When building a battery pack balance wires same length ?

[darkshirikens]

This thread is more out of curiosity cause i always wondered if it makes a difference in the discharge or charge of or each cell longer vs shorter balancing leads? ( Example:if the balance wire is shorter it will take more energy than the longer balance leads when discharging, or when the battery charges the shorter balance leads will charge those cells faster than the longer balance leads?

 

[dogman dan]

As I understand it, you do want balance leads the same length. DC voltage is affected by length of wire more than AC.

The problem I think, is that you want whatever device is using the leads to measure voltage to do it the same on all cells. So balance leads the same length will all have the same voltage drop when the current arrives at the bms or chargers bms.

By same length, I'm not talking about same length to a centimeter. I mean for example, you don't want leads on one part of the battery 6 inches long and the other 24 inches long.

 

[darkshirikens]

Thank you dogman for replying i was thought this would be an interesting thread cause i don't know many who would think about the length of balance wires while building their battery packs and if it was a benefit to having wires the same length but i know battery cells sometime have have their different way they discharge and take a charge depending on IR I believe. Hopefully i get more feedback to see how more people feel about the subject.

 

[docnjoj]

When I had to rebuild my 20 AH 48 volt Ping when some cells failed I decided to split it into 2 18 nominal volt packs using a new BMS. One new pack had the balance wires extended about 18" and the other side has the regular balance wires. I did up the wire gauge to about 18 from the 24 that came with the Ping on the extended side. The battery has been in excellent balance for almost 2 years since I did this and is used about 4-5 times a week, sometimes on rides over 20 miles. It seems that if you up the wire size (gauge) then you can extend them without a problem. YMMV.
otherDoc

 

[darkshirikens]

That basically sounds like you cut the loses through going up the wire gauge on the extended side which you could of had am i right ?

 

[SamTexas]

... if the balance wire is shorter it will take more energy than the longer balance leads when discharging,

balance wires are not used for discharging unless you have a very special setup. Discharging is done through the main neg and pos leads for the whole battery pack.

... or when the battery charges the shorter balance leads will charge those cells faster than the longer balance leads?

Unless you have a charger that consists of individual cell-chargers, the balance wires are not used for charging either. They are only used for balancing cells. Most balancing chargers balance cells at a very low current, so the actual length of the wires does not really matter.

But you're right. Theoretically speaking you want to keep length (and size) of the balancing wires as close to each other as possible.

 

[darkshirikens]

... if the balance wire is shorter it will take more energy than the longer balance leads when discharging,

balance wires are not used for discharging unless you have a very special setup. Discharging is done through the main neg and pos leads for the whole battery pack.

... or when the battery charges the shorter balance leads will charge those cells faster than the longer balance leads?

Unless you have a charger that consists of individual cell-chargers, the balance wires are not used for charging either. They are only used for balancing cells. Most balancing chargers balance cells at a very low current, so the actual length of the wires does not really matter.

But you're right. Theoretically speaking you want to keep length (and size) of the balancing wires as close to each other as possible.

I know that lol maybe my wording could of been a little different i should of known better being on this forum but has any anal person on here and i know there is some notice any different in balancing a pack with different wire length opposed to balance wire being the same ? Before anyone says i know some cells take a charge faster than others I lack experience not brains Lmao

 

[SamTexas]

What are you talking about? Are you offended by MY ANSWERS TO YOUR QUESTIONS? If so, why?

 

[docnjoj]

That basically sounds like you cut the loses through going up the wire gauge on the extended side which you could of had am i right ?

Yes that is what I tried to do and it seems to work fine. I'm afraid I really don't know what happend if you just extend some wires with the same gauge and leave others shorter. I hope this helps.
otherDoc

 

[darkshirikens]

Nope not offended at all Sam don't read into the words too much i appreciate all feed back and insight on this matter of my curiosity thats all.

 

[dnmun]

wire length of the sense wires does not matter. sense wire current flow is tiny at balance so delta voltage is insignificant.

 

[SamTexas]

I'm afraid I really don't know what happend if you just extend some wires with the same gauge and leave others shorter.

It would work just fine. Balance wires carry such a tiny amount of current that make the actual length and gauge insignificant.

 

[chilledoutuk]

Having different length cables might make balanced chargers take a little longer balancing as there might be a bit more difference revealed when the current is tappered off towards the end of the charge but in the end all the cells should be charged to the correct level otherwise the charger isn't doing its job.

As previosly said the bleeding type of BMS's will be using such a low current which also tapers off as they become balanced that the length of the cables is not mega important.

Honestly the connectors and connections used to wire the balance cables probably will not all have the same resistance anyways so its not like you can have perfectly equal resistance on all the balance connections.

Personally with balance cables there are other concerns as they are often a source of failure in some battery packs.

Keeping the wiring neat and tidy as possible and routed away from where they might get pinched from the wright of the pack is important also using a cable with a adequate gauge and insulation resistant to heat and pinching as possible.

PTFE coated cables would be my choice as they are very resistance to heat and mechanical damage. In fact so much so that you have to use a craft knife to strip them effectively.

If you wanted to be extra careful you could add a fuse to each balance lead that's lower than the current handling of wire used to prevent the pack bursting into flames should a short in the balance cabling ocure.