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--- electrical:12v:battery_monitor [2024/02/11 14:37]
frater_secessus [Victron SmartShunt]
+++ electrical:12v:battery_monitor [2024/04/26 14:37] (current)
frater_secessus [TL;DR]
@@ Line 1: / Line 1: @@
 ====== Battery monitors ======
-We want battery monitors be "gas gauges" but they are not. The monitor is more like the Range prediction on the dash that uses historical and present information to make an educated guess about how far you might get.
+===== TL;DR =====
+[[https://amzn.to/4bxDjw8|{{ https://m.media-amazon.com/images/I/71g0M+mQjIL._AC_UL320_.jpg?100}}]]
+  * Battery monitors start from a known State of Charge then count every Amp going into or out of the battery bank
+  * the current acceptance of the battery bank in Absorption can tell you how close to fully charged a lead battery is((applies to Li also but the numbers are much smaller and we are not chasing 100% necessarily))
+  * these measurements can go astray ("drift") over time and require recalibration
+  * drift will be more noticeable with lead chemistries than lithium
+===== overview =====
+We might want battery monitors be exact "gas gauges" but they are not. They do their best to keep track of incoming and outgoing current to estimate present [[electrical:depth_of_discharge|State of Charge]].
@@ Line 10: / Line 23: @@
   * a calculation on capacity used/remaining
-For the monitor to keep accurate count, power **from all charging sources** and **to all loads** must pass through its //shunt//, a relatively heavy piece of metal that reacts predictably to current. Typically the shunt is placed between the battery bank's negative post and the systems's "ground".((negative/return side of the circuit))  The monitor display is mounted remotely where it is convenient to view.
+Typically the shunt is placed between the battery bank's negative post and the system's "ground".((negative/return side of the circuit))  The display is mounted remotely where it is convenient to view.
+Heads up:  there are //uni//directional monitors sold for purposes other than battery monitoring; read the specs and reviews to make sure what you are getting.
+===== sizing =====
+The shunt should be sized to handle your highest expected demand comfortably. For example, a system designed for 80A of current would work well with a 100A shunt.
+A shunt that is **too small** a shunt will fail if exposed to current that exceeds its rating.
+A shunt that is **too large** presents different problems.  Firstly, it will usually be more $$$ than a correctly-sized shunt.  Secondly, each shunt is designed to measure a certain range of currents accurately.  A 1000A shunt designed to measure many hundreds of amps accurately will not do a good job measuring much smaller currents like our 80A scenario above.
-Most people will choose a "bidirectional" monitor (counts both charge and discharge amps); read the specs and reviews to make sure.
 ===== how they work =====
@@ Line 18: / Line 41: @@
   - you tell the monitor the battery's [[electrical:12v:battery_capacity|capacity]], usually in Ah and printed right on the battery.  Remember that lead batteries are typically only used to about 50% of their rated capacity.  You'll either be making that mental adjustment when you configure the meter or when you read it later.  Battery capacity will also degrade over time, requiring the capacity setting to be adjusted periodically.((every 6 months? 12 months?))
-  - the monitor will watch for your system to hit **the configured 100% state of charge voltage**, at which it assumes your bank is "full".  It will also stop at 100% if it counts enough amps going in.  Then,
+  - the monitor needs to know where it is starting from.  This is either 0% (empty) or 100% (fully charged).  You might [[electrical:12v:battery_monitor#drift_and_reset|manually reset]] the monitor at those points, or it might automatically reset to 0% or 100% based on high or low voltage setpoints.
-  - when your system drops below that voltage **it counts Amps/Watts in and out**, thereby calculating the percentage of capacity used and percentage remaining.  This is an imperfect science with lead batteries because 100% SoC is only reliable after a //complete// charge, and because different discharge rates will change the effective capacity.   Think of the monitor as general guidance for lead batteries rather than gospel. Lithium banks will align more closely with what the monitor shows.
+  - as your bank charges/discharges  **the monitor counts Amps in and out**, thereby calculating the percentage of capacity used and percentage remaining.  **Any loads that bypass the shunt cannot be counted.**
 Note:  [[electrical:solar:status|Watching amps trail off]] at the end of lead Absorption (endAmps) will also tell you when the bank is [[electrical:12v:charging|fully charged]]; you might manually reset to 100% at that time.
@@ Line 25: / Line 48: @@
 ===== drift and reset  =====
-Small counting errors accumulate over time and reduce accuracy.  Errors are more likely when current is either very low (<1A) or very high (the monitor's upper limit).  For this reason the monitor is regularly reset ("zeroed", "calibrated") when the bank is at a known state of charge.  This can be done at 0% or 100%; for practical reasons most owners will reset them when the bank is known to be at 100%.
