High Load interferes Drops Charging Voltage?

[DLTooley]

I did a test of running my new compressor fridge during solar daylight hours only in lieu of upgrading my battery bank immediately.

It didn't work, but not for the reasons I was expecting.  I've worked through a number of options based on moderate quality data.

My best guess is at this point is that the load, 2/3 of the upgraded panel, is dropping the voltage enough to prevent recharging.

Does that make sense?

 

[John61CT]

Volts should not be the problem if the charge sources are working properly.

You need more Power (watts, amps*volts) in order to charge the bank.

Don't run big loads until the bank is up above 85-90%, still need 3-5 hours of charging, but at that late Absorb stage the batt is only accepting low amps, so you have some to spare for these other loads.

But bottom line you need more power, with a 24x7 fridge plus other significant loads, I say 300+w. Others make do with 200, even less but depends on weather, tilting the panels etc.

Or maybe a little genny to be less dependent on weather.

Duplicate thread?

 

[jonyjoe303]

I have an ampmeter that monitors how much amps the battery is getting, if the panel is putting out 10 amps, and my loads are using 6 amps, the meter shows only 4 amps going to the battery. 
The panel will always put out the max that is required in whatever mode it is in, and will compensate by putting out more if it is float mode.

In your situation your panel isn't putting out enough amps to run your loads, the battery has to make up the difference, so instead of charging your battery you are actually discharging your battery. This situation happens to me in cloudy weather.

 

[John61CT]

I have an ampmeter that monitors how much amps the battery is getting

Ammeter, in this case also a shunt-based battery SoC monitor would be ideal.

The panel will always put out the max that is required in whatever mode it is in, and will compensate by putting out more if it is float mode

No, float means very low amps as well as lower volts, should only happen when bank is full.

Up to the controller how well it deals with heavy amp loads coming online, but you're right if greater than panel output the bank starts getting discharged, will usually trigger a new Absorb cycle next time current is available for charging.

 

[frater secessus]

I think jj meant that if you are floating and add loads then the controller will increase power (if available) in order to hold Vfloat. To a human observer the controller appears to step up power to run the load.

 

[John61CT]

OK.

The way I would put it:

The regulator is making current available at a certain voltage according to its configured algorithms.

Amps are determined as drawn by the loads, up to the power supply's current limit, above which point loads are pulling from the battery.

The higher the load, the more the "combined" voltage drops, until a new Bulk/Absorb cycle is triggered by the regulator.

But to the extent amps are drawn to the other loads, very little may be actually available to be accepted as charge to the bank.

 

[DLTooley]

Load is definitely less than the panel output, 2/3 exactly. A 100 watt panel with a 66 watt load. I think the question is whether my particular controller will maintain voltage under load or uses a non-direct estimate of same.

It looks like it is time to get an ammeter.

 

[John61CT]

Yes, never rely on measurements given by charge sources without independent verification.

Best way to monitor your bank getting to full is watching amps drop at the tail end of the Absorb cycle.

Also accurate DMM for voltages

Adjust the charge source settings until you see it doesn't drop to Float until current at Absorb is .005 - .010C, or .5-1A per 100AH.

And check for V drop across connections and over long distances.

 

[John61CT]

Load is definitely less than the panel output, 2/3 exactly. A 100 watt panel with a 66 watt load.

We usually talk in amps, but NP

33w left for charging is not much more than 2A, barely a trickle charge.

Just for some perspective:

For most House bank sizes, 20-30A is a small shore charger, many better served by 100+A.

Fridges and compressors are much higher than most van House loads.

But with large banks and inverters, some might run induction cookers and microwaves for short times.

 

[John61CT]

Load is definitely less than the panel output, 2/3 exactly. A 100 watt panel with a 66 watt load.

Note that most days you'd be lucky to get more than 50-60 actual output from a "100w" panel, and that only for a couple hours.

Depends on latitude, panel temp and cleanliness, weather, tilting or not, any shade even a twig, etc.

Actual measurements are key, don't rely on specs.

 

[tx2sturgis]

Load is definitely less than the panel output, 2/3 exactly.  A 100 watt panel with a 66 watt load.  I think the question is whether my particular controller will maintain voltage under load or uses a non-direct estimate of same.

100 watt panels only put out that level of power for a few hours a day if they are static roof mounted. If you can move them to track the sun, then you get more out of them for longer.

But lets say in mid-summer, with static panels, you are getting about 100 watts of power for about 4 hours...the rest of the sunlit hours the panel is derated by as much as half or more, average.

So if that 66 watts (2/3 of 100) is only for a few hours when the panel is at max power, then your '2/3' ratio is way off for the other 12-14 hours of sunlight. And even a thin layer of dust reduces the output even more.

As I said in the other thread...you don't have enough total solar input for your loads.

