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Old 18-06-2016, 06:54   #1
nwn
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Question Latching relays for protecting LiFePO4 batteries

I have been inspired by the many posts on this blog to exchange my ailing golf cart lead batteries with new LiFePO4 batteries from the firm ev-power in Prag. I have mounted an BMS of the brand Celllog8. That gives an alarm when the batteries gets too low in volts. I wanted to connect that alarm with a latching relay, so that the relay would switch off the power entirely when the alarm goes. For that purpose I need a latching relay that can handle up to 200 AMPs or maybe less. I want a relay that only uses power when it breaks. It may need power as long as the voltage is below the minimum because I take it for given that my solar panels will recharge the batteries at some point, but it would of course be better to get a real latching relay, that stays out until closed again manually or automatically.
I have found some relays on Google but have difficulties finding out where I can buy them. There seems to be very expensive and very inexpensive solutions around.
Since a lot of people must now be using LiFePO4 batteries, there must be some practical and updated experience among members?
Can anybody help me here?
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Old 19-06-2016, 10:01   #2
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Re: Latching relays for protecting LiFePO4 batteries

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Originally Posted by nwn View Post
I have been inspired by the many posts on this blog to exchange my ailing golf cart lead batteries with new LiFePO4 batteries from the firm ev-power in Prag. I have mounted an BMS of the brand Celllog8. That gives an alarm when the batteries gets too low in volts. I wanted to connect that alarm with a latching relay, so that the relay would switch off the power entirely when the alarm goes. For that purpose I need a latching relay that can handle up to 200 AMPs or maybe less. I want a relay that only uses power when it breaks. It may need power as long as the voltage is below the minimum because I take it for given that my solar panels will recharge the batteries at some point, but it would of course be better to get a real latching relay, that stays out until closed again manually or automatically.
I have found some relays on Google but have difficulties finding out where I can buy them. There seems to be very expensive and very inexpensive solutions around.
Since a lot of people must now be using LiFePO4 batteries, there must be some practical and updated experience among members?
Can anybody help me here?
I was looking at the Blue Sea https://www.bluesea.com/products/770..._-_12V_DC_500A It can handle 500A and no power consumption when latched or unlatched. The problem I have is figuring out how to get the bi-directional signal to the switch using the House Power BMS which only sends one signal of one polarity upon either a low voltage or high voltage situation. If your BMS will do both then you are good to go. I posted a question on the big LiFePO4 forum a couple of days ago on how to do this but no answers yet. Either it is too hard or not possible or a dumb question - not sure which. You can buy the switch on ebay.com and I am sure someone will ship to you for the cost of the shipping from the US if you cannot get it in Europe. I found the best price for a new Blue Sea switch (13mA power consumption) at https://inverterservicecenter.com/

Good luck.
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Old 19-06-2016, 11:23   #3
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Re: Latching relays for protecting LiFePO4 batteries

I used the blue seas latching relay on the load side of my LiFePO4 system .

The latch relay power needs to be powered from the load side of the relay. The trip (off) wire is all so powered by the load side but runs thru a normally open automotive relay that is controlled by house bms.

You have to manually activate the latch relay to turn it on. Forget about the momentary switch that comes with it. When the house bms wants to cut the load off it closes the automotive relay activating the latch relay like momentary switch would. Being powered from the load side of the relay the activation circut goes dead when the relay unlatches.

It's easy and eliminates the parasitic load to only the draw of the main master relay.
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Old 20-06-2016, 14:19   #4
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Re: Latching relays for protecting LiFePO4 batteries

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Originally Posted by toddedger View Post
I used the blue seas latching relay on the load side of my LiFePO4 system .

The latch relay power needs to be powered from the load side of the relay. The trip (off) wire is all so powered by the load side but runs thru a normally open automotive relay that is controlled by house bms.

You have to manually activate the latch relay to turn it on. Forget about the momentary switch that comes with it. When the house bms wants to cut the load off it closes the automotive relay activating the latch relay like momentary switch would. Being powered from the load side of the relay the activation circut goes dead when the relay unlatches.

It's easy and eliminates the parasitic load to only the draw of the main master relay.
Thanks for the suggestion. The manual activation of the latch relay sounds like the best approach. I thought of that but wondered if anyone had an automatic way of doing it. The manual on should work though.
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Old 20-06-2016, 16:02   #5
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Re: Latching relays for protecting LiFePO4 batteries

Both Stan Honey Here
And Entropy Here
describe different approaches on how to use the LVD/HVD contactors.

Entropy wants a disconnect to be independent of any failure (except maybe the contactor itself) and requires power to keep it closed.
Stan felt that that "powered closed" was too much of a power waste and trusts his BMS to send the signal to open the contactor.
Both have used basic circuitry to modify how the contactors interact with the BMS which you can find detailed in the links I've posted.

Toddedger: Your approach on the load side sounds great for a LVD. Does this mean your charging sources are on a separate bus and your main master relay also acts as a HVD? If so how do you use your BMS to power the second contactor closed yet also let it work as a HVD?

