Theoretical question - voltage to loads when BMS Charge mosfets open

[Barry Timm]

I'm curious, in a system where a solar panel may be connected directly to a Lithium battery with common port BMS but with BMS having separate control of Charge and Loads, and the panel Voc is substantially higher than the battery base voltage, what happens when the BMS over-charge mosfet is opened by the BMS to stop charging, while the BMS Loads mosfet is still closed. 

Does the fact that the solar panel voltage (which has then increased to near Voc due to the BMS opening the charge mosfet), then feed the loads with Voc level voltage as the voltage is much higher than the battery voltage and thus the loads will receive dangerously high voltage directly from the solar panels?

[sailingharry]

This is an ElectroDacus forum, and so this question doesn't apply.  However, the question you raise is one of the benefits of the 'Dacus -- there is no Charge MOSFET to open.

In the situation you are describing, with on-board FETS, it is critical that you don't have "a solar panel connected directly to the battery."  Those systems REQUIRE a charge controller, either a PWM type or better, a MPPT type, that can directly regulate output voltage rather than having unregulated voltages as high as Voc.  Of note, on my system my MPPT controllers feed to a Battery Protect (a switch), and when the Battery Protect is open, the MPPT output is still regulated to ~14V, not Voc -- and that is with ZERO load.

Again, as a proper BMS, the 'Dacus doesn't open a switch on the battery, it shuts down the output of the charge devices (solar, alternator, shore power, etc).

[Dacian Todea (electrodacus)]

Barry,

As sailingharry already mentioned you are likely referring to the case where charge FET and discharge FET are in series and that is where BMS is integrated in to battery like tools batteries, photo camera batteries where battery is either connected to charger or to load and never at the same time to both. That sort of BMS is no suitable for applications where both the charger and Load are present at the same time.

For the SBMS 60, 100, 40, 120 the charge and discharge mosfets where both connected directly to battery and not in series thus when solar was disconnected the Load was still seeing battery voltage not PV Voc.

[Barry Timm]

Sorry, I should have added more background. I know this is not an Electrodacus hardware related question. It is related to a question posed by someobody else where he asks why it is necessary to always use a solar controller in a system that uses a standard LifePO4 battery with built-in BMS where the BMS can be configured for Charge and Load individually. He posed the question because the BMS already provides a means to protect the 4S cells in a 12.8v Lifepo4 battery from over-charging and thus asked why the need for a charge controller (MPPT or PWM). I suggested that part of the reason is to ensure that the cells are not constantly kept as 100% full state (via the BMS) as, unlike with a solar controller, the BMS will never drop the solar voltage coming in to below the over-charge voltage limit (at least, nothing like a floar voltage level commonly used for the MPPT/PWM controllers).

An ongoing discussion in this same thread, was that a colar controller is essential in such a system as if one relies on the BMS to prevent over-charging, and the cell voltage reaches the BMS Over-charge voltage threshold and the charge mosfets in the BMS OPEN to stop charging, then the solar panel voltage will rise to around Voc voltage levels and be passed directly to the loads (12v lights, fans, etc) as even if the BMS Loads mosfets are closed to allow the battery to provide current to the loads, the higher solar incoming voltage would supply current to the loads and nothing will be flowing out of the battery due to the lower battery voltage, which would damage the loads circuits.

So, the question is.....if there is no solar controller and the sola rpanel is directly connected to the BMS and the BMS charge mosfets open to protect the cells from over-charge, would the connected loads circuits then immediately be exposed to the mush higher solar controller voltage, or would the loads continue to receive battery voltage level current?

I asked this "general" question here because I trust the knowledge of this community much more than most other sources of info.

[Dacian Todea (electrodacus)]

In this conditions the Load will see the PV Voc or close to it and likely the Loads will be damaged due to over voltage.

But those BMS where not designed for this type of application and not even with a solar controller will not work unless the solar controller is capable of having the battery disconnected while PV panels are still connected and not many solar charge controllers can do that without being damaged or damaging the loads.

Just posted about the new SBMS app that connects to SBMS trough USB and shows the data in a nice graphical format.