Latching relays for protecting LiFePO4 batteries

[isaacchacon]

I just got the blue sea 7713 for my
Costly lab. The disadvantage on it is that when open, it will still consume energy from either a or b main connections. That means my lvc won't be a true lvc. The power it sucks is small (5mA), but enough to void the warranty of my batteries from elite power solutions.


I'll try to workaround this on the weekend by adding a small relay that cuts the ground connection from the 7713. I think this should work. Concerned about the energy consumption, I even got a specialty relay (elk912) that consumes only 30mA instead of the wasteful 130mA approx that an automotive relay would burn.


If my experiment t doesn't succeed , I'll have to return the 7713 and maybe try the 7700, but it won't be as straightforward as it is now, since the 7700 requires only momentary currents, I may have to play and cascade Normally closed relays.


I spoke yesterday to a blue sea tech ( they are nice and knowledgeable), he said that the 7700 won't need power from a nor b connections

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[isaacchacon]

Originally Posted by exMaggieDrum

The Blue Sea 7713 remote battery switch I am (currently) planning to use has a manual override switch built in to it so it can be operated independent of the relay.

I may have a new issue after rereading Stan Honey's PDF. On the section on parasitic draws he says he used a BS 7712 switch. That is the previous version of the one I have ordered which is now the 7713. I think they are basically the same so they use the same relay (I would expect but don't know for sure). Anyway, he says that the BMS signal is (basically) the wrong kind. (I tried to cut and paste his paragraph but it was totally messed up here). He said he had to use a P-FET and resistor to convert the signal to a "12 volt pull-up control signal". I'll have to get someone to translate that for me and then decide if I need to do that. He says the HP BMS uses a "open collector" pull down output for solenoid control and that wouldn't work for his 7712.

I like the feature of the manual override on the 7713 which is why I wanted to use it, so there isn't a single point of failure on the main battery bus (like Entropy's system and as per the HP installation manual).

On my (current) design I am putting both of my alternator outputs to the Main Bus as opposed to putting them on the HVC bus. As per the discussion by Entropy I am going to take the risk of a problem from that since (a) the Main Switch will prevent overcharging just 0.005 volts higher, (b) I will have a alarm siren on reaching the warning level and can manually shut off the alternators, and (c) I am putting in two field disconnect relays for the alternators which will be driven by the HVC control from the BMS (one relay in front to change the "logic" so that the fields are closed circuit until a HVC warning event). See HP BMS manual and Entropy as to changing the "logic".

I will look into whether I have to use the P-FET and resistor to control my main bus "protection level" switch or whether is will work as per the BMS manual or whether I can use another solenoid relay to send the right signal to open it when the protection level is reached.

I hope all this complexity is really necessary and not just extra like Rich Boren says. It hasn't been a cheap exercise.

Ex Maggie that sounds too complex to me. I have the 7713 and the elite power solutions bms. I'll test the lvc connection to it this weekend and will let you know.

I think that 12v are required to close the relay, no matter if it's Generated by a mosfet or whatever electronic component as long as it's 12v. Though My knowledge in electronics is really basic..

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[exMaggieDrum]

Issac,
The ELK912 looks like a neat little relay. It will take up to 7A draw. The 7713 Blue Sea switch can draw "less than" 7A for 20ms. I found the ELK912 on ebay for a good price. I may get it as it is a much lower parasitic draw than the automotive ones. Another pucker factor is to replace a well-known and widely used automotive relay with one that I have no experience with and do not know anyone who has used it, but it has excellent reviews on a couple of sites.

Yesterday, I spent some time trying to figure out how to create momentary pulses from the steady on voltage from the BMS and I decided that I could probably do it but might have to build a little circuit with capacitors and resistors. I decided that is pushing the complexity out of my comfort zone (which is pretty far out there already). I also could not determine all the characteristics of a pulse from this and don't want to have to play EE and troubleshoot it later. This is so I could use the BS 7700 relay.

