Battery issues?

rossb · October 2015

Greetings.. Newby PV question.. I need some help.. I have 4 Renology 100 W mono panels feeding a 40A Renology MPPT charge controller. I currently have two 200 amp-hour flooded-cell RV/Marine batteries (less that 1 year old) for storage. I am only driving 12V LED strip lights (occasional use) and outside dusk-dawn night lighting. Per the MPPT charger aux meter, the batteries indicate 97% charged and the charge control flashes fully charged during the day and everything seems fine, BUT when the sun goes down the batteries quickly drop to only 44% charged with or without any load. The batteries are watered, connections are tight, and all the terminals are clean... Any suggestions as to what is happening?

Mountain Don · October 2015

Have you checked the cells with a hydrometer? A specific gravity reading is much more reliable than a meter. That is where I would start. If you don't heap enough at auto parts stores. This one is nicer, easier to use and has temperature compensation built in.

Also what is the size of the overnight draw; amps or watts? How many hours on? Name / model of the charge controller and batteries?

BB. · October 2015

In addition to the Hydrometer, I would suggest a DC Current Clamp DMM (like this inexpensive one from Sears)--It makes it much easier to check that everything is working correctly (each solar panel is sharing the the output current, parallel batteries are sharing load/charging current, etc.).

Next, you need to verify that the batteries are seeing full charging voltage... Flooded cell batteries need ~14.5 to 14.8 volts for a couple hours per day (more or less). And around 13.2 to 13.6 volts after the batteries are charged (float voltage).

Personally, I do not like to put two 12 volt batteries in parallel for a 12 volt battery bus. It makes it difficult to just use a volt meter to check each battery (or each cell) for proper charging/discharging voltages. With two 12v batteries in parallel, each battery reads the same voltage. You need a Hydrometer (to check each flooded cell) and/or a DC current clamp meter (to check for proper current sharing) to isolate a bad cell/battery.

If you (for example) put two 6 volt @ 200 AH golf cart batteries in series (for 12 volt @ 200 AH battery bank), GC batteries tend to be cheaper and work better than the typical RV/Marine 12 volt batteries (which usually are not very good "deep cycle" type batteries).

Check the cables (corrosion, bad connections) to the batteries too.

At this point, I would fear that one or both of your 12 volt batteries are now becoming sulfated (under charged--setting at less than 75% state of charge for weeks/months at a time) or possibly damaged by being severely discharged (taken near dead at some time in their life).

Before you replace your present batteries, using a hydrometer and Current Clamp+DMM to better understand your system's behaviour (are all the panels supplying appropriate charging current, are batteries reaching 14.x volts every sunny day by early afternoon, are your loads being replaced by solar charging--and you are not "deficit" charging--solar panels supplying only 95% of daily power usage and batteries are slowly getting lower and lower every day--especially with the onset of winter).

-Bill

Aguarancher · October 2015

I see your CC can be 12/24V. How do you have it set-up. How are your panels configured, parallel, series or a combo of both? How have you been keeping your batteries charged before you got your solar system?

rossb · October 2015

Thanks for comments. Batteries purchases at same time as panels. Running panels in 12v configuration as loads will be 12v devices. I will do hydrometer check next trip out. It looks like I need to invest in a dc current clamp. Thanks for the tips. Will check further and update next week!

BB. · October 2015

A couple of other quick comments.

Can you replace the dusk to dawn lighting with a motion detector controlled light? That will dramatically reduce the power usage--And, personally, I prefer the lights coming on when needed (and attracting attention) vs just always on lighting.

Second, the Sears DC current clamp meter does work fine--But the Current Zero function is a bit different vs other meters I have used--And can be confusing.

Basically, DC current clamp meters have "zero drift" -- You need to rezero every minute (or few minutes, depending on how accurate you need to measure the current flow). With the clamp removed from any wiring, press the Zero button to turn "zero" on in the LED screen. Now measure your DC current (only clamp one wire at a time, if you clamp both red/black or +/- wires at the same time, they will add up to zero current flow). After a few minutes, remove the current clamp, press button to turn off zero, then press button to zero again--Then reclamp the wire.

By the way, how many Watts does your security light take? In winter, assuming you get at least 2 hours of sun per day:

400 Watts * 2 hours * 0.52 system eff = 416 WH per sunny winter day
416 WH per day / 12 hours = 35 Watt light "break even"
35 Watt light / 12 Volts = 2.9 amps @ 12 volts

More or less, any light more than ~20-30 Watts kept on over night will run the risk of taking the battery dead during winter/stretch of poor weather.

