Getting Panel's Full Output When Charge Controller Goes Into Absorption Stage

[tvpierce]

Hi all. I’m new to the forum. I have a 200 watt portable system I built a few years ago to support my very modest power needs when dry camping/boondocking in my camper.

200 watts is way overkill for my needs and has worked very well for several seasons (we only camp from June – September). All interior lights are converted to LED. Only one or two lights are ever operated at one time. And for no more than for 30 minutes a day, right before bed. The rest of the camper has a constant 9 watt draw which runs the electronics for the gas fridge and a gas leak detector.

This past season I added a 12 volt fridge/cooler that uses 36 watts when the compressor cycles on. How frequently and for how long it cycles is of course dependent on ambient temperature, how often it is opened to access the contents, etc.. So its total watt/hours consumed in a day is variable, but needless to say it has significantly increased my power demand. My solar system still keeps up, but it's closer than I'd prefer.

My question is about extracting the most power from my panels during the day.

I have unobstructed sun exposure from about 7:45 am through 3 pm.

At midday, with unobstructed sun, my panels are capable of putting out about 9 amps max. (measured at the input terminals of the charge controller) By 11:00 am each day, the Bulk stage on my battery bank (two 6 volt golf car batteries) is complete and the controller has transitioned to the Absorption stage so is putting just 2-3 amps to the batteries. So each day there’s about 4 hours (11:00 am – 3 pm) where 6-7 amps of potential generation is not being realized because the batteries don’t need it. But my 12 volt fridge/cooler is still consuming power.

If I ran the fridge/cooler from the Load terminals rather than from the battery, would the charge controller recognize this and draw more current from the panels when the fridge/cooler cycles on?

Thanks in advance for any insight.

 

[AKGuy]

na

 

[ianganderton]

The problem is not with your charger but your lead acid battery. As the charge goes up above 80% a lead acid battery’s internal resistance goes up and it takes a lot more force to slowly get the last 20% in

So end of this story is the solution is to change your battery chemistry to Lifepo4 because it doesn’t have this problem and you could used a greater

 

[fratermus]

So each day there’s about 4 hours (11:00 am – 3 pm) where 6-7 amps of potential generation is not being realized because the batteries don’t need it.

In an offgrid context solar power is only generated when demanded. It's like worrying your car isn't making all 200 rated horsepower at a stoplight; it doesn't need to.

If you want to make more kWh for some reason then add opportunity loads. Heat water, cook with a crockpot, whatever.

If I ran the fridge/cooler from the Load terminals rather than from the battery,

The LOAD output makes no difference.

would the charge controller recognize this and draw more current from the panels when the fridge/cooler cycles on?

Already works that way. The controller keeps battery voltage at a setpoint. If that dips (increased load) it increases harvest to attain the setpoint again. If voltage increases (decreased load) the controller harvests less. Like cruise control going up or down hill: throttle is adjusted to maintain speed.

 

[12VoltInstalls]

My solar system still keeps up, but it's closer than I'd prefer

So then you say,

each day there’s about 4 hours (11:00 am – 3 pm) where 6-7 amps of potential generation is not being realized because the batteries don’t need it. But my 12 volt fridge/cooler is still consuming power

So you have plenty of overnight power with your batteries? If they are down to 12.4V each morning you should double up your batteries if you wish more days of storage if it’s cloudy or whatever.

If I ran the fridge/cooler from the Load terminals rather than from the battery, would the charge controller recognize this and draw more current from the panels when the fridge/cooler cycles on

The controller keeps battery voltage at a setpoint. If that dips (increased load) it increases harvest to attain the setpoint again. If voltage increases (decreased load) the controller harvests less.

So what I would do is actually add another 200- or 400W of panels, and add batteries. Sell the expensive cooler to offset the cost, and swap out the RVs fridge for a 120V one that costs $150-$250 depending on sales. Sell the RV fridge, they fetch good money.
Then you have a bigger fridge with a freezer, a lot more solar to make using lights etc less cumbersome, and a net system with a lot more power at the least expense for the upgrade

 

[tvpierce]

Thanks all for the input. Sorry in advance for the long post, just wanted to respond to everyone who took the time to help me out.

The problem is not with your charger but your lead acid battery. As the charge goes up above 80% a lead acid battery’s internal resistance goes up and it takes a lot more force to slowly get the last 20% in

So end of this story is the solution is to change your battery chemistry to Lifepo4 because it doesn’t have this problem and you could used a greater

Lifepo4 would be great to have, it would be overkill for my application and for me doesn't make financial sense for my needs.

In an offgrid context solar power is only generated when demanded. It's like worrying your car isn't making all 200 rated horsepower at a stoplight; it doesn't need to.

If you want to make more kWh for some reason then add opportunity loads. Heat water, cook with a crockpot, whatever.

