Solarizing our sauna at the cottage

My family has a sauna up at our cottage. It's a very cute little place - I'd say it measures 10 feet by 10 feet square. It's a great place to get toasty, jump in the water, and then get toasty again.

Currently, it has two light bulbs inside. They are both 12V bulbs, and they run off a 12V boat battery.The sauna is completely unheated - at least when we're not using it  

The sauna is completely off-grid right now. When the battery dies, my dad needs to carry it up to the cottage, recharge it, and bring it back down. Sometimes when we have family or friends up, people forget to turn the light off when they're not in there. Then the battery dies, and my dad grumbles about charging it. I get it - it's quite a load to be carrying back and forth!

It was this summer that I thought - could we not put a solar panel on it and get rid of the whole carrying situation entirely? Something powerful enough to run those two bulbs. There are no receptacles in the sauna. When my dad uses electrical tools in it such as his electric filleting knife, he brings his power pack down and runs off that.

The bonus of no receptacles and only DC loads is that we won't need an inverter. But here is where I get a little confused...

1) Do I still need a charge controller to protect the battery?
2) Should we upgrade to a different type of battery? I don't know if marine batteries are rated for the slow charge and discharge cycles of a solar application.
3) If I do need a charge controller, are there more rugged models out there that can handle working in an unheated/cooled space? I know it's bound to be hard on any machine, but I'm curious if there are tougher ones. Obviously everything is sheltered from the elements, but the temperature still fluctuates wildly depending on the time of year. I guess the same could be said for the battery.

Thanks everyone!

For solar a deep discharge battery is better than a car battery.
Today there are a range of batteries, starting with Lithium Iron as probaly the best through to special lead acid style.
A charge controller will be needed.
Battery may need to be protected against freezing.

Hello Cam

Most likely, your battery is already physically damage from being totally drained, so it's continued usability is questionable.  Lead-acid batteries that get totally drained get sulfinated, which over time permanantly reduces battery capacity.

Does your battery have caps for adding water?  If yes, you can check the acid density with a battery hydrometer.  First, fully charge the battery.  If your charger also has a desulfination setting, run that to.  After finishing, pop off the caps and measure the specific gravity of the acid.  A new battery in good shape will have a density of ~1.260 to 1.275.  Density drops the further down the capacity scale you go.  Anything less than 1.100 after full charging is not likely worth saving.

Chronic undercharging is the typical way that most batteries die, so if you want to add solar, you need to add enough.  You determine how much current you need by battery size.  An off-grid battery designed for slow discharge will be rated for Amphours.  Ignore cranking amps or other automotive parameters.  They are not useful for off-grid.

A general rule of thumb is that lead-acid batteries like charging in the range of 1/10th of C to 1/8th of C, with 1/8th being the better choice.  C is the Amphour rating of the battery.  Most 12V batteries for off-grid use have an Amphour rating of the range of 100-120Ah.  Let's say the battery you have right now is 120Ah.  You divide 120Ah/8 to get a charging rate of 15A.  To get at least 15A of current while charging with at least 12.5V you need at least 15A X 12.5V = 187.5W of solar.

Couple of problems though with real-world applications.  First, is that panels are tested in an artificial test chamber with artificial light, at exactly 1000W/square meter, at exactly 77 degree F.  In the real-world, sunlight is more variable, and the panels are hotter than room temperature.  So the power goes down.  I like to de-rate panels to 85% for real-world production.  So, that 187.5W becomes 187.5W/85% = 220W.

Second variable is the electronics you employ to control the panels.  Yes, absolutely, you MUST have a charge controller when connecting solar to the battery, unless you want to stand there all day with a voltmeter, deciding yourself when the panels need to be disconnected.

Older, cheaper PWM controllers only operate by shutting current off.  They let the battery drag the charging voltage down to battery voltage.  So, the typical 100W solar panel that puts out 5A at 20V, will only produce 5A at 12.5V.  So, that 100W panel becomes a 62.5W panel.  That means to get an honest 15A into the battery, you need three of those 100W panels.  Does that make sense?

