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 batteries have a maximum amount of current they will accept, typically 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.¹⁾

Deep-cycled lead batteries have a lengthy Absorption stage that cannot be sped up²⁾. 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 - MaineSail³⁾

MaineSail found that

• charging AGM at C/2.5 (max rate) resulted in the battery being 63.3% “full”⁴⁾ 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 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 configurations⁵⁾, as slower charging regiment would reduce the required Absorption duration.⁶⁾

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 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.⁷⁾

In commercial examples the BMS will limit current (typically to 1C) to protect the cells. A small 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.

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