 

[John61CT]

A shunt-based amp-counting SoC Battery Monitor would show that "big picture" very clearly.

 

[DLTooley]

The panel is mounted on the hood, which gives it a slight angle. I generally park facing approximately SW, into the prevailing wind. I perhaps should have gone with the extra expense of monocrystalline given the limited space.

I did install a voltmeter at the panel, which I can read from inside the vehicle. I have been at altitude which boosts the voltage. I am commonly getting numbers around 20 volts, more than 110 watts. When driving it approaches 22v. Shady/cloudy and dusk conditions reduce the voltage, but even at 13v the panel should be generating near 75 watts. Am I misinterpreting this - it's just volts times the amp rating of the panel?

I definitely need to check for voltage drop on my cabling. The controller is PWM and I don't believe adjustable.

I don't need to run a microwave, my needs are modest.

 

[tx2sturgis]

Am I misinterpreting this - it's just volts times the amp rating of the panel?

Nope.

Power ratings on panels are given for max volts times max amps...but panels in the real world can rarely or never operate that way.

Max power actually produced is always less than the rating of the panel.

 

[tx2sturgis]

I cut and pasted this here, from a solar website...if you get an ammeter inline along with a voltmeter and THEN calculate the actual power being produced THEN you will probably have the right numbers.


"It would be nice and simple if you could just use Equation 1 to multiply the rated power Wmp (80 watts in this example) of a solar panel by the total number of hours of sunlight to calculate the energy that panel can produce in a day. But, you can’t, and here’s why:

1)First of all, the solar panel ratings are not really correct. The power output of a solar panel is highest when the panel is not powering anything, but that is not really such a useful number, is it? When the panel is hooked up to a load, the voltage drops as it powers the load, and the total power output of the panel is less than the rated power. Unfortunately, solar panels are rated by the power they produce while they are not powering anything. The quick solution to this problem is to use the rated amps (Imp) and ignore the Wmp or Max rated power (like the 80 Watts in our example). This is because your typical charging voltage is more like 13V. 13V X 4.7A = 61.1 watts, not 80 watts).

2)The second reason is that the power produced by a solar panel depends on how powerful the sunlight is, and the power of the sunlight changes throughout the day from morning to night, reaching a peak at noon. It also changes with the weather, as well as through the seasons, and is different at different locations on Earth at any given moment.

In order to produce consistent ratings for solar panels, the solar panels are tested under what is called a Peak Sun. A Peak Sun is defined as 1000 watts of sunlight per square meter of surface, and is approximately equal to the power of the sun at noon on a clear day at the equator.

For the rest of us, the rest of the time, the sun is less powerful than 1000 watts per square meter.

{snip}


Imp * Peak Sun Hours/day * 13V = watt-hours/day

In our 80 Watt solar panel example:
4.7A * (5 hours as example) * 13V (estimate) = 305 watt-hours/day."


http://store.sundancesolar.com/how-much-energy-can-a-solar-panel-produce-in-a-day-1/

 

[frater secessus]

Nope.
Power ratings on panels are given for max volts times max amps...but panels in the real world can rarely or never operate that way.

To be clear:

Vmp * Imp <-- volts x amps at maximum power point
not
Voc * Isc <- max volts x max amps

Max power actually produced is always less than the rating of the panel.

High altitudes, cloud edge effect, etc can result in overperformance. But, yes. In general we derate the panels' lab rating.

 

[John61CT]

And size for a stretch of hot cloudy weather, not crisp clear days.

I like 200w per 100AH.

Basic rule of thumb for mobile, as many watts as you can get to fit. . .

 

[HumbleBeginnings]

And size for a stretch of hot cloudy weather,  not crisp clear days.

I like 200w per 100AH.

Basic rule of thumb for mobile,  as many watts as you can get to fit. . .

For this rule of thumb, is that 200w of solar per 100AH of total battery installed (meaning no more than 50AH of usage)?

Or 200w of solar per 100AH of potential usage (meaning most likely 200AH of battery installed)?

 

[John61CT]

installed

so using LFP would be 350+w per 100AH

Actually, since LFP's crazy low resistance means it would accept solar's full amps output right up to full rather than lead's "long tail", I guess I'd lower the ratio a bit, maybe 250?

And note others find 1:1 OK, I prefer as I said targeting bad weather, and put a higher value on getting to 100% full as often as possible.

You can always divert "excess" power in the afternoons to HWS or an icemaker. . .

 

[tx2sturgis]

High altitudes, cloud edge effect, etc can result in overperformance.  But, yes.  In general we derate the panels' lab rating.

lol...well yeah, if weird stuff happens like using a giant fresnel lens or mirrors, 14,000 feet altitudes,  or mounting them on a space ship...but I DID say I was talking about the 'real world'...meaning normal conditions for most of us...