LFP Battery Disconnect Solenoid Switches

[blondjohn]

I’ve searched the forum but did not find a specific discussion of solenoid switches for high and low voltage battery disconnection for DIY LiFePO4 (LFP) batteries. I’m hoping some of you experts can help me out.

I’m at the design phase for my first conversion from AGM to a DIY LFP house bank. I’ve read that “best practice” is to install separate load and charge bus bars, and place a safety disconnect (solenoid switch) between the LFP batteries and each bus. A BMS can be programmed to disconnect the charge bus for high voltage and disconnect the load bus for low voltage. I’ve also read that solenoids should be Normally Open (NO) meaning control current is required to keep the solenoid switches “closed.” Theoretically, if a BMS failed, control current would stop, solenoids would open, and the batteries would be disconnected “Fail Safe.” My current design follows this best practice” advice. I understand bus disconnects should never happen because of charging source control and audible alarms, but it does seem like good insurance.

In our 12 volt system we have a 290 amp Balmar alternator, and even though we are unlikely to run it that “hot” I’m sizing everything for minimum 300 amp continuous duty rating. I see lots of cheap solenoids advertised as “Continuous Duty 300 Amp” but when you read carefully you find they are only rated for 150 amp continuous duty and 300 amp intermittent. The Blue Sea Systems L Solenoid part 9012 is rated for 300 amp continuous duty when installed with 2/0 wire. The biggest problem I have with using two of these solenoids, even more than the cost of $170 each, is their continuous current draw which totals 6.2 amps per 24 hrs to keep the coils energized, the switches closed, and battery current flowing. I also suspect these will get hot, most solenoids do, and I would prefer to not add more heat to my battery /electrical compartment.

A bi-stable latching relay would be great from the perspective that it does not draw continuous current and does not get hot, but the ones I’ve seen are not “fail safe” because a momentary application of control current is required to cause the latching relay to change state from closed to open. Also, it is unclear to me which brand of BMS might have relay outputs that can be programmed to provide a momentary control signal instead of a continuous signal.

Has anyone found a better 300 amp solenoid alternative to the Blue Sea?

Does anyone have a better solution for automatic safety disconnects?

Any and all thoughts on this topic are appreciated.

Thanks, John

[more]

Originally Posted by blondjohn

first conversion from AGM to a DIY LFP house bank. I’ve read that “best practice” is to install separate load and charge bus bars, and place a safety disconnect (solenoid switch) between the LFP batteries and each bus. A BMS can be programmed to disconnect the charge bus for high voltage and disconnect the load bus for low voltage.

In our 12 volt system we have a 290 amp Balmar alternator, and even though we are unlikely to run it that “hot” I’m sizing everything for minimum 300 amp continuous duty rating. I see lots of cheap solenoids advertised as “Continuous Duty 300 Amp”

Thanks, John

John first watch https://youtu.be/tMUBBAiFX6c?t=236
secondly do you need in your life for boat 20-30 metre 300A charging

[lmxr]

Originally Posted by blondjohn

[...]discussion of solenoid switches for high and low voltage battery disconnection for DIY LiFePO4 (LFP) batteries. [...]

Perhaps you already know the article, in any case Nordkyndesign (https://nordkyndesign.com/protection...battery-banks/) discusses three options. In short, he prefers latching relays.

[...]I’ve also read that solenoids should be Normally Open (NO) meaning control current is required to keep the solenoid switches “closed.” Theoretically, if a BMS failed, control current would stop, solenoids would open, and the batteries would be disconnected “Fail Safe.” [...]

I disagree with "if a BMS failed, control current would stop". Assuming the driving FET kicks the bucket, then the FET fails shorted in 99% of the cases and control current will continue to run. Assuming the microcontroller locks up, then it will retain its last state, which is probably closed. While it may fail open in most of the cases, I wouldn't trust it to do so, and therefore wouldn't consider it "fail safe".

I understand bus disconnects should never happen because of charging source control and audible alarms, but it does seem like good insurance.

I agree, especially for the charging bus. There is some discussion about whether this is really necessary. Given the enormous amount of energy LFP batteries can store, that in case of severe overcharging they can release significant amounts of gas, that the value of the assets being protected runs in the hundreds of thousands, and the fact that human life is at stake (contrary to an RV, evacuation is rarely a reasonable option), in my opinion a double layer of protection against overcharging is justified.

[...]The biggest problem I have with using two of these solenoids, even more than the cost of $170 each, is their continuous current draw which totals 6.2 amps per 24 hrs to keep the coils energized, the switches closed, and battery current flowing.[...]

Same. 6.2 Ah daily is not much but still too much for my taste. What I dislike most is that it leads to a difficult dilemma's in case of an empty battery. If the battery is empty, then all consumers should be turned off, but the charge bus NO relay is also a consumer, so the chargers get cut off as well, and now how is the battery going to be recharged without manual intervention?

