Adding golf cart batteries.

[DaveVA78Chieftain]

A piece of the puzzle not clearly presented:

For solar you need the higher voltage in order to squeeze as much energy as you can out of the limited time the sunlight is available (roughly 5 hrs/day).  You pay for that extra punch through increased gassing of the battery.  Just means you have to keep an eye on battery water level.

The battery will still get charged at the lower voltages however it takes many more hours of charging to do that.  The boost stage (14.4 or 14.6) brings the SOC up to around 80-90%.  Current converter/charger design reduces the voltage to 13.6 for absorption stage so that gassing is almost nil.  The trade off is it may take up to 12 more hours of charging to finish off (top off) that last 10-20% of SOC.  This converter design compromise keeps the consumer happy by not creating battery dry out problems.  As long as your plugged into shore power, like the majority of RVr's are, it is a none issue.  If you are trying to use a generator for restoring battery power, then a battery charger maybe needed so you shorten the charge time.

Solar controllers like Morningstar Tristar keep the the absorption charging voltage at the stage trigger voltage (14.8 in this case rather than the converters 13.6).  This extra punch (fast charge) during absorption stage is how they decrease the time but like I said, the trade off is you have to watch battery water levels more frequently.  It is during the absorption stage that battery gassing is most prevalent (14.4VDC and above).  The battery is near full and is resisting the desire to continue charging which is evident by the increase in temperature (gassing).  The nearer you get to 100% SOC, the more resistance resulting in more heat created so, during the absorption stage the charger reduces the amount of current while maintaining the charging voltage (13.6 or 14.8 as appropriate).  This current reduction helps keep gassing under control and also prevents battery damage.

You bought a 100amp converter.  During the bulk stage lots of amperage can be pushed to your large battery bank fairly fast.  During absorption stage (from 80% to 100% SOC) you still can push a lot more most of time during that stage than a typical solar array can.  A 400Watt solar array (three 135 Watt panels) could only push a max of 30 amps (more like 20-25 due to all the loss factors).   So, you have to squeeze all the energy you can out of the solar array in order to maximize the short amount of time (roughly 5 hours) of effective sunlight.

On the next to the last page of John Mayers 2011 pitch (last web site I provided) he says one of the first things to do is get a battery monitor such as a Trimetric.   That device lets you see energy going into and out of the battery from the Amp-Hour perspective (SOC).  You will be able to visualize the rate of change (charge/discharge) with it.    So, from there you start to learn the effects of your lifestyle and if necessary, adjust your lifestyle (or system capabilities) to maximize your investment.

I hope that clears up some of the mystery and con fussion.  It is a matter of trade offs: time [slow/fast charge] vs minimum main tenace [to gas  or not to gas ]   Hm?

The MPPT vs PWM controller:  A MPPT controller can use a higher voltage array panel which can mean smaller gauge wire runs.  You could also wire two 12 volt panels in series (24 volts).  Array panels are like batteries.  Each cell produces approximately 0.5 volts.  Like two 6 volt batteries in series make 12VDC, you wire 36 solar 0.5VDC cells together in series to make a panel that produces approx 18VDC.  Even though it produces around 17-18VDC it is still reffer ed to as a 12VDC panel.  You have to have a higher voltage in order to charge a 12VDC system.  Anyway, a 12VDC PWM controller would use 12VDC panels (could be 2 or more wired in parallel).  A MPPT could use a series-parallel (multiple series pairs wired in parallel) or single higher voltage panels in parallel up to the maximum amperage of the controller.  Leave a amperage buffer for controller selection (i.e a 30 amp controller connected to 25 amps of panels).

One aspect of battery management is depth of discharge.  For one, you should not discharge a deep cycle battery below 50% SOC.  Discharging to less than 50% SOC on a regular basis will drastically shorten the life of a deep cycle battery.  Form a potential of 6- 8 years of life to 1-3 years.   If notice in Handy Bob's article he typically runs in the 25-30% / day range (70-75% SOC)  This means he only has to restore around 25-30% of energy / day.  The smaller the amount of energy used = less time it takes to recharge the battery.  However, the time to recharge a battery is 125% of the time to discharge.  So, if it took 2 hrs to discharge it will take 2 1/2 hours to recharge at the same rate (20 amps charge rate vs 20 amps discharge rate).  However, if discharge at 30 amps for 2 hours (2 x 30 = 60AH) and recharge at 20 then it will take 3 1/2 hours to recharge (3 hrs x 20 = 60AH x 1.25 = 3.75Hrs).  As you can see from this simple equation, in the space of 5hrs, you have to squeeze out all the solar energy you can.  And then, which is something we tend to forget about, that 20 amp charging value is reduced by any energy your are using during the charging cycle (aww geeze, I wanna run my laptop to yak on CWVRV today).  So the time to charge gets longer because the max charging amperage available is only 19 amps (1 amp for laptop).

