====== charging faster isn't always better ======


There is a common misperception that charging one's battery banks //faster// is //better// or that having more charging capacity guarantees faster charging.  Here are some reasons it is not always true:


===== lead-acid =====



**Lead batteries** have a **maximum amount of current** they will accept, typically [[electrical:12v:battery_capacity|C/3]] for AGM and C/5 for flooded. So if you have 100Ah of AGM the most it will want to take is something like ~33A.  Once the battery's limit is hit throwing more charging capacity at it can't charge it any faster.((it may help run loads, though))

** Deep-cycled lead batteries have a lengthy [[electrical:12v:charging|Absorption]] stage** that cannot be sped up((at least not more than a few percent, see below)). Depending on use, Absorption typically requires several hours.There is a saying that "it takes 9 months to make a baby, but two women cannot make a baby in 4.5 months".  Same with absorption - it takes what it takes and you cannot hurry it in the normal sense.

However:  

> charge current affects the SOC transition point from bulk to absorption charging - MaineSail((https://marinehowto.com/how-fast-can-an-agm-battery-be-charged/))

MaineSail found that 

  * charging AGM at C/2.5 (max rate) resulted in the battery being 63.3% "full"((amp-wise)) at Vabs and being fully charged in **5hrs, 30 minutes** (20min Bulk, 5hr 10mins Absorption)
  * charging AGM at C/5 (min rate) resulted in the battery being 77.4% "full" at Vabs and being fully charged in **5hrs, 42 minutes** (76min Bulk, 4hr 26mins Absorption)

Note that the **using 2x the charging current only sped up the whole process by 12 minutes (3.6%)** due to Vabs starting earlier but taking longer. 

  * **Getting to Vabs faster** could be [[electrical:solar:pwm_tweaking|preferable on PWM controllers]], as the system would be making max power for a longer period of time.
  * **Getting to Vabs slower** could be preferable on timer-based Absorption configurations((like 180 minutes)), as slower charging regiment would reduce the required Absorption duration.((180/266mins rather than 180/310mins))


===== lithium =====


**Lithium batteries** can slurp up current wildly, but shouldn't for their own health.  Lithium lasts longest in sub-C use (ie, less than 100A for a 100Ah Li bank).  0.5C (50A) is a more common manufacturer recommendation, and folks [[electrical:12v:drop-in_lifepo4#an_approach_to_greater_longevity|striving for added longevity]] might limit it to 0.2C (20A). There is some evidence that 0.4C rates are acceptable for long life when combined with gentle charging voltages like 3.45Vpc.((FIXME  - insert Rod's testing of 2011 bank here))

In commercial examples the BMS will limit current (typically to 1C) to protect the cells.  A small [[electrical:12v:b2b|DC-DC charger]] can help protect bare cells from their own greediness.



Limiting charging  current also makes it easier on alternators when they are used to charge Li.  






===== exceptions =====

  * solar charging - sunlight is only available during a short window, as when camped in a forest canopy with a clear overhead view.  Note that [[electrical:solar:output|poor insolation]] calls for [[electrical:solar:rationing#maximizing_solar_harvest|increased harvest]] / [[electrical:solar:overpaneling|overpaneling]], not faster charging necessarily
  * [[electrical:generator|generator]] charging - for efficiency purposes the generator is shut down after Vabs is reached.  Generators run most efficiently when fully loaded. 
  * [[electrical:converter|shore]] charging - if you only have a certain amount of time on the extension cord
  * [[electrical:12v:alternator|alternator]] charging - if you will driving for short periods only