New MPPT controllers act like a transformer, taking raw high-voltage solar and transforming down to battery-charging voltage.  The extra amps gets transformed into extra charging amps.  MPPT controllers are more expensive, but so are extra panels.  The break-even point seems to be around 200-300W, depending on what kinds of deals you can get.  So, with greater than 300W, MPPT is the cheaper way to go, whereas an application that needs only 100W is better served with the lower-budget PWM controller.  You decide, but I'd go with MPPT.

Here is a controller that might work well for you, Epever's Tracer 2210AN.  Don't spend extra money on 12V automotive panels.  You pay a price premium for 12V products.  You get more bang for your buck with large, high-voltage residential panels.  The kind they use for grid-tie solar.  You first connect your MPPT controller to the battery, and after it boots up on 12V battery power, you connect the solar panel.   The controller knows already that the battery is 12V, so it takes the 30-37V of the residential panel and transforms it down to battery charging voltage.

Let's say you buy a 30V 240W grid-tie panel.  I see them on Craigslist for ~75-80$ right now.  The incoming 30V gets transformed down to 12.5V for charging the battery.  So, what you get is 240W/12.5Vcharging = 19.2A.  With 85% de-rating, you are actually likely to see 19.2A X 85% = 16.2A.  Your battery will be very happy getting charged that rate.  So, with the 2210 controller going for 77$, and a grid-tie panel for 75$, for a total cost of 152$ you get a fully functional charging system.

Cam Haslehurst wrote:My family has a sauna up at our cottage. It's a very cute little place - I'd say it measures 10 feet by 10 feet square. It's a great place to get toasty, jump in the water, and then get toasty again.

Currently, it has two light bulbs inside. They are both 12V bulbs, and they run off a 12V boat battery.The sauna is completely unheated - at least when we're not using it  

The sauna is completely off-grid right now. When the battery dies, my dad needs to carry it up to the cottage, recharge it, and bring it back down. Sometimes when we have family or friends up, people forget to turn the light off when they're not in there. Then the battery dies, and my dad grumbles about charging it. I get it - it's quite a load to be carrying back and forth!

It was this summer that I thought - could we not put a solar panel on it and get rid of the whole carrying situation entirely? Something powerful enough to run those two bulbs. There are no receptacles in the sauna. When my dad uses electrical tools in it such as his electric filleting knife, he brings his power pack down and runs off that.

The bonus of no receptacles and only DC loads is that we won't need an inverter. But here is where I get a little confused...

1) Do I still need a charge controller to protect the battery?
2) Should we upgrade to a different type of battery? I don't know if marine batteries are rated for the slow charge and discharge cycles of a solar application.
3) If I do need a charge controller, are there more rugged models out there that can handle working in an unheated/cooled space? I know it's bound to be hard on any machine, but I'm curious if there are tougher ones. Obviously everything is sheltered from the elements, but the temperature still fluctuates wildly depending on the time of year. I guess the same could be said for the battery.

Thanks everyone!

you will need a charge controller. Most of the entry level PWM charge controllers also have a load control feature so when hooked up to the lights would turn off power to them if the battery gets too low. I like better gear like this one here: https://www.morningstarcorp.com/products/sunsaver/
but there are 30-40 dollar ones online as well but they die. No use in going in for a larger MPPT charge controler if its just a single battery and panel.
Cheers,  David

Thanks everyone for your helpful and speedy responses. I'll get back to you one by one.

David Baillie wrote:you will need a charge controller. Most of the entry level PWM charge controllers also have a load control feature so when hooked up to the lights would turn off power to them if the battery gets too low. I like better gear like this one here: https://www.morningstarcorp.com/products/sunsaver/
but there are 30-40 dollar ones online as well but they die. No use in going in for a larger MPPT charge controler if its just a single battery and panel.
Cheers,  David

David, thank you for the recommendation. I watched their product video, and wow that thing really does look rugged. I sent them an email!

John C Daley wrote:For solar a deep discharge battery is better than a car battery.
Today there are a range of batteries, starting with Lithium Iron as probaly the best through to special lead acid style.
A charge controller will be needed.
Battery may need to be protected against freezing.

Maybe the sauna will be a summer only place, and we can disconnect the battery for the winter. I'll talk with my dad about this. We could bring the battery up each time we wanted to use it instead. However that would be a hassle, and it would subject the battery to big fluctuations in temperature (room temp to whatever the outside temperature is).

I can do some more research into battery types if we decide on a new one for the system.

