[RE-wrenches] FW: Open Loop vs Closed Loop

Sat Sep 6 09:03:12 PDT 2025

Thanks Zeke – You bring up more issues & I appreciate the conversation and assorted directions this conversation might go.
This bucket of options for both inverters, chargers &batteries, needs some common communication rules kind of like the world finally adopting the “C” cable for small electronic devices

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From: RE-wrenches <re-wrenches-bounces at lists.re-wrenches.org> On Behalf Of Zeke Yewdall via RE-wrenches
Sent: Friday, September 5, 2025 2:28 PM
To: RE-wrenches <re-wrenches at lists.re-wrenches.org>
Cc: Zeke Yewdall <zeke at darkforestsolar.com>
Subject: Re: [RE-wrenches] Open Loop vs Closed Loop

For the definitions of open loop vs closed loop according to how we use those terms for lithium batteries:  the definitions you gave are according to engineering control systems theory.  But these terms have kind of been adopted by the solar industry without really applying them correctly in my opinion.

Open loop batteries there is no communications between the inverter (or charge controllers) and the battery.  The inverter/cc makes its decisions based on voltage and current -- just like we always did on lead acid batteries.  It might be more advanced, having a battery current monitor or an AH counting monitor on the battery, like some of the victron systems and I think Trimetric had a charge controller that worked with their meter, so adjust charging based not just on the voltage and current at the charge controller, but also voltage and current at the actual battery terminals or calculated SOC.  But there is still no communication with a battery BMS.

I would still say that there is feedback in this setup.  The inverter/cc is continuously adjusting the current based on what the voltage is doing.  If the voltage tries to go up above absorb setting, it reduces the current to keep it from doing so.  If a load turns on and voltage drops when it's in float, it increases current to keep it at float voltage.  So it's not really open loop in the strict sense of an open loop control.  An open loop charge control would be one that puts in 20 amps for 4 hours on a timer, and pays no attention to battery voltage, for example.

What we call closed loop lithium battery communication is really taking the control away from the inverter/cc and giving it to the battery.  Instead of the inverter/cc watching the voltage and deciding what to do... how much current to put into the battery.. the battery BMS sends a set of rules to the inverter saying how much it wants.   It tells the inverter -- give me as much power as you can.  or, limit your power to 5000 watts now, or I'm full, only do enough power to handle the load.

I'm not sure exactly who makes some of the decisions... for example does the battery tell the inverter to turn off for low battery when the BMS sees below a certain SOC, or does it just tell the inverter what the SOC is, and you program the inverter to turn off below that SOC.  Effectively it doesn't matter, as when we are programming it as a user, we don't actually know which of the devices is making the decision, but we just program the result of the decision... i.e. turn off when SOC drops to 10%.

There is still another level of control the BMS has, whether in open or closed loop.  If the inverter does not behave... either pushing the battery voltage out of acceptable ranges in open loop, or not following the commands of the BMS in closed loop ... then the BMS will turn off the charging and discharging contactor and turn off.  It's protecting itself... sometimes with disastrous consequences for the inverter and rest of the system.  I once had a battery turn off the contactor due to overvoltage from a 6kW DC generator that was not throttling back charging properly.  The DC generator spiked to 75 volts due to sudden loss of load, which fried everything in the system that couldn't take 75 volts.  Amazingly, the radian inverter survived it.  My theory of operation is that I never want the battery BMS to feel like it needs to open the main contactor to protect itself...  that often means a on site physical restart.  Worst case can be a black start situation where you have to manually bypass the BMS, or destroyed equipment from charging sources spiking without a battery to smooth out the voltage.  So, in open loop I always try to program such that the BMS never gets unhappy... stop charging before it hits high voltage.  Stop discharging before it hits low voltage.  Etc.    When in closed loop, you hope that the battery having control over the inverter prevents situations like this, but they can still happen, especially on the low end.  SOC indicated by BMS's can drift from reality, just as SOC from AH counting meters can over time, if they don't reach reset conditions regularly.  And many battery BMS's are blind to very small currents... less than 1 amp for most BMS's in the 100+ amp size, and less than 0.2 amps for lower amperage ones.  This means that a very low discharge from an inverter tare load but no AC loads on it, or DC LED lighting, can slowly discharge a lithium battery without the BMS counting down on the SOC.   After a few weeks, the actual SOC may be 30%, while it still reports 100%.  This means you can start discharging it and have a voltage dropout while SOC is still quite high.  At this point, the battery BMS will usually cut off due to low cell voltage, and suddenly recalibrate itself to 0%... when it may have said 60% just a minute before.  So, even with closed loop comms, you can still have issues if you don't reach a true 100% every week or two I find, due to SOC drift.
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