DC Negative Cables

[tpbrady]

After redoing my house back last summer, I replaced all the positive cables making them the same length from the battery to the bus bar and from the bus bar to the point where both port and starboard banks are combined. The negative cables are the same length to the negative bus bar but the negative cables from the bus bars to the ground distribution are different lengths. Should I make the negative cables the same length?

Tom

 

[RT Firefly]

Greetings,
Mr. tp. I see no reason to even up the cables. IF they are large enough to handle anticipated load, leave them be.

 

[O C Diver]

Greetings,
Mr. tp. I see no reason to even up the cables. IF they are large enough to handle anticipated load, leave them be.

I would agree with this.

Further, you can have 2 cables of the same size, length, and crimped lugs on the end, with different resistances (which is what you are theoretically trying to overcome). This is one of the reasons I prefer to oversize cable gauge.

Ted

 

[Ka_sea_ta]

It all depends, if you believe current flow is electrons, then electrons are a negative charge and flow toward the positive. In reality if you are really concerned about the length of equally sized cables in a 12 volt system, where you have no idea if the internal resistance of each cell is or the resistance of each connection of the battery cables I think you're good.

 

[STB]

If you are making the cables the same length for things in different locations, you'll end up with "extra" cable. That sounds messy to me and likely to get in the way, get damaged, etc.

 

[Comodave]

I always go oversize on DC wiring. Are the cables big enough?

 

[socalrider]

Your cables are probably sized for starting loads (300A or so on my boat for FL120s). I think the main concern behind unbalanced cables is that the battery voltages won't be equal when charging. But this always seemed unwarranted in most conditions because the time when charging voltage is important, and frankly 99% of the time overall the currents are very low (think 10A or less), so voltage drop across a cable sized for 30x that amount is negligible. Charging my bank at max is 100A (750Ah total), and I can't see it mattering too much if one bank is getting 30 and the other two 35.

That's my theory at least - please correct if I'm wrong! I tried to keep lengths the same where feasible but one of my banks is farther away from the bus and uses a 3' lead (2/0) versus the 2' leads on the other two.

If this is what eventually does in my FLAs, I'll be a happy man.

 

[jungpeter]

Hi Tpbrady (Tom?).

Should I make the negative cables the same length?

I think the answer to your question lies in what you're trying to achieve in your battery system refurbishment. If, indeed, you're simply attempting to achieve a balanced battery bank, with all batteries contributing equally to energy storage, then proper battery cable lengths (including the negative grounds) and configuration are vital. You may wish to read up on the benefits of proper battery bank balancing from sources such as SmartGauge Electronics - Interconnecting multiple batteries to form one larger bank and numerous related articles from https://panbo.com/ and https://marinehowto.com/ and go from there.

If, however, you're attempting to attain information sufficient to properly design, install, test and operate a sound marine electrical system from this forum, you'd be far better off to consult an ABYC-certified marine electrician for help. This is seldom a DIY project, at least to do it safely and correctly.

Regards,

Pete