Home Built "Lower Temperature Charge Protection" Circuit.

[rickst29]

Good morning, I'm a new member. I see a few Threads about BMS boards without temp protection, and BMS boards "with temp protection" cut-off temperatures set too low. I have built the following system for my own RV (stored in a COLD garage through the Winter, and subject to cold AM temperatures in early Spring Camping.

Independent of any BMS built into an existing battery pack, I use a cheap 12v "fridge controller" (thermostat with temp display) programmed upside-down: Its low-power Relay contacts switch "ON" when the temp Sensor falls below 38 degrees Fahrenheit, and then turns back "OFF" when the temp Sensor reaches 50 degrees. But I don't have an actual heater connected to the Fridge temp Controller. Instead, I have only a low-current 12v circuit (from the battery) which will drive the coil of a big 5-pin automotive "Power Relay".

The "common" power leg (pin 30) on this Relay comes in from my charging Source (a Power Converter). In normal conditions, when the relay coil is unpowered, power goes directly to the +12 Volt Battery Power Distribution Bus, and the batteries can be charged. (I have 3 somewhat small LFP batteries in the RV.) But, in cold temperatures, the "Power" Leg is switched away from the batteries and instead goes through some relatively cheap heating pads, made for motorcycle seats, which slowly warm the battery cases. When the entire compartment has warmed to 50 degrees, the heater pads are turned off and charging begins. Diagram:

 

[rickst29]

Unfortunately, the system does not work if the batteries must be charged only from Solar power. Most Solar Charge Controllers do not have separate "Battery charge output" versus "input power" connections, and will lose all of their settings when the battery is disconnected. In my RV, I currently have a separate MANUAL switch to "heat directly from the battery itself", and I manually shut off the "PV +" Solar Circuit Beaker. These switches could be replaced by more Relays (one following the "PV +" safety circuit breaker), again run by the temp controller. But they would "heat the battery" through the night, unless the control signal for the "heater relay" was also run through a timer.

For relatively small benefit, in comparison to switching by hand in the morning, those add-ons make things a bit too complicated for me. I'm staying with hand-switching, during mornings in "Boondock" camping.

 

[MisterSandals]

and will lose all of their settings when the battery is disconnected.

I have never heard of an SCC losing all its settings once unpowered.

What SCC are you referring to? That seems like a really poorly designed SCC ...

 

[rickst29]

I have never heard of an SCC losing all its settings once unpowered.

What SCC are you referring to? That seems like a really poorly designed SCC ...

Thanks, MS: You're probably correct - the settings in my Ep-ever "BN" series controller might survive. But would it actually function, and put out regulated 12v with no battery attached? (I have a bad feeling about that).

 

[MisterSandals]

But would it actually function, and put out regulated 12v with no battery attached?

I am pretty sure that like most other SCC's, that the battery is the first thing that should be connected. This is because it provides the startup power when the battery/system/charging voltage is sensed and set.

And, every manual that I've read says NOT to connect panels without a battery already connected.

So no, you should NOT power your SCC with panels and output 12v without a battery.

 

[rickst29]

My conclusion exactly. The "battery charging voltage" would be only be going through the heater pads, with no capacitance.

 

[HRTKD]

So no, you should NOT power your SCC with panels and output 12v without a battery.

Definitely agree here. Some of the PWM controllers may get by without a battery, not sure. On the RV sales lot, I see a lot of PWM controllers that are "on" without a battery.