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====== Voltage sag ======

[Note:  this article is written from the standpoint of the [[electrical:solar:charge_controller|solar charge controller]].  It affects all circuits to some degree and is not limited to solar. -- secessus]

In typical installs the charge controller doesn't know the actual battery voltage;  it only knows what it sees on its own BATTERY terminals where the wires are connected.   Wire sizes, junctions, current, etc, can cause this voltage reading to be inaccurate.  Which can throw off charging.  Which can affect battery perfornance and/or [[electrical:batterycide|longevity]].


Extreme example:  During heavy charging the controller might see 14v instead of the battery's 13v.  This 14v -> 13v difference is called voltage sag.  A similar thing happens in reverse when consuming power.  The battery might be 12.5v but an inverter running a big load might "see" 11.5v at its own terminals and shut off prematurely.((from the user's point of view)).  It also makes it difficult to assess [[electrical:depth_of_discharge|state of charge]] of lead batteries by voltage.

Sag presents special challenges when charging [[electrical:12v:drop-in_lifepo4|lithium]] battery banks.    Since the majority of lithium's usable power is found in an **extremely narrow voltage range**((about 0.5v wide)) sag can result in wildly unpredictable/inconsistent charging.
===== approaches =====


==== do nothing ====

This is the majority position since most are unaware of the issue, or don't judge it to be important.  And it may //not// be a problem since since

  - solar-only charging is often low-current charging, minimizing charge-induced sag 
  - lead-chemistry batteries often have a range of Absorption and Float [[electrical:solar:charge_controller_setpoints|setpoints]] (see below)


==== shorten the circuit ====

All other things being equal, a **shorter wiring run will have less sag** than a longer run.  For this reason charging sources are mounted **as close to the battery bank as possible**.  





==== beef up the circuit ====

Once the length of the circuit is established voltage drop along the circuit can be minimized:

  * run bigger wires to minimize sag - see [[https://www.bluesea.com/resources/1437|this BlueSea article]] with the well-respected wiring size chart
  * minimize the number of connections/junctions


==== adjust charging setpoints ====

If your sag is consistent you could adjust your setpoints upwards.  If you have 0.2v sag and want 14.0v you would configure the setpoint to 14.2v. 

Or if your battery manufacturer offers a range of charging voltages like 14.0v - 14.4v you could configure 14.4v and know it will be ≤14.4v in real conditions. 


==== voltage sense wire ====

A voltage sense wire is a separate wiring circuit running from the battery bank to the controller's voltage sense terminals (if so equipped).  

Since the circuit isn't carrying any real current there will be very little sag and even quite thin wires can be used to get an accurate voltage reading.  The controller will then adjust output voltage to put the desired setpoint voltage into the bank.

Example:  The absorption setpoint (Vabs) is 14.0.  The sensed battery voltage is only 13.8v, so the controller actually puts out 14.2v.((or whatever it takes to get 14.0v at the battery))

Some sense "wire" are wireless ([[https://amzn.to/3IbnnQg|Victron Smart Battery Sense]])


==== networked shunt ====
 

Some controllers use networked [[electrical:12v:battery_monitor|battery monitors]] to report back what the battery voltage is in reality.  

Examples:  

  * [[https://amzn.to/3tiqlhu|Victron Lynx shunt]]
  * [[https://amzn.to/3qbfcNJ|Victron VEbus smart dongle]]
  * [[https://amzn.to/3ijF6u9|Victron BMV-712]].




==== calibration ====

The controller may also have a **voltage calibration** setpoint.  If your sag is consistently -0.2v you could input that calibration.  You put in 14.0v or whatever you want and the controller increases voltage by 0.2v.  

Unfortunately, sag varies with current so the user might need to figure out average sag in their use case.
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