Problem: Following an underway with my boat with an already nearly fully charged battery, when plugging in shore power and the inverter/charger starting to charge battery, it stays in an absorption mode far too long (7 hours). Plus, the volt/current profile more often shows steadily lowering voltage and steadily increasing current over the 7 hours.
Details: I have a 24V/5000/120-100/100 120V Victron Quattro and six 12vdc Victron Telecom batteries in series/parallel connection for a 24vdc system. Most of the time the max current after the 7 hours runs around 6-7 amps, but once it was up to 15 amps and even set off the propane detector due to excessive current. The offending volt/current profile attached. The Quattro is new as of Aug 2024 and the telecom batteries were new Sept 2021.
Simply, what’s the most likely cause? I see the typical list of causes, but am wondering if the lowering voltage/rising current isn’t a tell tale of the problem.
Is theoretically not in absorption if it is not at target voltage. That would be bulk.
If it has reached target and slowly goes down, that would usually then be going to float lowering to float voltage.
A larger drop on shore loss if it is not a huge load may indicate weak batteries (but is also an indication of a lack of balance or DC wiring that could be improved.)
Do you have balancers in the bank. Have you ever rotated the batteries in the bank? Check them as individuals. Often in built banks a 12v separate charger is good to help with bank health.
You are overcharging your batteries and they are getting too hot. Do you have a temperature sensor on them and if so is temperature compensation enabled. If temperature compensation is enabled then the absorption voltage will be lowered as they get warmer. The trouble is when overcharging AGM they get hotter which reduces the resistance letting more current through so they never get to tail current setting and go to float.
This commonly happens when a boat leaves dock with full batteries then goes on a long passage under engine power and the alternator does not have an external regulator so massively over charges the batteries for hours. The CO monitor going off is caused by it picking up hydrogen being released by the batteries.
With a bank of AGMs consider reducing your absorption voltage to 14.1 or 14.2V, make sure max absorption time is set to perhaps 2-3 hours, increase tail current transition to float and get an external regulator. Also consider when docking after a passage to delay hooking up shore power so the inverter does not try charging with absorption.
Several other posts here on the topic.
I spotted the very early stages of it on my boat with Victron Telecom batteries a couple of times during hot days and long runs so I have fitted a Wakespeed WS500 alternator regulator.
Forgot you are 24V so reduce absorb to 28.2 to 28.4V to reduce likelihood of overcharging.
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Nice. Was coming back to add something similar. Definitely temps compensation or overcharging.
Another thing, if you have temperature compensation enabled did you leave it at the default setting (32mV/degC 18mV/degF for 24V batteries). AGM need a higher compensation so the voltage drops faster as they warm up. For my Telecoms I found Victron do not quote a value, so I used the value from the rest of the Victron AGM range of 48mV/degC 27mV/degF.
https://www.victronenergy.com/upload/documents/Datasheet-GEL-and-AGM-Batteries-EN.pdf
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