I am preparing a LFP battery install. I want the BMS to be able to shut down the MPPT (2x 75/15 and 2x 100/50). These small MPPT controllers have an unacceptable charge-enable mode (if the plug is loose or falls out, charge is enabled and external control is lost), so I want to put a Battery Protect as a solar disconnect (the BP has a positive charge-enable design – if the control wire is compromised, charge is disabled). My choice of BMS will requires that it control the MPPT at 100% SOC (the SOC reset is the same as the charge limit), so this will turn on and off frequently. I know Victron shows this in reference diagrams, but two questions I want to know for sure:
Is there any negative impact (physical harm) to the MPPT if I use the BP to disconnect the battery while charging at 100% of the MPPT capacity? I understand some MPPT brands do not like this. This could potentially happen many times a day.
Is there any operational impact to the MPPT by being disconnected from the battery? Victron seems to want the battery connected to the MPPT before the PV, so it can be configured. But the MPPT doesn’t actually “disconnect” it – it seems the battery can still feed milliamps backwards and keep the MPPT live, so this may not be an issue.
I mean you could put a spot of hot glue there?
I haven’t had any fall out, though i have seen complaints of such. But i do have in my set ups cable support and fixtures.
The smart battery protects are uni directional, so as long as it is only charging not supply that’s ok once in a while however multiple times a day- that is definitely not going to be good for anything. That being said, yes the mppts can handle it a few times buuut (and its a big one) the power needs to sink somewhere electronics won’t do well with repeated abuse like that.
The mppts if the batteries are full, since they have several charging stages (in absorption) are near full then usually (if they are charging only) are not actually producing tonnes of current.
When you set up the MPPT you need to set it to charge standalone with settings for your battery in case you lose communication. A battery protect between the MPPT and the battery protects the battery bank and connected equipment from overvoltage in case the MPPT internally fails and the voltage from the solar panels is seen on the output. That is why you see them in some installations.
The Battery Protect is for shutting down a circuit when the upstream battery voltage goes under the specified amount. At least the two Battery Protects that I’ve used work that way. I just checked in VictronConnect using the Demo Library to see if there was an over voltage setting. I don’t see any support for over voltage.
I don’t think using a Battery Protect between the solar charge controller and the battery is a good use case.
Does your BMS require closed loop communication? This is not a requirement that I’ve heard.
An alternative is to set the solar charge controller parameters to a lower voltage so that it never comes close to the over voltage limit in the BMS. This is how most of us do it. Most BMS have a 14.6v upper voltage limit. The Victron defaults of 14.2 or 14.4 are usually quite safe and is unlikely to ever trip the BMS.
I’m sorry it took so long to respond to your questions.
The BP has two modes. One is the classic under voltage battery protect operation – when the battery gets low, it disconnects. It has other features/operations, but since I’m not using it this way I haven’t dug in to it.
But it also has a “lithium mode.” In this mode, it is a remotely controlled relay. There are two plugs on top of the BP, one has two contacts with a loop of wire across it, this is a remote operator. In Lithium mode, it turns on when that loop is closed, and turns off when the loop is opened. There is no voltage sensing at all. This allows a BMS to turn it on and off. In my case, it will be a “safety” on the MPPT, turning them off if a single cell goes high (even if the entire pack is in the safe zone). It also can turn off two charge sources I have that cannot be adjusted – a charger that works from a towed propeller (called a Watt & Sea) and a Victron Centaur, a fairly dumb charger but one that works on many voltages – 110, 220, even 400VDC!
My BMS does not require closed loop communication. The concern is that in a mobile (vibrating) environment, if that little plug on the MPPT falls out, the MPPT fails ON, and will run even if the BMS is sending a “OFF” signal. The BP is the reverse – if it does not have a signal to charge, it stops charging. And the plug is much more secure as well.
Your point to set the charge value to a safe value is quite right. This “should” prevent all BMS shut down of charge sources. There are two issues with this. First, and most important, is it is based on pack voltages. A 14.6V upper limit is 3.65V per cell. If you have a bad cell, you could have 3 cells at 3.6V, and 14.6 would result in a single cell at 3.8 – in other words, the BMS would have tripped the battery at a pack voltage of only 14.4. This is hopefully a rare situation. In addition, my BMS directly modulates charge sources as it brings the battery to 100% ensuring balancing and that the SOC meter gets reset. I would like some different programming in this regard, but I do like the BMS.