LiOn Battery Charging, Use Store Switch

[Wross]

2013 25’ RB Flying Cloud
About 2 years ago we replaced the old lead acid batteries with 2 Lithium Ion batteries each 100 AH. I relocated from the outside battery box to the empty space behind the converter.

I also replaced the Converter power supply and battery charger with a LIon 14.4 Volt charger, and added a Victron Smart Shunt.

Recently we’ve had a charging issue with the Victron showing a maximum voltage of 13.4 volts, so the batteries are not fully charged.

I measured with my meter and confirmed 13.4 volts at the battery terminals, and measured at the converter and saw 14.4 volts at 40 Amps. Looking for the voltage drop I found a 1 volt drop across the Use - Store Latching Relay controlled by the switch by the door.
I thought it was an easy fix, ordered the exact replacement “solenoid” Latching Relay. I replaced it yesterday and tested the results.
I found the same 1 volt drop across the new relay! (40 Amps x 1 volt is 40 watts of power, heat across the relay contacts!)

When we connect to shore power the converter powers up and automatically switches the latching relay on to energize the 12 volt system in the Airstream. I don’t need to push the “Use Switch”, it just happens.

In this situation I can measure the voltage drop across the relay.
If I push the Use Switch by the door a couple of times the voltage drop decreases to millivolts, and the charge voltage to the batteries is 14.4 Volts, the batteries are properly charged and reach 100% State of Charge.

I saw this same thing happen with the old relay, and now with the new relay.

My working theory: in the power up process when connecting to shore power (or flipping on the breaker to the converter) the self latching relay does not get a strong enough pulse to properly latch, hence the resistance and voltage drop. Most times we don’t notice because the converter is powering the Airstream so everything works fine from the converter.

My ask: have you experienced this issue? What is the best solution?

I did add a rotary bright red battery disconnect switch in the battery compartment. I don’t think I could get spousal approval for the aesthetics of a bright red rotary switch in our Airstream 😉

Regards,

Walter

Austin, TX and Madeira Park, BC.

[Wazbro]

Since you have a rotary bright red battery disconnect switch in the battery compartment, you don't really need another disconnect you can just remove or bypass the store/use switch and relay.