Help with fridge / panel pairing (1 Viewer)

[TWILLY]

I seek help with the following questions from those who understand off-grid power better than I. Thank you in advance for your assistance.

I am trying to determine the adequate wattage of 12V solar panel to power this fridge, which has a built-in lithium battery. The solar panel alone must be able to power the fridge continuously, absent any other input power.

Specs on the fridge and battery:

- 42 amp hour
- 65 watt total input power
- 1.1 amp hour average power use

Questions:

Will a 100 Watt, 12 volt solar panel at full power exceed the charge that this battery can accept? Is there thus no charging benefit to using a panel with more wattage?

Am I also correct to understand that, given 12 volts at 1.1 amps per hour:

(a) the fridge would consume 13.2 watts per hour (12 volts x 1.1 amps per hour = 13.2 watts per hour), and

(b) under full charge of 65 watts, while operating, the fridge would have a net charge rate of about 51.8 watts per hour (65-13.2=51.8) watts per hour, or, in amps, 4.3 amps per hour (51.8 watts /12 volts = 4.3 amps), and

(c) it would require ten hours of continuous charging at 4.3 amps per hour during operation for this 42 amp hour fridge battery to fully charge (4.3 amps per hour x 10 hours = 43 amps)

Lastly, I understand that solar panels only produce maximum power under the most optimal conditions.

Assuming I were parked in mixed sun all day with a fixed roof panel, how large of a panel in terms of wattage would one need to reasonably ensure at least 65 watts of continuous output throughout the day?

 

[Mike6158]

Will a 100 Watt, 12 volt solar panel at full power exceed the charge that this battery can accept? Is there thus no charging benefit to using a panel with more wattage?

Solar panels require a charge controller. It prevents overcharge. Linear charge controllers do that resistively. Ie they dump the load into a high wattage resistor and it basically (I've made a few of those) converts the charge to useless heat. There are "smarter" controllers that handle the unneeded current from a solar panel better. MPPT is, as far as I know, the best charge controller to use. Make sure whatever use is specific to the type of battery you are using, The benefit to using a higher wattage panel is the decrease in recovery time.

Correct - solar panels don't produce max power all day. If you can keep them pointed at the sun all day then you can improve the efficiency. We probably have 1,000 individual solar panels, maybe more, where I work. 40W - 200W in size scattered over a million square miles, literally. They power our telemetry. They are pointed south and angled at or near the latitude of the location... basically 36 - 37 degrees. No tracking.

Once the fridge cools down it's power consumption will drop. Not sure how much. I've never measured mine.

 

[ducktapeguy]

Too many variables to give you a definite answer, but based on my experience a 100W panel will mostly be able to keep up with consumption for many days.

My fridge is very similar spec, 65W, 1.1 Ah (under certain conditions). I'm using a 100AH LifeP04 battery and a 100W panel. In reality a 100W panel gives about 60-70W max. Over the course of a week the panel can mostly keep up with the power draw on the fridge, enough that I could probably get many weeks from a battery charge. But that's under ideal conditions.

Power consumption is going to vary significantly depending on the delta between ambient temperature and fridge setpoint. Your fridge may only draw 0.4A/h at night when the temperature is cooler and if your fridge is set above freezing. It maybe draw up to 4-5 A/h if the interior temp of the car is 130F and the fridge is set to below freezing. Also, the fridge may consume an average of 1.1 amps/hour, but in reality it will be cycling between 0 and ~5-6Amps, the duty cycle being dependent on the temperature delta I mentioned above.

Since you're in AZ, having enough sunlight isn't an issue, you could probably count on at least 8 good hours of sunlight per day. However, the drawback is the fridge now has to deal with a hotter ambient temperature unless your panels are detached from the vehicle. The charge rate is also non-linear, meaning might charge faster when the battery is low, but will taper off as the battery gets charged. So you can't assume you'll be getting a 4.3A charge rate continuously.

Also, you might want to check to see if the battery can be charged while the fridge is running, a lot of power packs have a switch between charging and power, so you can do one or the other, but not both at the same time.

 

[e9999]

OP,

Comments:
- saying that the average energy consumption is 1.1Ah (per hour at 12V) is meaningless if it does not specify under what conditions and especially what temperatures. In AZ for example, if you are exposed to very high temps in the desert, you will likely use up quite a bit more. Or if you open it up more than usual etc. So I would take that number with reservations.
- It's unclear to me what that 65W would refer to, without checking the full specs. Is that the power consumed by the fridge when operating (about 5A, that sounds reasonable if a bit on the high side) or the max power allowed to charge the battery?
- I'm not fond of the idea of the fridge and battery being one unit, TBH. And you'd want to make sure how the battery can get charged (or not). Remember these batteries are consumables, you may need to replace it at some point.
- you are not stuck with a panel if it's too small, you can add more later on if the battery controller can handle it (and best if the panels are the same).
- a 100W panel is very rarely able to produce 100W, that would only happen typically around noon in the summer under ideal weather conditions.
- if you want to get a very good idea of how much power you can get out of a panel, there is free software out there to do that, even for very specific locations and times of day. But it'll probably take a few hours to become good at it.
- good thing is that desert AZ is just about the best place for solar power generation in the country. However, keep in mind that if you design your system for AZ , it may fall short if you go on far away travel.

as to your questions:
- a higher nominal power panel will of course produce more power and so charge things faster and will also have benefits if insolation were to be less than normal for example. The extra capacity may allow you to produce enough power still whereas a marginal panel may not. Con is that it's more expensive and physically bigger. But you don't have to worry about a panel being "too powerful", the charge controller will handle that. It's not like a 100W panel always produces 100W even at full insolation, it will only give out what the load requires, so you won't be frying things up.
(a) sort of, it's not exactly 12V so maybe a bit more Wh if your battery is not abnormally low.
(b) yes, sort of, on average, but that disregards inefficiencies
(c) yes, roughly speaking
- yes, you would probably be lucky if you got half of the nominal power averaged out over the day time.
- you will never get 65W of continuous output during the day. That is just not possible. The output will start at zero around sunrise -regardless of the size of the panel- and increase during the morning until you can get whatever max the panel has to offer. And then go down in the afternoon. Can't be constant. However, that's where the battery helps, it allows you to average things out (if big enough).
How much of a panel you'd need depends on insolation but personally if I had to guess I would go with at least 100W in the South of the US and that might even be marginal. But as I said, you can start with one panel and then add more as needed instead of just replacing it; and having several smaller ones may in fact be a more flexible approach than one big one. But -important!- you need to plan for that when choosing the controller, well at least if you don't want to have to buy controllers repeatedly.