A bi-stable latching relay would be great from the perspective that it does not draw continuous current and does not get hot, but the ones I’ve seen are not “fail safe” because a momentary application of control current is required to cause the latching relay to change state from closed to open. Also, it is unclear to me which brand of BMS might have relay outputs that can be programmed to provide a momentary control signal instead of a continuous signal.

I think TAO BMS supports latching relays natively. REC sells an external latching driver that I guess is compatible with other BMS's as well. Some latching relays have an internal driver that lets them be controlled as they were NO relays. There's a Dutch company that sells these and Blue Sea also has versions of their ML-RBS that can do this, I think.

[Steve_C]

Blue Sea RBS 7713 is a Bi-Stable 500 amp rated auto-release version. Its expensive but can be found on ebay often for less than $150. It uses only milliamps to maintain connection, but releases when the control signal stops. It is available in 12 and 24 volt.
The cost is very reasonable especially considering the quality and the fact it includes the ability to remotely open or close it with an LED to indicate its state. If you mount it remotely you can still have immediate control from wherever you mount the switch or switches.

https://www.bluesea.com/products/cat...ttery_Switches

[tkeithlu]

Continuous duty 12v solenoids are not exactly rocket science - every small airplane uses one. Here's the non-FAA approved version that seems to meet your current requirements for $19.00:

https://www.amazon.com/Continuous-So...8452717&sr=8-4

OK, you found those already. You need 300 amps continuous? What are you doing, welding from your house batteries?

[nwn]

I have been searching for the same and found the Chargery system (see http://chargery.com/ ) that uses Mosfet contactors which draw a very low current. The Chargery system has BMS and chargers all interconnected with the contactors. I have built a system with a contactor for charge and one for consumption for my 12V system and for my 24Volt system.
The 12 Volt system works very smooth. A display tells you continuously the state of the batteries and the BMS is balancing with more than 1 Amp per hour, which is more than many other BMS. I will recommend external voltage regulator of the alternator because the LiFePO4 batteries are quickly recharged. Else, the Mosfet contactor on the charge side gets too much work to do. For a boat use, it is essential that you choose separate charging and consumption contactors.
On the 24Volt side I have experienced that heavy use of the bow thruster because of a grounding in a shallow harbour made the Mosfets rated at 300 Amps to burn out. I have a post on that. The reason for the burn out was probably that I had underestimated the spike current every time you start a thruster. So now I have to get new contactors rated higher.
I should add that I am not connected to Chargery in any way, but I think it is good solution to the problem we all have with d.i.y LiFePO4 batteries .

[Cadence]

Originally Posted by tkeithlu

Continuous duty 12v solenoids are not exactly rocket science - every small airplane uses one. Here's the non-FAA approved version that seems to meet your current requirements for $19.00:

https://www.amazon.com/Continuous-So...8452717&sr=8-4

OK, you found those already. You need 300 amps continuous? What are you doing, welding from your house batteries?

Way to much current for welding.

[CaptainRivet]

Originally Posted by tkeithlu

Continuous duty 12v solenoids are not exactly rocket science - every small airplane uses one. Here's the non-FAA approved version that seems to meet your current requirements for $19.00:

https://www.amazon.com/Continuous-So...8452717&sr=8-4

OK, you found those already. You need 300 amps continuous? What are you doing, welding from your house batteries?

Simple induction cooking…300Ax12V=3600W…a 2 or 3 burner induction stove has that…i have a 5kw inverter, 3 stove induction and 1200W convection oven and if the wife is cooking for guests I sometimes have 400A load for 15-30min on the house

[Cadence]

Originally Posted by tkeithlu

Continuous duty 12v solenoids are not exactly rocket science - every small airplane uses one. Here's the non-FAA approved version that seems to meet your current requirements for $19.00:

https://www.amazon.com/Continuous-So...8452717&sr=8-4

OK, you found those already. You need 300 amps continuous? What are you doing, welding from your house batteries?

Looks like a starter solenoid from days of yore.

[GRIT]

When I first set up my system. I used two Victron "battery protect" mosfets. Within 6 months, one of them failed (closed), and I switched to the Blue Sea systems solenoids you mentioned earlier. It was very disappointing, as they cost as much as the Blue Sea Systems relays.

I don't mind the 6ah a day; it's so much better than having a mosfet fail closed, then needing it without knowing it failed.

I gave away all the other "battery protect" units I bought (all 200A units), and sleep better now.

I just checked to be sure of what I'm saying, and they don't get hot, or even warm. No issues there.

Cheers.
Paul.

[Philtao]

You can have a look at Gigavac GXL14 (350A) or Tyco (TE Connectivity) BDS-A latching that are much more affordable, unclear about the rating (190 - 260A)

I attach a spreadsheet listing the latching relays I found and the way they are controlled...

