Tristar45, MSVIEW and Battery SOC

Re: Tristar45, MSVIEW and Battery SOC

Welcome to the forum DIY.

Estimating battery bank state of charge purely from voltage at any point in time is difficult... Just to give you an idea:

leaf wrote: »

Am trying to upload the charts I am using...

Attachment not found.Attachment not found.

[note charts are from: I believe those charts are from Home Power #36, August- September 1993. Lead-Acid Battery State of Charge vs. Voltage ©1993 Richard Perez.
Here is a link: http://www.scubaengineer.com/documen...ing_graphs.pdf

vtMaps

I don't quite a agree with the resting voltage line (I think the voltage is a bit low)--But it shows how to estimate a battery's state of charge while operating.
-Bill

The battery bank voltage can vary from ~11.0 to 13.5 volts depending if battery is under load, resting, or under charge--And by how much current you are moving through the battery.

If these are flooded cell batteries, you should use a Hydrometer to measure the Specific Gravity of each cell (temperature corrected) and log the information. That is the "gold standard" for estimating battery bank state of charge.

If these are sealed batteries, I would highly recommend using a Battery Monitor (Victron is also very good, and out of the EU). Battery Monitors are also very nice for flooded cell batteries too.

In general, for a battery bank at normal room temperatures and with normal loads, I would suggest that you not go below 46 volts (11.5 volts @ 12 volt bank).

Batteries can be quickly damaged by discharging too deep and/or not recharging back over 75% state of charge quickly (batteries "resting" below ~75% state of charge will begin sulfating in days and can be killed in a few months). Note: You can cycle batteries between 50% and 80% state of charge daily--And recharge >90% SOC once every ~7 days (for true deep cycle batteries).

How much solar array do you have (in Watts)? Is it peforming the way you expect (enough current in full sun?).

Generally, we recommend 5% to 13% rate of charge. And 10% or more if the battery bank is heavily cycled.

Example: 440 AH * 10% = 44 amps

44 amps * 58 volts charging * 1/0.77 panel+controller derating = 3,314 Watt array "nominal"

At this point, you need to make sure your charging system is working correctly, you do not have too much loads (discharging current), and you may need to use a generator to help bring the battery bank back >90% state of charge... You are probably close to damaging the battery bank.

-Bill

Re: Tristar45, MSVIEW and Battery SOC

All you "know" with AGM/Sealed batteries is when they are "dead" (not helpful) or full--Basically when you have held 14.2-14.4 volts for 2-6 hours or so (you need to check the manual for the battery bank).

Another method to know when the batteries are full is to watch the charging current... You have a temperature corrected charging voltage (say 14.4 volts at 25C) and monitor the current into the battery bank... You will see the current fall until it goes no lower--At that point, the battery bank is "full".

The termination current can be be as much as 2% (for old flooded cell fork lift batteries) to less than 0.1% for AGM/Sealed batteries. (note, batteries that have a charge terminating current of >~2% are probably about to fail and should be replaced before they fail with flame/fire).

Be careful with over charging... AGM/Sealed batteries can be very sensitive to overcharging (most have a small catalyst like platinum or similar in the battery cap--The hydrogen+Oxygen is recombined back into water--Which creates heat too). If you vent (over pressure) a sealed battery, you permanently lose electrolyte.

Also, watch battery bank temperature... For every 10C rise in temperature (~25C is "spec." temp), the battery bank will age 2x faster. Over ~45C, you can permanently damage your battery bank. As always, read the manual for your batteries.

A battery monitor is not perfect either... They measure the Amps*Time of current going in to and out of the battery bank. Sort of like monitoring the fuel level in the car's fuel tank... And it will become "less accurate" over time. A battery monitor (typically) will reset if the battery is held (for example) at 14.3 volts for over 3 hours (reset to 100% state of charge reading).

There are alternative metering methods that monitor voltage:

http://www.smartgauge.co.uk/products.html
http://www.solar-electric.com/mnbcm.html

Each has its advantages and weaknesses.

Also, what have you seen as your typical maximum array current (cool/clear day, batteries needed charging, sometime around 12 noon):

• 2,160 Watts * 0.77 typical panel+controller deratings * 1/58 volts charging = 28.7 amps typical maximum current from array
• 28.7 amps / 440 AH battery bank = 0.065 = 6.5% rate of charge

Depending on how much sun you get in your area--4 hours of noon time equivalent sun is pretty good for many area (9 months of the year minimum sun on clear day):

2,160 Watts * 0.52 typical off grid 120/230 VAC system eff * 4 hours of sun = 4,493 Watt*Hours of AC energy per day

Typical battery bank design is for 1-3 days of storage and maximum of 50% discharge... Using 2 days and 50% maximum discharge, your battery bank would source:

• 440 AH * 48 volts * 0.85 inverter efficiency * 1/2 days storage * 0.50 maximum discharge = 4,448 Watt*Hours per day "nominal"

Of course, you should never plan on using 100% of an off grid system's available planned energy--Some days you will get more from the sun, on other days you will get less...

Any backup power source (grid/genset)?

How much AC (or DC) energy do you use/plan to use from your system?

-Bill