Things I’ve Learned About Solar Power

What follows are some things I’ve learned about solar power in mobile applications, meaning in a motorhome or some similar application. This is not an instructional on solar power but rather a collection of informational tidbits. That said, if you use or may wish to consider using solar power then knowing these things is important. Not knowing them may be likened to shooting at a target while blindfolded. Not only are you likely to miss the target but you may unintentionally cause damage or injury.

My comments are in regard to 12 volt systems using flooded lead acid batteries, the type I understand that is most commonly used in RVs. I use the terms battery, lead acid and flooded lead acid interchangeably. My remarks will be largely relevant to systems that use one or more 12 volt batteries wired in parallel, or two or more 6 volt batteries wired so as to form a 12 volt system. My comments will be mostly about batteries, charging batteries and the charge controllers used to do so and are written for those who know less than me which would mean both total newbies to solar as well as people who have been using solar, perhaps for years, but never bothered to really learn that much about it. I don’t know very much about solar panels themselves. I’m not sure they are as important in the equation as the charge controller or batteries. They may be, but since I don’t know much about them at the time of this writing I can’t write much about them. I’ve come across much more discussion about batteries, charging batteries and charge controllers than panels which may indicate that PV panels don’t require as much knowledge to purchase, maintain or use properly. Dunno.

When I bought my portable solar panel kit I really didn’t know anything about solar power. Now, over a year later, in large measure due to the generous help of HandyBob, I know quite a bit more. Much of what I have learned has to do with batteries and charging batteries. So, here are some things I have learned:

In a solar system, sometimes referred to as PV for photo-voltaic, the electricity generated by the solar panels is generally used to charge one or more batteries rather than powering electrical devices directly. When electricity is needed to run an appliance of some sort it is taken from the batteries, not directly from the solar panels.

There are different kinds of batteries within the lead acid family and within the flooded lead acid subgroup there are different kinds. For example, some batteries are meant for starting a vehicle’s engine. These need to deliver lots of power in short bursts. They are constructed in such a way so as to be able to do that. These are sometimes called Starting batteries. Other batteries are made to deliver lower levels of power over longer periods and to be more deeply discharged than Starting batteries. These are the batteries most typically used in RV solar systems and they may be called Deep Cycle, Marine, or Deep Cycle Marine batteries. There are differences in the construction of batteries based on their intended use and there are differences between brands of the same category of battery.

Batteries should be fully recharged as soon as possible after having been drained in order to maximize their useful life.

Batteries store electricity and the measure commonly used to express how much a battery can store is called ampere-hours, sometimes said amp-hours or abbreviated AH for ampere-hour or AHs for ampere-hours (plural). Simply put, if a load such as a lightbulb or appliance requires 1 amp to operate, then operating it for an hour will require and drain 1 AH of electricity from your batteries. If you let it run for two hours it will drain 2 AHs. If your battery can store 75 AHs of electricity then in theory you could run that lightbulb for 75 hours before your battery is dead, however, batteries should not be drained beyond half their AH storage capacity rating or you can drastically shorten their lives or damage them

Manufacturers and resellers of batteries use different methods to determine and advertise the AH capacities of their batteries and this must be understood in order to make an informed choice when selecting batteries. Using one method versus another can lead a consumer to purchase Battery One over Battery Two thinking he/she is getting more storage capacity from Battery One when in fact it’s the other way around. As it turns out, if you drain a battery more slowly, for example, under a load of 1 amp, you will get more AHs out of it than you will if you drain it at  a higher load such as 20 or 25 amps. A battery rated at 100 AH storage capacity when drained to dead at the rate of, say, 1 amp, may only have an 85 AH rating when drained to dead at a higher rate of, say, 20 amps. This is because you get less AHs out of a battery when drained at a higher load. Manufacturers that want to make their batteries look good test their batteries at lower loads. That way they can squeeze more AHs out of them and advertise them as having higher AH storage capacities than they would if drained at higher loads. When advertising material says, for example, that a battery has a 100 AH capacity, your first question should be, “at what rate of drain?” If one company advertises their battery as having a 100 AH capacity at C100 and another company advertises a battery as having an AH capacity of 100 at C20, buy the one with 100 AH capacity as measured at C20. C100 means the battery was drained to dead over a 100 hour period. C20 means it was drained to dead at a higher load in 20 hours. C20 is the more commonly accepted industry standard but some companies use other metrics such as C100 in order to make their batterise look better than they otherwise would. Again, the battery with a 100 AH capacity when drained to dead in 100 hours (C100) wouldn’t have as much storage capacity if drained to dead in 20 hours (C20). When comparing storage capacities of batteries always make sure you compare AHs of storage at the same C rating.

