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Discussion Starter #1
Some know that I've purchased Steve's van with lifepo4 batteries. One of the problems with lithium is they can't be charged in cold weather. Steve mentioned that he would just hook up a space heater when it was parked for a while during cold weather. I live in a condo building with a large parking lot but don't have anyway to hookup a heater to run during the winter with shore, so I'm looking for other ways to address charging in cold weather. He also has heating pads that were hooked up to the inverter with an AC controlled thermostat that would turn on at a set temp. While those are nice, they draw around 18-20amps. I can only bring in 12-14amps on a good day so I'd be at a net loss for the day, plus they would run all evening when I have no solar coming in. I think it would just be easier to come up with an automatic way of disabling the solar panels when it's below freezing.

I've got a 12V thermostat (
). I was talking to someone at work and he suggested using an automotive relay. Most automotive relays are Normally Open, so they would require a current all the time to keep it closed. This would be a drain on the system when most of the time, you'd like to keep it closed. 5Pin SPDT relays ([ame]https://www.delcity.net/images/linedrawings/Relay%20Diagram.pdf[/ame]) with an 87a pin can be wired to to be Normally Closed and would only require current when you'd want to open it when temps near the battery reach freezing temps (lets say 36 degrees). However while the coil won't be energized all the time there would be current going through it for half of the day every day when there is solar. I know that most relays aren't used all the time (horns and high beams aren't used continuously) so my concern would be burning out the relay. There are some that are rated for 40-60amps, but I'm still worried that they could burn out. I know they are cheap so i could keep spares and I've even found a base so I could easily swap them out ([ame]https://www.waytekwire.com/item/75290/Relay-Connector/[/ame]) but I'd hate to see them burn out when they would be needed.

Does anyone have any thoughts on this?
 

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This will switch 20 amps. Cheap. Try similar and save yours for monitoring. What you need is a thermostat. There are battery operated ones. You should be able to connect to normally open or normally closed contacts.

[ame]https://www.amazon.com/12V-Programmable-Mini-Digital-Thermostat/dp/B01N7PSSH5/ref=sr_1_10?s=hi&ie=UTF8&qid=1512188820&sr=1-10&keywords=12+volt+temperature+switch[/ame]
 

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It's been a while since I did any of this stuff and I usually had a EE looking over my shoulder but here goes...

There are solid state relays which don't burn out when either continually open or closed, but come with a higher closed resistance, lower open resistance and reverse leakage current as compared to a conventional relay.

Is there any way to can suppress your charger rather than trying to switch your solar? You'd think that an intelligent enough charger would be able to do this, and that they'd include a remote thermal sensor for this purpose considering this is a known design parameter for lithium batteries.

Other thoughts would be finding D.C. Battery heating mats that use less current or finding a way to cycle the ones you have with a timer (might involve another relay). Here's a water tank heater along those lines with a built-in thermostat: https://www.etrailer.com/RV-Fresh-Water/Therma-Heat/277-000164.html Most lithium batteries are small compared to lead acid, so I don't think you'd need a very big one.
 

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Discussion Starter #4
This will switch 20 amps. Cheap. Try similar and save yours for monitoring. What you need is a thermostat. There are battery operated ones. You should be able to connect to normally open or normally closed contacts.

https://www.amazon.com/12V-Programm...0&sr=1-10&keywords=12+volt+temperature+switch
Only problem with that (and the unit I have) is the terminals are very small and can't fit the larger cables that run from the roof. I also doubt their ability to handle that much current.

It's been a while since I did any of this stuff and I usually had a EE looking over my shoulder but here goes...

There are solid state relays which don't burn out when either continually open or closed, but come with a higher closed resistance, lower open resistance and reverse leakage current as compared to a conventional relay.

Is there any way to can suppress your charger rather than trying to switch your solar? You'd think that an intelligent enough charger would be able to do this, and that they'd include a remote thermal sensor for this purpose considering this is a known design parameter for lithium batteries.

Other thoughts would be finding D.C. Battery heating mats that use less current or finding a way to cycle the ones you have with a timer (might involve another relay). Here's a water tank heater along those lines with a built-in thermostat: https://www.etrailer.com/RV-Fresh-Water/Therma-Heat/277-000164.html Most lithium batteries are small compared to lead acid, so I don't think you'd need a very big one.
You'd think there would be a way to cut off charging at a certain temp on the MPPT, but surprisingly my controller and all the other controllers I've looked at do not do that.

As for different heating pads, I'd like to move away from using power to enabling charging. It just seems counterintuitive. As you said, I could wire up a timer as well, but it seems simpler to just cut off charging rather than attempting to heat the batteries.

I keep going back and forth if I should just bite the bullet and buy the blue sea latching relay. It's pricey, but so are the lifepo4 cells if I damage them. I've found another possible solution through a kickstarter, electrodacus.com has a BMS for lithium and solar control in one unit. It looks like a pretty nice solution but won't work with my inverter charger. I've thought about just using it to connect solar and use the BMS just to monitor cell voltage and disconnect the solar. It has a temp input so it can shut off it's own charger. But, it's the same cost as the blue sea solenoid and that has no software to control it. It either latches or it doesn't. I could also wire it to a physical switch to turn on or off the solar. hmmmmmm
 

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Discussion Starter #6
Found this cold charge disconnect.

Not that I'm advocating buying this as it's a pricy solution, but the wiring diagram might give you some ideas.

https://amsolar.com/rv-lithium-battery-banks/70-coldac
Thanks. I've come across that before as the system in my van is from AM Solar before they started using Victron. It's a pretty complex setup that basically will do the same thing as what I'm trying to do. It won't work for me as that requires a Victron battery monitor to act as the "thermostat" and a Victron inverter as that can accept a remote shut down on the charger portion. However I'm not really concerned with the inverter/charger charging in cold weather as any time I'd have it plugged in, I'd be running heat.

Though that Relay they they use I've come across before, but it's still pretty expensive ($130). It can support 200 amps, but I wouldn't need that much current as I'd only be using it to control solar current. In their setup they have all charging sources going through it which could be close to 200amps.
 

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Discussion Starter #7
I found a really good deal on an Tyco Contactor EV200. My BMS (and that cold weather disconnect Ziggy posted above) use the same one. Most people use the EV200AAANA where:

EV200 is the series for 500amps
A is for 9-36V Coil Voltage
A is for 15.3 in Coil Wire Length
N for no terminal connector
A is bottom mount.

Well I found a EV200AAACA where the C stands for a mini Molex connector. It's the exact same relay but it as a molex connector on the end! I guess since most people order without a connector on the end, this may have been old stock they couldn't move. Best part is I got it for $50. Sure it was more expensive than a simple relay, but these are high quality contactors and I could always re-purpose it in the future if I want to change my BMS. It is a normally open relay so it'll be drawing .13amps all the time, but piece of mind that it should be rock solid is worth the 3 amps per day ;)
 

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Discussion Starter #9
Does it get 'way cold' there? Do you have provisions for minus temperature battery protection?
It doesn't get that cold. We have some 20 degree days and some years some teens, but nothing negative.
 
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