Why I Sometimes Prefer Open-Loop Lithium Battery Systems

I want to clear up a misunderstanding that causes customers a lot of unnecessary worry. A lithium battery operating open loop is not running without protection, and it does not mean the battery or inverter is defective. Open loop is a normal control method that experienced engineers and installers still choose every day.

What open loop actually means

The difference is simple. In closed loop, the battery sends information such as state of charge and allowable current to the inverter through a CAN or RS485 communication cable. In open loop, the inverter operates from voltage and current settings entered by the installer.

The battery's internal BMS is still working in either case. It continues watching cell voltage, temperature, charge current, discharge current, and other protective limits. Open loop removes the data conversation between the battery and inverter. It does not remove the battery's protection.

Why a simpler system can be the better system

A communication cable sounds simple until a system refuses to communicate. Then we have to check the cable pinout, CAN or RS485 port, battery protocol, inverter selection, firmware versions, DIP switches, master-battery assignment, addressing, terminators, and sometimes a separate gateway.

Every one of those items is another place where a problem can develop. The battery can be perfectly good. The inverter can be perfectly good. The power wiring can be perfectly good. One wrong communication detail can still make the system act like something is broken.

Fortress sometimes chooses open loop—even with Fortress equipment

This is important because it clears up the idea that open loop is only used when two brands cannot communicate. I have had Fortress factory technicians and engineers recommend open-loop operation even when a Fortress battery was paired with a Fortress inverter and the equipment was capable of communicating.

The reason was not that the equipment was defective. Their practical judgment was that voltage mode would be simpler, easier to troubleshoot, and less likely to develop a communication problem. That does not mean Fortress always prefers open loop. It means their own technical people recognize that closed loop is not automatically the best choice for every job.

Pytes sometimes makes the same recommendation

I have seen the same practical decision with Pytes. In several situations, Pytes technical support has recommended an open-loop configuration even though the Pytes battery and inverter were capable of closed-loop communication.

Again, the reason was simplicity. Open loop was easier to configure, easier to use, and easier to troubleshoot. Pytes does not treat open loop as an improper shortcut. Its support material includes a dedicated section for E-BOX-48100R inverter open-loop settings, and its battery compatibility material separately identifies open-loop and closed-loop applications.

Compatibility is another reason open loop remains necessary

CAN and RS485 are connection methods. They are not universal languages. Two products can have identical-looking communication sockets and still use different pinouts, messages, protocols, or firmware.

That is why a battery may communicate with one inverter but not another. It is also why an inverter or battery update can change a combination that previously worked. When the manufacturers approve voltage-based operation, open loop can bypass that compatibility problem completely.

This does not mean batteries can be randomly mixed. Battery voltage, chemistry, current limits, and manufacturer requirements still have to match the design. Open loop broadens inverter-to-battery compatibility; it does not erase proper engineering.

The majority of lithium did not begin with closed loop

For years, most lithium battery systems operated from programmed voltage and current limits. That includes a huge installed base of off-grid, backup, marine, mobile, and legacy-inverter systems. Fortress Power itself wrote in 2020:

“Most batteries on the market still use open-loop communication.”Fortress Power, Why Closed-Loop Communication Matters

Closed loop has become more common, but that history matters. Open loop is not an experimental idea. It is a proven method with an enormous real-world operating history.

What the manufacturers actually document

Fortress Power says its battery storage can work without the communication cable and that “the battery will function just fine.” Its current integration material also publishes open-loop settings for supported systems.

Some combinations require open loop. Fortress states that its eVault Classic with Victron is supported in “open-loop communication (voltage mode) only.” Fortress has also officially recommended voltage mode as the solution during a communication disruption.

Pytes publishes open-loop setup instructions and model-specific voltage requirements. Discover Battery likewise publishes a complete open-loop application note and says its AES batteries can be configured for either open-loop or closed-loop operation.

Open loop still has to be set up correctly

I will defend open-loop systems, but I will not defend careless programming. A proper open-loop installation uses the settings for the exact battery model. The installer must correctly program:

• Maximum charging current
• Maximum discharging current
• Bulk or absorption voltage
• Float voltage or the manufacturer's no-float instruction
• Low-battery shutdown voltage
• Restart and recharge voltage

I also want the inverter's voltage reading checked against the battery terminals. If accurate state of charge is important, I prefer a dependable battery display or a properly installed shunt because lithium iron phosphate voltage stays fairly flat through much of its operating range.

The system should normally operate inside the programmed limits. The battery BMS remains the final protection, but it should not have to disconnect the battery every day because the inverter settings are wrong.