Mis-matched solar panels

[NanMWright]

Hi Folks,

I have two 100W Renogy solar panels on the roof of my Class C Sunseeker, wired in parallel and connected to  a 20A Rover Controller.

One of the panels is monocrystalline and the other is polycrystalline. (This was not intentional.)  I'm having a lot of trouble keeping my two 12V batteries charged with the solar and I wonder if the problem is because I'm mixing different types of panels.

If I need to change one of the panels out, do I stick with the poly or the mono?

On a very good winter day, the panel monitor shows 18V / 9A from the panels, but my batteries rarely stay charged, frequently falling to 11.5 V.

Thanks for your suggestions.  

Nancy Wright

[kaypsmith]

I think that your problem may be more addressed toward how much load are you pulling from the battery bank than the fact that the panels are mismatched.  How much load is being put on the battery? What is the stated output for each solar panel? Also remember that this wattage is in full sun, this time of the year most of us can only claim about 8 hours of sunshine, and with that in mind that 9 A should be divided by 3, which averages only 3 amps per hour, and at 12 volts, the actual battery stated output is only 36 watts per hour over a 24 hour period. In other words a probability for more panels to fulfill your consumption needs. If you let us know what max output wattage those panels are we can tell if they are functioning as expected or not.

[desertdeals69]

Check to see if the charge controller can take more than 45 volts on the input.  I think it will and you should wire the 100 watt panels in series.  The poly is not quite as efficient as the mono.  I have 8 100 watt Renogy mono panels wired 4 in series and the two banks in parallel as my 40 amp MPPT controller will take 100 volts max.  I think you need at least two more panels and a MPPT charge controller.

[NanMWright]

Thanks for your replies.

I'm re-doing my needs and running some calculators to better understand the math. The first fix seems to be running in series with monos. Then determining if I need additional panels and a larger controller.

I appreciate your input.

Nancy

[Lenp]

The 9 amps at 18 volts translates to 162 watts.  That is not too bad for winter sun angles (about 80% efficient which is good for low winter sun angles).

You didn't say how many batteries your rig has but if we assume two deep cycle marine/RV batteries with around 80 amp hours each capacity you have a total of 160 amp hour battery bank. Assuming you have totally discharged them it will take 17 hours or more at a nine amp charge rate.  Your only going to get the 9 amps for 3-4 hours at best and after that it will probably average 3-4 amps for another 3-4 hours for a maximum of 48 amp hours - you won't even recover 50% of your battery capacity.

The fact your dropping to 11.5 volts indicates a totally discharged battery bank, BUT I expect your still pulling a load from them which would cause the voltage reading to be lower than it actually is.  To measure the REAL State Of Charge (SOC) you need to remove ALL loads (and charging sources) from the batteries (disconnect them) for 10-15 minutes and then read the voltage with a multi meter.  Or measure the specific gravity of each cell in each battery.

My guess is your never getting your batteries fully charged and are using the bottom half of the charge (10-50% range).  

Suggest you also have your batteries load tested.  If you have been consistently discharging them to the 11.5 level they may be seriously damaged.  An article I read many years ago stated you can cycle a deep cycle battery from 90% to 50% at least 2000 times, however it will only last 200 cycles if you go from 90% to 10%.

You also did not indicate what type of battery(ies) you have installed.  I assumed above you have the typical Marine/RV battery but I may be wrong.  Regardless, the typical marine/RV battery is not a good choice for our use.  You would be much better off with a true deep cycle battery like Trojans.  Perhaps switch to 6 volt golf cart batteries - much better then the marine/RV batteries. 

One way to more accurately monitor your battery condition and charge level is with a battery monitor system like the Bogart Engineering Trimetric.  http://www.bogartengineering.com/products/trimetrics.html

FYI, here is a chart I have used for years to measure SOC via voltage reading.  As noted above, the battery MUST be at an "at rest" condition (disconnected at least 15 minutes and, if recently charged, have a small load place on it for the 10-15 minutes, like a 12 volt light bulb, to dissipate any surface charge).  Please note that a charger/converter will place 13.6 (or more) volts on the battery and when you first disconnect the charger this 13.6 volts will remain as a "surface charge" until a small load is placed on the battery.  A fully charged battery at rest will read 12.66 volts.

