Help Please: 12V battery and 24V panel

System · May 2009

Greetings folks, I'm a new member and this is my first post. I'm not going to pretend I know a lot about 12V power or solar battery recharging

so I'm just gona ask my questions and suffer the ridicule that comes with being a green horn,

but I am a quick learner so hopefully won't ask the same question too frequently. Here goes:

1) I have a 12 volt system (1 battery/Group 27) set up in my sailboat and want to use solar to resupply the battery bank. I have an opportunity to get an 18V or 24V 120Watt solar panel at a better than market price. Would an 18V or 24V solar panel be compatible with my 12V battery or is there a device that can be placed between the panel and battery to make them compatible?

2) If I were to put a second 12v battery in my boat and connect them appropriately to achive a 24V battery bank, will I still be able to run my existing 12V lighting, radios, vhf, and gps, or would I risk overloading those components?

and lastly,
3) I may have the opportunity to buy a couple of these 120W (5) panels and may want to apply them to a home use setup. Is there common sense, easy to read manual/book that someone could reccomend that would help me to design and setup a home based solar power battery bank and then use the stored power appropriately ?

Thanks, Larry

Kamala · May 2009

Re: Help Please: 12V battery and 24V panel

Larry,

First, don't worry about being ridiculed. I am new here too and have received valuable help. The people here are great and very experienced and knowledgeable. Heavier on the experienced.

You will get the help you need.

Second, what I think you need is a charge controller. It is a device that, as you guessed, goes between the panel(s) and the battery (bank). It ensures that the proper volts and current are delivered to provide a proper charge profile.

All others that respond to your post will, rightly, ask you for an understanding of your needs. That is, what are your loads and how much average usable sunlight do you expect to have available on a daily, or discharge/recharge cycle, basis.

Craig

mikeo · May 2009

Re: Help Please: 12V battery and 24V panel

You don't need to justify that you are a beginner, just ask away and read the archives using the search box.

1) I have a 12 volt system (1 battery/Group 27) set up in my sailboat and want to use solar to resupply the battery bank. I have an opportunity to get an 18V or 24V 120Watt solar panel at a better than market price. Would an 18V or 24V solar panel be compatible with my 12V battery or is there a device that can be placed between the panel and battery to make them compatible?

Go for the 18 volt panels and you can find several economical charge controllers like the Morningstar or others rated at 15 amps or less to handle charging your batteries.

2) If I were to put a second 12v battery in my boat and connect them appropriately to achive a 24V battery bank, will I still be able to run my existing 12V lighting, radios, vhf, and gps, or would I risk overloading those components?

Not a good Idea due to load balancing problems. Can be done but can get costly!

and lastly,
3) I may have the opportunity to buy a couple of these 120W (5) panels and may want to apply them to a home use setup. Is there common sense, easy to read manual/book that someone could reccomend that would help me to design and setup a home based solar power battery bank and then use the stored power appropriately ?

Search the archives and read about 12 volt PV systems on the web and you will garner a lot of information very quickly. There are a few books out there but I will let others comment on their value.

icarus · May 2009

Re: Help Please: 12V battery and 24V panel

There are a number of voltages that are relevant.

A "12 volt panel" might have a maximum voltage of ~25 volts (Voc) (open circuit voltage) and a voltage around 17-18 (vmp) You need a voltage significantly above battery voltage to be able to charge a battery,, hence the 17-18 Vmp (Voltage @ maximum power)

So in fact I have never heard of a 18 volt panel (though they may exist), are probably looking at the 2 voltage numbers of the same panel.

A 24 volt panel will have numbers ~ double those above.