I think I am going with the 7713. I am going to use an additional NO automotive (or the ELK912) relay to drive it with the BMS. At least the main battery bus will be shut off. I am wiring the other relays to the main bus (after the main contactor) and the BMS to the battery (before the main contactor). So if the BMS shuts down the main bus it will also shut down the relays so no power draw to them. I will be able to reset the BMS with it's reset button.

But (from what you said) I will still have the 5mA draw by the 7713 itself which must be wired to the battery side "24 hours". That is a worry if the boat is left for any time and the battery does go low enough for protection level to kick in. I calculate the draw would be less than 1Ah per week though so not very much at all. I guess at some point that might be a problem if the battery is really going flat.

Nothing about this project is really uncomplicated for the average sailor, or even for the average professional marine electrician. I have shown it to a couple of electricians I know here and they said they would not want to work on something like it if asked, or if they did, it would take a lot of hours for them to understand and troubleshoot it.

One of the issues on my to do list is how to do the initial, and subsequent, testing of the entire system after installation to make sure it will all work as designed (or intended since that could be different). The only way I can see to do it for the moment is to go through the entire charge/discharge profile of the installed cells to take them up to high voltage and then down to low voltage and on both sides to protection levels and see if it indeed works.

I would expect that Elite Systems would have a recommended and approved method that would meet their warranty criteria. But that may mean a manual disconnect of all drains on the battery. I guess you could have a manual switch to disconnect the relay if you were going to leave the boat for a while.

I will be "self-insured" and "self-warranted" on my own. I am hoping that I will have checked and double-checked the design and then checked and double-checked if I installed it as designed, and then test the heck out of it first before relying on it. In concept it is not overly complex but the actual bits are numerous with lots of room for error in mis-wiring or faulty components. But it is an interesting project and I think it is doable so long as the BMS works as advertised.

[toddedger]

Use 7700 draws no current in the On or Off mode.

[exMaggieDrum]

Issac,
A question on the 7713: Did Blue Sea tell you it will still draw 5mA even if it is turned off via the manual override switch on the top of the unit, or only draw 5mA if the remote switch is off and the 7713 is set to remote operation, i.e. when it is remote enabled?

[exMaggieDrum]

Originally Posted by toddedger

Use 7700 draws no current in the On or Off mode.

How do you reset the 7700 using the HP BMS, i.e. turn it off from on? The BMS sends an always on signal with normal operation which will turn on the 7700 and this signal will stay on, i.e. not momentary as expected by the 7700. When you first turn on the BMS it would energize "Relay" to "E" which is used to drive the main switch (or via intermediate relay). This stays on until protection level is dropped. The way I understand the 7700 you need to send a momentary signal to turn it off via another circuit. Just dropping the on circuit signal does not turn it off.

How did you do this?

[toddedger]

For protection level your suppose to use a 9012. It uses some power but is inherently more safe. See post #6

At the dock I switch my load over to my AGM starter battery and turn on the AC charger for it. Why use the expensive batteries when not needed or under not ideal conditions. When under way the small load of the contactor is relatively nothing for my 400 am bank.

[exMaggieDrum]

Originally Posted by toddedger

For protection level your suppose to use a 9012. It uses some power but is inherently more safe. See post #6

Not disagreeing with you, but just for information, why is it inherently more safe?

At the dock I switch my load over to my AGM starter battery and turn on the AC charger for it. Why use the expensive batteries when not needed or under not ideal conditions. When under way the small load of the contactor is relatively nothing for my 400 am bank.

Sounds like a good plan if you have a dedicated AC charger with an AGM charge profile. I don't but right now I plan on keeping the solar panels on with the HVC relay keeping them from overcharging the cells. But that would be a last resort on the charging side as I will set my solar regulator at 13.8V (subject to change). I am still waiting to get more info on the possibility of damaging the cells with constant current at any voltage as stated by one poster on the other thread.

I do have a concern if that event happens and the alarm goes off and I am not there to deal with it (alarm drain and urgent calls from the marina or dockmates, etc.). I have been thinking of putting in an alarm on/off switch in any case since they can be distracting after the initial alert and you can't solve the problem in a few minutes. The first thing I always check on returning to the boat is the batteries.