Hope this helps,

Lenp

Voltage	Charge Remaining
12.66	100%
12.61	95%
12.57	90%
12.53	85%
12.49	80%
12.45	75%
12.41	70%
12.37	65%
12.33	60%
12.29	55%
12.25	50%
12.21	45%
12.17	40%
12.13	35%
12.09	30%
12.05	25%
12.02	20%
11.99	15%
11.96	10%
11.93	5%
11.9	0%

[NanMWright]

Ugh.

I can't say I fully understand the math behind battery use and maintenance, but your explanation certainly helps. I believe I have (mostly) destroyed my batteries which are less than a year old, Interstate 12V.  I have too large a draw being supported by mis-matched and inadequate poly/mono solar panels which, when used together, are incompatible with the MPPT controller. The solution will require some education, several replacements, and better management of my load...an expensive bit of ignorance on my part. 

Thank you all for your input. 

Nancy

[wildebill308]

Nancy, unfortunately it looks that way. Can you run your generator and fully  charge your batteries? Do you have shore power where your RV is? It would be good to fully charge and test your batteries before you replace them. Check and see if what the warranty is.

Bill

 

[NanMWright]

I'm full-time on the road and I only boondock. 

My generator (Onan) does not really charge the batteries unless I run if for hours. I don't have shore power. I usually run my Ford 450 engine to recharge the house batteries.

I'll call the dealer where I got the batteries. Interstate has a 12 mo warranty and they are less than 1 year old. So, I will need to fully charge and test, per Lenp:

On 1/30/2019 at 10:50 PM, lenp said:

To measure the REAL State Of Charge (SOC) you need to remove ALL loads (and charging sources) from the batteries (disconnect them) for 10-15 minutes and then read the voltage with a multi meter.  Or measure the specific gravity of each cell in each battery.

I'm not sure that I have not voided the warranty by my use/abuse. Any suggestions on that?

Nan

[manholt]

Any Auto parts store or Interstate store can bench test your batteries, to see if one or all are bad.

[NanMWright]

Got it.  I have an Interstate near me. I'll start there, then figure out how to build out the solar, battery. load so I can function. 

Thanks, as always for FMCA help.  I couldn't be on the road without you.  

Nan

 

[wildebill308]

Just a thought. Have you measured the charge voltage when you are using the Onan? The "bulk charge rate" that's when you first start, should be 14.4 volts. If it isn't you could use an additional battery charger to make shure it is at 14.4 (that is the max they will absorb) just plug it into a 110 outlet on the generator or on the coach. You have the generator running you might as well get as much done as fast as you can. The charge cycles should adjust and step down the charge rate as required. 

Bill

[NanMWright]

Thanks again for your comments,

I had my 2 12V batteries load tested and they are still good batteries - testing in at 14.4V. I understand from lenp's analysis that I may have limited their life, but for now they are okay.

My plan is to expand to 400W monocrystalling solar panels with a 40A MPPT controller. Later on I'll upgrade my 2 12V batteries to 4 6V batteries and manage my daily load better. 

Thank you for these many posts. It has helped me manage my energy use more realistically...looking at the long game and not just what I need seasonally.

As always, I'm glad I'm an FMCA member, and grateful for your help.

Nan

[Donco234]

I note that you are in the Pacific Northwest, as I am. All solar panels will produce a less than their rated output the farther north you are AND we (in the Northwest) have had a lot of cloudy conditions lately. If hypothetically  both your cells are putting out 75 watts during bright times, and nothing at night, and very little during cloudy times, it would be very easy to use more than the cells can replace. Most likely, as others have noted, your batteries are never reaching full charge unless you run your engine for quite a while. 

The nature of battery charging  is that the first part of the charge goes in fairly quickly and then the charge rate tapers off quite a bit before full charge is reached. When you crank up your RV engine, it will charge low batteries at high rates at first but taper off at about 85% to a much slower rate. I suspect that you may be stopping this charge somewhere in this range.

