Solar newbie with questions

scorpius

Hi all, I'm a newbie to the solar scene. I've always loved the idea of getting power from the sun since I was a kid, and now that I'm looking at buying a travel trailer in a year or so, I've found reason to really get to know more about it. Anyway I will get to the point..

I'm looking for a suitable deep cycle AGM 12V battery to power two 5V 1A devices continuously off of a 50w solar panel. Two small IP cameras.

I was previously using a 60400 mAh, 123wh lithium battery but started to run into surge issues as there was no charge controller. I didn't even know I needed one until I started to read more about solar.. One of the devices became inoperative because of power surge damage. I am hoping to avoid this issue by using a 12v deep cycle battery that is compatible with a commonly available charge controller. I'll appreciate any feedback about PWM vs MPPT for such a small solar application..

However I am unsure of what Ah of battery I would need as I know that deep cycle lead acid batteries are much different from lithium, and that going below 50% of power on a deep cycle battery should be avoided.

Thanks for your help and patience..

Estragon

A 10 watt load running 24x7 is 240 watt-hours / day, plus a bit of loss in the 12v-5v dc conversion. A 50w panel will make maybe 50 x .75 (temperature derating) x 4hrs (full sun equivalent) or ~150wh/day in most climates. Depending on location, if the trailer is only for summer use, sun hours might be a bit more - with an approximte location, this could be estimated more accurately.

Assuming solar is the only power source, it looks like you'll likely need more solar and/or reduced operating hours.

A 50w panel is likely a 12v nominal with a Vmp rating of 18v or so, which would make pwm a better choice. MPPT like more headroom in operating voltage. Pwm also typically draw less power just being on overnight.

scorpius

Thanks for your response. I'm looking forward to learning a lot more from the forum..

Chris

BB.

I will run through the math quickly here... They are rules of thumbs that we use to setup a relatively reliable/optimal (cost/performance) system for an off grid cabin. Your assumptions may be different--But it does give you a good place to start.

I will do this with Flood Cell Lead Acid batteries--AGM (which are lead acid) will work fine too, and be a bit more efficient (80% for FLA, vs 90%+ efficiency for AGM). Nominally, for lead acid batteries you want to cycle them between ~90% and 50% state of charge. >90% recharging every day is actually kind of hard on Lead Acid (tend to gas more, use more water, run warmer when charging, errode and corrode plates more). Below 50% to 20%, batteries will function, but require recharging >~75% state of charge ASAP, and will have less of a cycle life. With Solar, you get charging current when you get sun, so below 50% during bad weather (unless you use a genset), best to avoid doing very often.

Battery bank sizing... Assuming battery supports full load for 2 days (no sun) and 50% maximum discharge:

• 10 Watts * 24 hours per day = 240 WH per day
• 240 WH per day * 1/0.85 DC to DC converter eff * 1/12 volts * 2 days storage * 1/0.50 max discharge = 94 AH @ 12 volts

Your first choice, go with an ~94 AH @ 12 volt battery (deep cycle storage battery, not car or marine type)--Or choose 2x 6 volt @ 200 AH "Golf Cart" batteries--Connect them in series for 12 volts @ 200 AH. A bigger bank than you need, but if you want some more power (charging a cell phone, running LED lighting, running a laptop, etc.)--GC batteries are relatively cheap and rugged. And easy to find.

Next, there are two solar calculations to make. One is based on the AH storage capacity--Larger AH battery banks need more charging current. So, there is another "cost" with large battery banks (5% to 13% rate of charge typical for solar, 10%+ for full time off grid solar loads suggested).

• 94 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 89 Watt array minimum
• 94 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 177 Watt array nominal
• 94 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 230 Watt "cost effective" maximum
• 200 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 188 Watt array minimum
• 200 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 377 Watt array nominal
• 200 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 490 Watt array "cost effective" maximum

So, just based on storage, a 2x larger battery bank really needs a 2x larger solar array. In the old days, batteries were cheap and solar panels were expensive. Today, batteries are expensive, and solar panels are cheap--So adding panels is usually a good bet to keep your battery bank happy (and your loads powered).

