AGM subsystem for battery temperature maintenance.

[SParker42]

I am imagining a temperature maintenance solution for a LiFePO4 battery bank in a van to protect them from low temperature charging.

The scenario I am trying to protect the big (for a van, only about 10KWH)24V LiFePO4 battery bank from is one where a van is left parked outside for a few days and the weather gets cold. Maybe even a bit of snow for a day or two that temporarily interrupts charge from the solar array. The LiFePO4 batteries are disconnected from the charge controller as a precaution (I know the picture shows them connected, but I wanted to emphasize that the two solar charge controllers are working in parallel from the same array). In "normal" operation the van is warm, the LiFePO4 batteries are connected as shown and the heating pad is not doing anything because its temperature control circuit has it turned off.

There's a rectifier to isolate the load output and charge outputs from the AGM's charge controller. There's also an under voltage discharge protection circuit on the AGM. However, the solar array is very oversized for the AGM and the sub-freezing response operation would keep the LiFePO4 warm with help from the load output of the charge controller during the day, and then the AGM would keep the LiFePO4 warm all night by itself. The AGM would charge during the day

I've been told that charge controllers can be run in parallel without misbehaving, but never specifically if a 12v AGM charge controller can run in parallel with a 24V LiFePO4 charge controller. I figured it would be good to isolate the load and charge outputs from the AGM charge controller, and I can get my hands on great big rectifiers, and thought they could do the trick.

I've spent a couple hoirs going over old posts to see if my questions could be answered and did not see anything. That doesn't mean it's not there, just that I did not find it in this forum. My feelings will not be hurt if there is something glaringly obvious and you point it out to me. That's kinda why I am asking this question.

 

[cs1234]

Solar charge controllers can run in parallel if connected to their own solar panels. They can't share solar panels. You'll either have to use a DC to DC charge controller from the lifepo4 to the 12v agm battery, or give the agm battery bank it's own solar panel.

At least if you want to use off the shelf equipment that doesn't involve custom built circuits to somehow switch the solar panel array back and forth between charge controllers.. which sounds horrifying.

 

[SParker42]

Is that the case if one of the controllers is MPPT and the other is PWM? or both PWM?

 

[cs1234]

I'm afraid I will have to defer to the experts here that have more experience with sharing an array between multiple charge controllers.

I've seen some dual battery bank solar charge controllers, but none of them that I can find that do separate voltage ranges to each bank (12v/24v). You could always put a dc to dc charger on one of the outputs from such a charge controller, allowing you to charge the 12v bank.

I'm going to bow out of this one for now, and proceed to learn from the masters.

 

[SParker42]

I greatly appreciate your input cs1234. I've also read that the "competition for PPT" between two MPPT controllers is not an issue if the voltages (hence the PPT) is significantly different between the two banks. I wonder if the 12V would be enou8gh less than the 24volt system that there might not be "competition for PPT". I use quotes as I'm not sure about what this process is or if I am far out in I-Don't-know-crap-about-this-land for even mentioning it.

 

[cs1234]

If it were me doing it, I would have a small separate array for the "heating" battery, the AGM, with the panels for that one facing the early morning sun at a steep angle. Then you can heat the LiFePO4's up before they would normally even get much sun on their array. It really depends on your number of panels, controllers, and available physical space.

 

[SParker42]

In a van setup it is less likely that a particular array will be pointed in the desired direction like "early morning sun" any given day. Using something like a switch that chose one controller over another might be the best approach here if the controllers cannot play together nicely. This could both disconnect the LiFePO4s to prevent cold when it connected to the AGM charger and disconnect the AGM charger when the van was all warmed up and occupied. It might not even limit the autonomy of the system much if I was planning on manually disconnecting the LiFePO4s anyway.

 

[cs1234]

My apologies, I forgot about the van part. I should glance back at earlier posts in a thread before replying to later posts.