is off - isolating the battery cells from the connector for safety. When the charger connects, it expects to find the battery voltage at the connector. If voltage is absent, the charger cannot distinguish between "BMS protection is active" and "no battery connected."

When a cell drops below ~2.0V (deeply discharged state), the BMS itself may have shut down to prevent over-discharge damage. With the BMS unpowered, it cannot communicate over CAN or close its MOSFETs. A small activation pulse from the charger can supply just enough energy to boot the BMS, which then closes MOSFETs and initiates normal communication.

ACTIVATION TRIGGER THRESHOLD

<= 2.0V

per cell | BMS may be offline below this voltage

NON-CAN ACTIVATION THRESHOLD

<= 15V

total pack voltage | triggers automatic activation

ACTIVATION WINDOW

10 sec

charger waits for BMS response before fault

The Four Activation Scenarios

Every charger connection results in one of these four outcomes depending on battery and BMS state.

Normal Charge Start

BMS is healthy. CAN messages arrive within 5 seconds of AC connection. Battery voltage is above minimum threshold. Charger reads BMS setpoints and immediately begins charging at the requested current.

AC ON -> Listen 5s -> CAN OK -> V OK -> CHARGING

BMS Communicating, Voltage = 0V

BMS is communicating but the charger cannot sense voltage at the DC output - because the BMS discharge MOSFET is open (protecting the cells). The charger must pulse Vmax/Ilow to activate the discharge MOSFET and close the circuit to the battery.

CAN OK -> V=0V -> ACTIVATE 10s -> V seen -> CHARGING

Deep Discharge - BMS is Offline

Battery cells are below 2.0V/cell. The BMS has shut itself off to prevent over-discharge. No CAN messages arrive in the 5-second window. The charger enters Activation Mode, outputting Vmax/Ilow for up to 10 seconds. The activation energy boots the BMS, which then starts communicating. After CAN is established, normal charging begins.

No CAN -> 5s -> ACTIVATE Vmax/Ilow -> BMS boots -> CAN OK -> CHARGING

Note: BMS typically takes 5-15s to start communicating after power is restored. The 10s activation window must be long enough to bridge this gap.

Activation Fails -> Fault Mode

The charger pulsed Vmax/Ilow for the full 10 seconds but the BMS never communicated and no voltage was sensed. This means either the battery is beyond recovery, the wiring is faulty, or the BMS hardware has failed. The charger disconnects output and enters Fault Mode.

ACTIVATE 10s -> No CAN + No V -> FAULT MODE -> LED error

Activation Timeline Visualised

Safe Activation Procedure

Follow this sequence exactly when waking a deeply discharged battery.

Inspect the DC connector before connecting. Confirm polarity every time: Red or Brown = Positive (+), Blue or Black = Negative (-).

Connect the DC output connector to the vehicle inlet first.

Connect the AC input plug to the wall socket and switch on.

Do not interrupt the first 15 seconds. This allows the 5-second listen window and up to 10 seconds of activation to complete.

If the charging LED begins flashing, activation succeeded. Leave the charger connected for a full charge cycle.

If a fault pattern appears, move to the LED Guide and follow the matching troubleshooting tree.

If charging does not begin after 3 correct activation attempts with 30 seconds AC-off rest between tries, stop. The battery needs professional assessment.

Do not energise an open DC connector. During activation, the charger can fire a live Vmax / Ilow pulse into an unconnected circuit if AC is applied before the DC connector is seated in the vehicle inlet.

Activation Mode Risks & Precautions

No Reverse Polarity Protection
During the activation pulse, the charger's reverse polarity protection circuit is bypassed. If the connector is plugged in with reversed polarity, the forced output will directly damage the battery cells and possibly the charger. Verify polarity (red/brown = +, blue/black = -) before every connection to a deeply discharged battery. Reverse polarity damage during Activation Mode is excluded from Solterra's warranty.

No Short Circuit Protection
Activation Mode also disables short circuit detection. A connector that bridges + and - will receive the full Vmax output with no cutoff. Never connect to a shorted harness or damaged connector in this state.

No-Load Activation (10s)
If the charger is not connected to any battery (open circuit at the DC output), the activation pulse still runs for 10 seconds at Vmax. This is a live voltage output with no load. Ensure the output connector is never left open and energised - secure it safely before powering the AC input.

Best Practice for Deeply Discharged Batteries
1. Visually inspect the connector for correct polarity before connecting. 2. Connect the DC output first, then connect AC mains. 3. Allow the full 10-second activation window without interruption. 4. If charging does not begin after 3 attempts with the correct connector, contact Solterra service - do not repeatedly attempt activation on a potentially damaged battery.

Activation Implementation Options

Solterra and OEM customers have discussed the following implementation philosophies for activation. Each involves trade-offs.

#	Approach	Behaviour	Risk Level
1	Continuous regardless of CAN	Power output regardless of whether CAN is established	High
2	10s pulse when CAN OK	Pulse output for 10s only after CAN comms are confirmed. Resume after voltage is sensed.	Medium
3	Output only after CAN + MOSFET closed	Output begins only after CAN is OK AND BMS confirms MOSFET/contactor closed	Low
4	Pulse on AC connection, verify then continue	Upon AC ON: pulse output for 10s. Continue only if BMS comms OK and MOSFET closed within window.	Recommended

Solterra current standard (Option 4): Upon AC connection, passive listen for 5s. If no CAN -> pulse Vmax/Ilow for 10s monitoring bus continuously. If BMS communicates and voltage is sensed within window -> switch immediately to normal charging. If timeout -> Fault Mode.

Next: LED Indicator Guide