Then there is sizing the system based on hours of sun per day and seasons... If you don't have backup power (genset/mains), and you have a 24x7 base load, you really should have 1/0.65 or 1/0.50 more solar arrays (~2x larger) to keep the batteries charged and loads running during bad winter weather (you may still run out of power in bad weather that goes more than 2-4 days--Really "dark" days, you may be 5% to 1% harvest--Or almost nothing).

Let's say you are around Raleigh NC. Fixed array, facing south:

http://www.solarelectricityhandbook.com/solar-irradiance.html

Note, you will have to enter United States, N.C, Raleigh, south, best year round harvest at 54 degrees from vertical (we have a bug in the forum software that does not let me copy and paste the tables):

December is 3.67 average hours per day of sun... February is 4.00 hours of sun per day. If we use the "worst case" December average (seasonal):

• 240 WH per day * 1/0.52 off grid system eff * 1/3.67 hours of sun (Feb) = 126 Watt array "break even" for Feb.

And ~2x that for full time off grid winter base load (suggested) at 252 Watt array to minimize genset usage.

And the nominal solar array (based on 10% charging rate) is ~177 Watt array for 94 AH battery bank, or ~377 Watt array for 200 AH battery bank... The "relatively" conservative 252 Watt array easily supports the 94 AH battery bank, and if you round up to a ~377 Watt array, a 200 AH battery bank would be "happy" too.

You talked about Surges for charging your battery bank. Generally, you do not get surges when charging with solar... However, connecting your array to a LI-Ion battery bank, Li-Ion cells "do not like" over voltage charging... You could have take the battery bank way over the desired battery bank voltage. And you may have cooked "or worse" your LI-Ion bank. Depending on Li-Ion chemistry and stuff, it can be very bad to over charge or over discharge for Li-Ion cells.

Lead Acid batteries will begin to gas and get hot if over charged--And for a period of time, they can take 5% or so over charge and survive (few hours to a few days of solar). Long term, lead acid batteries do not do well with over charging either.

In the end, for most systems, you want a solar charge controller to prevent battery damage (and reduce the chances of fire, chemical spills, etc.).

If you are just running USB adapters... There are some that will take 12-24 volts very happily:

https://www.amazon.com/Charger-Compatible-Devices-Anker-PowerDrive/dp/B07H4LH6P7 (Anker 2x USB runs 12-24 volt power).

Off grid storage batteries, especially Flooded Cell Lead Acid can have a pretty wide voltage range for a 12 volt bus--From 10.5 v (dead) to 16 volts (charging in cold weather, equalize charging).

Many "12 volt" nominal devices really only do well at ~11.5 to 14.4 volts or so...

Questions, corrections? The above is just a starting point for the conversation.

-Bill

Photowhit

Before you go buying things, it might be good to get a real idea of how much energy the cameras use. I doubt they take a 'full 5 watts all the time.

scorpius

Bill,

Thanks for that breakdown. I was surprised to see that thorough of an explanation, and also in a way that a newbie like me can understand. I appreciate it!

I will be primarily hanging out in desert areas initially, Vegas area and other desert areas, so abundant sun won't be the problem. It did snow in Vegas recently though. Right now I live in Houston and the weather changes at anytime. I am however concerned about overcharging the battery because of how much sun is there in Las Vegas, as well as giving ample airflow to the panel(s) during especially hot days when I will be losing efficiency.

Thanks for the link to the Solar Electricity Handbook (on the way) and the Anker 2x USB charger. That was what I was looking for when testing the panel with the lithium ion battery. Now I am more concerned about potentially overcharging the battery than I am about damaging the devices. I had noticed that during sunset the battery display was flashing on and off repeatedly, with the camera responding in the same fashion. After that it no longer functioned and emitted a high pitched sound when plugged into a standard outlet. After that and your explanation of lithium vs lead acid, I feel that lead acid is the way to go with a charge controller. I'll study your post some more.

Photowhit,

Both IP cameras running at the same time showed average about a 6.5w energy consumption.

Chris

Photowhit

6.5 watt hours per day? 6.5 watts per hour per day for 6.5 x 24 = 156 watt hours per day?

The charge controller job is to ensure that the batteries aren't 'over charged'. A battery temperature sensor would be nice or a charge controller with an internal sensor in the same eviroment as the batteries.