wiring 12v solar panels to charge 24v

seayfam · May 2013

Hi all, I'm new here and have a question in regards to wiring my solar panels.
I have three 75w 21v solar panels on the top of my coach that are wired parallel to charge a 12v system. I'm wanting to convert it to charge a 24v system. I purchased a new ProStar 30M controller that will handle12 or 24v. My question is... can I wire three 12v solar panels to charge 24v with the controller I have?
My thought is wiring two panels parallel and then series to the third panel. Do you all think this will work?
Thanks in advance
Gary

niel · May 2013

Re: wiring 12v solar panels to charge 24v

welcome to the forum gary,
the arrangement you propose will not work. the controller you speak of is a pwm and if you had chosen an mppt cc then 3 of those pvs could've been made to work. this would have been with all 3 pvs in series. one of the smaller mppt controllers that would have worked is this one,
http://www.solar-electric.com/mosumpsochco.html

being you have the pwm cc then you have 2 choices here. either expand the array with a 4th like pv or eliminate one. it would be better adding one of course and you would have 2 in series paralleled to 2 in series. the other option of eliminating 1 pv would leave you with 2 and those would be in series.

generally when paralleling pvs you want to match up the voltage within 10% and i prefer within 5%. for series pvs you need to match the current within the same percentages and you proposed 2 pvs putting double the current through 1 pv. this would limit the output current to that that the single pv would give so it would resemble 2 pvs in series even though there's a 3rd involved.

BB. · May 2013

Re: wiring 12v solar panels to charge 24v

Welcome to the forum Gary.

In general, for a PWM charge controller your only option is to wire the panels in series pairs of two (wire two panels in series, then wire each series pair in parallel with the next string).

Your only option with your three panels is to connect two, or get a fourth one and then have two series pairs for 4 panels total.

The other option is to get a new MPPT (maximum power point tracking) charge controller and put the three panels in one series string. Unfortinatually, MPPT charge controllers are not cheap. The least expensive small mppt charge controller that would work for you (that I am aware of) is a Morning Star 15 amp 12/24 volt MPPT charge controller:

Morningstar SunSaver 15 Amp MPPT Solar Charge Controller

And that will cost ~$225 plus a remote temperature sensor (highly recommended for this controller) and possibly a remote LCD display and/or computer interface (not cheap).

-Bill

seayfam · May 2013

Re: wiring 12v solar panels to charge 24v

Will any 75w solar panel work in series with my existing panels? I can't seem to find anything on line that are the exact specifications. Also the fourth panel will have to be mounted in a separate location on the other side of my roof air conditioner. Will that cause any problems if one isn't getting the same amount of sunlight as the other depending on the suns angle?

niel · May 2013

Re: wiring 12v solar panels to charge 24v

for putting in series you need to find a pv with the same general characteristics for the current. now the twist is that the string that that is in will need to be in parallel with the other string and that voltage will now need to be within the percentages mentioned.

putting the 4th pv into a ddifferent location could be detrimental as it could have a differing angle or could become shaded when the others aren't.

if the controller you have is fairly new then see if you could get the dealer to accept it back on the premise you could obtain an mppt cc that will handle the 3 pvs in series such as the mppt cc i linked to.

do you know the specs to your pvs or can give the make and model otherwise?

BB. · May 2013

Re: wiring 12v solar panels to charge 24v

Before you lay out any money... You should do more research and do a couple paper designs vs your load requirements.

Under 100 watt panels are usually pretty expensive these days vs the 200 watt or larger panels. But, for the money you save with larger panels, you usually need MPPT type charge controllers (which are not cheap either).

I like to go for a "balanced" system design. The loads drive the battery bank sizing. And the Loads+Battery Bank drive the solar array sizing.

For an RV, many times folks use a larger battery bank with a smaller array as the trips may be only a few days and there could be AC power at an RV park or backup genset for the (less often) longer trips.