Tony

PS Welcome to the site,,, there are some very smart folks here. The real common sense is here,,, and perhaps home power magazine. Folks here have reinvented the wheel here so that others don't have to over and over again.

niel · May 2009

Re: Help Please: 12V battery and 24V panel

i would like to know how you know it is an 18v pv? are you measuring the voltage when it's aimed at the sun without any batteries or other loads connected? what is the make and model # of the pvs and your batteries too?
as was mentioned a 12v pv can have around 17-18v nominal as the excess is needed to impress a charge to the battery and over varying conditions. this is compatible with your battery voltage, but the charge controller (regulator) is needed and preferably with a battery temperature sensor.
in addition, you should be certain you will have enough pv amps for the charge percentage to roughly fall within 5-13% of the battery amphours (ah) based on a 20hr rate. this is not cold cranking amps as that is a rating for starting batteries and starting batteries should not be used in solar applications. you also need to know that you will be replacing all of the power you are expecting to use from the battery with pvs or you will have a constant undercharge condition and destroy the battery through sulfation. one last note on a battery is that it should not be drained beyond 50% as that will hasten the death of the battery and many opt to not exceed 20 or 25% dod (amount discharged) to improve the battery lifespan.
in going with 2 batteries in series (batteries should be identical and about the same age) you will need more than 1 12v pv as 2 would then minimally be needed with a controller that is operating at 24v. this will not run 12v items and a converter will be needed to downconvert from 24v to 12v and should be able to supply enough current as your 12v loads would maximally require.
you can parallel identical batteries as well and eliminate the converter's need and this would double your battery capacity if going with 2 of them. this would also double your charge requirements so this means doubling your pvs.
fusing should also be used to and from the batteries and proper wire size to carry your power with little loss. switches are an option, but must be rated for the dc current you will pass through it.
i generalized this somewhat, but even at that it's allot. hope i didn't leave anything out.

peterako · May 2009

Re: Help Please: 12V battery and 24V panel

Hi there,
If the sailboat is sailing at sea ( salt water ) then be aware to use only marine pv panels. the price is higher but the standard equipment is not salt water proof.

Greetings from Greece8)

mike95490 · May 2009

Re: Help Please: 12V battery and 24V panel

2) If I were to put a second 12v battery in my boat and connect them appropriately to achive a 24V battery bank, will I still be able to run my existing 12V lighting, radios, vhf, and gps, or would I risk overloading those components?

If you attempt to power a 12V device from 24V, it will fail quickly.

You could run matched loads off each 12V battery, such as Port & Starboard lights You run the risk of not depleting both batteries evenly, and then while rechargeing, overcharging one, and not fully charging the other.

Instead of having to buy all new 24V devices, you could from the onset, get a pair of 6V, group 27, batteries, wire in series, and have a large 12v system. You will need to scale up your charging capacity too, to keep from undercharging the batteries.

System2 · May 2009

Re: Help Please: 12V battery and 24V panel

Greetings and thanks for being so kind and responding to my questions so quickly:D.

It sounds as though in theory my hunch was correct in that I could in fact use a 120W 24V solar panel to charge the 12V battery on my sailboat as long as I put a converter between the panel and battery to "down convert" the 24V to 12V. I think it was MikeO who reccomended a Morningstar charge controller. If I have completely misunderstood the theory I hope someone will corrct me.

I guess I could go with the 24V panel and 2- 12V batterys wired in series (I think series is the correct term) but all my electrical components on the boat are rated for 12V and I would probably be sending to much power. Does that sound reasonable?

Let me try to answer a couple of the questions that were asked of me.
1)I live in North fForida so we get a heafty amount of sunshine most of the year although you couldn't tell it based on the last 2 weeks of rain and overcast skies.
2)I'm not sure of the estimated power comsumption for the boat as I have just aquired her. Bilge pump, 12v fans, vhf radio, am/fm cd player, led interior lights at night, anchorage/running lights, thats about it. I will need to figure hours and amperage but certainly noting elaborate will be in use.
3) Not sure of my battery make but do know it is a 12V Group 27 deep cycle marine battery.
4) In regard to the 18V panel, the only proof I have that it is in fact 18V is the manufacturers spec sheet. The manufacturer is a Chinese company named "Suntary". His specs are as follows: Module Type SM120-18 - Rated Power 120Wp - Voc 21.4v - Vm 17.5v Im 6.86A
5) His specs for the 24V panel are: Module Type SM120-24 - Rated Power 120Wp - Voc 29.4v - Vm 23.5v Im 5.11A