In actuality I am thinking of just turning off the house anyway when I am away for anything but a short time. The only think I need to sort out is bilge pumps and alarms. The start battery should be fully charged in most scenarios with an EchoCharger when the house batts are in good working order.

Thanks for your suggestions. They are helpful.

[toddedger]

(Why is a latching relay inherently unsafe for a main protection circuit )


If anything goes wrong like a BMS failure the system defaults to off

. My house loads bus has a 1, 2, or both battery switch with 1 being LiFePO4, and 2 being the engine AGM. I can turn my bms off and go to both on the load switch and change my charger profile. But a small dedicated charger on the engine AGM would be a small price to pay for insurance when away from the boat.

[exMaggieDrum]

Originally Posted by toddedger

(Why is a latching relay inherently unsafe for a main protection circuit )


If anything goes wrong like a BMS failure the system defaults to off

. My house loads bus has a 1, 2, or both battery switch with 1 being LiFePO4, and 2 being the engine AGM. I can turn my bms off and go to both on the load switch and change my charger profile. But a small dedicated charger on the engine AGM would be a small price to pay for insurance when away from the boat.

Good answer. I'll have another look at it. At least my load and charger bus switches default to open. I have a separate battery charger on my wish list already as I don't want to rely on the older combined inverter/relay. Just haven't been able to afford it yet. Nice to have the insurance, and as you say, it would be good to isolate the house battery the way you have.

I am also putting in manual off switches for my alternators. I can turn off the solar via a remote already. Of course, nothing helps unless you turn them off before you leave the boat. I am going to set up a separate alarm via a separate battery monitor so I don't have to rely on the BMS to tell me the voltages are going south.

[isaacchacon]

Originally Posted by exMaggieDrum

Issac,
A question on the 7713: Did Blue Sea tell you it will still draw 5mA even if it is turned off via the manual override switch on the top of the unit, or only draw 5mA if the remote switch is off and the 7713 is set to remote operation, i.e. when it is remote enabled?

Yes. I just made that test and my multimeter says it draws 4.3 mA from A or B , even when manually switched off.
I'll test now my elk912 relay which arrived yesterday. I'll tell you how it goes later

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[isaacchacon]

I just tested the following setup with success:

I used the wired the negative cable of the 7713 through the Normally OPen ELK912 and then to the ground.

I also wired the positive output 'B' connection from the 7713, so that it would actuate the ELK912 andclose the circuit.

With this setup, it was possible, when the 7713 was open, to completely prevent any current to be drawn from the main battery.

To close the circuit from an open state is a bit tricky but i'd rather have to have some discomfort that way than having the 7713 drawing energy from my batteries even when open circuit.


To close the circuit, i put a momentary switch 'bypass' between hot and the postiive + of the elk912. without releasing this switch, i turned on the 7713, and then i released the momentary switch, since the elk912 was now getting fed from the 7713.

Hopefully i explained myself if not ill share a diagram hopefully tomorrow.

[exMaggieDrum]

Originally Posted by isaacchacon

I just tested the following setup with success:

I used the wired the negative cable of the 7713 through the Normally OPen ELK912 and then to the ground.

I also wired the positive output 'B' connection from the 7713, so that it would actuate the ELK912 andclose the circuit.

Not sure I understand what you are saying here but your diagram should make it clear. See below how I am tentatively designing for the issue.

With this setup, it was possible, when the 7713 was open, to completely prevent any current to be drawn from the main battery.

To close the circuit from an open state is a bit tricky but i'd rather have to have some discomfort that way than having the 7713 drawing energy from my batteries even when open circuit.


To close the circuit, i put a momentary switch 'bypass' between hot and the postiive + of the elk912. without releasing this switch, i turned on the 7713, and then i released the momentary switch, since the elk912 was now getting fed from the 7713.

Hopefully i explained myself if not ill share a diagram hopefully tomorrow.

I just heard back from Clean Power (House Power BMS vendor) and he says that Stan Honey was correct. You can't drive the latching Blue Sea 7713 or 7700 with the BMS. You either need to use the P-FET/resistor circuit as a relay or put in another relay.

