the problem[s] with 1:1 solar charging

this is a draft

1:1 rule of thumb

There is a rule of thumb that says a 1:1 ratio is the minimum amount of solar needed to charge a deep cycle bank.  The long form is 1w:1ah, which means 1 watt of solar panel for every amp-hour of lead chemistry deep cycle battery capacity under good insolation.  So if you had 150w of panel and 100ah of battery it would be 150w:100ah, or 1.5-to-1.  Yes, I know the case on the unit abbreviations is wrong but it’s easier to read.

As with all rules of thumb, it serves as a way to “sanity test” a flooded bank configuration.  If it’s not at least 1:1 then the solar is obviously insufficient as a sole charging source.  Many of use have been uneasy with the rule of thumb, fearing it was hiding problems.  I think I’ve figured out what those problems are.

Note:  although the ratio refers to Ah, for clarity battery capacity will be expressed in Wh below,  using 12.5v as a constant.

hours in the day

Claim 1.  there aren’t enough hours of Full Sun Equivalent (FSE)  anywhere in the U.S. at any time to fully charge

  • lead deep-cycle batteries
  • drained to to 50% depth of discharge
  • using 1:1 setup
  • flat-mounted (non-tilted) panels.

To make things simple, we will use a 100w:100Ah lead configuration, assume excellent conditions and MPPT controller.

After considering ~15% charging inefficiencies, it will take 718.75Wh to fully charge the battery the next day.    Under near-perfect conditions (5% dc-dc losses, 10% temperature derating) an mppt controller will extract 85w from the panel.   It will take 8.46 hours of FSE (718.76Wh/85W) to replace those amps.  There is nowhere in the US with that kind of sun, not even Las Vegas in the summer (7.69hrs), which is an extreme outlier.

Note that FSE is a measurement of solar harvest, not literal charging time.

So what do we do?


The most obvious solution would be to add additional panel to make the 718.75W in one’s location, and we will discuss that in a bit.

But what if we are stubborn and want to make 1:1 work?

cycle less deeply

Cycling to only 75% DoD halves the amount of Amps that have to be replaced, which halves the hours of FSE required.   Are there places in the U.S. with 4.225 hours of FSE?  Sure, most places during the summer, and a bunch of places average that across the year (Little Rock, San Francisco, Phoenix, Birmingham, Atlanta, New Orleans, Albuquerque, Tulsa, Las Vegas, San Antonio, Houston, Salt Lake City, and Casper Wyoming).

tilt the panels

FSE is expressed in terms of a flat area.  Carefully and consistently aligning the panels throughout the day yields ~30% more power from the panels in locations/times when the sun is hitting the panel at an oblique angle.

Example:  tilt can bring FSE in the Seattle winter up from 0.9hrs of FSE to 1.17, a meaningful change.  It can’t bring Vegas in summer from 7.69 to 9.99hrs.

add in another form of charging

Alternator is the most likely source for off-grid folks in vans.  Generator is likely for off-grid cabin folk, and shore charging is possible for folks who weekend and can boost the bank just before leaving the house.

Alternator charging will be difficult to quantify without a coulomb counting.  We do know it can provide massive amounts of current for the dollar.

There are several factors at play:

  • battery chemistry / internal resistance – lithium is capable of accepting power >=  1C.  AGM accepts up to ~C/3.  Flooded p to C/5.
  • different alternator charging methods will charge at different voltages.  Isolators will charge at alternator voltage (13.8v for the purposes of this discussion).  DC-DC chargers will provide Absorption voltage (Vabs, 14.4v for the purposes of this discussion)
  • Isolators pass current in huge amounts, often maxxing out the ability of the battery bank to accept it.  DC-DC charger output is dictated by the electrical components inside.  Common outputs are 20A-50A with the most popular bing 20A.   Note that isolator and DC-DC charging can be paralleled, as CTEK does with the SmartPass product.
  • charging approaches vary;  a commuter might be driving for 30 minutes.  A road-tripper might drive for 3 hours.  A stationary camper might idle for 5 minutes in the morning.
  • in a combined solar-alternator charing system, we are not concerned with fully charging the bank from the alternator — we only want to get a solid contribution from the alternator so  our solar can finish the job.

Luckily for us, in Bulk charge lead batts charge at almost 100% efficiency.   Alternator charging the battery for 30mins in the morning (driving or idling) will put about 162.5w into AGM or 106.25w into flooded batts due to their different resistances.

  • AGM now requires  6.54 hours of FSE instead of the unassisted 8.46 hours.  (718.75w – 162.5w)/85w)
  • Flooded now requires 7.20 hours of FSE instead of the unassisted 8.46 hours.  (718.75w – 106.25w)/85w)

2:1 overpaneling

Doubling our panels to 200w:100Ah drops hours of FSE required to 4.23, the same as halving our cycling to 75.

Let’s do the alternator charging math again.  The alternator puts the same power out.

  • AGM now requires  a reasonable 3.27 hours of FSE instead of the unassisted 4.23 hours.  (718.75w – 162.5w)/170w)
  • Flooded now requires 3.4 hours of FSE instead of the unassisted 4.23 hours.  (718.75w – 106.25w)/170w).  This might be counterintuitive, but flooded accepts less current from the alternator so solar has to carry more of the load.

