What About Termination Resistance?
As a marine electrician I have to work in the real world of actual measurements based on the as installed system.. Frustrating as it may be the marine industry, most books and even the ABYC have let us down on properly sizing our systems for voltage drop. The key word here is “system“. As related to voltage drop the “system design” should really include, the additive drops across all switches, terminations, fuses, isolators etc. etc. and were we are “sensing” the voltage.
Here is a prime example:
Marine tech John Doe sized for a 3% wire voltage drop for a customers new “high output” alternator. The 3% figure he used was based solely on the wires voltage drop. A 3% drop at face value, is already a -0.44V drop at 14.7V and will lead to slower charging. This is far from ideal for an expensive performance charging system where the owner expects short run times and maximum energy storage. To further complicate matters the tech apparently did not understand the correct way to install the performance voltage regulator and he was sensing voltage at the alternator itself. For more on this subject see:
Alternators & Voltage Sensing – Why It Matters
Boaters, including far too many professional techs, often forget the fact that the voltage drop calculation is based only on the wire drop. They ignore all the other things that add voltage drop in the “installed system“. For most circuits a 3% wire drop can be suitable, or is marginally acceptable, but it’s not adequate to size like this for a performance charging system. It’s not just the off-shore sailboat racer who needs to cram the maximum Ah’s they can, back into the bank, in the shortest time frame possible. Let’s face it, no one likes to listen to the engine or generator run any longer than it has to. In the end this owner spent big-bucks and the system did not meet his design expectations, for offshore racing, or what the tech led him to believe it should do.
That is when I was called in…
“Real world“, in this case, meant that when the wires were finally run, the length, according to the owner, “was a tad longer” than the tech has guessed it would be. The wire passed through multiple lugs/terminals, a fuse, switch, busbar etc. and each and every connection adds more voltage drop to the wire loss. The tech also used sub-par crimp tooling for each termination.
The way this was wired meant there were 16 connection points on the negative side and approx 6 on the positive side including a fuse and an alternator service isolation switch.
“RC, how do you figure the drop of all these connection points?”
Over years of field examples and tests. I tend to use a sizing factor of *0.00025Ω per connection point, as the resistance.. *This number assumes the connections are clean, tight and not made with a Dollar-Store crimp tool. You can see better performance than this with good tooling but I find 0.00025Ω pretty safe for sizing purposes.
Each Lug/Terminal = wire to lug as #1 & lug to XX point as #2.
This means two connection points per lug or terminal. This 0.00025Ω seems to bear out quite often but more importantly it means more accurate up-front voltage drop sizing. It would be quite stellar to see a 1.37% voltage drop (-0.2V @ 14.6V) but as installed that rarely happens especially when you size for 3% and fail to take all the terminations, switches etc. into account. In the image to the left we can see that the simple path from the “C” post to the #1 Post of a typical battery switch results in six connection points alone.
Let’s run the numbers on just the connection points at 80A:
22 Connection Points at 0.00025Ω = 0.0055Ω
80A X 0.0055 = -0.44V
This -0.44V drop is on-top-of the 3% wire voltage drop of -0.44V for a grand total of -0.88V. At absorption voltage, & 80A of alt output, his batteries were only seeing 13.82V instead of 14.7V.. Ouch! Voltage is the “pressure” that allows the current to flow into the battery. Lower voltage means less current can move from the charge source into the battery and results in slower charging.
If you don’t want to do all this math, you can just size for 1% to 1.75%, based on wire, and you’ll end up in a much better place for your charging system. When sizing for voltage drop, in a voltage critical system, use every tool you can to arrive at the lowest acceptable voltage drop you can reasonably estimate and do the math before you purchase the wire..
Good luck and happy boating!!