outdoorsinnature
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# Posted: 19 Sep 2026 07:23am
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I have a 12vdc system.
When the battery reaches full charge I notice a surge, hearing a surge in the fans blowing stronger, as soon as the battery is full.
That is not the only indicator. I have a digital display for my MPPT. What the voltage shows is a drop after full, which is expected when it goes to open circuit voltage, shutting off the panels, but then it will have sudden surges, now and then when 'floating' between off and trickle charge, sometimes as high as 18V volt range, which causes my lifepo4 battery's BMS (shutoff voltage is 16V, to cause a total shutdown to protect itself for a second or two before it goes back down again.
I asked the battery manufacturer what this might be and they didn't have any more information to tell me other than what I already knew "it sounds like you have something causing a power surge in your system".
So any ideas what may be causing this surging? Conversely is there a device I could get to regulate the voltage so it won't go above a point where it would cause the system to shut down? thus circumventing the need to find the cause?
Also how bad is it to leave it on with this surging happening?
At the moment I do not feel comfortable leaving it on and going out where I will be away from it for a long time - which sucks long term having to be around all the time while it is charging.
Currently the battery will fully charge within a few hours, and so I just switch the solar disconnect off when I would be going outdoors anyway but it is not going to charge that quickly as winter approaches and sets in.
So would like this resolved so I can feel ok to leave it unsupervised again.
Just to make clear that the surge does not happen when the battery is charging; only when it reaches full and the mppt goes to float/shutdown.
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Steve_S
Member
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# Posted: 19 Sep 2026 11:53am
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If this is 12V LFP Battery you are overcharging & causing BMS to disconnect, resulting in the surge/spike.
The working voltage range for a 12V LFP Battery Pack is from 2.800V to 3.450V per cell (11.2V to 13.8V)
Max Voltage going into the battery should NOT exceed 3.500V per Cell X 4 Cells means 14.0V MAX. This si your Bulk Charging Voltage.
FLOAT should be set to 13.7V (3.425V per cell) which will allow the cells to balance up which is dependent on the BMS within the pack, if it has balancing capability or not... Normally at this voltage the cells will comfortably balance out.
BMS will shut down once a cell reaches 3.650V in the pack. This is for safety and should never be reached.
Low Voltage Cutoff for your Inverter needs to be set to 2.700 VPC (volts per cell) or 10.8V
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You will have some line loss & resistance in the wires, so you do need to do a voltage test at the Solar Controller Terminals, the Inverter/charger terminals & at the battey pack terminals. Note the different voltages and adjust accordingly. Remember the Battery Voltage is the most critical & therefore is the target to adjust your equipment for.
* Use a good quality Digital Multimeter that is at least 2 decimal accurate showing you 12.24V for example. 3 Decimal accurate DMM are available but they are more $$$, 2 decimal are more common and much less $.
AMPERAGE IS CRITICAL !
When Charging do NOT exceed max amps allowed by the battery. With LFP that is typically 0.5C-Rate or in other words, a 100AH Battery can be Discharged up to 100A but can only be charged up to 50A (0.5C) Max Current.
As the battery reaches FULL Voltage (working range) of 3.400-3.425VPC (13.6V-13.7V) the amperage will fall as the cells top off. This can drop to 0A but usually it should kick off @ 5% or 5A of total capacity in AH. * Solar Controllers & Inverter/Chargers usually have a setting for amperage cutoff points.
WITHOUT KNOWING WHAT GEAR YOU HAVE, IT IS NOT POSSIBLE TO PROVIDE MORE DETAIL.
* I am familiar with Tier-1 Products like Midnite, Samlex, Victron and so on...
SPECIAL NOTE !
The BMS is a Failsafe System & Cell Manager. It is NOT supposed to control the battery charge/discharge unless there is a fault. Allowing it to do so IS causing potential harms and increases risks. It is importat to properly configure your components (Solar Controller, Inverter/Charger) so they do their job safely & properly.
--- I check in here randomly, not regularly. I can answer some questions but I have responded to numerous points on such here & left an extensive trail of documents, guides, tables & charts pertaining to solar systems, batteries & more. I can try to answer some questions but I woudl need far more detail in order to properly answer. See below:
Req Info:
* Battery Types, Make, Model, Capacity.
* Solar Controller(s) Make, Model etc
* Inverter/Charger, Make Model
* A Diagram/Schematic of setup is helpful.
* Wiring used from Solar Controller to Battery to Inverter/Charger... Gauge, Lug Types, DIY or Commercially supplied. IF Commercial, links needed.
* What Fuses/Breakers are used & where are they.
!!! (link to any/all products used if possible) !!!
ALTERNATELY, You can goto https://diysolarforum.com/ and you can post your information as I suggested you provide, plus the problems you are experiencing and some will chime in (too many opinions) including from some of us hard core types. I have left a vast trove of guides, docs, manuals etc on that forum that you can use as well. *WARNING* There are many "Bench Experts" there and lately some who just love to "mess with folks" use critical thinking... I, like many have backed away from there when it got "dark", no point in dealing with that stuff, real life has plenty.
Good Luck, hope it helps
Steve
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