8 replies to this topic

[Mark G]

I have a 175w solar panel charging a 15AH SLA battery through a $30 charge controller.  I'm running a Dometic 18L fridge and lights.

 

My theory was that the panel would provide enough power to the battery to run the fridge while the sun is up and stay cool enough for me at night.

 

The plan isn't working.  The fridge runs a couple of hours and shuts off.

 

Any suggestions?

[PaulT]

If the fridge is the CF 18, it requires 3.1 amps while running. Assuming a 50% running time/day, that is 3.1 amps * 12 hrs ==> 37 aH per day. You have less than half that available in your 15 aH battery.  If the battery is lead acid, you should not draw it down more than 50% or about 7.5 aH. 
You likely have enough solar panel for your needs but insufficient battery capacity. Try a battery of above 60 aH as a better match.

You don’t mention the current handling capacity of the charge controller, but it should exceed the current supplied by the solar panel.
 

This is a simplistic analysis of your situation but points out that at the least, you need more battery capacity.  Keep in mind that your solar power needs to supply The 3.1 amps of the fridge plus enough more current to recharge what was used the night before in the amount of time the sun is shining full on the panel. The time available is shorter in the winter And shorter still in overcast or rainy conditions. 
 

A battery monitor is also a good addition so that you will really know the state of charge of your battery.

 

Paul

[Happyjax]

Agreed!

[larryqp]

agreed, your battery capacity is much lower than is required just for the refrigerator, the refrigerator is shutting down because has a battery protection circuit, which stops the refrigerator when the battery gets to the set level.

 

Based on PaulT's calculation of 37 aH used per day, I would consider at least a 75 aH battery, therefore at 50% DOD you'd have the 37.5 aH, but do you really want to be at 50% every day, so why not a 100 aH battery and build in some cushion.

[Mark G]

Thanks all.  I figured that was the problem.  Time for more upgrades!

[CoreyTrevor]

LiFePO4 battery. Expensive but cheaper in the long run.

 

Much lighter, smaller, no maintenance, can be discharged to 80% or more DOD over and over. Doesn't hurt it to sit at low SOC. Mount in any position. Lasts many times as long as lead acid and you don't need nearly as many AH to do the same job.

 

Did I miss anything?

 

They look cool.  

 

Oh, yeah. Voltage doesn't drop off as it discharges like lead acid, so the fridge won't shut off from low voltage.

Edited by CoreyTrevor, 29 June 2020 - 04:35 PM.

[craig333]

Nothing wrong with LiFePO4 but I'm not sure its cheaper in the long run. We haven't been using them long enough to really judge how long they'll last. 

 

This past week I ran my regular fridge and a second fridge set to freezing. With 250w of solar and 250ah of agm batteries over five days worst I got down was to 77% S.O.C. 

Of course if were running a little truck then the weight advantage of Lithium would be a big consideration.

[underpowered]

LiFePO4 battery. Expensive but cheaper in the long run.

 

Much lighter, smaller, no maintenance, can be discharged to 80% or more DOD over and over. Doesn't hurt it to sit at low SOC. Mount in any position. Lasts many times as long as lead acid and you don't need nearly as many AH to do the same job.

 

Did I miss anything?

 

They look cool.  

 

Oh, yeah. Voltage doesn't drop off as it discharges like lead acid, so the fridge won't shut off from low voltage.

This is an old thread, and there are many discussions of the pro's and cons of li batteries across the internet, but for me, one of the big advantages for me was thermal performance. lithium batteries do not loose anywhere near the capacity in low temperatures. This is further compounded by the fact that it is safe to put them inside the conditioned space as there is no chance of outgassing, where lead acid based batteries can produce hydrogen and therefore must be mounted outside the conditioned space.

[camelracer]

You must also remember you have to turn off all charging sources when the battery temperature falls below freezing unless you have a BMS that that does this.