Information about batteries that is not available at point of sale can sometimes be found by going online or calling the manufacturer or reseller. This includes information about the AH ratings (some manufacturers will provide C100 and C20 ratings for example), and how to best charge the batteries. (More info in the Charge Controller section below.)

About 1/3 of all automative lead acid batteries are made by the same company, Johnson Controls, and sold under different brand names. Die-Hard, EverStart, Interstate and many others.

Different kinds or brands of batteries should be charged differently according to the maker’s specs. (More info in the Charge Controller section.)

RVs with built in battery chargers (separate from any solar charge controller) that charge the house batteries (the batteries that run the appliances as opposed to the one that starts the engine, if there is an engine) may not do such a good job. Information I’ve come across suggests they do not and that you may have to stay hooked up to shore power or run your generator for days in order to fully charge your batteries.

Batteries are temperature sensitive and should be charged differently at different ambient temperatures. Another aspect of temperature is that batteries function best at moderate ambient temperatures. If it is too cold they lose capacity. People who have gone outside to start their cars in very cold weather only to find their battery acting dead or almost dead have experienced this. Batteries can become damaged from excessive heat and the life of batteries operated at higher temperatures is shorter than those operated at more optimum temperatures.

The lead plates in a flooded lead acid battery should always be covered with electrolyte (the acid-water mixture inside the battery). If they aren’t they will become damaged. When the level gets low add distilled water, not more electrolyte. You have to remove the battery caps and look to check the levels in each cell. (Learn how to do this safely before attempting it.) I’ve seen visual indicators built into some battery caps that are supposed to change color when the electrolyte level is low. I don’t know how good these are, if they vary in accuracy or reliability from one to the next, etc. I wouldn’t rely on them unless I knew they were reliable. Batteries that need to have water added should be filled with distilled water after charging, not before. The exception is that if any portion of the plates is not submerged in electrolyte enough distilled water should be added to cover the plates plus a little before charging. Batteries should be filled at most to the bottom of the filler tube. Adding too much water dilutes the electrolyte and in hot weather it can be forced out of the batteries as things expand.

Battery charge requirements and charge controller settings may not be in sync. In fact they probably are not. (More info in the Charge Controller section below.)

Undercharging batteries reduces not only their performance but their life.

How a battery needs to be charged changes over its life as well as with ambient temperature.

Flooded lead acid batteries perform at their peak after a number of discharge/charge cycles, not when they are brand new.

Lead acid batteries should be stored in a fully charged state. Longer storage periods may require a trickle charger to keep them topped off, fully charged.

Buying a lead acid battery with the thought of saving it for use later on is not a good idea.

When lead acid batteries are used in groups they should be of like kind and condition.

Electrolyte is dangerous. It is a mixture of sulfuric acid and water. Always wear eye protection, rubber gloves and protective clothing or clothes you don’t care about because it will ruin them if splashed or spilled. Baking soda neutralizes electrolyte. Keep some readily at hand when working with batteries. You can make a paste by mixing some with water in a saucer to keep handy before you start working with your batteries. Do not get in your batteries as it will neutralize the electrolyte. Familiarize yourself on safety precautions and first aid procedures before working with lead acid batteries.

Batteries are dangerous because of all the electricity they may contain. Here too, familiarize yourself with precautions and procedures before working with them and things which may be connected to or disconnected from them. Hire a professional if you are not absolutely confident in your own abilities.

Sometimes batteries develop a moist looking appearance on them. This can be conductive just like a wire and can be dangerous. It should cleaned off, safely. Learn how to do this safely before attempting it.

The approximate state of charge of a battery can be determined with either a voltmeter or a hydrometer. A hydrometer will check each cell individually (12 volt batteries have 6 cells; 6 volt batteries have 3 cells). Using a voltmeter will not alert you to state of charge or condition of the individual cells and this can be important to know. Using a hydrometer requires removing the battery caps and drawing up electrolyte into it. This means dealing with battery acid and one should be educated about how to do so safely before attempting it. The process is simple enough in theory but can be made more difficult if, for example, the batteries are difficult to access or there are obstructions such as straps holding the batteries in place that first need to be removed in order to remove the caps, or wires attached to the batteries block access to the caps.

Some display panels in RVs that show the state of charge of batteries may be better than nothing, but rather than blindly relying on them it is probably much better to use other methods such as those described above. Some of them show the state of charge in very course increments such as 25%, 50%, 75%, 100%.

