Charging 24v battery bank with 12vg

[Chris1]

This may be a stupid question but here goes - Can a 24v battery bank (two 12v, 100Ah AGM batteries) be charged with a 12v solar controller?

Here are the details: This is for a small sailboat that will have a 24v electric outboard installed. There is room on the boat to install two 50 watt, 12v panels - I'm thinking these would be Monocrystalline panels with bypass diodes, VOC is 22.7.

Because there will be some shading from the sailboat's rigging I'd like to wire the panel's in parallel to minimize shading loss.

I'd like to use the Renogy PWM 20a, 12v controller as it is waterproof and fits the available space well.

If this won't work I guess it would be no big deal to get another 12v controller and set up two charging systems. Is that right?

In addition my plan is to set up two 100w panels on the boat's dock. These will plug in to the boat's system when it is tied up there (most days, all summer).

Is there a better way?

Thanks in advance for any thoughts. You guys are a wealth of information.

[Sunking]

This may be a stupid question but here goes - Can a 24v battery bank (two 12v, 100Ah AGM batteries) be charged with a 12v solar controller?

Absolutely it can be done, but why would you? It can be done if you charge each 12-volt separately. Good luck with that.

But why if you do not have to. Wire you panels in series and set charger up for 24 volts. Personally I would not bother with putting up 2 small 50 watt panels on a boat. Shade will always be an issue, and with no fixed tilt or orientation to optimize solar harvest, you can fart more usable energy then the panels will produce in poor conditions. Wind Turds work better for boats.

If it were me I would buy a good High Voltage Grid Tied of 250 to 300 watts which cost less then your 2 x 12 volt battery panels, use the savings to buy a good MPPT charge controller configured for 24 volts, and forget about solar panel dead weight in the boat to slow you down with no meaningful power generated. At 250 to 300 watts into a 24 volt battery will require a small 15 amp controller. Morning Star makes a SunSaver MPPT 15 amp charge controller which should be perfect.

Here is some math to crunch. If you used PWM controller with your 2 x 50 wat panels on its best day with the panels facing directly into the sun at high noon on June 21 might briefly output 60 watts or roughly 2.4 amps of charge current or 60 watts. The farts from a can of beer will generate more energy and not near as big and heavy. Want to use those 2 x 100 watt panels for shore power. Well they will give you up to 4.8 amps of charge current or 12 watts.

Use a single GT panel of say 250 watts, a MPPT charge controller and you get 10.4 charge amps or roughly the full 250 watts. Who on earth would have thought you could get 250 watts from a 250 watt panel? You can if you toss those antiquated PWM Controllers over board and send them to Davy Jones Locker where they belong. Anyway you would have to be a serious alcoholic to generate more beer fart power than aa 250 watt panel with an MPPT controller. Not to mention less expensive than PWM. To get 250 watts from a PWM controller will require 375 watts. With your idea of using 100 watts from the boat, plus another 200 watts from shore only gives 8.3 amps of charge current or roughly 200 watts from 300 watts of panels.

[littleharbor]

Absolutely it can be done, but why would you? It can be done if you charge each 12-volt separately. Good luck with that.

But why if you do not have to. Wire you panels in series and set charger up for 24 volts. Personally I would not bother with putting up 2 small 50 watt panels on a boat. Shade will always be an issue, and with no fixed tilt or orientation to optimize solar harvest, you can fart more usable energy then the panels will produce in poor conditions. Wind Turds work better for boats.

If it were me I would buy a good High Voltage Grid Tied of 250 to 300 watts which cost less then your 2 x 12 volt battery panels, use the savings to buy a good MPPT charge controller configured for 24 volts, and forget about solar panel dead weight in the boat to slow you down with no meaningful power generated. At 250 to 300 watts into a 24 volt battery will require a small 15 amp controller. Morning Star makes a SunSaver MPPT 15 amp charge controller which should be perfect.

Here is some math to crunch. If you used PWM controller with your 2 x 50 wat panels on its best day with the panels facing directly into the sun at high noon on June 21 might briefly output 60 watts or roughly 2.4 amps of charge current or 60 watts. The farts from a can of beer will generate more energy and not near as big and heavy. Want to use those 2 x 100 watt panels for shore power. Well they will give you up to 4.8 amps of charge current or 12 watts.

