Diversion load inverter sizing

UpNorthMan

Hello everyone,

My agm batteries are on their last leg at my remote weekend cabin. I am thinking about going to 8S golf cart batteries. My system is currently 48v and my requirements figure out to 220ah@48v. So the golf cart batteries match up perfectly. Since I am going to flooded batteries, I am also expanding my panels from 1000w to 2000w. I am going to build a well insulated battery house.  Our temps can easily get colder then -40F/-40C. With temps that low, I need to have a heat source to keep the batteries ideally 32F/0C.

I am planning on using the diversion feature on my CC to operate a SSR once my batteries are in float mode. The SSR will operate a mod-sine inverter. I am thinking most likely switch the AC side of the inverter . That decision hasn't been made yet. From there, heating a 20 gallon hot water heater filled with a anti-freeze mixture. Two thermostats in series for the heater. One baseboard heater thermostat on the wall set for something like 50F/10C to keep from over heating, and the standard thermostat on the heater set to something around 100F/38C to maintain a min fluid temp. In northern MN, It isn't uncommon to have many days in a row with no sun in the winter. The idea of the water heater is to have some thermomass to carry me over on these cloudy days. I also will be using a low battery voltage cutoff switch set at a high value to guarantee I don't freeze my batteries in case of a system failure.

With the above Ideas, are there any suggestions or different ideas out there?

If I have a 750W or smaller heating element in the heater. Is there a formula or rule of thumb as what size of a inverter I need?

Thanks in advance,

Ed

wellbuilt

Hmmm if you are not staying at the cabin you should just drain any water and turn off your system once you are @ full charge the battery’s won’t freeze 
 my battery’s  stay above freezing all the time 
 and once I charge them the temp will rise to 45 o in a couple days 
  

wellbuilt

Can’t run heaters off your system with out Killing the battery’s . 

 I was down around -15 for a couple weeks and 
  I t was all good , my battery’s can sit for weeks and when I turn it on the voltage stays up around 51.7/8 volts so it’s still above full charge .

UpNorthMan

The batteries are currently in the cabin and warm up as we get the cabin to comfortable temp with our wood store. We've walked in on a Friday night with the inside temp 0F/-18C. At that temperature, the batteries only hold approximately 45% capacity. That doesn't leave much usable capacity without the threat of freezing the and permanently damaging them. They will generate some heat with charging and discharging, but I don't believe enough. I currently leave only the CC on upon departure. By the time the cabin has cooled down,the batteries are back at float. I sized the system for about 1800w usage a day. When it is just my wife and me,our usage is less. When our kids visit or the neighbors come over to play cards and talk smart, our usage goes up. 5 days without sun isn't uncommon in the winter. At day 5, our batteries are getting to that 50% soc range. If they were in a unheated building, they would surely be froze. See the link below for conditions I do have to plan for. It is the extreme side of things, but -45F is real. Tower is only 30 miles a way.

https://www.fox21online.com/2017/02/02/coldest-place-small-minnesota-town-earned-states-frigid-title/

One suggestion from a few years ago was to build a bunker in the dirt and let mother nature help keep them warmer. I haven't given up on that idea yet.

To prevent over discharging, I would only allow the heating circuit to energize when the CC is in float mode. I also plan on setting the lvd to a very high value.

I also am planning on using this same circuit in the summer to operate a hot heater in the cabin.

Is there a guide or general rule for sizing starting power requirements for heating elements?

Wellbuilt, where do you store your batteries? Separate shed, or garage?

What type of insulation for your battery box?

What do you do for venting?

Thanks

Ed

wellbuilt

I have my battery’s in this box in my garage the box is large for the golf cart battery’s but I made it bigger for l16 if I need more power . 

 If I don’t go up for a few weeks the garage can get below zero -10 but if I hit the battery’s with my infer red thermometer I’m only seeing 25 to 35o most of the time . - 40 is cold at that temp this box mite be down to 0 o 
   But You are not think about you capacity the right way , you lose Capacity in the cold put your acid Dosent  get diluted 
 so you can’t freeze 🤗 . Problem solved . 

 If you get to full charge and turn off  your system it would have to get down to -80 f to freeze the battery . 

   You don’t take a dog sleds to the cabin I’m sure you drive and your car battery dosent freeze out side .
     If your solar panels get covered with snow your solar controller can kill your battery’s a heater will not work it will kill your battery’s .
   I have a vent fan that is powered by my out back fm 80 cc when I hit float voltage the vent fan turns  on 
   There is a fan behind the lid that is powered by my fm80 cc it turns on when the battery’s hit absorb voltage 
  Off when the absorb charge is done . 

 There a 5 4” vent holes in the bottom of the box in the winter they have 2” foam board in the holes and caps .
  There is a 2” pipe that runs into the house to draw warm air thru the battery box but I don’t need it . 

 Don’t use a larger fan it will draw all the heat out of the house .
   In the summer I leave the battery box lid up 2 “ or so for extra venting . The battery’s stay 70o most of the summer

   Last week it was about 10o in my garage -5 at 12 noon when we walked up the Mountain 
 the battery’s where 30o in there box . 

 The system was off for over 3 weeks when I turned the power switch on I had 51.7 volts this means there is still .6 of service charge in the battery .
 The next day my battery’s are at 90% at 7am by 200pm I’m down to 85% so I start the genarator and run for a hour and turn it off , at this point my battery temp is 35o the next day I discharge for 36 hours then charge for 2.5hours and the battery temp was 43o at 4 charges I was allmost 50o
 the rule of thumb for inverter sizing with heating elaments is. DONT do it  
    Depending on the size of the cabin  you mite use a propane water heater to run a heat loop . 

   But you would really need a pump and that uses power. 

     I have a 2800 sq foot cabin with 1300’ of Radiant heat in a 4 “ concrete slab  and I’m going to try to use a regular water heater to run it but it will be off when I’m not there . 

 It takes me 3 /4 days to get the house temp to level off burning wood at full blast 💥 but on day 4 I can heat the place with 2 fires a day . 

  When I leave I have 1” foam board from lows I put over the windows it keeps the inside above freezing .
    A few buddy’s stoped buy for a beer there is lots of snow and cold up there , the solar panels have been covered since Christmas 

UpNorthMan

Wellbuilt,

Thanks for the insight. I have time to think about it before I proceed. Nice setup! I like your surrounding roofed porch.

Ed

wellbuilt

It keeps the snow back from the doors . 

 Snows every day here all winter 
   My battery’s are the 89doller sams club wonders I’ve used them a few times a the seam to be good for the money  I have 6 years on 2 in a 12v system . 

 They do like to be charged hard to stay balanced .