I was doodling yesterday and drew up a tentative design using another relay, either the automotive style or the elk912. I ordered five of the elk912's yesterday after your recommendation and my own research. I may use them for all the relays in the system. They are inexpensive at 8$ per.

If I were to use the 7713 with its 2145 on/off switch I drew the following: Instead of providing B+ from the battery direct to the 2145 switch (24 hour always available before the main switch) I put the battery B+ to the new additional relay B+ (30 on an automotive relay).

This relay would be normally open and the switch part of the relay would be driven by the BMS - Relay + and "E" ground, 86 and 85). This is always on so the relay would be always closed until a protection level is reached. The output of this relay would go to the 2145 switch "3" and "8" (where the 24 hour B+ from the battery would go otherwise).

The control part of the switch would drive the 7713 as normal and both the switch and the 7713 would be grounded per spec. That way the 2145 would have to be on (with corresponding LED light) to close the 7713 and the 2145 switch could be used, per spec, to open the 7713 by pushing it to off. The LED indicator would go to "off".

This would break the B+ from the switch to the 7713 as it will revert to open with no power (unlike the 7700/7701 which requires a second momentary pulse to open from latched).

There will still be the small parasitic draw by the 7713 as that must come from the battery side main cable and the ground the switch requires to operate.

This may be what you are describing. When I am pretty sure it will all work I will do a final diagram neat enough to share.

Note: I didn't find a schematic for the elk912 relay but it may use a P-FET to drive its operation.

[isaacchacon]

OK.

I got the circuit.

Sorry that i couldn't make it simpler, but i put together my master design and then i may work on a simpler version that shows only the 7713 part.

this configuration drains 0 amps from the battery when opened. ( ALready tested).

IN my circuit pay attention to the 7713 on the left, and the ELK912 that is closest to the 7713. Also pay attentions to the conexions of those 2 and the involved switches / diodes / fuses.

I am using another ELK912 relay for cascading Over Voltage but that gets out of this topic.


To provide power :

press Normally OPen momentary switch 1 don't release it. you'll hear how the elk912 closes the ground for the 7713.

press Normally OPen momentary switch two. you'll hear a loud blast for the 7713 closing itself.

release the Normally OPen momentary switch 1.

In my case, i have to wait about 5 seconds until the BMS initializes and changes its under voltage from zero volts ( alarm state) to +12 v (Normal state)

ONce the LCD of my BMS powers , then i can release the Normally Open momentary switch 2.



Now, to completely turn off the system, so it doesn't draw any power, i'll just press the Normally Closed Momentary switch 3 and the 7713 opens itself which in turns opens the elk912 .


Hope this explanations helps to understand my already tooo complex circuit.


P.S. when i decided to go lithium I have not a single clue of the COMPLEXITY that it is. It's beeen a rouughhhhh road but i finally got my system 100% working !!!!!!!!!

[senormechanico]

I must be a caveman using LiFePo4.
I leave my solar on all the time (440 watts).
I leave the fridge on all the time (insulated with Aerogel).
I leave my ultrasonic barnacle system running all the time (average 800 ma).
I leave the LED backlight for my MPPT controller on all the time (mounted in the companionway steps which makes a great stair nightlight).
I leave indivdual LED cell voltmeters running all the time (under companionway steps adding to stair nightlight and readable from the V berth).
Total draw average? I really don't know exactly. Maybe 1.5 amps or more, but the 200 aH battery bank hits float almost every day. 13.8 bulk, 13.8 absorb time zero, 13.2 float.
CleanPowerAuto BMS just sits there doing nada but insurance only.
This is at the dock, unplugged for months at a time.
When we're cruising, we have lots of electrical stuff to use power including radar. watermaker, wifi, computer, wife's 1650 watt hair dryer, VitaMix (1400 watts for daily smoothies) and an ElectroScan every time we flush.
We never take a shorepower cord, it's just extra weight.

Never a problem for several years.
Are you guys really worried about 5 milliamps?