asked
Protecting LiFePO4 batteries from all sorts of things
If I connect my MPPT, DC2DC charger and a EHU charger to one side of a Battery Protect and the other side to the battery, and connect my BMV712 relay output to the Battery Protect, would that then disconnect my battery bank to prevent being charged when the temperature drops cold (as well as if it gets too hot) and disconnect in the event of low (or high) voltage events? Or is it not so simple as that?
@brassmonkey001, it's not quite as simple as that. The BatteryProtect is a one-way device only, which means that if you want to use it as a charge disconnect, you need to connect it in "reverse" - that is, with the charge sources connected to the "in" post and the battery connected to the "out" post; you may then configure it to disconnect in the event of high voltage.
To use it as a low-voltage load disconnect, you need to attach the battery to the "in" post and the loads to the "out" post. You cannot have a single BP serving both purposes. Current applied to the "out" post flowing to the "in" post will immediately and irreparably damage the BP, and can pose a very significant fire hazard.
It's also worth noting that to configure the BMV relay to trigger based on temperature, you will of course need either the BMV-702/712 temperature sensor connected to the shunt PCB or a Smart BatterySense on a VE.Smart Network with the BMV.
Please see the updated BatteryProtect user manual here: https://www.victronenergy.com/upload/documents/Manual-Battery-Protect-12V-24V-65-A-100-A--220-A-EN-NL-FR-DE-SE-SV-PT-TR-IT.pdf
Thanks @JustinCook. I have the BMV-712 temperature sensor connected between the battery positive terminal and the shunt. So if I understand it correctly, you can have DC2DC charger, MPPT controller and EHU charger all connected to the "in" post of the BP and the "out" post connected to the battery +ve and that will cut any charge going to the battery in the event of low (or high) temperature and high battery voltage? Could you then perhaps add a second BP between the battery terminal and the load to take care of low voltage disconnect duties? And if so, would I then use preset 6 for example to cut current flow when the voltage drops below 11.5v until voltage increases above 12.8v? Or would I need some external control to tell the BP what to do?
@brassmonkey001, correct on the charge side; setting it up properly will be a lot easier if you have the Smart BatteryProtect, so I strongly advise you pay a little extra for that so you can just configure everything on your phone.
On the discharge side, yes, you can have another BP (preferably the Smart BP) connected to the POS terminal with "in" connected to battery and "out" connected to your loads and you can configure it to shut down your loads at your desired voltage. Again, I strongly recommend the Smart version because you can dial in the shutoff/restart voltages to 1/100th of a volt, rather than being limited to the factory presets.
One important thing: the two BPs have to be on separate lines... ie, your charging cutoff BP connects to your battery POS via one cable, the disharge cutoff BP connects to your battery POS via a separate cable; you can't have the two BPs on the same line because then you're back to running reverse current through them.
As for external control, you can of course configure it to turn on/off via an external relay, but it's a lot simpler to just get the Smart versions and then configure them to do whatever you want. Obviously the temperature trigger will need to be an external trigger source, but all your voltage triggers can be set via the device interface itself.
I've done that, but only with my DC-DC charger and MPPT. I have a basic BMS with no temp control so I run the 'allow to charge signal' through the 712 relay and have that configured for temp control. So when the 712 temp sensor is too cold, it shuts off charge by disconnecting the BP100 I have for the chargers (50 amp DC-DC and 300W of solar through a 100/50). For the AC charger, I have a multiplus, so I use the two wire assistant to turn off the charger. I wired in an optoisolator to the same allow to charge signal after the 712, so it can also shut off when it's too cold.
Always have a BMS, don't use the BMV low or high voltage alarms for lifepo4 as that doesn't account for individual cell voltages. I'm only use the BMV to make up for the shortcomings in temp control on my BMS.
My batteries are Valence U27-12XPs which have an inbuilt BMS to balance the cells but there is no provision to balance between batteries. Valence's own external BMS is not available to the general public. Like you, I'm hoping the addition of a BP100 in conjunction with the BMV will make up for the shortcomings of the Valence BMS.
