How few Watts of solar to charge a 55 Amp/Hr AGM in 5 days?

[BigT]

Let me explain.  

I've got a 55 Amp Hour AGM battery that I'd like to charge with the smallest panel that will work.  
I'll have 5+ days (weekdays, plus possibly one weekend day) each week to recharge the battery between uses (weekends only, and only a couple of hours).  

Back when SternWake was posting on this site (is he back yet?), I remember him saying that 1 to 2 Watts of solar per Amp/hr. of battery capacity was best, but I understood that to mean if you needed it to recharge as quickly as possible.  
I'll have about 5 days to get it recharged, direct sunlight permitting.    

Can I use a 10 or 20 Watt panel, given I'll have several days to recharge the battery, or do I still need the aforementioned 1:1 - 2:1 Watt:Amp/hr. charge?

 

[RvNaut]

2 volt or 12 volt AGM? one or two?


Here is a link to Will Prowse's website with so much great solar and electric knowledge to share..
https://www.youtube.com/playlist?list=PL7bvIB2TPsCjTUIr9WISFg8JcAPrJfzxz

 

[abnorm]

Hey Big T..........you've been gone for a while...Welcome back

You can still find Sternwake at Gary's forum:

https://vandwellerforum.com/

 

[MrNoodly]

You might want to read this:

AGM Battery Charging: Rates, Currents and Times

https://workshoppist.com/agm-battery-charging-rates/

 

[jonyjoe303]

A 20 watt panel will only give you about 1 amp of charge current when the sun is overhead. Thats about 6 to 8 amps a day. Since a 55ah agm only has 27 ah usable power (50 percent) you might get it charged back up in 5 days if you only drained it 50 percent.
Just make sure there is no voltage drop from controller to battery, when your dealing with such low charge amps, you want max amps going to the battery all day long.
I wouldnt even consider a 10 watt panel, but a 40 watt might work even better especially in winter when the sun is lower in the horizon.

 

[wayne49]

Also, a couple of gloomy days could happen during those five days.

Smoky haze from wildfires can tint the sun to a shade of red well before sunset.

Stuff happens.

 

[Markw]

Here is a link to Will Prowse's website with so much great solar and electric knowledge to share..
https://www.youtube.com/playlist?list=PL7bvIB2TPsCjTUIr9WISFg8JcAPrJfzxz

  Love Will Prowse...VERY little fluff, just tons of good information.

 

[BigT]

2 volt or 12 volt AGM?  one or two?


Here is a link to Will Prowse's website with so much great solar and electric knowledge to share..
https://www.youtube.com/playlist?list=PL7bvIB2TPsCjTUIr9WISFg8JcAPrJfzxz

Oops, sorry, forgot that little detail.  

A single, 12V Power Sonic AGM.

Thanks all for the replies.  Good to know what I'll need to make it work.  

I also have the option of bringing the battery home and plugging it into a smart charger all week, but it's a pain, literally, to lug that thing around.  I just thought it might be easier to leave the battery in the boat and let the sun do the charging for me.

 

[AuricTech]

Just out of curiosity, why is having the smallest possible panel part of your mission statement?

 

[jimindenver]

2 to one is the bare minimum.

Some things to consider

Lead acid batteries self discharge. It takes a certain amount of current at a sufficient voltage to keep this from happening. It's called float and while it can be done with a trickle charger on a fully charged battery, most people buy to small of a trickle charger or think that they can actually charge with it.

Batteries resist being charged. So not only does your system have to replace overnight self discharge, it has to push past the resistance to get the charge in. You will know if you need more power if your battery gets to 13 volts and hangs there. It needs to get into the 14s to get a good charge.

So between countering self discharge and pushing past resistance taking part of your systems output, a small panel isn't going to have anything left to charge with. Even a 100 to 120 watt panel isn't going to realistically put a lot of power in every hour. that means it will take multiple days to charge the battery. That brings us to the next point.

Lead acid batteries hate being undercharged. The longer they are not at full charge the more damage is done to them. Slowly they succumb to what's called progressive capacity loss. One day you will hook up a fully charged battery and it's just not going to last very long.

In my line of work I can tell you that this is how most batteries die. Very few make it to a ripe old age and truly wear out.

Your best bet is always going to be getting the battery recharged as soon as possible and keeping it that way until you need it again.

 

[BigT]

Just out of curiosity, why is having the smallest possible panel part of your mission statement?

Paranoia, mostly.  

I have a little, 16' sailboat that I keep in a dry storage yard at the local marina, and while the yard is gated and has lights, security cameras and guards, I'm not super comfy leaving an expensive solar panel sitting out in the open all the time.  
It's not the sort of boat I can bolt the panel to.  It would just be sitting on top of the cover, in plain sight, easy to grab.  

To be honest, I've never heard of, or experienced, anything being stolen from the yard, and I've kept boats there for years.  

Besides the risk of being stolen, I didn't want to spend a ton of money on the system, given I have a decent little Noco "Genius-5" smart charger I can use, that seems to do a good job.

 

[AuricTech]

Well, considering that Amazon has this 50W panel kit with a 30A PWM charge controller for $89.99 (before applying the $5 coupon currently available), you wouldn't be out too much if it did wander off.

ETA: The brand name for the system linked above is SUNSUL.

ETA2: For comparison, the same brand's 20W system with 5A PWM charge controller is $40.99.

 

[BigT]

2 to one is the bare minimum.

Well, considering that Amazon has this 50W panel kit with a 30A PWM charge controller for $89.99 (before applying the $5 coupon currently available), you wouldn't be out too much if it did wander off.

Thanks for the link, but jimindenver says 2:1 is needed and I have a 55 Amp/Hr battery.   :huh:   :-/

I currently run a 200W panel, through a 30A MPPT CC, to the 75 Amp/Hr AGM I have in my van (on SternWake's advice), and it works great.  
I've been running it since 2015, and it's still going strong.

 

[sithigh]

Let me explain.

I've got a 55 Amp Hour AGM battery that I'd like to charge with the smallest panel that will work.
I'll have 5+ days (weekdays, plus possibly one weekend day) each week to recharge the battery between uses (weekends only, and only a couple of hours).

Back when SternWake was posting on this site (is he back yet?), I remember him saying that 1 to 2 Watts of solar per Amp/hr. of battery capacity was best, but I understood that to mean if you needed it to recharge as quickly as possible.
I'll have about 5 days to get it recharged, direct sunlight permitting.

Can I use a 10 or 20 Watt panel, given I'll have several days to recharge the battery, or do I still need the aforementioned 1:1 - 2:1 Watt:Amp/hr. charge?

No sense trying to skip your math. Need to calculate from what you want it to do backwards and consider the local weather and solar for your location. But it will die if you don't keep a charge on it. Get batteries for how you will use them. You will prefer hauling Lithium ion on Anderson plugs to heavier lead acid batteries with all bolted lead clamps. But my buddy only needed a little ~12V for his trolling motor and light. So he just borrowed his car battery. Every time.