PD or Iota for my upgrade?

KendallP wrote:
A thirsty battery connected to a converter trying to output 14.8V at a typical RV current will force the converter to operate at somewhere more in the mid 13s until the battery approaches somewhere near 80%. This is the absorption phase where a voltage regulated charger will begin to taper its current as voltage climbs at the battery.

Study up on Peukert for more. A 12V FWC battery is a box of chemical reactions that doesn't allow linear charging from zero to 100% State Of Charge.

I previously did extensive reading on Peukert's Law, but as I understand it, it relates to discharge rate and the effect on AH capacity. I've also read up on lead acid charging and understand the different stages.

Nonetheless, Let me see if I've got this right:

The converter current limit will kick in for a "thirsty battery" regardless of the voltage limit at the converter. This is bulk stage charging. The voltage at the converter minus the voltage at the battery will tell you the current the converter is trying to supply through the cabling resistance. If that exceeds the converter current limit, the converter will drop its output voltage to limit the current. The PD9200 series has a current sensor which tells the Unitrode chip when to limit the current. It does that for each cycle. You want that limit to be no more than C/10 (C is C-20 measured as the 20 hour AH capacity) to C/5 depending on your battery mnfr recommendations.

Trojan says C/10 to C/8 in their literature, but the tech line said C/5 is OK if you watch the water closely and need to optimize rate, as I do.

As the battery charges, the voltage at the battery rises. The charger will increase its voltage towards its voltage limit to stay above the increasing battery voltage and hold the constant maximum boost current set by the current limit of the charger. Again, in the PD9200 series design, it does this automatically. The charger will only increase its output voltage up to the voltage limit.

Again, the patents on the PD9200 show that the Unitrode chip has a voltage sensor input that monitors the charger output. The charger will increase output voltage up to its limit (set by a resistor divider and some 1% resistors) to try to hold the current limit.

Because it's still the difference between battery voltage and charger limited voltage that sets the charger current, by increasing the charger voltage limit, the max boost current should start to taper off later. The charger will hold the higher current limit longer, which is what I want.

Once the battery voltage has risen sufficiently the charger voltage limit will stop any further voltage rise by the charger. The charger current output will begin to taper. The battery will remain at roughly constant voltage. This is the absorption stage (constant voltage), as contrasted with the bulk charge mode, which was constant current. Tojan recommends that the charger limit voltage to hold no more than C/5 current (literature says no more than about C/8) in constant current bulk charge mode until 90% SOC then shift to constant voltage at 14.8 volts in the absorption stage until current drops to about about 1-3% of C-20. Then they recommend going to 13.5 float. It's hard to get comparable recommendations for Sam's Club batteries by Johnson Controls.

In both the Bulk (constant current) and Absorption (constant voltage) stages there is a benefit to shifting to above 14.4 volts.