Outdoor battery box: Insulation? Airspace/holes? Inverter placement below batteries?

[dontreignoorme]

Hi all, and thanks in advance for any suggestions,

I'm building an outdoor solar/battery box to power a Christmas tree that floats on a raft on my town's pond. It'll be 3 @ 100 watt panels charging 2 deep cycle batteries.

It will be cold (in open, in Mass), and I gather cold is the enemy of battery efficiency. It will only be used for two months in the winter, from Thanksgiving to the feast of the Epiphany.

So, the questions are four:

1 - How should I insulate the box? I've seen bubble wrap mentioned, and regular household insulation. I also wondered about using sand. Thoughts? Suggestions?

2 - How much air space I need to leave above the batteries - 2 inches do it?

3 - Would be useful/OK to placed the inverter _below_ the batteries? It seems they give off heat which is an issue in the summer but I'm wondering if I can harness that for my benefit in the winter.

4 - I've seen mention of needing to vent the battery box, but I can't find guidance on how much venting is needed and I'm kinda wondering if I really need to drill holes in the box which will let heat out/cold in. It won't be absolutely airtight, so I'm kinda wondering if it's really needed since most of the guidance I've read is mainly talking about safety for inside-the-home setups.

Thanks for any thoughts,
b

[Sunking]

OK before you jump in, from the sounds of things you have no idea f it will even work or not. What you have not done is determine how many watt hours you will need in a day. My hunch is you have grossly undersized the systems, and if I am correct you will destroy the batteries and go dark.

No need to worry about ventilation in this application. My advice is STOP right now. In your location at that time of year at best all you can generate with 300 watts of panels is roughly 300 to 500 watt hours. That will not be enough to support two batteries, just one very small 12 volt 125 AH battery. I suspect you will need a lot more power than your plan can supply.

[dontreignoorme]

Thanks for the thoughts, Sunking.

RE Watt hours, the aim is to power

• 2 @ RPGT 1000 LEDs 336ft Fairy String Tree Twinkle Lights. The specs say they use 6 watts/30 volts. They'll be on 10 hours per day.
• The inverter is a Xantrex 1000w, and it's spec sheet says it draws .3 amp. Not quite sure how to incorporate that in calculating total draw.
• The timer will have some draw as well; no idea what, but hoping and presuming it'll be slight.

I'm total newb to electricity this so please do correct me if I'm off, but from what I could find online I think that means the calculations are:

6w x 30v = 180w,
180 x 10hr = 1800 total watt hours per day
4 @ 100w panels would need 4.5 hours optimal sunlight (typo)

Daylight at Lat 42ish is 9:04 hours per day on the shortest day during the usage period of 11/26 - 1/06, 12/21.

So, I was hoping that should suffice to power the daily draw, with the additional hours of light going towards making up for cold battery and sub-optimal sun. I was thinking that the second battery would would catch additional charge beyond what's needed daily for days when conditions aren't good.

Thanks again to anyone who may chime in with thoughts!,

b

PS, also still interested in advice on the original questions about insluation/spacing/venting/inverter placement!

OK before you jump in, from the sounds of things you have no idea f it will even work or not. What you have not done is determine how many watt hours you will need in a day. My hunch is you have grossly undersized the systems, and if I am correct you will destroy the batteries and go dark.

No need to worry about ventilation in this application. My advice is STOP right now. In your location at that time of year at best all you can generate with 300 watts of panels is roughly 300 to 500 watt hours. That will not be enough to support two batteries, just one very small 12 volt 125 AH battery. I suspect you will need a lot more power than your plan can supply.

[Sunking]

Daylight at Lat 42ish is 9:04 hours per day on the shortest day during the usage period of 11/26 - 1/06, 12/21.

No Sir that is not Sun Hours, that is daylight hours. Fatal mistake. Example in Boston in the month of December with the panels at Latitude + 15 degrees oriented solar south you only receive 3 Sun Hours aka 3 Kwh/meter^2

However your math is all wrong, and you will be relieved.

I'm total newb to electricity this so please do correct me if I'm off, but from what I could find online I think that means the calculations are:
6w x 30v = 180w,
180 x 10hr = 1800 total watt hours per day
4 @ 100w panels would need 4.5 hours optimal sunlight (typo)

Is complete nonsense.

Watt Hours = Watts x Hours.

6 watts x 30 volts = BS
6 watts / 30 volts = 0.2 amps.

You have 6 watts x 10 hours = 60 watt hours.

This part you will like me for beating you up. Assuming you will use a inexpensive PWM Controller you need to generate 120 watt hours per day because the system is only 50% efficient.

Panel Wattage Required = Daily wh x 2 / Sun Hours = 60 wh x 2 / 3 h = 40 watts. You came up with 400 watts. You only missed it by a factor of 10.


12 Volt Battery Capacity = Daily wh x 5 / 12 volts = 60 wh x 5 / 12 volts = 25 AH a very small battery.

So if you were to hire me here is what I would use and charge you.

50 Watt Panel
5 to 10 amp PWM controller which is overkill, but the smallest you can buy.
12-Volt 30 to 40 AH battery.
50 to 100 Watt Inverter is way overkill, but you DO NOT want anything higher because it would drain your battery to fast. A larger Inverter would use more power then the load is using

Materials = $250 which is roughly my cost plus 20%
Labor and consulting fee, $400 for 1/2 day of work.

If you do this should only cost you $175 to $200.

I just saved you at least $1000 and you can thank me later for beating you up.

[Mike90250]

so, here's my opinionated answer

1) 12V 80ah deep cycle marine battery [ about $90

2) your solar panels, wired parallel to a :

3) Morningstar Sunsaver 20a 12v charge controller https://www.morningstarcorp.com/prod...unsaver-gen-3/

4) Morningstar sure sine inverter (very low power) https://www.morningstarcorp.com/products/suresine/ [ about $ 300 but solid and money well spent

5) whatever timer you use for your light time (most consumer timers will fail in extreme cold) Maybe better to plan on lighted 24/7 ?

You can get a cheaper charge controller, but I have confidence in the MS gear, because it''s very ruggedly built, and in a decent protective box, I'd expect no trouble from the gear.


Problems:
Maintaining good sun angle on the panels. Will the raft twist or blow off axis and point the panels north ? Will snow cover them ? Who will row out to clear them ?

the battery will be cold, so you oversize and insure it never gets to ice up.

It's hard to build a box to contain hydrogen gas, any gaps or cracks will allow it to safely vent

Skip the inverter and locate a 12VDC LED light string, that could be easily controlled by the charge controller for dusk/dawn operation. Skip the inverter expense and 120VAC on the water.

[thastinger]

Not sure if this is built yet but I agree that 12V lighting would be better for the reasons that Mike pointed out.