So, what are you loads and what is the size of your present/"ideal" battery bank, and is this a weekend trip or months at a time off grid type usage. Also, where/when will the trips be made... Southwest US anytime vs far north in winter for skying, etc... (how much sun).

-Bill

seayfam · May 2013

Re: wiring 12v solar panels to charge 24v

Here is what I've got. I have 4ea 8D batteries 2ea in series. Then the two banks are parallel to the 24v side of a Ventron 100A battery equalizer. I then have a 12V wire tap between the two batteries at each bank to bring 12V to the 12V side of the equalizer.
My solar is currently set up for 12v and is hooked to the 12V side of the equalizer. The problem is... I'm only charging one battery at each bank. I would like to keep all four batteries the same is the only reason for wanting to change. I was under the assumption that the equalizer was supposed to keep them all the same (but it's not) I usually only am out for a week and I'm never at a pole. I live in Alaska and the sun in the summer is out all day when it's not cloudy. I've been debating on just wiring two in series for now just to keep things balanced. I'm sure I'll be running the gen some also.
Thanks again
Gary

niel · May 2013

Re: wiring 12v solar panels to charge 24v

first of all never tap a larger voltage battery bank for a smaller voltage. this creates imbalances and could lead to the demise of your batteries. do you have any 24v loads for those 24v battery banks? if not then they should be paralleled for 12v or at least separate 12v battery banks. no equalizer will stop the imbalances you are creating between the batteries. the equalizer you have will only isolate the differing 24v charges for differing 24v battery banks. it does not equalize tapping those individual batteries for their 12v content.

2ndly, you are severely undercharging the batteries for at a minimum you would want a 5% charge rate. an 8d battery is roughly at about 250ah and would need at least 12.5a minimally for proper charging. now the fact that you are putting them in series for 24v would still require 12.5a min, but you have doubled the voltage so double the power is still warranted for this minimum charge. keep in mind that one should not go by the stc rating as that would not be the norm as those conditions are not usually met. this could add 23% more on average to the charge needs using the stc ratings. as an example those 4 batteries in parallel would be about 1000ah at 12v. 1000ah needs at a minimum 50a. from pv this might be an extra 11.5a for a total of 61.5a stc minimum. ideally, this would be higher as in the 10% range of charge (best general rate without loads applied) and may need to be higher depending on the loads. now that would put you at 123a.

at this point you need to stop everything and get those batteries properly charged individually. no more tapping for 12v loads either. in the meantime you should tell us the goals you have in mind here and at what voltage and current these loads would represent.

seayfam · May 2013

Re: wiring 12v solar panels to charge 24v

My coach is a MCI and is a 24v coach with 24v charging. The house systems are 12v for the lights and water pump and your typical RV appliances. Then the rest of the house systems are 110v AC from two separate 24v inverter/chargers.

The the instructions from the 100A 12v/24v battery equalizer show the correct wiring to have a wire tap from the lower battery in the series to go to the 12v post on the equalizer.

So is this the wrong way to accomplish this? How should I get 12v from a 24v coach? Should I add a separate 12v charging system to my engine and a separate 12v battery bank?

Everything in the system has been fine until I tried to solar charge the 12v side.

Thanks
Gary

niel · May 2013

Re: wiring 12v solar panels to charge 24v

i can't comment too much on what they are doing as i haven't a clue. maybe some with an rv have a better idea, but what i said generally does hold true. i'm just not up on how they handled that or what it is they did.

BB. · May 2013

Re: wiring 12v solar panels to charge 24v

A 12/24 volt battery equalizer should work OK--Assuming you do not exceed its ratings. I assume it is something like this:

www.vanner.com/markets/work-truck/ (listing of battery equalizers and other equipment)
www.vanner.com/manuals/70-60CAN.pdf

When you say it is not working--What is happening. Are the batteries not being properly balanced? Or is it using so much power itself, the batteries just do not last very long?

