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Post by learner on Oct 5, 2016 19:41:22 GMT -8
I have the 50 watt kit and a added 35 AH SLA battery and 300 watt pure sign wave inverter. I notice when the battery is charging it does not charge in a steady straigt line. Like it reads 40% charged, then 35%, then 45%, then 55%, then 40% etc. When using the battery / inverter as soon as you plug something in it goes from 100% charged to 60%. I turn it off at 50%. But after unpluging everything the battery returns to 80 or 90% of full charge. Is this normal? How do you know the correct charge level of the battery? If the battery capacity is not large enough for the task, will it cause this behavior? An y information is appreciated. Thank you.
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Post by jsb2000 on Oct 18, 2016 5:05:08 GMT -8
What you're seeing is normal and easily explained once you understand how SoC (State of Charge) is being determined in these systems.
Most controllers, meters, etc. use voltage to determine SoC. However, true SoC requires that a battery rest in an open circuit state (IE no charging or loads being drawn) for at least FOUR HOURS. Some lead acid battery manufacturers recommend 24 hours! Any disturbance during this time will distort the reading and give you an inaccurate indication of the SoC.
Obviously it's impractical to do this in an active system. With solar, the battery is always connected and charging/discharging, often at variable rates as the sun goes in and out. It's no surprise that you're seeing the percentage of charge vary like that. Most controllers and battery chargers estimate the SoC as best they can by measuring the voltage level and how long the battery has been at that voltage.
The same thing happens when you place a load on the battery. The voltage drops and the controller guesses at the SoC based on that voltage. When you remove the load, the voltage rebounds and the SoC indication goes back up as well.
So...what to do? I've found that nothing works better than knowing your system and using human intuition to determine how well charged your batteries are. Every battery charges and discharges differently. Every solar installation has a different charging profile based on local conditions. Every person drains their batteries differently. With experience, you get a "feel" for how well you're doing. By watching how your battery's voltage changes, where it sits with no load and so on, you can tell when things are healthy and when they aren't.
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Post by learner on Oct 19, 2016 13:03:59 GMT -8
You're right, makes more sense after its explained! Thanks!
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