Use a single GT panel of say 250 watts, a MPPT charge controller and you get 10.4 charge amps or roughly the full 250 watts. Who on earth would have thought you could get 250 watts from a 250 watt panel? You can if you toss those antiquated PWM Controllers over board and send them to Davy Jones Locker where they belong. Anyway you would have to be a serious alcoholic to generate more beer fart power than aa 250 watt panel with an MPPT controller. Not to mention less expensive than PWM. To get 250 watts from a PWM controller will require 375 watts. With your idea of using 100 watts from the boat, plus another 200 watts from shore only gives 8.3 amps of charge current or roughly 200 watts from 300 watts of panels.

How many watts does a typical beer fart contain? More than a popcorn fart, but less than a pickled egg fart is my guess.

[Sunking]

How many watts does a typical beer fart contain? More than a popcorn fart, but less than a pickled egg fart is my guess.

That is how I smell it.

[Chris1]

Absolutely it can be done, but why would you? It can be done if you charge each 12-volt separately. Good luck with that.

But why if you do not have to. Wire you panels in series and set charger up for 24 volts. Personally I would not bother with putting up 2 small 50 watt panels on a boat. Shade will always be an issue, and with no fixed tilt or orientation to optimize solar harvest, you can fart more usable energy then the panels will produce in poor conditions. Wind Turds work better for boats.

If it were me I would buy a good High Voltage Grid Tied of 250 to 300 watts which cost less then your 2 x 12 volt battery panels, use the savings to buy a good MPPT charge controller configured for 24 volts, and forget about solar panel dead weight in the boat to slow you down with no meaningful power generated. At 250 to 300 watts into a 24 volt battery will require a small 15 amp controller. Morning Star makes a SunSaver MPPT 15 amp charge controller which should be perfect.

Here is some math to crunch. If you used PWM controller with your 2 x 50 wat panels on its best day with the panels facing directly into the sun at high noon on June 21 might briefly output 60 watts or roughly 2.4 amps of charge current or 60 watts. The farts from a can of beer will generate more energy and not near as big and heavy. Want to use those 2 x 100 watt panels for shore power. Well they will give you up to 4.8 amps of charge current or 12 watts.

Use a single GT panel of say 250 watts, a MPPT charge controller and you get 10.4 charge amps or roughly the full 250 watts. Who on earth would have thought you could get 250 watts from a 250 watt panel? You can if you toss those antiquated PWM Controllers over board and send them to Davy Jones Locker where they belong. Anyway you would have to be a serious alcoholic to generate more beer fart power than aa 250 watt panel with an MPPT controller. Not to mention less expensive than PWM. To get 250 watts from a PWM controller will require 375 watts. With your idea of using 100 watts from the boat, plus another 200 watts from shore only gives 8.3 amps of charge current or roughly 200 watts from 300 watts of panels.

Ha. Okay thanks Sunking. It's going to take me a bit to decipher what you've written. Two quick thoughts: I don't have shore power available at the dock, and wind power is as good as non existent in the cove where the dock is. Good thoughts though for sure. Now that I think about it though the wind generator would be doing some work when the boat's out sailing, hmmm... got to mull the trade-offs over. The MPPT controllers look like lovely things alright. A little pricey but okay. I don't have a good place to put it where I can guarantee no occasional salt water splash. Remember this is a SMALL sailboat. That's why I was drawn to that waterproof PWM controller. But I'm a boat builder, I can make places for things that need to stay dry if I have to. From the cooling fins I see on those MPPT controllers I'm guessing they need to breath a bit.

That SunSaver unit looks good. I guess I shouldn't sweat the price but jeez, it starts to add up. Anyway you've given me a lot to think about. I really appreciate the help.

If you are still willing, can you tell me why some battery mfgs. list a minimum solar panel size with which to charge their AGM batteries? I'm thinking of Vmax in particular. While Renogy wrote to tell me just today that it doesn't matter, it just takes longer with a little solar panel. What to believe?

[Chris1]

Absolutely it can be done, but why would you? It can be done if you charge each 12-volt separately. Good luck with that.