A balancer, in its simplest form, just keeps the voltage of Battery A equal to Battery B. If you draw power from A (12 volt battery connected to ground on minus terminal), then the equalizer should take power from Battery B (the high side 12 volt battery in a 24 volt system) and "recharge"+supply current to DC loads. Likewise, if you "charge" Battery A with a 12 volt solar power system, the equalizer should move current (energy) to the high side battery and recharge it too.

Theory of Operation

In many 24 volt electrical systems it is desirable to tap into the battery system to obtain power for 12 volt loads. This method, while seemingly simple, causes a charge imbalance resulting in Battery B (see diagram) being overcharged, and possibly boiling, while Battery A discharges. To solve this application problem the Vanner VANN-Guard is connected to the battery system at the +24 volt, +12 volt, and ground points. The VANN-Guard makes the batteries look like they are in series and inparallel at the same time. The VANN-Guard maintains the voltage balance and therefore the charge acceptance rate of each battery. The VANN-Guard holds the Battery A and B voltages to within 0.05 volts under light loads and to within 0.1 volts at full rated load. When the voltage of Battery A is higher than or equal to Battery B the VANN-Guard is in the standby mode, i.e., it is not transferring power from its 24 volt input to its 12 volt output. When a 12 volt load is present, and Battery A's voltage decreases to just below the voltage of Battery B, the VANN-Guard activates and transfers sufficient current from Battery B to Battery A to satisfy the load and maintain an equal voltage and charge in both batteries.

A key advantage of a system containing a Vanner VANN-Guard , compared to a DC to DC converter, is that if the 12 volt load requires a momentary surge current which exceeds the rated capacity of the VANN-Guard, Battery A will supply the extra current to the load. The VANN-Guard will then replenish the energy to Battery A after the surge has passed.

If there is a significant voltage difference between the low and high side battery banks, then why is that? Too small of battery cables, poor connections, too much DC loads on the 12 volt side of the bank, etc...?

From reading their manual above, it appears that they may not capable of bi-directional power flow from the low side battery back to the high side battery.

If you can get a "cheap" DC current clamp meter (like this one from Sears) may be a good start in helping you to understand what is going on with your system.

The "simple fix" for the system would be to reconfigure your solar charging (and any backup chargers) for 24 volt operation only. If you are satisfied with the operation otherwise, then this should be fine (assuming the remote sense leads, current sense connections, configuration, etc. for the VannGuard are all OK).

So, this gets back to the solar power system. Many controllers will work on either a 12 or 24 volt battery bank--Does yours?

Next, configuring of the solar array. For a PWM controller, you need Vmp-array to be in the Range of ~35-40 volts for optimum operation. For a MPPT charge controller, you need Vmp-array>~35 volts and Voc-array to be less than the Vpanel-input maximum voltage for the controller.

Solar panel's output voltage rises significantly in sub freezing weather--So that is one thing you will have to watch out for--That your "cold panels" Voc-cold (voltage open circuit cold) do not exceed the maximum solar array input rated voltage for the MPPT controller.

Yes, you can put three panels in one spot, and another panel elsewhere. Just need to use a bit heavier cable (possibly--depends on array current and cable lengths involved). And you realize that any shading on one panel will take out the rest of the series connected string--energy wise). I.e., 4 panels, 2 in series, then two strings paralleled. One panel with shade will take out the entire 2 panel series string (i.e., shading even a part of one panel on a 4 panel 24 volt setup will cause a 50% reduction in array output--no damage, but shading is important).

How you wish to use your present panels (three only in series with a MPPT controller) or 4 panels (or more) of same panels with a PWM or MPPT controller is your choice.

I would also look at getting 150 watt or larger panels (looking for good $$$/Watt pricing) and a MPPT charge controller, and see if it would make sense for you or not.