A battery must be “rested” in order to measure its state of charge with any reasonable degree of approximation. This means it must sit without being charged or drained for a period of time. This may not be so easy in an RV because some batteries always have a load on them–hard wired smoke and LP gas detectors and refrigerator control boards for example, even when the fridge is set to run on propane. Some say to rest a battery at least two hours, some say 24 hours before checking its state of charge. When checking battery state of charge with a voltmeter 12 volt batteries in good condition measure about 12.7 volts when fully charged and rested. Charts are available online that show the approximate state of charge at different voltages. These charts vary somewhat in the information provided. A battery that is being charged may have a voltage reading significantly higher than it will once it is rested. Conversely, a battery under load may have a voltage reading significantly lower that it will once it is rested.

More information about battery charging may be found in the Charge Controller section below.

Charge Controllers
My comments here relate to charge controllers typically used in RV solar systems. A charge controller sits between your solar panels and you batteries. It’s job, as its name implies, is to control how your batteries are charged.

Modern controllers typically have several stages or phases which may include: Bulk, Absorption, Float and Equalization. These are common names for these phases although other names are sometimes used. The reasons for different phases include charging batteries rapidly, fully and safely.

When a battery has had some of it’s ampere hours drained it is able to be charged more rapidly, meaning more ampere hours can be put back into it more quickly than when it is more fully charged such as at later stages of charging. The first stage of charging a battery is called Bulk and the charge controller is sending as many ampere hours to the battery as fast as it can. When more amperes are being sent to the batteries charging voltage is typically lower.

As a battery becomes more fully charged it develops resistance to being charged further and it requires a higher voltage to force the ampere hours in. This is called the Absorption phase. It typically begins when your batteries are in the neighborhood of 80% charged. In the Absorption phase the charge controller raises the voltage because it’s higher voltage that forces more ampere hours into the batteries. Absorption voltages are often in the 14.4 to 14.8 volt area, or should be. The proper voltage depends on the battery and the specs for that battery should be obtained from its maker. Battery age, condition as well as ambient temperature are also factors.

Once the batteries are fully or vey near fully charged the charge controller should switch to the Float stage. Here the idea is to keep the batteries at full charge without overcharging them. Batteries lose a little voltage even if there is no load on them but in many mobile applications there is always some load on them, for example LP (propane) gas, carbon monoxide and smoke detectors that are hard wired into the coach’s electrical system as opposed to being battery powered. Another example is the control board for refrigerators which uses electricity even if the fridge is running on propane. Float voltages are generally closer to 13 or 13.5 volts, again, depending on the specs for the particular battery.

Periodically some batteries may require Equalizing. Some charge controllers are programmed to do this once a month for an hour or two. During this phase a high voltage is sent to the batteries, sometimes in the neighborhood of 15.5 to 16 volts, with the intent to cause the electrolyte to gas-out, making little bubbles come to the surface like they do when you boil water. This stirs up the electrolyte which may become stratified over time–the sulfuric acid concentrating near the bottom of the cells. This is more of a concern in situations where a battery hasn’t been jostled around such as it is in an RV that is driven. Equalization may also help the individual cells level of charge become more on par with each other, more equal. The more equal in charge and alike in physical condition the individual cells are the better a battery will perform and the longer it will last.

While the voltage needed to charge a battery safely, quickly and fully–from here on I’ll refer to those things as one and call it “properly”–is dependent on the state of charge of the battery, other factors also effect how a battery should be charged and these include the battery’s age, condition and ambient temperature. It may not be possible to determine with certainty the age or condition of a battery, not easily anyway, but temperature is easy to know. As the ambient temperature gets warmer batteries should be charged at lower voltages and visa versa. Some charge controllers have the ability to adjust charging based on ambient temperature but in order to do this effectively they must be located somewhere that is the same ambient temperature as where the batteries are or have a temperature sensor that is attached to the battery via an electrical lead that goes to the controller wherever it is.

Charge controllers are best located as close to the batteries as safely possible and connected with wire heavy enough to keep voltage loss to a bare minimum. (Safety note:  locating a charge controller in the battery compartment may not be a good idea since batteries expel hydrogen gas at times which can be flammable or explosive under the right conditions and charge controllers may spark and ignite the hydrogen. The area where batteries are kept should be ventilated.) Wire offers resistance and causes losses. Heavier wire causes less loss. Shorter wire runs cause less loss. There are charts and online calculators available showing how much voltage is lost over a given distance with a given type and gauge (thickness) of wire. The longer the wire run between the controller and the batteries the more voltage will be lost between the two. If, for example, a charge controller is set to output 14.8 volts and there is a wire run of 30′ between the controller and the batteries the voltage may drop to 14 volts by the time it gets to the batteries due to resistance in the wire and the battery won’t get charged properly even though the controller thinks it’s doing its job. The controller won’t know about the voltage loss. It can’t tell. If, on the other hand, the controller is within a foot or so of the batteries the voltage loss between it an the batteries will be much less. There will still be voltage loss between the panels and the controller over a long wire run but typically panels are sending more voltage than needed to the controller. It’s what gets to the battery that’s important, not so much what gets to the controller. I also wrote about this in It’s 10 O’clock, Do You Know Where Your Solar Charge Controller Is? Similarly, the thinner the wire the more voltage is lost.

