Protect your Charge controller from battery disconnect while connected to the PV

Fabian

Is there a way to protect your charge controller when there's a battery disconnect while being connected to the PV panels?

I am currently using a  lifepo4 battery and i am thinking that suppose one day my BMS cuts off the output because of a overload or because it suddenly develops a fault of its own so essentially the charge controller connection to the battery would be cut while the PV is still connected to the charge controller. I string my panels in high voltage string of around 100v coming from the panels going to the CC.

Is there a way to protect my charge controller from seeing the high voltage input coming from the panels when there is no battery connect to it?

BB.

IMHO, the first step is to design the rest of the system (charge controllers, inverters, etc.) to operate within the specifications of the Li Ion battery bank... Circuit breakers, BMS, etc. should only "trip" when uncontrolled or unplanned events occur. Breaker and BMS trips should never occur if the balance of the system is operating correctly.

Having good quality equipment. reliable wiring connections, ensuring no sharp edges of sheet metal can cut wire insulation.

"Over designing" the wiring and components. For example, in the US, circuit breakers and fuses are designed to never trip at 80% or less of rated current. And to (minutes to hours) trip at over 100% of rated current. The idea is if you design your system wiring/breakers to run at no more than 80% of maximum load, then the over current protective devices never trip--Except when there is an uncontrolled fault. That is 1/0.80 or 1.25x max continuous current is one such design "rule".

Another is the choice of "open loop" vs "closed loop" BMS... Open loop is just that. The BMS sits there and monitors the specifications. And if something is exceed (high/low temp, high/low voltage, etc.), then the BMS "trips".

Closed loop is when the BMS (for example) can communicate with other major components in the system (charge controllers, inverters) and inform them to gracefully "shut down" when there is a BMS "soft warning" before the hard trip point is met. Close loop communications is more complex, and requires the Hardware mfg to talk with the BMS mfg. There are a few out there, but not many.

And there are different ways to design "shut down" circuits. One way is for the BMS to open a relay or transistor... That is less than idea as "removing" the battery bank (which really controls surge voltage/current issues)--Which can allow the remaining devices (solar charge controller over voltage the DC battery bus disconnected from the batteries). This type of disconnect may "protect" the battery bank, but the "uncontrolled" battery bus voltage may take out an inverter or charge controller (charge controllers are not designed to hold 14.75 Volt output if there is no battery connected--Many times, the controller will simply over voltage the output faster than it can "control" the output voltage).

Another way is to choose a different method to disconnect the battery bank... For example one Mfg (Schneider on one of their Li Ion systems--perhaps an Alpha unit--Don't know the details) put a large fuse between battery bank and the DC bus. And then had "crowbar" (large relay or transistors) on the DC bus side of the fuse. If there was an "emergency" shutdown required, trip the crowbar and basically short out the battery fuse. DC bus goes to "zero volts" and fuse disconnects battery from balance of system.

While "all of these alternatives" are possible when an engineer designs a system, most of these are left to the customer to pick and choose their system components. You cannot connect to a non-existent communications path to make a close loop BMS if there is no path/software to use those "soft" notifications. You can only pick hardware that has those features you want--Limiting your choices, probably higher cost for hardware, compatibility issues/questions, etc...

Or just design the system like a car... The seat belts are always available and will do a darn good job of keeping the passengers "safe". But there are airbags that can supply an extra level of safety--But usually deployment happens because of a serious accident--Car is badly damaged, and the cost of replacing deployed airbags can cause the insurance company to "scrap" the vehicle... But at least you and your passengers are still "safe" (i.e., major damage to your solar equipment possible, but the house has not burned down).

-Bill