For a 4D 24 volt battery bank with 4x batteries at ~250 AH per battery... That would be a 24 volt @ 500 AH battery bank. The smallest to largest (cost effective array) using our rules of thumb for a solar PV powered off grid system (assuming you want most of your power from solar) would be:

29 volts charging * 250 AH * 1/0.77 panel+controller derating * 0.05 rate of charge = 471 Watt array minimum
29 volts charging * 250 AH * 1/0.77 panel+controller derating * 0.05 rate of charge = 942 Watt array nominal
29 volts charging * 250 AH * 1/0.77 panel+controller derating * 0.05 rate of charge = 1,224Watt array "cost effective" maximum

So, you can see from our rules of thumb, you can put a pretty large array on your RV Bus and probably make good use of the power. Whether such an array will fit, or be cost effective is another whole can of worms.

Are we being helpful here--Or have I lost the thread of your questions?

-Bill

vtmaps · May 2013

Re: wiring 12v solar panels to charge 24v

seayfam wrote: »

So is this the wrong way to accomplish this? How should I get 12v from a 24v coach? Should I add a separate 12v charging system to my engine and a separate 12v battery bank?

Everything in the system has been fine until I tried to solar charge the 12v side.

Bill speculated that you have a Vanner equalizer. Is that correct? If so, and everything was working fine, It would seem the simplest thing would be to add 24 volt (rather than 12 volt) solar charging.

An independent 12 volt system (battery and charger) would also work.

--vtMaps

seayfam · May 2013

Re: wiring 12v solar panels to charge 24v

Bill, that was some really good information!
My equalizer is almost identical to the Vanner, but it's a Ventron model EQ-100 100A. I've tried to find information online for it but can't seem to. When I had the panels charging at 12V side, it was only bringing up batteries A at each bank. I'm not sure if this particular equalizer will do exactly what the Vanner will?

All I'm trying to do is make good use out of the panels that are already mounted on the roof. Obviously the panels aren't enough to run more than a few lights. I'm mainly hoping to keep things charged up when we are out and about. Right now I have two of the panels wired in series and it seems to be doing ok at maintaining the batteries equally when we are just letting it sit.
Hopefully it will cut down on gen time?
Gary

BB. · May 2013

Re: wiring 12v solar panels to charge 24v

Gary,

A 150 watt of solar panels is not a lot of power... Roughly, say you get 5 hours of sun (noon time equivalent--a good amount of sun in spring/fall for most places):

150 watts * 0.52 system efficiency * 5 hours of sun = 390 Watt*Hours of AC power per day

Or, about the equivalent of 1/10 gallon of gasoline in a Honda/Yamaha inverter generator worth of energy (say an hour of run time at ~400 watts or 1/4 load for a Honda eu2000i or equivalent genset.

So--It really comes down to how much power you use in a day... That is not a lot of power for most people--Just a few lights, a computer, radio, charging cell phone type application.

Also, I see I forgot the link to a lower cost DC Current Clamp meter that is "good enough" for our needs--Here it is:

http://www.sears.com/shc/s/p_10153_12605_03482369000P

Note that DC Current Clamp meters can determine the direction of current/power flow--So you can quickly figure out what is going on with the equalizer system if needed... Also, a current clamp meter is real handy for debugging battery bank issues with parallel wire runs, etc.

-Bill

seayfam · May 2013

Re: wiring 12v solar panels to charge 24v

Bill,
Say I wanted to add a little more power to the coach... I've been looking for a fourth panel to match the 75w panels I have with no luck. Can I just add two new 100w or so panels in series and parallel them to the two I have?
Gary

BB. · May 2013

Re: wiring 12v solar panels to charge 24v

If you get pairs of panels in the Vmp~17.5 to 18.6 volt range--Yes you can add parallel pairs of panels of any wattage to your system...

Of course, you have to not exceed the current limits of your existing PWM controller. And as you add parallel strings, you should put a fuse/breaker in each string (a combiner box with fuse/breaker per + input string, and a common bus for the negative leads).

The fusing/breakers prevent a shorted panel/string from being feed with too much current from your other connected panels.