But why if you do not have to. Wire you panels in series and set charger up for 24 volts. Personally I would not bother with putting up 2 small 50 watt panels on a boat. Shade will always be an issue, and with no fixed tilt or orientation to optimize solar harvest, you can fart more usable energy then the panels will produce in poor conditions. Wind Turds work better for boats.

If it were me I would buy a good High Voltage Grid Tied of 250 to 300 watts which cost less then your 2 x 12 volt battery panels, use the savings to buy a good MPPT charge controller configured for 24 volts, and forget about solar panel dead weight in the boat to slow you down with no meaningful power generated. At 250 to 300 watts into a 24 volt battery will require a small 15 amp controller. Morning Star makes a SunSaver MPPT 15 amp charge controller which should be perfect.

Here is some math to crunch. If you used PWM controller with your 2 x 50 wat panels on its best day with the panels facing directly into the sun at high noon on June 21 might briefly output 60 watts or roughly 2.4 amps of charge current or 60 watts. The farts from a can of beer will generate more energy and not near as big and heavy. Want to use those 2 x 100 watt panels for shore power. Well they will give you up to 4.8 amps of charge current or 12 watts.

Use a single GT panel of say 250 watts, a MPPT charge controller and you get 10.4 charge amps or roughly the full 250 watts. Who on earth would have thought you could get 250 watts from a 250 watt panel? You can if you toss those antiquated PWM Controllers over board and send them to Davy Jones Locker where they belong. Anyway you would have to be a serious alcoholic to generate more beer fart power than aa 250 watt panel with an MPPT controller. Not to mention less expensive than PWM. To get 250 watts from a PWM controller will require 375 watts. With your idea of using 100 watts from the boat, plus another 200 watts from shore only gives 8.3 amps of charge current or roughly 200 watts from 300 watts of panels.

Now it's coming clearer to me. I could put all my solar resources into a shore based power station at the dock. This boat will almost never be out for more than a daysail so it wold rarely need to produce it's own power. And if the batteries where dead and there was no wind there are still oars. I like that. And like you say, leave the dead weight of the solar panels off the boat. Genius!

[J.P.M.]

How many watts does a typical beer fart contain? More than a popcorn fart, but less than a pickled egg fart is my guess.

One of my mentors who happened to be the dean of engineering at one of my Alma Maters once went through some quasi tongue in cheek calcs and came to the conclusion that a flea fart was ~ (within about an order of magnitude +/- a bit) equal to about 1 electron-volt or ~ 1.6 EE-19 Joules. So, maybe to a first approx., a beer fart ~= (1ev*(1 human mass/1 flea mass)).

Now, by googling "how much does a flea weigh", a decent approx. of a male flea's weight ( because, c'mon guys - who ever heard a female bust a fudgie - at least an unmarried female ?) after biting an animal is ~ 36 mg. Now, assuming 90 kg. human male.

--->>> (9 EE7 mg/36 mg)*1.6 EE-19 J = ~ 4*EE-13 J/beer fart.

To be clear, that's probably only lost Pv work associated with unrestrained expansion into a vacuum. Available PV work at sea level will be less because of the lost availability due to atmospheric back pressure.

Determination of any of that PV work as may be expressed as velocity head is left as an exercise in orifice flow characteristics for the student. That seems the type of usefulness most graduate work amounts to these days anyway.

Also, none of the above is to be confused with any energy resulting from thermochemical conversion processes should any of the beer fart be flared off. See the Koch brothers for further information on such matters.

As usual, take what you want of the above. Scrap the rest.

[littleharbor]

One of my mentors who happened to be the dean of engineering at one of my Alma Maters once went through some quasi tongue in cheek calcs and came to the conclusion that a flea fart was ~ (within about an order of magnitude +/- a bit) equal to about 1 electron-volt or ~ 1.6 EE-19 Joules. So, maybe to a first approx., a beer fart ~= (1ev*(1 human mass/1 flea mass)).

Now, by googling "how much does a flea weigh", a decent approx. of a male flea's weight ( because, c'mon guys - who ever heard a female bust a fudgie - at least an unmarried female ?) after biting an animal is ~ 36 mg. Now, assuming 90 kg. human male.