I fully agree with you on the control side, in case of a failure you definitely need to send the close pulse. But if your BMS is not responsive it may not do so. I my opinion this relay control function is better to be outside the BMS.

Having the same need I have designed and just tested prototypes for a latching relay driver that converts the on/off (or open/close) BMS output into the proper pulses. Pulses are sent at the change of state. So if your BMS fails in an NO state (open/off), the close pulse is sent (as long as the relay driver is powered!). See attached picture (the switch is where the BMS output connects)

Jus before the load disconnect happens you may also want the BMS to close a relay to parallel a backup battery (starter, bow-thruster...). that may save you from being in the dark with no VHF, navigation light, bilge pumps.... A small latching relay would be enough just to power the safety equipment

[blondjohn]

Thanks so much to all of you who have taken the time to help me and others who will read this thread. I think I’m pointed in the right direction now with more research to do, and I’m sure more questions to follow.

Imxr, Steve C, and Philtao you all share my award for “most helpful.” (Sorry, no cash prizes, just accolades.) Thanks for educating me and providing recommendations. I will do further research on the TAO BMS, REC latching drivers, and the Blue Sea Latching relays. It appears I did not understand the full range of potential functionality of the latching relays from what I read on the Blue Sea website and I need to dig deeper. I also did not understand the availability of drivers for integrating latching relays. I have read all the information from Nordkyn Design but I need to go read it again. There is a lot to absorb.

Philtao, thanks for the spreadsheet. I’m impressed with you designing your own latching driver, but electronic circuit design is beyond my “current” abilities so I will try to purchase a ready-made solution if needed to integrate disconnect switches with the BMS I eventually select.

CaptainRivet, thanks for coming to my defense that 300 amp circuits are not ridiculous and only useful for welding. I don’t have big loads like you, but I have a Balmar 96-275 alternator I installed in 1999 with 3/0 wire on a Yanmar 4JH2-TE. 22 years and I haven’t welded anything yet. I’ve never even blown a Class T fuse. The Balmar spec sheet says 290 A max and I’ve seen it put out 280 A for a moment until the batteries were at voltage. For charging I normally use a 160 amp alternator, turned down to 130 amps, on a single cylinder Kubota diesel used as a DC generator. Still, all electrical system components must be sized for the maximum possible load. Even a dummy like me knows that.

Grit (Paul), thanks for sharing your experience that the Blue Sea relays do not get hot. Very good to know. Maybe I should upgrade my 35 year old DC refrigeration compressor relay that does get hot to the touch. I saw one cheap 300 amp rated relay that said to expect operating temperatures in excess of 200F!

And finally, nwn, I’m glad to hear you are happy with your Chargery BMS. I hope it gives you many years of good service. I crossed the Chargery off my list of BMS contenders for several reasons, but let’s save that discussion for a future thread on BMS selection.

Thanks, John

[Philtao]

Originally Posted by blondjohn

Philtao, thanks for the spreadsheet. I’m impressed with you designing your own latching driver, but electronic circuit design is beyond my “current” abilities so I will try to purchase a ready-made solution if needed to integrate disconnect switches with the BMS I eventually select.

That product will be available for sale in a couple of months. We are adding a terminal block to connect a SPDT switch (3 positions) to have a manual override (Auto - OFF - ON)

When selecting a driver for latching relay, be sure to check what it requires as input and that it matches the BMS output. It can be:

• low side drive: the input pin must be connected to ground to activate
• high side drive: the input pin must be connected to 12/24V to activate

The BMS I know does both, but the latching relay driver I designed requires a high side drive (although it is more complex circuitry, in my opinion it is better as power is only applied to the input when the relay is activated... and a choice had to be made!)

[blondjohn]

Originally Posted by Philtao

The BMS I know does both, but the latching relay driver I designed requires a high side drive (although it is more complex circuitry, in my opinion it is better as power is only applied to the input when the relay is activated... and a choice had to be made!)

Thanks for that. I just went to your website for the first time. Now I have more to study!

John

[hjohnson]

Originally Posted by blondjohn

A bi-stable latching relay would be great from the perspective that it does not draw continuous current and does not get hot, but the ones I’ve seen are not “fail safe” because a momentary application of control current is required to cause the latching relay to change state from closed to open.

It kind of depends on how you rig it, but I believe you can achieve that with the Blue Sea 7700 and at least the REC BMS (fail safe with the latching relay).

Using CatNewBee's circuit (basically a couple of capacitors, LEDs, and resistors), you can connect what amounts to two pulse connections to the two NO and NC relay outputs from the BMS. If the BMS fails, and lets go of its own internal relay, that will cause it to switch back, generate a pulse on the "open" line on the 7700, and voila, it goes open. The LED consumes significantly less current than a traditional contactor would.