Guru
Joined: 13/10/2014
Location: AustraliaPosts: 1860
| Posted: 06:14am 28 Jul 2024 |
I've been thinking about Inverter low battery voltage Ramp down and restart, obviously some of us have various ways to handle this outside of the Inverter - but not all external monitoring allows for control of a large DIY Inverter.
Having this setting in the inverter is common in almost every commercial Inverter I have seen, it's up to you if you want to employ it, if you think this is a bad idea, please stop reading.
I'll repeat -> If you think this is a bad idea for ANY reason, please stop reading!!!! you are missing the point.
Soooo for anyone who wants to employ this setting in a basic off-grid system for Inverter control, I have the following settings:
Before I start and get dumped on, I guess I must state the bleeding obvious for some:
If an installation has not been correctly built, you will have more trouble than just the battery Cutoff settings - knowing your setup and monitoring various state of charge and voltages under system loading is something we need to do and record if we plan to set it up correctly.
Low voltage cutoff will be problematic if the battery to inverter DC path is not wired with appropriate gauge cable and correctly terminated at every point - no weak links allowed in any large inverter.
Back to the Inverter Controller:
Low voltage ramp down and restart voltage points are what I have been testing and pondering over, and the implementation of Low voltage Cutoff logic, it's obviously not as simple as it appears because of different Loads, battery capacity and technology employed in out individual DIY off-grid Solar and Inverter builds.
Large LFePO4 bank voltage differences from charged and running under "average loads", to running at a lower SOC, is often measured in millivolts down to 40% SOC. In battery types with slightly higher internal R, the delta is often measured in volts from a running charged state down to around 40% SOC.
There is voltage sag under heavy loads, and there can be high transient startup loads or short duration increased loads, such as a fridge, freezer or electric jug, and all voltages are impacted by the current state of charge and system build type and quality.
The transients are easy to handle with a simple short timeout period for Low voltage cutoff setting, but you may want to extend the timeout period for that fridge or Jug, especially when batteries are getting down and operating near the cutoff voltage, this may not be to much of a problem with a good large LFP battery bank, but could pose a problem, depending on size and SOC, with older battery technology's.
I've come up with a workable solution for my Inverter and I have implemented the following:
These additions do not complicate the code or the operation of the controller in any way.
An adjustable timeout setting "In Setup" for low battery voltage cutoff, and in extending that timeout period, I also reduce the timeout period if the voltage drop is more than 10% below the normal cutoff voltage setting.
If 10% below cutoff, the inverter will stop almost immediately and wait for the restart voltage set point, an adjustable restart timer "In Setup", is then implemented to allow the batteries to attain some SOC.
The restart point could be tricky if set low, and even then, with LFP and lower charge rates, or high charge rates with high current chargers, the inverter could restart with a low OR no change in SOC.
Yes !! we ALL know that voltage is a very poor indicator of SOC, and especially in a dynamic situation when charging, I don't need to have that pointed out for the umpteenth time as though I'm some noob - Steps off soapbox.
1: I've implemented "low AC current" logic for Low battery voltage under 3A ac, if less than 3A and below cutoff, stop inverter.
2: If AC current is above 3A, and if the voltage drop is more than 10% below the low voltage Cutoff, stop the inverter.
3: Once the Voltage is above the restart voltage, (it's charging) and an "adjustable" Delay period "In Setup" starts a countdown before the inverter is allowed to ramp up again. The delay period can be set from 1 minute to many hours.
This at least gives the batteries some time to charge, or it can simply be be used to stop a possible charge to load low voltage trip cycle condition. Again if interested, the Controller Hex code has been uploaded.
Download
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Edited 2024-07-29 10:25 by KeepIS