Battery Cabling Best Practice

Here are a few pointers when considering wiring up your BMS and batteries.

1. Put your Fuses near the battery terminal

A fuse is there to protect the wire, have it as close as possible to the battery terminal to protect the circuit beyond the fuse. If wires rub through and short circuit inside the loom or other circuits are then they are protected beyond the fuse.  In some jurisdictions (regulatory bodies)  there are specific maximum lengths allowed and requirements for fusing on all wires directed attached to a battery. This applies to both the power carrying circuits and the balancing sensor wires. This is a good idea even where it is not a requirement for safety.   

We suggest fuses be placed typically less than 10-15 cm (8 in) from a battery terminal, the closer the better.

ALWAYS put the fuse nearest to the battery, It can save your project from turning into a BBQ

2. Ceramic DC Rated Fuses are Preferable

We typically recommend hooking up your typical  26  AWG hook-up cable to a in-line 3A fuse ( preferably  ceramic sand filled) to extinguish the DC arc. Typical fuse holders of  5x20 mm or 6.3x32 mm ( diameter / length) are suitable for low voltage circuits.

3. Tinned Multi Stranded Wire

Multi-stranded versus Solid-core
  • Multi Stranded wires are far more tolerant  to mechanical vibration, movement and flexibility, which means it is going to last longer and less likely, in its lifetime, to fail.  
  • Solid-core whilst an excellent strong conductor is less flexible and can easily break with repeated movement. If they break it can be hard to detect intermittent faults when flexed in certain directions due to the insulation holding the conductors together. 
Tinned versus Raw copper 

Most wires have raw copper under the insulation which is fine, however  tinned copper will add extra protection to corrosion at the exposed terminals.  It is all about durability, exposed wires at the connectors are tinned to make them last.

4. Smallest cable size possible 

For balancing wires they ideally use 26 AWG rating cable, it will blow at 10 Amps (i.e. self-fuse) and we choose to use this size cable on our LongMons and BlockMons for that reason.

If you are interested in looking at the fuse rating of different wire gauge/size here is one example but there are other sources

We frequently see photos of DIY projects running balancing cables with medium duty 16 AWG wire from their batteries without fusing (According to the charts it will handle 398 A for 1 second). 

Do not only consider the continuous power rating of the wire but also the rating of when it will fuse and fail.  

5. Use Crimping Tools when Necessary 

Wires need to be crimped, do not use pliers or your fingers, when working the proper crimping tools allow a tight joining of wires which keeps them solid and strong.  

If not crimped properly then the cables can fatigue over a period of time and the connection will fail.  It takes but a moment to carefully crimp your cable wires which gives you solid mechanical and electrical connections and allows them to last long into the life of your project.

6. Ferrules ensure better connection at terminal screws 

Make sure you crimp your wires for efficiency and longer lasting cable life for your project. Ferrules help ensure that you have good connections to terminals ensuring mechanical durability and preventing the strands from being pitched.  

Ferrule or Not to Ferrule? 

Read more about what is a ferrule and why stranded wires with ferrules perform much better than without them.

Source: Weidmüller Interface GmbH & Co. KG

7. Adopt connectors that shield the wire 

  • Always consider the risks of accidental short circuits. Multi-core connectors have a lot of pins in close proximity making it very important in your selection and pin assignment.
  • Consider sufficient air-gap (typically referred to as pitch) and those with separators between pins.
  • Connectors which fully enclose the pins for both the Male and Female housings are ideal as if dropped reduces the chance of a fault occurring.

8. Colour Coding of Wires to Prevent Confusion

  • Colour Coding can make troubleshooting easier and avoids mistakes
  • Each colour can be used for specific identification or purpose
  • Adopt typical colour coding conventions and avoid mixing them up 
    • BLACK is Negative - SN on our Watchmon Supervisor terminal strip
    • RED is Positive - SP on our Watchmon Supervisor terminal strip.

9. Add extra housing or conduit especially outside enclosures 

Whenever combining multiple wires together always consider putting them into a split-core conduit or some form of secondary protection or shielding so that they cannot be punctured.   

Attach cable ties around cables and anchor at enough intervals to ensure that they are properly mounted to structures, preventing them from catching or getting tangled.

10. Use cable Glands or Grommets when passing through enclosures

Any time a wire passes through an enclosure, cabinet or box  it needs to have protection with Cable Glands to prevent insulation from being pinched, causing as fault.   

Typical questions we get asked

What size fuse to use?

Depends on the wiring gauge and the power required on the circuit, for balancing cables we typically recommend 22-26 AWG hookup wire with an in-line fuse of 3 Amps, ceramic (preferably sand filled) so that it can extinguish the DC arc.  

Can I just fuse the positive wire?

Every wire that is attached to the battery needs to be fused as any core has the potential to fail. Not just the positive but also the negative and the balancing / sensor wires.

Can I Extend the Communication LongMon / BlockMon Wire?

For the communication wire use twisted pair less than 5 meters, here are a few extra considerations:
  • Do not get the polarity wrong
  • Do not extend whilst connected and powered
  • Do not use solid core cable
  • Do not use thick cables we use  22-26 AWG for a reason
  • Do not use Speaker Figure 8 cable
  • It is OK to use CAT5 network pair (use multi-strand patch lead not the in-wall solid core variant)
  • For extra connectors that is your choice, just do not put both the TX pair and RX pair in the same combined housing

Can I extend the power wires on the LongMon / BlockMon?

  • If you are going further, carefully consider adding a fuse
  • Do not use thick cables, we use 22-26 AWG for a reason
  • Do not get the Polarity wrong
  • Do not extend whilst connected and powered
  • Do not use Solid Core Cable
  • Ask yourself why are you relocating the cell monitor away from the battery? 
  • Maybe reconsider the WatchmonPlus WM5 / WM7 / MM8 range.

WatchmonPlus ( WM5 ) Preparing Balancing Wires Video

This VIDEO shows the step by step process of preparing balancing wires for the WatchMonPlus.

Example Wiring Diagram of WatchmonPlus

Note each 3A fuse is as close as possible to the battery and not near the BMS terminals.

Always remember that proper cabling methods should be the normal practice with the WatchMon supervisor using the correct fusing at the battery terminal and not the load (WM5 and WM7).

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