3:1 serious overpaneling

Tripling our panels to 300w:100Ah drops to 2.82 hours of FSE required, remembering that actual charging time will be more than that.

  • AGM now requires  2.18 hours of FSE instead of the unassisted 2.82 hours.  (718.75w – 162.5w)/255w)
  • Flooded now requires 2.4 hours of FSE instead of the unassisted 2.82 hours.  (718.75w – 106.25w)/255w)


Battery health

Claim 2: 1:1 charging is insufficient to meet minimum charge rates for lead batteries.

Flooded batteries require a minimum of C/10 bulk charge rate to stay healthy.  To do this with an mppt charger requires a 1.25-to-1 ratio. PWM requires 1.5-to-1.

AGM requires a minimum of C/5, twice what flooded needs.  To do this with an mppt charger requires a 2.5-to-1 ratio. PWM requires 3-to-1.


My own system is 570w:220Ah, or 2.59-to-1 charging a bank of flooded golf cart batts.

Since I camp in two-week outings, the alternator is rarely spinning.  Solar-only charging does a great job charging.

I typically discharge to 12.3v-12.4 by morning (60-70% state of charge).  I hit 50% state of charge if I run an electric blanket all night or similar, or if I play Minecraft too late.  🙂

  • On normal days absorption is complete by ~noon.
  • In challenging insolation like today (light rain, full overcast), it will be late afternoon before it’s complete.
  • under terrible insolation conditions (snow, thunderstorms) it may only reach float voltage without alternator assistance


I think the 1:1 minimum stands as a sanity check for flooded batteries charged under excellent insolation/FSE, or batteries partially cycled.

2:1 would be a safer default for those running normally cycled (50% DoD) flooded, or for running AGM with alternator assistance.

3:1 would be a good default for those running AGM without alternator charging.

In tabular and more granular form:

W:Ah ratio flooded / MPPT flooded / PWM AGM / MPPT AGM/PWM
good insolation
(Phoenix, AZ)
1.25 1.5 2.5 3
average insolation
(Salt Lake City, UT)
2.5 3 5 6
poor insolation
(Seattle, WA)
3.75 4.5 7.5 9


idiot permanence

Object permanence is the understanding that objects continue to exist even when they cannot be seen, heard, touched, smelled or sensed in any way. — wikipedia

I propose idiot permanance, which is the understanding that idiots and jerks continue to be idiots and jerks even when they are out of your direct observation.  That swervedriving guy who just cut you off in traffic with no blinker?  Probably not the first time he’s done that today, or even in the last 5 minutes.

Continue reading “idiot permanence”

laptop cooling update

The thermal paste fix has made a huge difference.  I played Minecraft in 92F ambient the other day (hottest day in Twin Springs just before I bailed).  No issues.

Normally I use the chromebook because of the low power consumption, but I’m getting such good sun here in Boise NF that I am running the laptop this morning.  This means I can do some extra stuff:  Continue reading “laptop cooling update”

field mouse

I picked up a field mouse in the Twin Springs area.  I saw some of them running around on the ground.   About the size of a domestic mouse, but more intelligent-acting and with a furred, tuft-ended tail like a gerbil.

not actual sizeThe easiest way in would be to jump about 10″ up to the stepstool, then another 10″ into the van.  Good job, buddy, but ya gotta go.

Walmart didn’t have the normal metal “cheese pedal” ones like this:

and only had this bit of heresy:


The holding mechanism is iffy at best.  One could not be set at all.  The second one in the package could be set with only a few false positive snaps.

I placed the trap and caught the mouse within an hour.

BN:  I’ve used this style before in rat traps and they worked fine.  Never used the mouse sized one:

Twin Falls, ID

It was scheduled to rain last night, so I gathered up the campsite.  Since I was planning to head out in the next couple days anyhow, I topped off water from the campground and headed out.

The historic bridge as one crosses the Snake River and enters the city:

Stopped at Wally World for provisions.   Sunday morning, which is Continue reading “Twin Falls, ID”

a walk to the west

I’ve been wanting to walk up the hill to the west but an attentive bull has been hanging around the last several days, trolling back and forth on this side of the hill, vocalizing softly but constantly.   First day he appeared he looked straight at me and now he’s been hanging around.

Today he wasn’t there so I grabbed the daypack, the dog, and the Multipurpose Walking Stick and headed out.

The other day when I was looking on Google Maps I saw what appeared to be irregularly-shaped denuded spaces but thought it was an artifact of image stitching.  But I saw today they are really there:

looking south

digital zoom of the above: Continue reading “a walk to the west”

chirp, cranking up the Baofeng UV-5R

When I was reorganizing the van the other day I found the little dual-band transceiver and the charger.  The charger was DOA but the HT was fully charged.  It is the only piece of ham gear I own now after downsizing into the van.

I ordered a replacement charger off Amazon.  Because of the weird economics of these things, it would only cost $10 more to buy an entire replacement radio with charger, but I have limited spac.  I wanted a USB version anyhow.   That package will meet me in Twin Falls, ID, mid-week.

In the meantime, Continue reading “chirp, cranking up the Baofeng UV-5R”