Michael Qulek wrote:Hello Cam

Most likely, your battery is already physically damage from being totally drained, so it's continued usability is questionable.  Lead-acid batteries that get totally drained get sulfinated, which over time permanantly reduces battery capacity.

Does your battery have caps for adding water?  If yes, you can check the acid density with a battery hydrometer.  First, fully charge the battery.  If your charger also has a desulfination setting, run that to.  After finishing, pop off the caps and measure the specific gravity of the acid.  A new battery in good shape will have a density of ~1.260 to 1.275.  Density drops the further down the capacity scale you go.  Anything less than 1.100 after full charging is not likely worth saving.

Chronic undercharging is the typical way that most batteries die, so if you want to add solar, you need to add enough.  You determine how much current you need by battery size.  An off-grid battery designed for slow discharge will be rated for Amphours.  Ignore cranking amps or other automotive parameters.  They are not useful for off-grid.

A general rule of thumb is that lead-acid batteries like charging in the range of 1/10th of C to 1/8th of C, with 1/8th being the better choice.  C is the Amphour rating of the battery.  Most 12V batteries for off-grid use have an Amphour rating of the range of 100-120Ah.  Let's say the battery you have right now is 120Ah.  You divide 120Ah/8 to get a charging rate of 15A.  To get at least 15A of current while charging with at least 12.5V you need at least 15A X 12.5V = 187.5W of solar.

Couple of problems though with real-world applications.  First, is that panels are tested in an artificial test chamber with artificial light, at exactly 1000W/square meter, at exactly 77 degree F.  In the real-world, sunlight is more variable, and the panels are hotter than room temperature.  So the power goes down.  I like to de-rate panels to 85% for real-world production.  So, that 187.5W becomes 187.5W/85% = 220W.

Second variable is the electronics you employ to control the panels.  Yes, absolutely, you MUST have a charge controller when connecting solar to the battery, unless you want to stand there all day with a voltmeter, deciding yourself when the panels need to be disconnected.

Older, cheaper PWM controllers only operate by shutting current off.  They let the battery drag the charging voltage down to battery voltage.  So, the typical 100W solar panel that puts out 5A at 20V, will only produce 5A at 12.5V.  So, that 100W panel becomes a 62.5W panel.  That means to get an honest 15A into the battery, you need three of those 100W panels.  Does that make sense?

New MPPT controllers act like a transformer, taking raw high-voltage solar and transforming down to battery-charging voltage.  The extra amps gets transformed into extra charging amps.  MPPT controllers are more expensive, but so are extra panels.  The break-even point seems to be around 200-300W, depending on what kinds of deals you can get.  So, with greater than 300W, MPPT is the cheaper way to go, whereas an application that needs only 100W is better served with the lower-budget PWM controller.  You decide, but I'd go with MPPT.

Here is a controller that might work well for you, Epever's Tracer 2210AN.  Don't spend extra money on 12V automotive panels.  You pay a price premium for 12V products.  You get more bang for your buck with large, high-voltage residential panels.  The kind they use for grid-tie solar.  You first connect your MPPT controller to the battery, and after it boots up on 12V battery power, you connect the solar panel.   The controller knows already that the battery is 12V, so it takes the 30-37V of the residential panel and transforms it down to battery charging voltage.

Let's say you buy a 30V 240W grid-tie panel.  I see them on Craigslist for ~75-80$ right now.  The incoming 30V gets transformed down to 12.5V for charging the battery.  So, what you get is 240W/12.5Vcharging = 19.2A.  With 85% de-rating, you are actually likely to see 19.2A X 85% = 16.2A.  Your battery will be very happy getting charged that rate.  So, with the 2210 controller going for 77$, and a grid-tie panel for 75$, for a total cost of 152$ you get a fully functional charging system.

Talk about a detailed response. Thanks Michael!

I'll have to check if the battery has caps for adding water or not. When I was peeking around the sauna last, I didn't check to see. If it does, I can look into a hydrometer.

Thank you for the tip about residential panels. I don't see any on Kijiji (Canada's craigslist) near me, so I may be purchasing new instead. I see what you mean with the 12V panels - wow!

I'm thinking two small residential panels might work - but the answers to my questions only confirmed that I need to do more research before taking any action.

Thanks everyone for your help!