Conversion Process - Electrical - DC to DC converter wiring modifications .....

Am using a Sevcon 622/11086 DC to DC converter (72v Input/13.4v output @300 watt) to charge a small PowerSonic deep cycle accessories battery (as required by law here for EV conversions). But I have found out the hard way that due to the large surge current and voltage at switch on, that switching the converter on/off via a solid state relay is NOT the thing to do - the SSR can’t handle the high in rush currents on start-up of these converters and after the first switch on the SSR is a piece of wire! They do not have a “soft-start” feature in their design. This could be overcome by adding say a .1 ohm resistor in series with the input of the Sevcon but I decided to look at a few other issues.....

The questions?
1 - Is it actually necessary to disconnect the DC to DC unit from the main 72 v battery pack or can one simply hard wire the input and leave it on continuously? I see some conversions do this….
2 - When fully charged the Sevcon goes to a “float” condition (at 13.4 volts) thus maybe it makes sense to leave the converter connected as complete charging could take longer than the driving time would allow - for example under extreme load conditions on the aux battery it might not recharge completely in subsequent short drives if it was only being charging during vehicle on time.
3 - Once charging the aux battery is completed, how much input “standby” current would be required from the main pack?

So I set up some test conditions and made some measurements.....

The results are:

1 - Under some conditions the 300 watt Sevcon would need to be ON (after the vehicle was used) to completely charge the aux battery

2 - The standby current once the aux battery reached 13.4 volts was less the 30ma and not a major concern in discharging the main battery pack

 So given the Sevcon is a quality product (meets ip67 standards) I have decided to simplify the wiring, not use the solid state control relay and go with a permanent hardwired solution.

Please note MightyboyEV Motor Bay Wiring Diagram version 1.11 onwards reflects these wiring changes

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