Help, please

Okay, lets work on basics from your post...

...and the basic equation used in solar Amps x volts = watts and it's cousins watts divided by amps = volts and watts divided by volts = amps.

DebbieK said:

I have a 400 watt Renogy array with a Renogy MPPT40CC Controller, a Model MT-5 Meter, a Xantrex PROwatt SW 2000 inverter and 2 VMAX MR127 12 volt batteries. This entire set-up is old (6-7 years) and everything with the exception of the batteries, appears to be working properly. I had my mechanic check the batteries and they charged, but I don't think they are holding a charge.

Not bad stuff in general.

I search for the MT-5 meter, looked at the instructions and they don't mention 'shunt' so it's basically a voltage based meter. Though since it give a SOC (State of Charge) of 32% at 14.1 volts perhaps it's getting info from the shunt in the charge controller.

Tracer-Meter-MT-5-For-MPPT-Solar-Charge-Controller.pdf (renogy.com)

Yes 6–7-year-old batteries are likely dead but go ahead and work and learn with them.

So there is a cycle to charging batteries, I wrote this for flooded lead acid batteries, but it all applies to AGM batteries like you have;

These are good questions, I'll bet your bog garden is helping to oxygenate the water!

Might be you could run the pumps a bit in the morning and evening to help keep the levels high enough to sustain the pond eco system.

Often systems get built without balance in mind. I think the inverter you have is very large for the balance of your system. I wouldn't buy batteries until you have a good idea of where you want to be when you have a system to meet your needs.

Let's do some math again...

You have a 400 watt array, which will typically produce about 300 watts per hour of direct sunlight near solar noon. Most places have reduced hours November through January. Maybe average about 3 hours a day. Producing 900 watt hours allow you to store maybe 750 watt hours in the batter bank (inefficiency charging at 14.5 volts but discharging at 12.5 volts) 

Your VMAX SLR 125 AGM Sealed deep cycle 12V 125aH batteries could store 12v x 125 amps = 1500 watthours each or 3000 watthours total for 2. The 400 watt array allows you to charge at about 10% of the battery bank capacity, not a bad fit!

Your 2000 watt inverter can deplete your battery bank in less than an hour, (due to something called peukert effect and that we try not to discharge our battery banks to less than 30% ever and less 50% on a daily basis.) I ran my house on a 1800 watt inverter, including fridge, window air conditioner and front load washing machine....

So, it might appear to be oversized. BUT it might have been sized to start your pumps which might draw 8X their running load on initial startup. 

Bigger inverters usually come with bigger base/idle loads, My old inverter ran about a 25 watt idle load, you can likely look up that info in a manual for your inverter. A 20 watt load 24 hours a day would be 480 watthours. about half watt your array is producing in much of the country during the short days of winter.

You can evaluate how much sun you get on average this time of year at this site;

PVWatts Calculator (nrel.gov)

Do you have additional means of charging the battery bank?

DebbieK said:

I turned the inverter back on and plugged in the pumps when the SOC was at 100% and the controller was blinking indicating the batteries were fully charged. The SOC went down immediately to about 54% and would go lower (33%) when the sun went behind clouds and back up in full sun. The inverter said 11.9 and 0.19 when the pumps were plugged in and 12.7 and 0.0 when they weren't. I think that means that the pumps were drawing 190 watts; is that correct? Also, one of the readings on the controller said "0.0 AH and 0.2 WH" so is that indicating that the power being is used is 200 watts? Still trying to figure out what the readings on 

Debbie, you are not understanding what I am saying about the SOC with your monitors and charge controller.

They are likely just voltage measerments. So when you begin charging and your batteries are say 40%, the system volt, which is all they can see, is above the voltage of a fully charged battery at rest.

This may help or confuse you... This is the rough voltage range of an AGM battery at rest;


Visually your monitor looks like this;


Say your battery is 40% full and at rest it's 11.95 volts. Your charge controller limits the battery voltage to something like 14.5, and your battery is taking in all the current it can but it's only 85-90% efficent, so the voltage goes up the extra 10% to allow the current to flow from the array to the battery. 11.95 x 1.1= 13.4 volts. The battery is still only 40% full, but the 'dumb' meters see 13.4 volts and 'say or suggest that the battery is full.



The lights are only displaying the system voltage.

If you turned off the array/charging for a couple hours the voltage would return.

DebbieK said:

From what you are saying, if the meter for my battery (which is just a simple gizmo that shows volts) and the one on the inverter and the controller, for example, say 12.9, then my batteries may be or may not be adequately charged? 

Sorry, not much time this morning. but quickly, YES! Though a voltage type monitor can be designed to determine ascent to absorb/bulk voltage, but often this doesn't work well either.

DebbieK said:

Then,what would be the best way to determine if the batteries are maintaining a charge and what their capacity is? Should I buy a multimeter or just disconnect the batteries from the controller (and consequently, the panels) and see if it holds steady?

With AGM batteries, It's difficult to do without a shunt-based battery monitor. They measure the current going into and out of the battery bank. They aren't particularly inexpensive. A couple are the Trimetric and Victron BMV-700. Renogy has a shunt based one now, I've not seen one in use, but I can't think it wouldn't work.

To see the current state of charge using voltage, you would want to disconnect everything and see where the voltage settles at usually a couple hours. You might apply a light load for a few minutes to help clear a 'surface charge'.  I would use a handheld multimeter, they are useful for a few things but connecting/switching on the inverter would work fine, it's minimal load would be negligible.

DebbieK said:

This is my most important question: Don't both the controller and the inverter have automatic cutoffs that won't allow the battery to go below 50% capacity?

NO!
This is a misconception by many. The charge controller rarely has any interaction with loads/output of the batteries. Some can be used with small DC loads. If your inverter is connected to the loads of the charge controller, your system is NOT setup correctly!

The inverter will have a Low Voltage Shutdown, this is to protect the inverter and not the battery bank! 

If it has a settable LVShutdown sometime people will set it to help prevent discharging the battery too much, but any preset value is to protect the inverter it's self!

DebbieK said:

Sorry I'm being so stupid. When I said I had the electrical knowledge base of a 5 year old, I wasn't kidding. It would be wonderful if the batteries were functional; they've gotten so expensive.

You are doing great!
Asking the right questions! 
Heck, I'm being school about things I haven't thought about in years, forgot some charge controllers have a 'loads' connection. 

Usually batteries lose capacity over time. So perhaps they can be charged to have 100% capacity at the end of a sunny day, but certainly have less capacity than new. If they aren't charging quickly, often this will be a good sign that they have a good bit of capacity.

Do your panels have good direct sunlight?

There have been pond aerator discussions here in the past. Might do a search and try to gleen some info from them.