Friends:
This case is reminiscent of a not so recent experience I had trying to integrate Fortress E-vaults into a Sunny Island system. When the batteries approached 100% SOC there would be a spike in the battery voltage that caused a system shut-down. My theory is that lead acid batteries present a more constrained impedance to a charging source than do batteries with a BMS system. When approaching full charge the BMS will try to curtail charging by increasing the impedance as seen by the charging source. William Bryce’s description of the process seems to ratify that theory. The work-around seems to be to limit the top tier of charging, maybe any charge above 90% SOC. This will reduce the overall efficiency of the operation. Leaving 10% on the table every day is not a great option. Although there are plenty of anecdotes on this forum that lithium technology can work with legacy inverters (those designed for connection to lead-acid batteries) my policy is to avoid the uncertainty and specify systems that can work closed-loop. I have successfully paired a Soul-Ark with Deka duration and Fortress inverters with Fortress lithium batteries. My experience on the subject is limited-- I am not pretending to be an expert on this. William William Miller Miller Solar www.millersolar.com CA License C-10 77398 *From:* RE-wrenches [mailto:[email protected]] *On Behalf Of *William Bryce via RE-wrenches *Sent:* Sunday, January 25, 2026 10:21 AM *To:* RE-wrenches *Cc:* William Bryce; Zeke Yewdall *Subject:* Re: [RE-wrenches] Strange battery overvoltage issue (Midnite AIO / Lithionics Battery) This is more of a programming / battery issue than an inverter inverter. In looking at the system settings, you are pushing the voltages of LFP to the max. This will cause the BMS to protect the battery. The "Spikes are caused by the BMS operation not the inverter (the current has to go someplace), the BMS FET's open under load the voltage will spike. The BMS should be the last resort in regards to battery charging protection. All LFP batteries (cells) are mostly the same, and are made by 5 companies. The upper and lower voltages are set for LFP cell makers. The battery (pack) builders then program the BMS, and post on specification sheets the recommended setting voltages. With *lower quality cells and lower warranties*, you will see the voltages are set *higher.* When using A+ cells and long warranties you will see* lower* top voltage settings. Here are the recommended voltages for a 48V LFP (16S) pack when using A+ cells from CATL, BYD, and EVE (this is from the cell manufacturers recommendations) *Charging range:* *~52 V to 56 V* · *Full resting:* *~54.6 V* · *Nominal operation:* *~51–53 V* · *Stop discharging (resting):* *~48 V* *Recommended MAX voltage is 56.8V (for normal use) 58.2 Absolute MAX (chemistry limit) Cell damage* You need to update the MNP AIO system firmware to V7. It is now the only AIO that has true charging control for LA (that works) the changes made were recommended by a highly qualified LA battery rep who is also a part of this list. Set the absorb setting as you need, but be aware that it does not start the adsorb until the current drops below ~ 10 amps or so. On Sun, Jan 25, 2026 at 12:48 PM Zeke Yewdall via RE-wrenches < [email protected]> wrote: I see on the Lithionics spec sheet it says a charged voltage of 57.6 volts This seems very high for a 16 cell LFP battery. I am used to using 55 to 56 volts, maybe 56.5 volts at most on a 16 cell LFP. I suspect that the 57.6 is the protection voltage of the BMS, not the appropriate absorb voltage setting. I have seen this error a lot on lithium battery spec sheets, where they give the overvoltage protection level, which is not an appropriate absorb setting for typical solar equipment. Not everyone will agree with me, but the way I set up absorb voltages when doing open loop lithium battery systems is to NEVER let the BMS do anything. if the BMS shuts off charging, that means that I already failed... the charge controller or inverter should have stopped charging before the BMS feels the need to protect the cells. Same with low voltage disconnect... if the BMS shuts off due to low voltage, I already failed -- I should have shut the load off before the BMS felt the need to turn of discharge to protect the cells. The BMS is like an airbag...only to react in cases of emergency when the rest of the system doesn't work properly. Open loop equipment (all of the traditional outback/magnum/schneider, etc stuff, plus any AIO's in open loop setup) is designed to always have a battery in the system to stabilize the DC bus voltage. Without the stabilizing effect of the battery, voltage spikes can occur. I find that many charge controllers, especially the midnite classic and victron ones, can actually keep the system fairly stable without batteries, but charging from inverters is more questionable, and especially charging from any DC generators can cause severe spikes if the battery disconnects. I have fried equipment when a lithium battery disconnected from a large kohler DC generator and the generator couldn't react fast enough to keep it from spiking to 75 volts or more. This is why you never want the BMS to disconnect... which means setting absorb at a level where the BMS never feels threatened. Lithium battery manufactures giving the voltage at which the BMS disconnects as the "charge to" voltage does not help the situation. That may work when charging it with a dedicated lithium battery charger, without anything else connected to the battery while it's charging. But in a functioning solar system with charging from multiple sources and loads all occurring at the same time, we need to be smarter than the battery, IMO. Zeke _______________________________________________ List sponsored by Redwood Alliance Pay optional member dues here: http://re-wrenches.org List Address: [email protected] Change listserver email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org There are two list archives for searching. 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