what is the right way to wire a battery bank?
Before we start, some terminology. When connecting batteries together to form a bank the cables between the batteries are called loops.
The best way to connect batteries together is a way that makes sure that the power is drawn from and sent to all the batteries in a bank identically, so that all the batteries get used equally and charged equally and live their full lives. Incorrectly wired banks cause batteries to have a shortened life.
First lets look the cause of the problem.
The cause is that rogue Mr Ohm otherwise known as Resistance. He makes it more difficult for the power (Volts x Amps) to reach the other end of a cable (loop) at the same level they started at. None of the wires and cables that we use in the boating world is a perfect conductor. So the power (Watts) going into the cable reduces the further it goes down the cable and the higher the current (Amps) the greater the drop.
In order for the batteries in a bank to be used and charged equally the voltage drop across each battery needs to be same otherwise one or more batteries in the bank gets over used and undercharged resulting in an earlier failure of the batteries. The inverter, pump or whatever looks up the cables has to see each of the batteries equally.
Lets start with one of the common. but understandable mistakes, the batteries looped together and the boat’s supply taken off from the first battery in the bank.
Battery 2 has a voltdrop of 0.0156V because the power passes through the first positive loop and the first negative loop.
Battery 3 has a volt drop of 0.0234V as its power has to pass through all the loops to reach the battery.
The majority of the power is taken from Battery 1, some from Battery 2 and even less from Battery 3. Roughly Battery 1 supplies double the power that Battery 3 does. The result is that battery 1 gets heavily used and has a short life, batteries 2 & 3 don’t get fully charged because the power does not get to them fully and gradually die of sulphation.
A commonly met way of combining batteries into a bank is a version of the above but with the connections to the boat being made to the diagonally opposite corners of the battery. It is an improvement but still not completely balanced so that all the batteries are used and charged equally.
Battery 1 has a voltdrop of 0.0117V
Battery 2 has a voltdrop of 0.0156V
Battery 3 has a voltdrop of 0.0117V
It is not unknown for Battery 2 to fail before the others.
It is better but not totally balanced.
To get the best out of your batteries I would suggest the following method; the power drawn and the charging are balanced for each battery.
Battery 1 has a voltdrop of 0.0078V
Battery 2 has a voltdrop of 0.0078V
Battery 3 has a voltdrop of 0.0078V
Every battery is used evenly and charged evenly and they should give equal lives if charged correctly
This also works for the four-battery bank
Battery 1 has a voltdrop of 0.022425V
Battery 2 has a voltdrop of 0.022425V
Battery 3 has a voltdrop of 0.022425V
Battery 4 has a voltdrop of 0.022425V
Fully balanced all equal
For those with a five-battery bank the only way, so far the only way to have the bank fully balanced is to use Star Wiring. This can be used on any bank of batteries but because each battery has to be fused at the battery it can cost more to do.
I am still working on the six-battery bank :o)
© Graham Mills 2018
I am a professionally trained engineer having undertake various formal technical qualifications. In my non-boating life career, I worked as an engineer for the BBC where I designing and installed the BBC studios around the country, worked for Hewlett Packard when it was a full electronics company as the UK Technical trouble shooter, that side was sold off in 1997, also worked overseas for the Kenya Broadcasting Corporation as Engineer in Charge of a transmitter site.
I have owned boats since I was 10 and have helped other boaters with their electrics for over 50 years, sixty years in about 18 months. I sold my last boat in October 2016 as I can no longer singlehand a boat. The result is more time to help others and that has expanded. I have never charged for my help, although now a days as a pensioner I ask boaters to pay my expenses to get to them which can be anywhere in the UK. I help boaters on their boats with the sorting of virtually any electrical problem be it installing new batteries, sorting poor charging, installing a new inverter, helping and guiding self builds designing their electrics system for them; anything basically. I seem to have developed specialism designing and installing lithiums batteries systems for liveaboard boaters; doing an average of one installation a month, all I can take on physically. I spend on average three days a week working on someone's boat. The rest of my time is caring for a young lady who has Autism & ADHD who lives in my house.
The aim when I leave the boat is for the problem to have been solved, the owner to know a lot more about their electrics how they work and feeling a lot more confident about taking on small jobs sorting small problems with their electrics. Teach the owner as we repair their boat.
Graham Mills is the founder of a Facebook electrical group and helps boaters with their electrical problems. If you wish to contact him, please do so through the Facebook group:
BELOW IS AN ARTICLE WRITTEN BY GRAHAM MILLS WARNING BOATERS THAT WHEN MAKING AN ELECTRICAL INSTALLATION IT IS IMPORTANT TO READ THE MANUFACTURER'S INSTALLATION GUIDE.
Please read the Installation Manual
There are occasions when I get a message and photo that causes me to drop everything and get in the car and drive. The attached photo, received a while ago, was one such occasion. There are some that will say they know the problem and I could have dealt with it by messenger but things are not always as simple as they first seem.
The symptoms were the fuse holder getting hot to the extent that the plastic melted, cables getting over warm and the insulation starting to deform and melt. They were feeding a 3kVA inverter from the boat’s 12V batteries.
Any cable carrying a current has a resistance and that resistance causes heat in the cable and the higher the current the more heat that has to be dissipated. In this case the stud on the fuse and the cable were getting hot, not just warm, but such that the insulation had started to deform and going soft. The query in my mind was, was this a simple case that the connection on the stud was causing the problem or was there something more going on and if I got it wrong then there was a possibility of a fire.
So yes the obvious fault is the nut between the cable lug and the fuse. It should not be there, the cable lug should be straight onto the fuse lug, nothing between them.
The stud on the fuse holder is a 8mm stud and the inverter has two 8mm studs on each of the DC connection pads, positive & negative. A total of four studs why?
The inverter a 3kVA inverter with two 8mm studs on each of the DC connection plates. In the back of my mind I knew there was a problem with this sort of connection; particularly when the cables were being expected to run for more than a very short time at full power, for that inverter a current approaching 250A. The current is within the stated current carrying capacity of the cable being used 70mmsq (485A); but it is over the continuous current carrying capacity of a 8mm stud.
Checking the manufacturer’s manual, it states that for the DC connections, for runs up to 2 metres that two runs of 50mmsq should be used for both the positive and negative, why?
The area of a 70mmsq 8mm hole cable termination lug including the hole is 313.5mmsq.
The area of a 50mmsq 8mm hole cable termination lug including hole is 261mmsq and for 2 is 522mmsq.
This means that by using two 50mmsq cables and two fuses the contact area of the surface contact of the lug to the connection plate is increased by 1.6 times and the current through the fuses and their studs is halved. Therefore there is a better and easier current path, less resistance etc. Hence why the manufacture has specified twin cables and twin fuses.
Just for those that are thinking the boater should have used 95mmsq cable the area of a 95mmsq 8mm hole cable termination lug has an area of 498.15mmsq, but it is still over the continuous capacity of the 8mm stud and the 50mmsq cable solution is still better and deals with all the problems.
Please please read the installation instruction carefully. If the manufacturer has provided twin studs to connect an inverter please read the manual and if the manufacturer has specified/recommended a similar arrangement follow it. They are doing it for good reasons to stop you ending up with a fire from overheated cables etc.