--->>> (9 EE7 mg/36 mg)*1.6 EE-19 J = ~ 4*EE-13 J/beer fart.

To be clear, that's probably only lost Pv work associated with unrestrained expansion into a vacuum. Available PV work at sea level will be less because of the lost availability due to atmospheric back pressure.

Determination of any of that PV work as may be expressed as velocity head is left as an exercise in orifice flow characteristics for the student. That seems the type of usefulness most graduate work amounts to these days anyway.

Also, none of the above is to be confused with any energy resulting from thermochemical conversion processes should any of the beer fart be flared off. See the Koch brothers for further information on such matters.

As usual, take what you want of the above. Scrap the rest.

Well, now we're getting somewhere!

[J.P.M.]

Well, now we're getting somewhere!

I'd add that the popcorn farts probably have a lower dew point so I'd use the higher heating value for the flaring process calcs.

SK: Since you're the one who broached the flatulence barrier in this thread, and as a retired Dolphin, maybe you can enlighten me. What happens or how long does a fart stay in a submarine ? Length of the cruise ? Just wonderin'.

[Sunking]

If you are still willing, can you tell me why some battery mfgs. list a minimum solar panel size with which to charge their AGM batteries?

Certainly. Batteries have minimum charge rate requirements which is directly related to panel wattage.

[Sunking]

Now it's coming clearer to me. I could put all my solar resources into a shore based power station at the dock

Well have another cup of coffee and the rest should make sense. You are absolutely correct, if the boat is only out for a day, you have no need for solar on the boat. Solar Systems are designed to have 3 to 5 day battery capacity in the event you run into cloudy spells. Size the battery correctly, and you are good for 2 days with no charge. I am an honest injun, me no lie.

[sdold]

Certainly. Batteries have minimum charge rate requirements which is directly related to panel wattage.

Even AGM? I thought that only applied to FLA.

[Sunking]

Even AGM? I thought that only applied to FLA.

Doubled down on AGM. Typically AGM batteries require much higher charge rates which is still directly related to panel wattage. Use a good oure lead AGM like Concord or Odyssey require C/4 to C/2 charge rates You need that high rate to make them recombine O2 and H back to water.

[sdold]

Was it that AGM could be charged more slowly without risk of sulfation?

[Sunking]

Steve AGM does not sulfate like FLA. That is not to say they do no sulfate because they do. The main difference is FLA sulfate accelerates at 50% and lower DOD, AGM can go down to 80% DOD before sulfate accelerates.

Please no one take that as meaning you should or should not use AGM because it does have its application. For Solar the main advantage is you can charge and discharge AGM 2 to 5 times faster. So Steve for example your neighbors to the north in gloomy doomy Seattle and Portland receive less than 2 Sun Hours for several months out of the year. That means you must use much higher panel wattage and run Charge Rates up to 1C. Example you want 1 Kwh of of usable power in a day. You have 1 site in Seattle and 1 in Tuscon, Seattle needs a 1000 watt panel and into a 12 volt battery is 80 Amps. Tuscon only needs 400 watts of panels and 35 amps of current. Both need the exact same 12 volt 400 AH battery but not the same type. In Tuscon you can use a FLA battery that cost half as much as AGM and last twice as long. You cannot do that in Seattle with a C/5 charge current.

Most deep cycle AGM's need to be charged fast, that is what they are designed to do and are cousins of SLI batteries. They need to be pressurized to make them recombine O2 and H with the catalyst used. Bottom line is check what the battery requirements are before you make a selection. It all has to fit and be designed to work with everything.

FWIW Steve AGM batteries were developed specifically for military aircraft where inversion and G forces are encountered and no spills and excess weight can be tolerated. Concorde to this day made the first AGM's and today still make the best AGM's. They are one of the very few that use 100% virgin lead with no alloys like antimony, calcium, tin, or silver. The lead used is not from recycled material contaminated with impurities and alloys. I can only think of two other manufactures that do that, and they are not tailored to RE, rather motorsports made by Odyssey and Optima. You can tell a lot about a battery by looking at the Term Post. When you see Auto Term Post, you know it is made for motorsports. Cycle application batteries have Flags you can use 1 or more bolt through hardware. Now these are AGM batteries with dual Term Post.