Solar power charging

[INA]

Newbie question here. We have 180 watts of solar installed. We started the day at about a 75% state Of charge. We spent about 6 hours traveling in the Florida sun and ended the day at 85% charge. There was nothing much on in the trailer except the fridge. Inverter was off. I’m looking for suggestions/recommendations.

[BayouBiker]

So 6hr in the sun times 160 Watts divided by 14.4 volts = 67 Ah but let's subtract for average sun angle 25% loss and battery and wire resistance losses 15% so about 43 Ah. Now I'll assume you traveled with 12V power on so subtract another 2 amps times 6 hours or 12 Ah and we are down to 31 Ah. You have 200-240 Ah capacity so the 10% is say 22 Ah that leaves 9 Ah unaccounted....

Its not too far off but not great either. Maybe I forgot to subtract for something.

[AirMiles]

I always use three times solar panel wattage as an estimate of daily watt hours on a good solar day. In this case its 3 x 180W = 540Wh. Then I divide this 540Wh figure by 13.5V, which I assume is the average voltage while charging. This 540W / 13.5V = 40Ah of battery charging. Then we need to reduce this by the 2A draw of an Airstream times six hours, or 12Ah. So the 40Ah of battery charging gets reduced by 12Ah which was used and therefore 28Ah would be actually added to the batteries. I would say 28Ah should add more than 10% to your battery charge state, but then you may have had some cloud conditions that would affect charging. In addition to this, the tow vehicle should have added some Amps to the battery, but my experience is that I'm lucky to get mid-single digit Amps from the tow vehicle. Even if that was 7A, that would have been an additional 42Ah of battery charging.

I don't know what device you are using for your percentage state of charge, but it appears to me that it is not very accurate. What device is telling you your state of charge? What was the voltage reading before and after the trip? What kind of batteries are you using?

You are probably using factory solar. The factory solar controller may be less efficient that my aftermarket solar installation on which I base my estimates. I know the factory solar controller is much less efficient than the Victron solar controllers I have used. I know the wiring between the factory solar controller and the busbars is of much smaller gauge than I used in my installations and therefore would suffer more voltage loss. Many have upgraded the factory solar controller with a high grade solar controller and have gotten much better results.

So even though we used different formulas, BayouBiker and I are pretty much in agreement with our estimates of charging. I think the lost Amps we are looking for are a result of the factory solar controller and the thin wire between the solar controller and the busbars. I would upgrade the solar controller to a Victron 100/30 and increase the cable size between the new Victron solar controller and busbars to 6 gauge to improve performance.

[jeffmc306]

Apologies to the OP but I have a follow up question for AirMiles.

AirMiles, any idea where the factory solar controller is located on a 27’ Globetrotter? Based on your observations I’d like to replace the factory Atkinson PWM controller with a Victron MPPT but so far haven’t stumbled across the physical Atkinson unit. Any hints on where they installed it?

Thanks!
Jeff

[INA]

Factory installed AGM batteries. The controller is also factory-installed. State of charge was derived from the factory installed display.

[BayouBiker]

So we can assign the missing ~10Ah to phantom voltage, or more inefficiencies than the 15-20%, or we could say the readout is inaccurate and the battery charged 15% and not the 10%. Perhaps the before and after readings were not taken precisely at the beginning and end of the drive.

INA do you have any thoughts on where the 10Ah may have gone?

[bbandeddie]

If by “derives from the factory installed display” you mean that you’ve converted from a voltage reading to a battery percentage then this is a nearly meaningless method of determining battery percent state of charge with accuracy.
Assuming the battery was actually at 75% SOC it would have very soon entered, if not already be, into the absorption phase. At this point in the charge cycle you will not receive a linear charge. Ie, the solar panels may be capable of providing X Amps, but the batteries are only able to accept X-y Amps as the charge amps tails off.
Add in the fact the factory solar panels are a long way from ideally aligned to the sun and quite possibly, even likely, impacted by obstacle shadows from objects on the roof and the inefficiencies inherent in most factory installations and you have plenty of opportunities for loss of charging inefficiencies.
Many more, and better controlled, charging scenarios are needed to assess your concern further.

[BayouBiker]

bb makes some good points. realize though that the controller has displayed the percent of charge based on its voltage and current sensors and its firmware. Still the numbers are likely not that accurate...

Consider, most modern chargers will transition to absorption charge between 80% and 85% where current profiles decline rapidly from max of 40+ amps. But the 160 watts is quite low to begin with so it will enter absorption mode a bit later. Then consider late in the afternoon its probably about 7-8 amps max. Figuring two batteries gets us to 3 amps max per battery after subtracting trailer load. Most charge profiles won't approach 3 amps till about 95% charged.

More food for thought.