As others have said, you really need to be able to estimate your daily electrical use before you can know just how large of solar panels you need. I have 450 watts and they do a good job of "prolonging" the time before I need to recharge with shore, engine, or generator power. 

You may find that a good/quiet portable generator with a smart charger would do a better job for a lot less money. 

[jbell]

I would suggest the trimetric system as mention above to really get a handle on what's going on. Well worth the price and actually gives you %charge and amps going in to your batteries and amps going out.

[kaypsmith]

Nan, if you have room for more batteries, you can continue using the 2 12V batteries in addition to adding more 6V batteries in pairs, ie. 2 6 or 4 6V in addition to the 2 12 V's already in place. If you decide to do this just ask here how to wire them correctly.

[Donco234]

Doesn't matter how many batterires you have, if you take more out than you replace, you will run out. 

 

[manholt]

Donco234.

That is a profound statement.  

Nan is a lady we all admire.  She spends a lot of time off the grid, not many will do that...she asks questions and we try to answer...the last thing she needs, is basic math!  Most people on this Forum, are very well educated and your not the only Electrical Engineer here...but you can help, we are always looking for that.  

Carl C.

[Donco234]

Roger that

[riverbend]

I'm new here and certainly late to this discussion, but if the OP is still working on her 12V/solar electrical needs, I think it worth mentioning that there is still a 30% fed tax credit for solar installations, and our motorhomes qualify. Plus if she were to upgrade to LiFePo4 batteries with her upgraded panels, that cost can also be claimed for the tax credit. We just bought a used 2019 Isata 3 and the AGM batteries had been deep cycled to death, so I installed two 100Ah BattleBorn LiFePo4 batteries and couldn't be happier with their performance. If I were to add another 100W panel (currently 200W), and I may, I could also realize the credit on the entire upgrade, batteries included. Sure, their cost is up there, but for what they give in return, it deserves consideration IMHO. We boondock a lot, our batteries are incredible and they provide so much more usable power than FLA. Not trying to start a debate on FLA vs LiFePo4, cost or otherwise, just trying to offer options to the OP.    

[richard5933]

Riverbend - Welcome to the forum. When you get a minute please add a little information to your signature, like maybe a name and specs on your coach.

What else did you have to change out when you went from AGM to lithium batteries? Was your charger/inverter/etc able to be reset to the lithium?

[desertdeals69]

Since my previous post Jan 30 I have changed my 6 AGMs for 4 100 amp Battle Born lithium batteries.  I reset the charging parameters on the Xantrex SW 3012 and the Renogy 40 amp MPPT charge controller.  The performance is outstanding.  Full charge last at least twice as long and it takes about half the time to recharge.

[riverbend]

22 hours ago, richard5933 said:

Riverbend - Welcome to the forum. When you get a minute please add a little information to your signature, like maybe a name and specs on your coach.

What else did you have to change out when you went from AGM to lithium batteries? Was your charger/inverter/etc able to be reset to the lithium?

Thanks for the welcome! 

Regarding my LiFePo4 upgrade, I had to change the charging module in my PD4060 converter/charger, and my solar controller was able to handle the change so that stayed. The only other thing I did was to add a Victron BMV-712 monitor for an actual look at SOC.  These batteries are AMAZING, plus the two together weigh less than one AGM, saving me a considerable amount of carrying capacity. Ten year warranty, just about twice the available power...without the amperage drop of a FLA...and they charge much faster.   

[desertdeals69]

This is my lithium battery install.  Used 1/4 x 1 inch copper bars.  These batteries have the same footprint as a golf cart battery.

[wildebill308]

I would protect the 1/4 x 1 inch copper bars with at least some shrink tape to prevent accidental arking or shorting across them.

Bill

[richard5933]

39 minutes ago, WILDEBILL308 said:

I would protect the 1/4 x 1 inch copper bars with at least some shrink tape to prevent accidental arking or shorting across them.

Bill

You could also take a length of heater hose and slice one side of it open lengthwise - then just slip it over the positive bus bar and hold in place with some zip ties. Would add a measure of safety. If that were mine, eventually I'd drop something on it that would cause some fireworks.