question

brassmonkey001 avatar image

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?
MPPT - Solar Charge ControllerLithium Batterybattery chargingBattery Protectcharger
10 |3000 characters needed characters left characters exceeded

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

2 Answers
Justin Cook - Bay Marine Supply USA avatar image
Justin Cook - Bay Marine Supply USA answered ·

@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

10 comments Share
10 |3000 characters needed characters left characters exceeded

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

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.

brassmonkey001 avatar image brassmonkey001 Justin Cook - Bay Marine Supply USA ♦ ·
Oh, I fully intend to go for the Smart option! And yes, I was intending separate cables with the battery +ve terminal being the junction point between the two. So charge source>BP>battery terminal>BP>Load. So can I just set it all up on my phone and not need any external control? Is it smart enough to know that one BP does one job and the other BP does the other? I've just had a play in the Victron Connect App demo but it doesn't appear to have the ability to add one BP to the VE network, never mind two, so I'm not sure how you'd go about configuring it to control two BPs? What do you mean by "Obviously the temperature trigger will need to be an external trigger source"? Will my BMV-712 temp. sensor not do the job?

Right, so the "charge" BP will be externally triggered by your BMV; you'll set your BMV relay out to trigger based on temp (you set this in the app, connected to the BMV over bluetooth), and run the trigger wire down to the "charge" BP and into the remote switch pins. I think Victron even shows something very similar on the diagrams included with the new updated manual.

The "charge" BP won't be on a VE.Smart network at all because it's getting the trigger command via hardwire from the BMV. (this is the "external trigger source" I was referring to)

The "load" BP won't be networked either, you just set the voltage cutoff/restart points in the app and you're good to go.

You'll set up each device separately in the app, so they actually won't be "talking" to each other at all... one is doing one thing and the other is doing something else entirely, so they don't need to share information.

brassmonkey001 avatar image brassmonkey001 Justin Cook - Bay Marine Supply USA ♦ ·
Ah, right! I get it now! That makes perfect sense, thank you Justin.
How do you get around the problem of the solar panels providing current to the MPPT when the BP cuts the connection to the batteries? Won't the MPPT get damaged in this event?
brassmonkey001 avatar image brassmonkey001 Justin Cook - Bay Marine Supply USA ♦ ·
How do you get around the problem of the solar panels providing current to the MPPT when the BP cuts the connection to the batteries? Won't the MPPT get damaged in this event?

Well, that's always been a concern in the back of my mind, but the reality is that it's no different than if the battery BMS itself cut off charging due to temp/voltage, and that's a supported function, so... I tend to not worry about it. I've actually tried forcing an MPPT to fail by repeatedly disconnecting the battery while the MPPT was going full-bore and I've never been successful in causing a failure, so my takeaway is that while the instructions to always disconnect PV first serve a viable purpose and do protect against possible controller damage, I think the likelihood of it actually causing damage to the controller to disconnect the battery is effectively zero.

Show more comments
brassmonkey001 avatar image brassmonkey001 Justin Cook - Bay Marine Supply USA ♦ ·
Hi Justin, I'm finally getting around to installing the Smart Battery Protects! I'm installing the Load SBP first as it's the simpler of the two. On the actual physical installation & connection side of things, if I understand the instructions correctly, I connect the 'In' pole and the negative wire first, then set the program and then connect the 'Out' pole? Or I can connect the 'In' and the negative, set the Load output switch in the app to 'Disabled', then connect the 'Out' pole and set the program and then set the Load output switch to 'Active'? What is the difference between Li-ion mode and Normal? Is it simply different voltage parameters or is there more to it? In Li-ion mode, it says Shut down at 10v and Restart at 13v. I feel that might be a little low for comfort and quite a wide range, can I put it on Normal mode and set my own limits e.g. 11v shutdown and 12v restart? Or is it best set to Li-ion mode? @Justin Cook - Bay Marine Supply USA ♦
nebulight avatar image
nebulight answered ·

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.

1 comment Share
10 |3000 characters needed characters left characters exceeded

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

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.