I’ve also written a User Report on the GoPower GP-PSK-120 portable solar panel kit.




6 thoughts on “Things I’ve Learned About Solar Power”

  1. Great tidbits about batteries Russ. On a separate note, when I turned my iPad horizontally (landscape orientation), I wasn’t able to find a link on the menu bar to this page (the drop-down menu didn’t show up). When I turned it in a portrait orientation, then I could find this page in the main menu.

    1. Glad my post was useful and thanks for the bug report! I see what you mean.

      I found that if you press and hold on a menu bar navigation item with a triangle next to it indicating that it’s a drop down menu, then wait for the browser to pop up its options about opening in a new link or new page, then tap outside of those options to dismiss them, that you can then tap on the menu item to reveal the drop down items.

      It shouldn’t be this way and I only discovered this by accident. The drop down menu should appear with a single click on it.

  2. Just a word about the C-rating on batteries, which you have understood totally wrong.
    C1 means the battery is discharged completely in 1 hour.
    C20 means the battery is discharged completely in 20 hours.

  3. Hi Russ,

    You have done a very good job in your explanations. Here are a few corrections to make you perfect!


    Correction: All power from the PV modules will power loads first and any left over will go to the batteries. If you consume more than what is being produced, that extra power comes from the battery. This rule applies to your battery charger as well.

    Quote: “…however, batteries should not be drained beyond half their AH storage capacity rating or you can drastically shorten their lives or damage them.

    Correction: This is a HUGE internet myth. ALL deep cycle batteries are designed to use 100% of the stored energy. Using less will lengthen battery cycle life but don’t be afraid to use it because of this myth. If you regularly deep discharge, I recommend 80%DoD as the maximum discharge level. Some battery companies have also recently changed their discharge recommendation from 100% to 80% maximum to minimize damaging the battery during recharge and provide longer cycle life.

    Quote: “…if you drain it at a higher load such as 20 or 25 amps per hour…, say, 1 amp per hour,…of, say, 20 amps per hour.”

    Correction: This is a common nonsense term used by well meaning people. There is no such thing as “amps per hour” just like you would never say my engine has 250 horsepower per hour. It’s Amps or Horsepower. Drop the “per hour”.


    Correction: Reverse that. The slower you discharge a battery, the greater the usable capacity. Peukerts Law describes this. (BTW, his name is pronounced “poy’-kurt”

    Quote: “In the Absorption phase the charge controller raises the voltage…”

    Correction: The bulk stage is unregulated voltage and current. The absorption or acceptance stage, aka CV stage, is when the controller regulates the battery voltage. Float stage is also a CV stage but regulates voltage at a lower setting.

    Quote: “…Charge controllers are best located as close to the batteries as safely possible…

    Correction: With PWM controllers it does not matter where they are placed in the series circuit as they are only switching voltage off and on (PWM) to control it. MPPT controllers likewise can be placed anywhere but keeping them close to the battery minimizes the controller to battery wire size.

    Helpfully yours,

    Starlight Solar Power Systems

    1. Thanks for all the corrections and information Larry. Much appreciated. I’ll incorporate them into the article ASAP. Meanwhile, did you get the email request I sent the other day asking for your assistance with my inverter installation?

      Where I wrote that electricity is taken from the batteries and not solar panels I think I was maybe half right. People with no knowledge of solar power setups often think that power flows directly from the panels to appliances and they are unaware that an integral part of RV solar setups is a battery or battery bank. My statement oversimplifies things but in essence communicates that appliances there are panels and batteries involved. You clarified by adding a fine point which is true during the day, but at night when no solar electricity is being generated then, as we both know, all electricity is drawn from the batteries.

      You quoted me as writing “…battery with a 100 AH capacity when drained to dead in 100 hours (C100) wouldn’t have as much storage capacity if drained to dead in 20 hours (C20).” In reply you offered “Correction: Reverse that. The slower you discharge a battery, the greater the usable capacity. Peukerts Law describes this.” I think you misread what I had written which agrees with the correction you offered.

What do you think? Leave a Reply


You are commenting using your account. Log Out /  Change )

Google photo


Twitter picture


Facebook photo


Connecting to %s

RVing America

HandyBob's Blog

Making off grid RV electrical systems work

Wheeling It


Live Laugh RV

Our Next Chapter

%d bloggers like this: