backchannel: Solar Systems Charged and Challenged

from this article in Truck Camper magazine.

What these folks don’t realize is that many RVers don’t need a solar panel system and would be better advised to upgrade their battery banks.

No warning bells, but wakes me wonder how he’s proposing to fully charge lead batts offgrid without solar.

Is a solar panel system a cost effective option for truck camping?....the answer in most cases is no

Solar is expensive per watt, but if it’s the only effective solution to a particular problem then cost effectiveness is not paramount.

All too often, truck camper owners have not realized the full potential of their existing battery banks.  They overlook the simple upgrade of bigger and better batteries, and aren’t properly charging, monitoring, and maintaining the battery bank they currently have.

Good, at least we are talking about proper charging.

solar power is worthless without a corresponding battery bank that can take full advantage of the power that the panel system can generate.

Incorrect.  Panels can run loads directly.  Also, panel installs are typically undersized and cannot charge at the minimum current required by the battery manufacturer.

Anything more than your battery bank can handle will likely be wasted and/or diverted.

Excess panel, to the extent that such a thing exists, is not wasted.  It is insurance against poor insolation.  It is power for proper charging.  It is power for running opportunity loads.

For example, a camper with two Group 27 or Group 31 batteries will produce between 100 and 240 AmpH battery reserve.

100 and 240 Ah of capacity.

Of that reserve, approximately 50-55% is available to use before the batteries will need to be recharged to maintain their full battery health.  To compliment this capacity, 200-watts of solar is needed to satisfy a typical 20-40 Amp-Hour daily need

First of all, if discharged to 50% DoD we are talking about replacing 50-120Ah, not 20-40h, and we aren’t even considering charging inefficiencies.

Second of all, here are the minimum panel sizes for charging the batteries for those capacities, and the minimum required panel wattage to meet it in excellent insolation and a very generous assumption of 6 hrs of full sun equivalent (FSE), and 15%

  • 100Ah -  replacing 57.5A with 6 hrs of FSE requires 137.8w of panel.

  • 240Ah  - replacing 138Ah with 6 hrs of FSE requires 330.6w of panel.

Third of all, here are the minimum charging rates for those capacities.  Hopefully a truck camper will be meeting this with a generator (alternator assistance is less common in truck camping).  Assuming 85% yield from the panels with MPPT (again, very generous):

  • 100Ah -  10A min - 143.8w

  • 100Ah AGM - 20A min - 287.5w

  • 240Ah flooded - 24A min - 345.1w

  • 240Ah AGM - 48A min - 690.2w

anything more than 200-watts of solar would be unnecessary, under the right conditions

Unfortunately, humans don’t get much input into whether or not there are “right conditions”.

The amount of solar that works for one truck camper owner may not be the amount of solar that will work for you


.  To determine how much solar power you will need, you need to survey the amount of power, measured in Amp Hours, you use while truck camping.

Battery chemistry matters, as does historical data about average insolation.

Like a floodgate, a charge controller will divert excess power from the solar panels to prevent over charging.

Solar charge controllers don’t work that way.  MPPT controllers move Vpanel up near Voc to produce the correct amount of power.  PWM turn the panel on/off very quickly to regulate the correct amount of power.

Solar panels, cable insulation, charge controllers, battery monitors, and batteries all have an expiration date

No, they don’t.  The electronic components have a failure rate and batteries have a duty cycle life.

From the minute you start using solar panels and batteries, they begin to slowly degrade over time.

No, they don’t.  Panel degradation is not a function of use, and correctly maintained batteries will last longer than batteries that just sit on a shelf.

Maximize Your Battery Size

only if

  1.  you need the capacity

  2. and can charge the batteries correctly

Consider Upgrading to AGM Batteries

AGM is not an upgrade, IMO.  Yes, they are more vibration resistant.

Completely Charge Your Batteries


It’s just as important to fully charge a battery bank when recharging.  This means recharging to 90-100%.

No, it means charging to battery manufacturer recommendations, typically Vabs until acceptance falls to C/200 to C/100.

A few hours of driving is often all that’s needed to completely recharge a battery bank from the previous day’s power usage.

Isolator charging typically fails both requirements of full charging.  If a person had a DC-DC charger and was  driving 6+ hours, then yes.

...perhaps you’ve run your generator for hours to top off your depleted batteries.  It’s actually more efficient and more effective to charge a battery for 1.5 hours, allow the battery to rest for 1.5 hours, and then charge them again for 1.5 hours.

No, it’s actually more efficient to Bulk charge with the generator shut it off and let the properly-sized solar config handle Absorption and Float.

Surface charge occurs when the surface of the lead plates inside the battery become fully charged, slowing the rate of charge deeper into the battery plates.

This is a misunderstanding.  Lead batteries accept less and less current under constant voltage (CV) charging.    It is not a function of surface charge.