You can, if you ever decide to do it, add additional solar charge controllers in parallel (connect each set of controller+panels directly to your 24 volt battery bus, with proper wiring, protective fuse/breaker/etc.).

As to exactly what you will want to do... It will be based on your research for the "right size" panels that fit your space (and budget).

A few notes--Panels over ~140 watts have limited shipping options (i.e., truck vs UPS/Bus/etc.) because of their physical size. And shipping small numbers of panels can be just as expensive as shipping a pallet of panels after repackaging/handling charges are taken into account.

Also, being you are in the great white north--Have you setup your panels to tilt--Especially if your travels are year round (shedding of snow, pointing towards low sun in sky, even get added power from refections of snow/ice on ground).

From our host (Northern Arizona Wind & Sun), some possible panels that would work on a 24 volt battery bank with a PWM controller:

Solartech SPM130P-S-N 12V
Solartech SPM130P-WP 24V (Vmp~34 volts, a tiny bit low, but should work nicely in cold weather, plus you can install "odd numbers" of panels if you wish)
Solartech SPM140P-S-N 12V
Kyocera KD140GX-LFBS

The above panels are around ~$2 per watt...

If you can use larger panels at >200 watts (around $1-$1.30 per watt--based on physical size of panels and shipping options to you)... You can get about 2x the amount of power, and with the cost savings, get a really nice MPPT charge controller.

I know that there are 2 states were cost of goods/shipping is much higher in costs... You might be better off getting some large format solar panels locally from an installer (>200 watt) and then get a Nice MPPT charge controller for ~$300 to $ 600 plus S&H.

None of us moderators are in the solar business or have any relationship with NAWS other than as volunteer moderators here... So, we cannot address the shipping/handling/insurance costs for solar panels delivered at your door (or freight yard).

Try picking some different panel options, and we can help you with some controller options to go with the panels you have selected...

I know the 75 watt panels were not cheap--But you may be hurting yourself by trying to keep them in your present setup--Especially if you are giving up "prime space" to support the small panels when it would be better if filled with 2-4x larger format panels.

-Bill

Dragonlady5 · May 2013

Re: wiring 12v solar panels to charge 24v

There are DC to DC converters available on the market which will convert your 24 VDC to 12 VDC. Amps out of the converters are typically up to 60 which should be plenty for the RV applications (or use multiple converters). Using a converter is recommended rather than what I have seen in some installations, where 12 Volts is tapped off part of the 24 Volts, -this over time will cause an imbalance in the bank (as neil mentions).

seayfam · May 2013

Re: wiring 12v solar panels to charge 24v

Dragonlady5 wrote: »

There are DC to DC converters available on the market which will convert your 24 VDC to 12 VDC. Amps out of the converters are typically up to 60 which should be plenty for the RV applications (or use multiple converters). Using a converter is recommended rather than what I have seen in some installations, where 12 Volts is tapped off part of the 24 Volts, -this over time will cause an imbalance in the bank (as neil mentions).

The only problem with converters is there not as efficient as equalizers.
Most bus manufacturers install the equalizers from the factory. Even many fire trucks use them.
My solar system right now is doing a good job as a battery maintainer.

nsaspook · May 2013

Re: wiring 12v solar panels to charge 24v

seayfam wrote: »

The only problem with converters is there not as efficient as equalizers.
Most bus manufacturers install the equalizers from the factory. Even many fire trucks use them.
My solar system right now is doing a good job as a battery maintainer.

Like Bill said before a equalizer is a (cheaper,needs 1/2 the power of a full voltage converter) DC to DC converter but instead of a fixed output it tracks the load voltage of one battery and adjust the current draw on the other battery to supply a matched voltage in parallel so in theory it could maybe be 1 or 2 percent more efficient than the normal 96% efficiency seen in a good converter. A potential problem is if the health of one battery is really different from the other this system will tend to make that difference increase over time.
http://www.ase-supply.com/v/vspfiles/downloadables/21060E00.pdf

wiring 12v solar panels to charge 24v

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