TalkBaja.com » Baja Mexico Forums » Off-Grid / Solar, Wind or Generator?

I am going to go out on a limb and assume that many of you living off-grid obsess as much as I do as to their battery bank SOC (state of charge). Although I don't have my system all put together or panels all up, I did wire up my battery bank fed by six 150 watt panels. That gives us a fair amount of power to operate during the day and even watch the big screen on HD for several hours at night. Of course I have to check the battery status one last time before turning off the lights and again first thing in the morning.

Has anybody else noticed that different sources give different battery SOC specs/suggestions? Here are some that I found - it can be really confusing to neophytes like myself...



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Volts are only one part of the picture in measuring the state of the batteries. Volts is more a measure of the potential, or active force that the electricity is pushed by the batteries. Amps is really a better measurement of how full the battery is. (Volts in a wire is similar to water pressure in a hose, amps being used is similar to the volume of water coming out of the hose, amp storage in a battery is like the amount of water stored in a cistern).

The problem using volts to measure charge is that voltage readings are affected by the load on the system, temperature and other factors. If you have something on, it will draw down the voltage and give an inaccurate reading. It's best to measure volts with no load or active charging going into or out of the batteries. It's a good measurement, but doesn't give the whole picture. Your battery mfgr. may have a voltage chart specific to your batteries.

Using a meter that shows the amount of amp-hours being used or "amp hours below full" is a much more accurate measure of the state of the batteries. Trimetric is one of the least expensive ($140.00) and will show an accurate % of charge, how many amps you are currently using, how many you've used, and a lot of other useful figures.

Trimetric gives the figures below for using voltage as a state of charge, as well as some of the variables. One more chart for your collection

I used to obsess over the system, but once you get a better handle on what you can and can't do for the given conditions (cloudy, foggy, sunny) you'll find you'll be more comfortable and might even go two or three days without looking

Now were talking Solar....

Obviously the SOC voltage will be different based on whether you are charging/discharging the batteries and the temperature at which you measure them. What struck me was how folks like Trojan, Surrette and other respected sources differ so greatly on the charge % levels.

I really like your amp-hour meter suggestion Wooo and will start looking online for one today...

Originally Posted by BajaGringo

I ... will start looking online for one today...

http://store.solar-electric.com/tm2020.html

This is the on I use. So far it's been very reliable.

I was on their website and had added that to my cart until I saw this...

PentaMetric Three-Channel Battery Monitor System

Since you're running with two different battery banks, that would probably be a good choice. I believe the Trimetric can only monitor one bank.

I have always felt that knowing a battery's state of charge in a working solar system is difficult to determine, and for the most part is not very useful information. In a working system your batteries are always either being charged or discharged, so the state of charge is always changing. Sure the charts show all sorts of different numbers for determing the charge but unless your batteries have been sitting idle for a number of hours, the voltage that your batteries are at, at any given time doesn't mean much.
I have always felt that knowing when your batteries are fully charged and stopping the charge cycle then and going to float is more useful info. I charge my batteries until they reach the absorb voltage, then my controller holds them at that voltage, and the charge current tapers down for X amount of time. I then watch the controller and when the amps going into the batteries reaches 1% of the amphour capacity of the bank, I tell the controller to go to float charge. I my case, I have a 1600 amphour bank, after about 2.5 hours the charge has tapered down to 16 amp (1%) going into the batteries so at that point I let the controller go to float. Actually my controller is only programable in 1 hour increments so I let it absorb for 3 hours. Using that method and a good battery monitoring system will help to keep your batteries healthy for a long time.
Hope this helps someone,
Larry
PS: I use agm batteries, but if you have flooded lead acid batteries then a temperature compensated hydrometer woud be a good tool to have also.
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You are right Larry. I probably fell victim to reading "too much" info before I got my system started and I was a bit paranoid about my batteries when I first put them online.

I have been slowly expanding my system and been running some tests on a small, separate battery bank with my wind turbine. I am now up to 1.5kW in panels and have added a few more batteries so that I now have 1200 amp hours at 24 volts. I am building what will be my main control board / panel now. It will have all the fuses and breakers I should have put in the first place as well as a few shunts for monitoring data to my Pentametric. I will also be installing the new, updated diversion controller setup for the wind turbine and dump loads.

I do feel a lot more comfortable now with my system and it is true that you get a real "feel" for your batteries and their state of charge over time. The wind turbine test has gone very well and I am looking forward to get it incorporated into the main system in the next week or so. Adding an average of 5 to 8 amps at 24 volts into my battery bank all night after the sun goes down has to be a good thing. It will definitely go a long way to relieve any remaining concerns about my SOC when watching the big screen or running my audio gear at night once I get that set up...