+Small problems accrue over time, reducing the monitor's usefulness:
-One rule of thumb is to reset the monitor 1x/week.  There is no harm in doing a reset more often if you happen to notice it's sitting at 100%.
+  * the **measurement can be inaccurate** because current is very high or low relative to the shunt's working range.  To reduce these errors the shunt should be big enough to handle your largest expected simultaneous currents comfortably without being grossly oversized.
+  * the measurement and counting is **accurate but is not reliable for estimating changes in state of charge**.  This isn't a significant issue with lithium, but lead chemistries have substantial charging inefficiencies((10-20% in Absorption and 100% in Float since that stage is just offsetting self-discharge)) and consume more capacity than the count would suggest at higher currents (Peukert effect).
+  * or a combination of the above
+For those reasons **the monitor needs to be reset regularly** ("zeroed", "calibrated") when the bank is at a known state of charge.  This can be done at 0% or 100%:
+  - automatically by the monitor when a certain voltage setpoint is achieved.((14.0v = 100%, for example))
+  - manually by the user.  For practical reasons this reset is usually performed when the user observes the bank to be 100% SoC.
+One rule of thumb is to reset the monitor 1x/week.  There is no harm in doing a reset more often if you happen to notice it's sitting at 100%.  [Hopefully you won't regularly be encountering 0% - secessus]
 ===== gotchas =====
@@ Line 49: / Line 79: @@
 ==== Victron SmartShunt ====
-[[https://amzn.to/4bxDjw8|{{ https://m.media-amazon.com/images/I/71g0M+mQjIL._AC_UL320_.jpg?125}}]]
+[[https://amzn.to/4bxDjw8|{{ https://m.media-amazon.com/images/I/71g0M+mQjIL._AC_UL320_.jpg?100}}]]
-The 500A SmartShunt is available in [[https://amzn.to/3IfWai1|as the bare bluetooth-enabled shunt]] or [[https://amzn.to/4bxDjw8|bundled with a display]].
+The 500A SmartShunt is available in [[https://amzn.to/3IfWai1|as the bare bluetooth-enabled shunt]] or [[https://amzn.to/4bxDjw8|the BMV package which includes a display]]. The BMV also adds programmable visual and audible alarm and programmable relay.((https://www.victronenergy.com/upload/documents/Datasheet-SmartShunt-EN.pdf))
 Both setups can provide voltage, current, and temperature information to the app or directly to other Victron gear like solar charge controllers.((the Orion-TR is a notable and much-lamented exception))
@@ Line 72: / Line 102: @@
 === AiLi ===
-[[https://amzn.to/49wa9eV|{{ https://m.media-amazon.com/images/I/51-5-oQoDAL._AC_UL320_.jpg}}]]
+[[https://amzn.to/49wa9eV|{{ https://m.media-amazon.com/images/I/51-5-oQoDAL._AC_UL320_.jpg?100}}]]
 The most popular inexpensive shunts are probably [[https://amzn.to/49wa9eV|the round AiLi]]. They are somewhat cheaper off ebay and can be found for ~$25 shipped off AliExpress if you can wait that long.((Feb 2024 prices))
@@ Line 91: / Line 121: @@
 By default the backlight is ON when  discharging and FLASHING((slow pulsing)) when charging.  The backlight can be toggled OFF/ON by holding the LEFT+RIGHT buttons at the same time. With the backlight disable charge vs discharge is still displayed by small icons on the left side of the display.
+== quirks ==
+The decimal point is small and moves as numbers increase.  Examples:
+  * 1**.**234A (also 0.123A)
+  * 12**.**34A
+  * 123**.**4A
 === Renogy ===
@@ Line 101: / Line 140: @@
 === other examples ===
-[[https://amzn.to/2BO0Eto|{{https://images-na.ssl-images-amazon.com/images/I/41ZuQEfExvL._SY90_.jpg }}]]For folks who are [dis]charging at 50A/75A, a cheaper shunt is available for 1/5th the price of a Bogart.
+[[https://amzn.to/2BO0Eto|{{https://images-na.ssl-images-amazon.com/images/I/41ZuQEfExvL._SY90_.jpg }}]]For folks who are [dis]charging at lower currents like 50A/75A, even cheaper shunts are available.
 Heavier-duty shunts are available (up to [[https://amzn.to/2XBeDiN|at least 350A]]).

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