Actually, that's kind of how I arrived at adding the fridge/cooler. I knew I had the potential to generate way more power than I was consuming. I was purchasing ice every other day for my outdoor drink cooler, so decided to use the excess power to eliminate the need for ice. It's worked really well. And since it's just a drink cooler, then if we ever have several dark rainy days when power generation is severely decreased, I suppose I could simply unplug it -- although I've never had to do that. Plus I have a full size fridge/freezer inside the camper that runs on propane.

The LOAD output makes no difference.

That's what I was wondering.

Already works that way. The controller keeps battery voltage at a setpoint. If that dips (increased load) it increases harvest to attain the setpoint again. If voltage increases (decreased load) the controller harvests less. Like cruise control going up or down hill: throttle is adjusted to maintain speed.

To be honest, I've never thought to monitor the output of the charger with the cooler/fridge cycled on and off. So let me ask a follow-up question if I may. Let's say it's midday and the batteries are at 85% so the charger is in the absorption stage, outputting 2 amps. Then my fridge/cooler kicks on adding a 36 watt load to the system (36 watts/14 volts= 2.5amp). So I should see the output of the charge controller go up to 4.5 amps?

So you have plenty of overnight power with your batteries? If they are down to 12.4V each morning you should double up your batteries if you wish more days of storage if it’s cloudy or whatever.

Yes, we have plenty capacity to power everything we need overnight. 12.4V should be 80% on FLA batteries, unless I'm calculating something wrong. My SOC is usually 12.3V - 12.4V in the morning. By mid/late morning, I've completed my bulk stage of charging and I still have 4+ hours of unobstructed sun for absorption charging. I think I'm doing pretty well in terms of capacity... but admittedly I'm a noobie so I'm open to suggestions and feedback.

So what I would do is actually add another 200- or 400W of panels, and add batteries. Sell the expensive cooler to offset the cost, and swap out the RVs fridge for a 120V one that costs $150-$250 depending on sales. Sell the RV fridge, they fetch good money.
Then you have a bigger fridge with a freezer, a lot more solar to make using lights etc less cumbersome, and a net system with a lot more power at the least expense for the upgrade

The fridge/cooler (it's actually a freezer too because it can maintain a temperature into the low 20s) is just my outside drink cooler. These 12V units have really come a long way in the past few years. Mine is a 53 quart capacity and was $310 delivered -- that's less than a comparably sized Yeti, never requires ice, and if my calculations are correct, only requires about 220 watt/hours per day. (Estimated run-time of 5 minutes every 20 minutes = 15 mins/hr x 24hrs = 6 hrs per day @ 36 watts) BTW, the 5 mins every 20 mins is a high estimate... I don't think it runs anywhere near that much.
I already have a full-size fridge/freezer inside the camper that runs on propane.

 

[fratermus]

Let's say it's midday and the batteries are at 85% so the charger is in the absorption stage, outputting 2 amps. Then my fridge/cooler kicks on adding a 36 watt load to the system (36 watts/14 volts= 2.5amp). So I should see the output of the charge controller go up to 4.5 amps?

Short answer: yes

Longer answer / caveat: short-duration loads + slower MPPT algorithms can mean exact equilibrium isn't found before the load shuts off again. My own cheap controller (EpEver) has a notoriously-slow algo. If we were to change the example above to "running a laptop" then the EpEver would find the right power point after a couple minutes.

 

[12VoltInstalls]

12.4V should be 80% on FLA batteries, unless I'm calculating something wrong

I consider 12.1V as essentially fully discharged with lead acid batteries. And in my own small system don’t like seeing below 12.4V.

 

[tvpierce]

Short answer: yes

Longer answer / caveat: short-duration loads + slower MPPT algorithms can mean exact equilibrium isn't found before the load shuts off again. My own cheap controller (EpEver) has a notoriously-slow algo. If we were to change the example above to "running a laptop" then the EpEver would find the right power point after a couple minutes.

Makes sense.

 

[tvpierce]

I consider 12.1V as essentially fully discharged with lead acid batteries. And in my own small system don’t like seeing below 12.4V.

Got it. I'm a relative noobie, but have done a bunch of research and go by the numbers from the following chart, which seem to be pretty universally accepted for FLA batteries. As I understand it 12.1V would be about 50% discharged -- which is the absolute lowest recommended to go without incurring long-term damage to the batteries. (and is actually NOT recommended as a regular practice) And that 60% (12.18V) is probably a safer/more conservative low limit.
By my estimation, my daily charge/discharge cycle is 100% - 75%. I'm comfortable with that range given my use-case: camping for up to a week at a time (although usually only 3-4 days). It's easy for me to reduce the load on my system by taking non-essentials off line if conditions dictate: I can unplug my drink cooler (single biggest draw), don't charge the iPad, etc.. After all, I'm "camping" (and only for a few days at a time)... so it's not much of a hardship to give up some conveniences.
If I used my system for everyday life, my expectation for consistency would be much higher, and therefore I'd probably want twice the battery capacity, and want to cycle it less deeply, as you suggest.
Again, I'm open to thoughts/suggestions. I'm here to learn.