So, in simple terms:
Lower charging voltages (13.6 absorption stage) = longer time to complete recharge but greatly reduced gassing issues

Lower charging amperage's = longer time to recharge

Higher charging voltages (14.1 to 14.8 [L-16 battery up to 15.5]) = faster charging but produces gassing above 14.4 [don't forget possible corrosion effects also; connectors, cables, battery tray, etc.].  An automotive alternator runs at 14.1VDC in order to a.) charge the battery and b.) ensure system voltage as seen by the components (radio, ignition, etc.) is close to 12VDC (negates cable/connector loses as well as loading of high current devices [heater/wiper motor]).

Higher charging amperage (max of 25% bulk rate [220AH battery * .25 = 55Amps]) = faster charging but can damage battery if to high.

13.2VDC Charging value (float) = counteracts the self discharge value of a normal wet cell battery (a balance of energy in = energy out)

>15VDC Sulfation charge = Voltage needed to break down crystals and boil electrolyte in order to mix (de-stratify) the acid/distilled water mixture.

Charging Voltage/At rest Voltage/Specific Gravity readings are affected by temperature.  Baseline voltages are typically referenced to either 77 or 80 degrees F.  The colder a battery is, the higher the voltage has to be in order to charge it (14.8 can increase above 15VDC).  The higher the battery temp is, the lower the charging voltage has to be (14.8 may drop to around 14.4 at 110 degrees F).

Dave

[ibdilbert01]

In the past few years I've tested a ton of charge controllers by various major and not so major companies.  I have to say my favorite is the one found on this site.   

http://www.roguepowertech.com/

I actually tested the first generation of this product, and will buy the new model before springtime.  In the pic below I had it connected to two US Solar 64 panels, it was one of those days where it was a mix of sunshine and sprinkles, and I was still getting a good 5+ amps of charge. 



Next in line would be any of the Xantrex controllers, I honestly don't think you can kill them if you tried.

[DaveVA78Chieftain]

I been wondering when the real resident solar guru would jump in!   

I sure hope my ramblings have been true and correct.

Please note in that picture that the input amperage from the array is 4.3 amps.  The output amperage (to battery) is 5.2 amps.  This clearly shows what advantage MPPT technolgy provides.  It's wrapped up in DC to DC converter technolgy which I will not get into.  Just wanted to point out what I mean by squeezing the energy out of the array.
Also, it is running at 13.2VDC which, while I do not know what he has it programmed for, is typically a float mode voltage (batteries at 100% SOC).

Dave

[ClydesdaleKevin]

Thanks again Dave, and Dilbert too!  I'm going to start with a really good battery usage monitor, which I'll purchase and install once the AZ renfaire starts.  And then we'll go from there. 

For our purposes, I don't foresee the need for an inverter/charger, since the small one we have runs the TV and Patti's blow dryer and curling iron and whatnot.  If we have to run the AC or the microwave, I turn on the genny for those short times, and barely use any gas.  Since the genny will have to run a couple of hours a day anyway to charge the battery bank, the laptops and phone and tablet can be charged up during those times, or off the inverter as well in a pinch...and of course the phone and tablet can be charged off the cigarette lighter port in a pinch as well.

But as far as solar goes, we are definately going to invest in the monitor first.  That way we have almost a year to really determine our power usage, and get just what we need to do the solar right. 

Thanks again guys!

Kev

[ibdilbert01]

Dave, your knowledge and expertise is well above my head and I'd trust your ramblings over mine any day of the week!   

Kevin, your on the right track, if you want to do it right the first time, research is the key word.   

[DaveVA78Chieftain]

Thanks Tim however, many times a lot of what I say is theoritical but at least meets the common sense test.  If you have been testing charge controllers then that practical experience is invaluable.  That is the only way quality of material, workmanship, and overall ease of use can be measured.  Add in the 'gut feel" and it becomes priceless.  I am failrly certain you know more than your letting on!   

Dave

[ClydesdaleKevin]

The new Powermax Boondocker converter arrived today since the one we bought in November went bad, and it was only a 4 day turnaround!  Thanks Randy at BestConverters!

Its all reinstalled and working flawlessly.  Apparently I bought the only one that EVER went bad, so hopefully I'll have better luck with this one...lol!

So, to make sure it wasn't user error, I won't be charging the batteries again with an external charger AND the converter while boondocking.  If the genny is running, I'll let the converter do the charging, period.  A 100 amp charger pushing out 14.8 volts peak outperforms my external charger with a max output of 14.6 volts anyway, and I don't want to damage the new converter, period.

If I absolutely have to charge my batteries with the external charger, I'll turn off the breaker to the power supply that goes to the converter...but they cancel each other out, so its a WAY better idea to use just the converter to charge the battery bank.  The external charger is just a backup for emergencies.

I am now once again reading a nice 13.2 volts.  SWEET!

Kev