Question of usage/battery life

hillside · April 2020

Thanks for all the help in setting up our basic system. Without going into a long question (that was just erased as it was about to be posted thanks to some internet gremlin), is it better to use our system part time, turning off the inverter every morning and only using a small amount of it's capacity at night and by doing so keeping the batteries at a higher SOC or will they last longer being drawn down and used more?

We were without power for 4&1/2 months after hurricane Maria and although it's unlikely to happen again we do have occasional power outages when we need they system at max.

hillside · April 2020

Edit, 8 panels now.

Dave Angelini · April 2020

Yes Maria was nasty. I have quite a few in PR. Your best thing that you can do is make sure you are not undercharging that DEKA AGM battery. If by limiting it's discharge you can get it charged fully, then do it!

More solar will make it easier. It is a good balance also to use the battery but undercharging is the enemy and easier to do because you can't use a hydrometer. Good Luck

BB. · April 2020

Another (or a second to also try) option is to "use the system" one full (24 hour) day a month and see if the system still performs as expected... Once the system fails to meet your test/needs, then it is time to replace the battery bank (most likely, but additional debugging too to ensure all other equipment is working correctly).

Since you have several 6 volt batteries in series--Checking their voltage (resting/charging/discharging) with a DMM is a very quick and easy method to find a battery behaving "differently"--Typically a battery that is about to fail, or poor wiring connection, etc.

If you have a DC Current Clamp DMM, and parallel battery bank connections, checking that current is being shared between parallel strings (roughly equally), is also a quick way to find problems (during charging/discharging).

Lastly, checking the solar array (reading your charge controllers when the sun is up, and especially if load on battery bank/somewhat discharged battery bank) is also helpful--You want to find any problems while you have time to address them (and before damage is done to the batteries). DC Clamp meter if you have parallel solar panels strings is also a good/quick check on panels+array wiring.

Checking once a month (detailed checks) may become a bit much--But doing these checks a month or two before hurricane season once a year (running system for a few days "off grid", etc.)--Saves "trying to recover" after the storm, much less stress.

-Bill

hillside · April 2020

Thanks guys.-.Dave, they're flooded batts. not AGMs though.

Don't have a DC Current Clamp..will check it out. We only use the system for about 5 hrs. a night for TV (180W and receiver 35W) and 2-3 loads of clothes washing p/ week with a top loader washer and a propane dryer. The controller shows a fast recharge with morning sun. In the early AM it shows 25.5 volts and by noon or 1 PM showing full charge at 27V and as high as 29V.

I think I will hook up a small fridge which is near the inverter and only draws 1.5 Amps. (need to check it with the kill a watt) and run it for a couple days and see what happens.

softdown · April 2020

Be careful with the small fridges. Many, perhaps most, use almost as much power as a full sized fridge. Since you don't need a full sized fridge you could use an Energy Star chest freezer controlled externally. I finally found a simple, sturdy external controller that I like. But it only goes down to 40F for odd reasons. Not being a big beer guy - I can live with 40F. Would do so in PR for sure.

Freezers have twice as much insulation as fridges. 2" vs. 1".

hillside · April 2020

Thanks again. I'll check the wattage on it and will try what BB suggested. The batteries are fairly new and I want them to last a long time.

BB. · April 2020

You have two different (major) failure modes... One is cycle life (say 2,500 Cycles to 75% SoC--Will probably last ~1,250 cycles to 50% SoC, etc.)... Another way of looking at it... Say you expect 50% SoC charge (during "major events"), and you expect (pray) for a 7 year battery life (good quality battery).

1,250 Cycles to 50% / 7 years (aging life) = 179 "useful" cycles (to 50% discharge) per year

Then look at the aging life... Temperature dependence is a huge issue. Batteries are usually rated 25C/75F... If you are in a hot climate (and/or do a lot of cycling/charging and raise the bank temperature--Hot room, etc.). The engineering rule of thumb is 1/2 life for every 10C/18F over "room temperature"...

So that is the other thing to watch... If you can keep the battery bank cool(er), then all is good. If you are in a hot climate/sun heated room, not good... Running at ~35C/95F "average temperature", then that 7 year "planned" life, is all of a sudden a 3.5 year life...re

Anyway... Some ways of looking at battery life, issues that affect life, and how you can use the bank "optimally". I.e., if you have a "hot bank", looking at reducing temperatures (better ventilation, move to basement, etc.) can help.

Or, if they "run hot" because of the installation, then look at more cycling or "cycling to get "free power" anyway (batteries age out before cycling out, then why not use it or lose it).

:Some other equipment to think about (links are for reference, the first for Hydrometer is our host NAWS)... A hydrometer (always rinse with a few squirts of distilled water before putting away, glass hydrometers and very easy to roll of tables/break):

https://www.solar-electric.com/search/?q=hydrometer

And just an example of a couple of DC current clamp meters... Note, there are AC only current clamp meters (very nice, but not useful for DC systems). And DC current clamp meters (really AC+DC clamp meters). Here are a couple (low cost, good enough and a mid-priced unit):

https://www.amazon.com/UNI-T-Digital-Current-Capacitance-Multimeter/dp/B0772FYF5M/ (low cost, "Good enough" for our needs)
https://www.amazon.com/gp/product/B019CY4FB4 (mid-priced unit)

-Bill

hillside · April 2020

good info, Bill. I have them in as cool a place as possible, shaded in a cove with concrete walls on 3 sides and a white steel cover over the top so they never see the sun. But it is the tropics and similar to that of south Florida or the rest of the South. Thanks

Dave Angelini · April 2020

Definitely get the hydrometer that Bill suggested, or similar. EQ's and your data can make it last a long time. Wipe down the moisture/dust on the bat tops. You probably get enough outages that you don't have to do practice much with loads

It is a good point to see how quickly it can recover. My guy's in PR all say high line voltage also knocks the inverters off line, not just the outages. The high line also kills your loads and a mini-split is not an inexpensive repair. They are told it is going to be years to get the utility up to a decent level of quality power. Some area's are especially bad. Good Luck!

hillside · April 2020

I clean the tops regularly and have a hydrometer..and they get distilled water. the inverter is off the grid...this is a backup system.

Our problem with the electric grid has been low voltage. I've measured ours in the house at 108V (occasionally it'd be normal) for about the last 4-5 years. Before I figured out the problem, I was replacing the gas oven igniter (twice) thinking it was defective--but it needs >115 or more just to light. (Piezo electric start) Our area has too many homes on one transformer.

Question of usage/battery life

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