Re: [electricboats] Common DC ground with multiple banks

The devil is always in the details.  Grounding for home construction is not an option.  For boats, historically, like cars, we tend to ground the negative and also tie that to any shore power ground that is brought onboard.

Beyond that, there's APYC and perhaps other rules/guidelines.

Beyond that, there's what you actually have chosen for components---and those can drive you directions you may not have thought about.

 

Take my boat for example:

 

DCDC: For 12v cabin power, I chose to use a 36-48v to 12v 30amp DCDC.  And while I initially thought I'd have a 12v pack somehow attached to the output of the DCDC.  As time went on, that need and even the idea pretty much died and I've never added a battery there (now 20years running).  Having said that, I'm guessing I'll add a 5-7ah 12v lithium battery there for kicks sometime soon.  Anyway, that's immaterial.  The DCDC that I am using is an old GE DCDC probably designed for golf cart or similar use.  Now here's the thing: This DCDC does use a common----the PLUS side of Input and Output are shared!  So, the 12v minus for my house power is not only NOT grounded to the NEG of the pack, nor is it floating, but rather is 12v less than whatever the Pack+ is at the time.

 

CONTROLLER: DC brushmotor controllers like those long used for golf carts have similarly driven the motor by sharing the "+" side and switching the battery's "-" side to drive the motor's other lead.  So here again, there's no way one of the motor leads can be grounded to pack minus, but indeed it is connected to pack plus (thru fuse, solenoid, switch) during operation.

 

Anyway, in general, if you can ground the negative side of your pack(s) and ground that to shore power, it is likely advisable.  However, there are circumstances and configurations in which doing this not only does not make sense but would be absolutely dangerous.

 

-Myles T.  www.evalbum.dom/492

 

 

From: electricboats@groups.io [mailto:electricboats@groups.io] On Behalf Of Jack Agarwal
Sent: Wednesday, March 15, 2023 12:24 PM
To: electricboats@groups.io
Subject: [electricboats] Common DC ground with multiple banks

 

In my planned design I have two 48v propulsion Lifepo4 banks (one in each hull) and a 24v Lifepo House bank.

The propulsion banks will be grounded by their respective pod motor which will be fixed in the water.

How should I handle the negative cabling? Is it okay or advisable to have a single common negative bus which all banks negatives are connected to? 

The 24v house bank is quite far from the propulsion banks, and I plan to charge it with a simple isolated DCDC converter from each propulsion bank. In this case is it necessary to have a common ground between the two banks or can I essentially operate them as two separate systems?

any advice appreciated!
Jack