There are two different problem scenarios but proper procedure handles both of them:
- disconnecting battery from solar charge controller while charging; and
- connecting panels to charge controller before battery
the estalished procedure
- connect battery bank to BATT terminals of solar charge controller (controller hereafter)
- connect solar panels to PV terminals of controller
- disconnect panels from controller
- disconnect battery bank from controller
- disconnection process above
- wait a bit
- connection process above
Best practices help steer us away from both obvious and non-obvious risk. “Look both ways before crossing the street” doesn’t mean you will definitely get killed if you don’t look, but rather that looking both ways is an easy and effective way to avoid risk.
because the manual says so
If the manual gives an order of operations you are well-advised to follow it.
Example connection order, Victron first:
- Connect the battery: allow the solar charger to automatically recognise the system voltage (wait 10 seconds). 2. It is recommended to verify system voltage: use VictronConnect or an external control display. 3. Connect the PV. – source (pdf)
NEVER connect the solar panel array to the controller without a battery. Battery must be connected first. – source (pdf)
Example disconnection order, Morningstar first:
WARNING: Risk of Damage: ONLY disconnect the battery from the TriStar MPPT 150V AFTER the solar input has been disconnected. Damage to the controller may result if the battery is removed while the TriStar MPPT 150V is charging – source (pdf)
IMPORTANT: Disconnect the system in reverse order – source
Some other examples are listed in this reddit post.
Controller manufacturers are not forthcoming about the nature of failures due to sudden battery disconnection while charging, but there are informed discussions.
A couple of ways it could happen, depending on how conservatively designed/spec’d the charge controller and how careful user is about Voc: 1) Input capacitors see full PV voltage. But transistors only see Vpv - Vbat. If Vbat = 0V, transistors hold off more voltage. 2) MOSFET Vgs(max) is something like +/-20V. If MPPT hasn’t seen Vbat and established suitable Vgs before PV is applied, the circuit could be pulled to a level that blows the MOSFET gate. – source
A victron community manager in contact with engineering says:
Disconnecting a battery from a charge controller that is charging is not recommended. What occurs is the battery side voltage of the MPPT will spike, as it is raising the voltage and expecting the battery to draw down the voltage. When that is suddenly removed, for a moment there is nothing to constrain the MPPT’s battery side voltage. It will shoot up, and there may be a false over voltage alarm generated and the unit may shut down. The MPPT may also handle it more gracefully, that would depend if there were other DC loads on the bus drawing down the voltage, or if the wasn’t a lot of current going into the battery at the time… The MPPT is not designed to be a power supply, it is a battery charge controller. So fast load variations may lead to voltage dips and spikes when the battery is disconnected. source
The problem comes down to voltage spikes and is not particular to SCCs. If you take a simple buck converter there is a series inductor that is used to limit the current so that voltage can be regulated. If the load suddenly goes away, or drops, it’s up to the control circuit/program to react quickly and cut back the on time of the transistor… So while SCCs do not need a dump load you still need somewhere to throw the energy if the battery goes away. – source
The risk of SCC destruction is higher when the PV array is oversized relative to the SCC’s max short circuit current rating – source
Victron claims their controllers are immune to Missing Battery Syndrome (in the link above) but at least one MBS-related failure has been reported.
a failure and field repair
I BURNT UP our SOLAR (YT video)
to ensure voltage stability
I have witnessed a PWM controller passing damagingly-high voltage to the LOAD output after the BATT wiring was accidentally disconnected. Reconnecting the battery returned the controller to the configured setpoint voltage.
to ensure correct voltage selection
Some controllers auto-detect system voltage upon boot. If only PV is connected and the implementation poor this could result in the controller overvolting the LOAD output. And overvolting the battery bank when it is eventually connected.
Yurtdweller puts it this way:
the reason to not connect panels before battery is that the inconsistent voltage coming from the panels can cause the cc to incorrectly identify the system voltage. It could decide that it is connected to a 24v system, when the battery is 12v or vice versa. Once the cc is powered, and has properly identified system voltage, this is no longer a concern. source
There are also [solar charge controllers] that autosense battery voltage continually because of poor firmware etc and they will, from direct observation, go completely loopy if the battery goes away. – source
Following the procedure ensures we are all on the same page: the controller was definitely unpowered, was able to boot and ID bank voltage, and start seeing the panels.
Even if your particular controller is immune to a particular failure mode, there is no downside to following standard connection/disconnection procedures.
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