I personally like the 24V panel better as the deminsions are a beter fit for my boat.
Not sure if this information is helpful or is enough for someone with a lot more knowledge than me to give a solid opinion but the info was asked for so I felt obliged to provide it Would anyone be willing to estimate a fair price for each of the above panels? That would be helpful for me to know if the manufacturer is negotiating in good faith or trying to over charge me.
Thanks and looking forward to your knowledgeable replies...Larry:D

icarus · May 2009

Re: Help Please: 12V battery and 24V panel

Your "18 vdc " panel is a 12 vdc panel, as evidenced by the ~17.5vdc vmp. The Vmp for the 24vdc panel is too low to power a 24 volt system,,, you could wire a string of batteries (3x6) to be 18vdc nominal and it might work for that.

Tony

niel · May 2009

Re: Help Please: 12V battery and 24V panel

sorry, but that 24v pv as you called it is not going to cut it with a vmp output of 23.5v when you have a 24v battery system. this may be a good 18v pv, but the other pv mentioned is a 12v pv that has the 18v out rating. the controller regulates the pv output to properly charge the battery. if the battery is a 24v battery and is to operate 12v items, that is when a converter is used in addition to all of the other stuff.

dwh · May 2009

Re: Help Please: 12V battery and 24V panel

Hi Larry, welcome to the forum.

The unit that goes between the PV and the battery is *not* a converter - the unit is called a "Charge Controller". You will need a PV that can put out a higher voltage than the battery, and then the CC will have adequate voltage available to charge the battery fully.

12 volts is what is known as a "nominal" voltage - 12v batteries are actually at around either 13.6 volts (or so), or 14.4 volts (or so) (depending on battery type) when fully charged. So, the charge controller likes to have at least 17 volts available so it can "top off" the battery fully.

Don't sweat the 24v vs. 12v thing - just make sure you get a charge controller that is rated to handle the input voltage and amperage. Of course, also make sure that it's rated to charge a 12v battery.

If you add a second battery, wire it parallel to double the run time of your 12v stuff. Forget about wiring the batteries as 24 volts - with the proper CC you won't need to.

The CC doesn't "down convert"...what it does is monitor the state of charge (SOC) of the battery, then it adds voltage to the battery as needed. A CC will add more voltage when the battery is low, and less as the battery is topped up.

Let's say you have a single panel - for an example I'll use the Kyocera 130w panel from our host's website:

The rating shows that it puts out a little over 7 amps max power at a bit over 17 volts - so you would need a CC like the MorningStar SunSaver 10amp, 12volt CC:

http://store.solar-electric.com/ss-10.html

Which, if you look at the operator's manual from the MorningStar web site:

http://www.morningstarcorp.com/en/support/library/SS.IOM.01.EN.pdf

It says: "The SunSaver is rated for 12 volt (or 24 volt) PV systems. Do
not connect to a PV array with open circuit voltage (Voc)
greater than 25 (or 44) volts."

Thus, it will handle up to 24 volts on the input, and will use that to charge a 12v battery (on the 12v version).

A perfect match for the 130 Kyocera panel and a 12v battery (or a couple of batteries as long as they are wired as a 12v bank).

There are tons of different charge controllers, in these forums you will see that most people here *definitely* recommend use a CC with a *remote* temp sensor to get a true reading from the battery. I don't think the SunSaver has that, so you might want to go with another charge controller.

BB. · May 2009

Re: Help Please: 12V battery and 24V panel

There two major classes of charge controllers... PWM which just "turn on and turn off" to charge or not charge a battery bank (typically many times a second). PWM works most efficiently when the solar panel Vmp is 2-3 volts or so over the maximum battery voltage during charging (~14-15 volts).

