Lights turning on and off

[powerdaddy]

I am a foreman with plenty of experience is the commercial line volt area and next to none in solar. I thought I'd give it a shot for some pathway lights at home. 100w panel (just a cheap kit with a controller) with a 8ah lead acid battery (the type of battery was dictated by the controller). 5 3w 12v pathway lights automatically switched by the controller. All was dandy for the first few months but towards the end of summer the lights started coming on and off in 4 or 5 second intervals. I thought with the sun getting lower maybe the battery wasn't charging well so I adjusted the panel with no change. Could the battery have frozen and been damaged in the sub freezing temperatures? 8ah should be more than enough to run the lights for 4 hours every evening. Is it as simple as a short? Or did I miss something in my ignorance?

 

[42OhmsPA]

5 x 3w x 4h =60wh
12v x 8ah = 96wh x 50% = 48wh
(I realize it's more than 12v but I'm keeping the math easy)
I'd say your battery is undersized and many over discharge cycles killed it.
Am I missing something?

 

[powerdaddy]

That makes sense. Does the 50% rule apply to Li as well? I have a 10ah Li I could use if I upgraded the controller. Or am I just chasing geese there?

 

[42OhmsPA]

That makes sense. Does the 50% rule apply to Li as well? I have a 10ah Li I could use if I upgraded the controller. Or am I just chasing geese there?

80% for lithium from what I have read but the cold (below freezing) is no good.
Why do you need to upgrade the controller?

 

[powerdaddy]

No need, really. It was my understanding that Li did better in cold than SLA. I have seen temperatures as low as -46f here, but it typically gets as cold as -30f in the dead of winter. On the flip side it approaches 100f in the full heat of summer. A temperature resistant battery will be necessary. From what I read, Li was more hardy in extreme temperatures than SLA. I personally have no real knowledge on the subject though.

 

[HRTKD]

LiFePO4 cells do not like being charged below 32°F. You can discharge them below 32°F, but life is better if you keep the battery above that for both charge and discharge. There are many threads on the forum about battery warmers for LiFePO4.

 

[MarkSolar]

I have automatic gate openers that run off a solar charged 12v flooded battery. We don't get as cold as you do, but I've come up with a strategy to keep them warm enough in the winter here at the same latitude as Chicago. I upsized the battery and use a 12v battery heater mat under the battery. They sit outside on the ground, I put down a 2" piece of pink foam with piece of plywood on top. I sandwich the heater mat between two pieces of ceramic tile to avoid direct battery contact and to provide some thermal mass. Battery sits on top of the ceramic tile. I have an old insulated cooler I turn upside down over the battery that sits on top of the plywood. I experimented with the temp inside the box using a infra red temp sensor and a hole in the cooler that I plug with a piece of rag. I tried mats with a thermostat and they either failed or didn't seem safe to me. I installed a timer that cycles the mat on/off so the battery has enough power to keep it working. I ended up sizing the mat at 60W and using an on/off switch that I mounted externally so I know when it gets below 20F I have to turn it on and it won't get too hot. So I generally turn it on most nights and off most days, especially when the sun shines on the cooler.

The freezing point of flooded batteries goes up really quick with the state of discharge, I would guess in your climate there's no way a battery will survive without a heater because you may get a combination of cold weather with overcast/snowy skies. So your battery will be sufficiently discharged that it will freeze, then it's generally ruined because there's physical damage internally. You could try just putting it in a really well insulated box that sits in the sun, that might be enough to help it survive.

 

[powerdaddy]

Battery heater on a 12v tstat. Easy enough. Sounds like I need to put some more thought into a better design. Right now I have battery and charger mounted in a 12x12 nema 3 metal j-box in the most shaded area I could find to help prevent overheating in the summer. Bigger battery, heater, tstat, possibly an inverter or xfmer to feed the equipment. I knew this all seemed too easy to be true. I appreciate the insight.

 

[MarkSolar]

You're right to worry about the heat. In the summer I use a little plywood box that has a sloped roof to keep the sun/rain off but both sides are open so the air can flow through and it's up off the ground about 6" so I don't hit it with the mower or string trimmer. The other thing you can do is monitor the charge state of your battery. If it's charged 95% or more I think the freeze point is something like -70F. If it's not staying charged, either turn off the lights for awhile or get a bigger panel. I also have my charge controller mounted in the shade of the panel so it doesn't get direct sun in the summer. It's in a black plastic sealed box that must be 200 degrees inside if it's in the sun. I don't see how consumer electronics could survive that for long.