Good morning, on a small boat I have problems with swelling of the Victron AGM super cycle 12V 170Ah batteries charged by a Multiplus 12/1200/50 with a Skylla ip65 12/70/3 in parallel. Questions:
can the 2 battery chargers be connected with the outputs in parallel?
what could be the causes of the swelling of the batteries? What checks could I do? (when they are not recharged by the 2 battery chargers, during navigation the batteries are recharged by a 14V alternator). There is no temperature control on the batteries.
Thanks
Battery swelling is a sign.of overcharging, continuing charging once they are full. This is often seen in boats that can have long engine runs with batteries in poorly ventilated spaces. The engine alternator is normally regulated to 14.4V to 14.6V, so once the batteries are full, they alternator does not go to float but stays at 14.4V and overcharges the batteries. You need to consider an alternator regulator that goes to float such as Wakespeed or Zeuss etc. This problem is made worse if a boat is at dock on shorepower with full batteries at the start of the passage.
The problem is further compounded because you do not have temperature compensation. As the batteries get full they warm up and resistance decreases, allowing more current to flow. As the batteries are full all this current goes into heat, accelerating the overheating. A proper alternator regulator will reduce charging voltage as the batteries warm up.
The other common feature on boats is battery banks made up of multiple parallel batteries. In this case, one or two batteries will have lower resistance or the wiring interconnects have lower resistance so those batteries are affected.much more than the others.
One thing that can be done is set your charge voltage more conservatively at say 14.2V. However, with a basic alternator you do not have this option.
Thanks for the detailed information. I will try to measure the charging voltages while sailing with the boat to investigate the problem. Regarding the parallel of a skylla and the multiplus that simultaneously charge the battery bank, is this a correct configuration?
I missed the parallel charger question, yes it is OK to have them in parallel, make sure that they both have the correct absorption and float voltages. Once absorption is reached one will probably reduce current first rather than both together. Just make sure that in bulk, both chargers are below the max allowed charging current for your batteries. You do not state your total battery capacity, but you have 120A of charging available, which at max charge rate of 0.2C equates to a battery bank of 600Ah. If your bank is smaller than this then your charging rate will be higher than the max rate.