Like NWN and exmaggiedrum I am having a hard time wrapping my head around this part of the system without great expense (in $ or Ah).
Thanks
Gary
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Old 20-06-2016, 17:42   #6
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Re: Latching relays for protecting LiFePO4 batteries

To answer Garys question, l still use a EV200 contactor for both HVC and LVC "protection" level event. (House power BMS lingo)

The latch relay interrupts house loads at LVC "alarm" level (low pack level)
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Old 20-06-2016, 17:57   #7
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Re: Latching relays for protecting LiFePO4 batteries

Quote:
Originally Posted by gpeacock View Post
Both Stan Honey Here
And Entropy Here
describe different approaches on how to use the LVD/HVD contactors.

Entropy wants a disconnect to be independent of any failure (except maybe the contactor itself) and requires power to keep it closed.
Stan felt that that "powered closed" was too much of a power waste and trusts his BMS to send the signal to open the contactor.
Both have used basic circuitry to modify how the contactors interact with the BMS which you can find detailed in the links I've posted.

Toddedger: Your approach on the load side sounds great for a LVD. Does this mean your charging sources are on a separate bus and your main master relay also acts as a HVD? If so how do you use your BMS to power the second contactor closed yet also let it work as a HVD?

Like NWN and exmaggiedrum I am having a hard time wrapping my head around this part of the system without great expense (in $ or Ah).
Thanks
Gary
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.
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Old 20-06-2016, 18:11   #8
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Re: Latching relays for protecting LiFePO4 batteries

The House BMS basically connects your relays and alarms to ground during pack level events.

It only provides a small amp load DC voltage to a main contactor that it cuts for cell level protection .

Hope that helps
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Old 21-06-2016, 10:40   #9
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Re: Latching relays for protecting LiFePO4 batteries

These contactors sold in Europe from china are damn cheap... 12V latching relay rated at 100Amps for 48V seems like it would work for most boat systems and for around 20-30 bucks they seem too good to be true. Does that mean they suck? If not where can we get them in North America.
GWL DC Latching Relay
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Old 21-06-2016, 10:52   #10
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Re: Latching relays for protecting LiFePO4 batteries

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Originally Posted by gpeacock View Post
These contactors sold in Europe from china are damn cheap... 12V latching relay rated at 100Amps for 48V seems like it would work for most boat systems and for around 20-30 bucks they seem too good to be true. Does that mean they suck? If not where can we get them in North America.
GWL DC Latching Relay
100 amps @48 volts equals about 20 amps @12 volts. Not much help for most boats. These are made for switching AC loads.
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Old 21-06-2016, 11:08   #11
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Re: Latching relays for protecting LiFePO4 batteries

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100 amps @48 volts equals about 20 amps @12 volts. Not much help for most boats
If the relay takes 100 amps at 48v, it'll take 100 amps at 12v. Amps is a count of the electrons passing a single point per second. Doesn't matter the pressure (voltage). Voltage rating for a relay has to do with how much voltage the relay can interrupt

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Old 21-06-2016, 11:41   #12
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Re: Latching relays for protecting LiFePO4 batteries

I thought a switch got de-rated as the voltage increases. So a contactor rated 100A @48V should be able to handle more @ 12V... I dont think its 400A as some people think but 125A @12V seems reasonable... I wish I knew how they calculated this...
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Old 21-06-2016, 12:19   #13
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Re: Latching relays for protecting LiFePO4 batteries

I think relays are rated at amps @ a specific voltage or small range of voltage. A relay rated at 10A @ 12V is not rated for 10A @ 48V. It might actually work at the higher voltage but it will not last as long. The inherent rating of Amps @ Voltage is a measure of the Power which is really how the relay is rated. 10A @ 48V packs 4x the power of 10A @ 12V. At higher voltages the relay contacts can arc more and this dramatically shortens their life.

I found several references but this one sums it up: http://www.relaypros.com/choosing_proper_amperage.htm
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Old 21-06-2016, 12:37   #14
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Re: Latching relays for protecting LiFePO4 batteries

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Originally Posted by exMaggieDrum View Post
I found several references but this one sums it up: http://www.relaypros.com/choosing_proper_amperage.htm
According to that link, the contactor rated 100A @48V will run 400A @12V... Really? The GWL relay looks like it has wire terminals that are half the size of the similarly rated Blue sea Latching Relays.
I remember reading an article somewhere that showed different relays and the effects of putting their supposed rated current through them (some caught fire). I will see if I can find that reference again.
Gary
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Old 21-06-2016, 23:41   #15
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Re: Latching relays for protecting LiFePO4 batteries

No matter what voltage you are running at the relay will have a certain resistance across the terminals causing a fixed voltage drop across those terminals for a given amperage. That resistance creates heat. Heat is the true restriction in almost all electrical devices. You can show this through the equation Volts = Current x Resistance. and it's counterpart Watts = Volts x Current. Therefore Watts(heat) = Current x Current x Resistance. The watts used in the relay is solely dependent on the amps flowing through the relay x the resistance of the contacts of the relay. Therefore until we reach arcing conditions in the relay(due to overvoltage), the amp rating of the relay is fixed.
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