Also, do you have 2x 200 AH 12 volt batteries (400 AH @ 12 volts) or 2x 100 AH for 200 AH @ 12 volt battery bank?

If you have a 400 AH battery bank, a 400 Watt array is a bit on the small side for a daily operating system. We recommend a 5% to 13% rate of charge (typically) for a flooded cell off grid system. 5% is good for weekend/seasonal use... 10% or more is good for a daily use system:

400 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 377 Watt array minimum
400 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 753 Watt array nominal
400 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge =979 Watt array "cost effective" maximum

And, just to finish the sizing question... If you had a 3 amp bulb (36 watt) on 12 hours per night, recommending 2 days of storage (no sun) and 50% maximum discharge:

3 amps * 12 hours * 2 days storage * 1/0.50 max discharge = 144 AH @ 12 volt battery bank "nominal"

A 400 AH @ 12 volt battery bank would be a bit large for this application.

-Bill

wellbuilt · October 2015

The CC state of charge is way off .

I have the same type of setup with 210 watts solar and 2 230ah GC 6 volt battery.

The meter is way off , I watch tv, run water , charge phones and computers and run lights off a inverter all night and don't even get in to my stored batt power .

If I turn off all my power and let it rest for 1/2 an hour the Volts are 12.6 I never even see 12.5 (even on a rain day) 12.6 is about full charge .

I checked all my cells after 3 months and I added 1/8 oz of water each , the reading where about the same, im all good.

BILL likes to see 14.5 v + for a couple of hours a day to charge flood battery, but my CC never gets that high.

I think its because im all ways @ close to full charge.

If I run every lite , TV, radio and pump water in full sun my CC will give me 14.5+ volts and up to 9.7 amps .

most of the time im at 13.6 to 13.9 2 to 5 amps ?

I do run a 30amp ranch and farm charger every month until I hit 15 volts to mix up the battery , it takes less then 1/2 an hour .

I was off grid jun to sept building my house ,with maybe 15 days off at home. John

BB. · October 2015

Yes, you will not want ~14.5+ volts for 2 hours per day unless the battery bank has been discharged by upwards of 20% per day.

If you discharge less than 10% over night--You are really shallow cycling, and you may not get (or need) the higher 14.5 volts "absorb" charging cycle.

-Bill

rossb · October 2015

All the cells test fine. It will probably remain a light usage system until I retire in a few years. Batteries are exide 27MDC. I am probably under Cycling as the night lights are minimal users and I only use my 12v LED strip lights (20amps if they are all on) a few times per week. I think it must be the CC meter as have never seen it below 40% No matter how long I run the strip lights. I will add another battery when my usage increases in the next year or so, and will consider adding more draw to the "night shift"! Thanks for all the suggestions!

jcheil · November 2015

Be very careful trying to add new batteries to old batteries. If you must, do it within 6-12 months of the old batteries and make sure they are the EXACT same battery. Mixing too old and new batteries and different makes/models can be disastrous. I have seen it first hand at a friends.

Vic · November 2015

Hi rossb,

Just as a point of clarification;

Your Exide batteries are Group 27, and EACH battery is speced as 105 AH. If you have then in parallel (they simply must be 12 V batteries), then you have a nominal AH Capacity of about 210 AH total, between the two of them (this may have been what you were trying to say, above).

Agree with wellbuilt, SOC Meters, particularly on inexpensive Charge Controllers are usually quite inaccurate, as their SOC readout is commonly based only on battery voltage, and might not even use a Temperature-Compensated battery voltage, at that.

Measurements that you take, carefully, with your Hydrometer are really the Gold Standard for SOC of Flooded batteries (like yours).

Vic

Johann · November 2015

rossb said:

All the cells test fine. It will probably remain a light usage system until I retire in a few years. Batteries are exide 27MDC. I am probably under Cycling as the night lights are minimal users and I only use my 12v LED strip lights (20amps if they are all on) a few times per week. I think it must be the CC meter as have never seen it below 40% No matter how long I run the strip lights. I will add another battery when my usage increases in the next year or so, and will consider adding more draw to the "night shift"! Thanks for all the suggestions!

Did you let the batteries rest for while/without charge or load before you done the test?
I had a similar problem not to long ago and my charge controller gave me a red light with a lower voltage reading just over night with only a few LED strip lights that I use only an hour a day, .
I checked the voltage at daytime and all was ok. So I disconnected the two 12volt batteries from the system and from each other and let it set 24 hrs. After 24 hrs sitting there disconnected from anything I done my testing and I figured out that one of my batteries was bad and sucked the power from the better battery which went down to a very small capacity and was just about useless also.

rossb · November 2015

Thanks Johann, I will try that check as well.

Battery issues?

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