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Old 05-21-2018, 10:01 PM   #1
Join Date: Jan 2018
Posts: 52
How reliable are my charge controllers 'State of Charge' Indicators?

Hi everyone,

We've just had our solar system set up, however I am running into an interesting situation.

We are plugged in to shore power all night right now which is charging our batts to basically full. Then we have had direct sunlight all day (unplugged).

Our Tri-star PWM 45 amp controller flashes green and the manual indicates that means we are at 80-95% SOC (state of charge)

However, within maybe a few hours of the sun going down the charge controller is indicating 0-35% state of charge.

Has anyone experienced this? I'm struggling to find information online.
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Old 05-22-2018, 08:35 AM   #2
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Join Date: Feb 2017
Location: The West
Posts: 1,198
Year: 1998
Coachwork: MCI
Chassis: 102 EL3
Engine: DD 60
Anything that uses voltage to indicate SOC for a battery that is in use is a waste of time.

The only time that voltage is an approximate indication of SOC is after the battery has been resting for many hours.

A good battery monitor is a very useful tool. These use a shunt to measure all electrons flowing into/out of the battery and, based on a some calculations/configuration data, give a reasonable approximation of SOC.
JD - Full timer out west
Missy - 1998 MCI 102-EL3 - 1.7kW Solar - 10kWh Lithium
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Old 05-22-2018, 02:02 PM   #3
Bus Geek
Join Date: May 2016
Location: Eastern WA
Posts: 6,380
Year: 2002
Coachwork: Bluebird
Chassis: All American RE (A3RE)
Engine: Cummins ISC (8.3)
Rated Cap: 72
If you are running flooded lead acid (FLA) batteries the most accurate method of measuring SOC is with a temperature compensating hydrometer like this:https://www.amazon.com/OTC-4619-Prof.../dp/B0050SFVHO

The battery monitor is a handy tool but not the best indicator of SOC.
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