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f200

New member
Joined
Aug 13, 2021
Messages
1
Hello, been reading the forums and faqs etc. I would like some help figuring out the fusing. I have:

3S4P setup,; using HX-3S-01 BMS
Sanyo 18650 3350mAh 7A batteries

Batts are placed in 3x 4 cell holders then 30awg tinned wire connected to 12g solid copper bus wires for cell level fuses.
Read you should fuse the leads from BMS to each battery pack, and between each packs. What amp rating should I use for the balance leads and where to place? Also would I need fuses between each parallel pack?

I have an assortment of Littelfuse Pico fast blow fuses to use. Any help much appreciated.
 

OffGridInTheCity

Active member
Joined
Dec 15, 2018
Messages
1,736
Personally, I don't fuse balance leads because the amps are so small and the wire is small (would melt easily) and is short/integral to the physical pack/battery. So the balance current is too small to do anything and a dead short will simply melt thru a sense wire. I do fuse the power wire from the BMS to the load.

But, I've noticed that Batrium K9 (sense) leads from the control unit to the battery are fused in a recent @LithiumSolar install youtube.

Maybe it's an issue of the wire-length?
* My Batrium longmons are physically attached to the packs with the longmon +/- soldered directly to the pack - e.g. 4inch length - and in such a way that short is very unlikely and if they 'melt' there's not much chance of dangerous issues.
* My Chargery BMS installs are 7s and 14s wires soldered out to the cells out to a 8pin / 16pin connector that plugs into the control unit - but again the length is short as the Chargery control unit is right at the battery.
* In the Batrium K9 youtube install by @LithiumSolar - the wires run a considerably longer distance / are exposed beyond the physical battery.

I'll be interested to see other comments :)
 
Last edited:

italianuser

Member
Joined
Feb 25, 2020
Messages
358
This is new to me, too, ...fusing balancing wires :unsure: I'm now curious to see @LithiumSolar 's video.

Going by what I know (ehm, not much, sorry LOL):

1) You'd fuse balancing leads to protect BMS from a reverse current coming from the battery;
2) Extra fuses surely don't harm; weird things can happen when different devices move current around;
3) On my small nominal 40A Daly BMS balancing current is 30mA, that's the maximum I'd expect flowing in the wires.

For point 2, I had a case where my 26AWG wires melted, although load was only 40mA, current going from MPPT controller to two 12V small Volt/Amp displays (using controller's LOAD connector). So, I did add a protection circuit for over-current, over-voltage.

For point 3, ... After what happened at point 2 I could think about protecting BMS leads, yes... But most probably I won't for now.

Going to see LithiumSolar's video right now...
 

daromer

Moderator
Joined
Oct 8, 2016
Messages
5,659
When having those longer wires from the BMS its common to fuse yes. The shorter wires that some BMS systems use ahre so thin so they act as the fuse if something goes wrong.

It have been discussed quite alot and when having that long leads to the bms you need to have thicker leads for not having to much of an error meassuring voltage and also due to heat created. So thus its easier to fuse them :)
 
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