The other is an MPPT (maximum power point tracking)--this controller does actually down converter using a switching power supply (typically a buck mode converter). It can take panels with Vmp>100 volts and efficiently (90-95% or so) down to the 14-15 volts needed to charge your battery bank.

And for small panels (~200 watts of 12 volts) there is a very nice MorningStar 15 amp MPPT controller (though--not cheap).

You can read about charge controller here in this FAQ.

A couple of suggestions... Stick with 12 volt battery bank unless you have need for lots of 24 volt power (as other say--you cannot reliably get 12 volts from a 24 volt battery bank unless you use a DC to DC down converter).

Also, stick with the "12 volt panels" (aka 17-18 volt Vmp panels) in parallel. A sail boat has lots of shading issues with panels You might get more power from two panels in parallel vs two panels in series (which would need a MPPT controller to do the "24 to 12 volt conversion"). In general, solar panels on a sail boat are a hit and miss issue--many folks mount them of the stern to be clear of lines and railings--but you still need to point the stearn at the sun for power.

You can use either a good qualty PWM converter (not as expensive) or the MorningStar MPPT (more expensive and flexible--you could do your own series/parallel panel connection tests and see what works best for you). Your choice--either will work fine. Make sure there is a remote battery temperaure sensor.

Look at getting a Battery Monitor--critical to know the state of your battery bank without having to measure specific gravity to get an accurate estimate of the state of charge of your battery (and know when to conserve power and/or fire up a genset).

AGM batteries are real nice for your application. You can mount them one with cells "up" or on their sides. They stay much cleaner, less hydrogen gas release (unless overcharged), and do not need watering. You should really get a Battery Monitor since you cannot measure their specific gravity. Issues with AGM--very senstive to over charging (need good quality/accurate charge controllers) and probably 2x as expensive.

-Bill

System2 · May 2009

Re: Help Please: 12V battery and 24V panel

Thanks Guys,

this is great informationand much appreciated. Now all I need to do is soak it in and understand it all. I'm sure there are more questions to come so bear with me. Larry N.

TnAndy · June 2009

Re: Help Please: 12V battery and 24V panel

Larry N wrote: »

I guess I could go with the 24V panel and 2- 12V batterys wired in series (I think series is the correct term) but all my electrical components on the boat are rated for 12V and I would probably be sending to much power. Does that sound reasonable?

If everything is 12v, you need to stay with 12v on your batteries.....you can add more batteries in parallel to up your amp hours, but not in series ( which keeps the amp hours the same, but doubles the voltage ).

Larry N wrote: »

4) In regard to the 18V panel, the only proof I have that it is in fact 18V is the manufacturers spec sheet. The manufacturer is a Chinese company named "Suntary". His specs are as follows: Module Type SM120-18 - Rated Power 120Wp - Voc 21.4v - Vm 17.5v Im 6.86A

That is a 12v panel for charging purposes.....it takes 14.5-15 volts to actually charge a 12v battery ( depending on temp, wire resistance, corrosion on terminals, etc )..then you loose a little in wiring from the panel to where you actually use it to charge......so anytime you see "Vm=17.5" ( or close to that ) you have a 12v panel.....for the purposes of charging batteries.....

Larry N wrote: »

5) His specs for the 24V panel are: Module Type SM120-24 - Rated Power 120Wp - Voc 29.4v - Vm 23.5v Im 5.11A

That is NOT a 24v panel for charging battery purposes.....as stated above, a single panel, or panels in parallel, will NOT charge a 24v battery bank....you would need to see "Vm=35v" ( or right close to that )

You could use it ( and probably even then want to string with more) in a grid tied system with NO batteries on your house....unless....

Unless you used an MPPT type charge controller with changeable set points....then you could string a couple of them ( or more ) together, and the charge controller would take the 47 or so volts that two panels in series would produce and step it down to the 28-29v the battery bank needs for charging, converting the extra voltage to amperage......MPPT charge controllers are really cool that way.....but as usual, you're gonna pay more for coolness....ahahahaaa

Help Please: 12V battery and 24V panel

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