BMS question

[Cisco-man]

I don't trust cheap BMSs (as reiterated by others) so I'm rebuilding my recovered 18650 cells from a failed Swytch battery as a twin 4P4S set (ie 2 x 12V). I'm dispensing with the integral BMS and incorporating an RC type monitor (8 inputs) to dynamically check the voltages on each of the parallel sets of cells. Each of those paralled sets will be recharged separately with a simple 1A 3.6/4.2V Liion charger. The resulting battery packs are for general 'cave' use eg desk lighting and occasional friction drive booster use (24V) on one or other ebike. The build is going slowly because of the need for care in handling/charging and the manual balancing of paralleled cells - I will let you know the overall outcome in due course. All a bit of a faff of course but an interesting learning exercise.

Yes, if I were to rebuild a pack I think that I’d like some insight as to what was going on inside, and also the ability to influence the balance proactively. I can see reasoning both ways but personally I‘m rather a micro-manager with my things, so it would suit me to bring the sense wires out and balance them with an intelligent charger. Still, I’m not at the stage of battery rebuilds yet. I’m looking for a cheap(ish) spot welder on eBay at the moment!

 

[Solarbake]

I think the max battery uses 21700 cells x 10 so the bms will charge each cell to full.
I think the cells must be a decent quality as I witnessed 15.5 amp discharge through my swytch controller.
I reckon there using the samsung 50s in there pack as the pack is rated at 180watt or 5.0a/h and could give 20amp discharge if needed.
My home made pack of 20x samsung 50E 10s2P configuration are rated at 10amp discharge per cell .
My pack in theory has a 20amp max discharge and I still noticed some voltage sag.
I making a new pack from 40x Molycell P42 in a 10s4P configuration rated at 30A discharge per cell so 120amp max dicharge in this pack would be possible.I report back when its built and tested but am expecting much higher performance from the pack.

 

[Sturmey]

........ bring the sense wires out and balance them ......

There is a potential risk with extra wiring that is unprotected (not fused) and can carry full battery, cell current. I think its especially risky if the balance wires are heavy as they can carry a large current if they short circuit. Light thin wire (but well insulated) could be safer. 24 awg silicon coated is often recommended for balance wires as it will usually fuse inside the silicon sheet at less than 15 amps and gives some protection.

 

[Solarbake]

Most balance wires are tiny as even with a big 6 amp charger its going to be split between the cell group so individual wires are not going to see more than probably .5 amp passing through.The bosch battery I replaced with a new BMS had the thinnest wire I seen yet on the cell groups and as you said if there a short the wires will burn out pretty quick hopefully

 

[saneagle]

There is a potential risk with extra wiring that is unprotected (not fused) and can carry full battery, cell current. I think its especially risky if the balance wires are heavy as they can carry a large current if they short circuit. Light thin wire (but well insulated) could be safer. 24 awg silicon coated is often recommended for balance wires as it will usually fuse inside the silicon sheet at less than 15 amps and gives some protection.

Balance wires carry about 100mA. The current doesn't matter. It's the voltage difference across any short that determines how big the sparks are.

 

[Sturmey]

Balance wires carry about 100mA. The current doesn't matter. It's the voltage difference across any short that determines how big the sparks are.

To give an example, a permanent short or contact between 2 adjacent balance wires will result in full discharge of the relevant parallel cell group around the loop.

 

[Cisco-man]

I can see the sense in all the warnings of consequences if external balance wires get shorted out. Do the wires actually need to appear outside the cell pack? Can the pack case not feature chassis connector of some sort? Time was when banana sockets were used - but I’m not suggesting those in this case!

 

[saneagle]

To give an example, a permanent short or contact between 2 adjacent balance wires will result in full discharge of the relevant parallel cell group around the loop.

The wires would probably blow like a fuse before any serious discharge happened.

 

[Cisco-man]

I think the max battery uses 21700 cells x 10 so the bms will charge each cell to full.
I think the cells must be a decent quality as I witnessed 15.5 amp discharge through my swytch controller.
I reckon there using the samsung 50s in there pack as the pack is rated at 180watt or 5.0a/h and could give 20amp discharge if needed.
My home made pack of 20x samsung 50E 10s2P configuration are rated at 10amp discharge per cell .
My pack in theory has a 20amp max discharge and I still noticed some voltage sag.
I making a new pack from 40x Molycell P42 in a 10s4P configuration rated at 30A discharge per cell so 120amp max dicharge in this pack would be possible.I report back when its built and tested but am expecting much higher performance from the pack.

Hi Solarbake. Have you opened a Swytch Max battery pack? I’m guessing the the front panel prises out of the hard rubber case surround?

 

[Sturmey]

The wires would probably blow like a fuse before any serious discharge happened.

That's why I am saying to use (the normal) thin balance wires and not use thick wire if externally wiring out the balance wires.

Fusing Currents versus American Wire Gauge for copper, aluminum, iron, and tin (Cu, Al, Fe, Sn), wire melting currents

Wire size table and fusing currents for design engineers including copper (Cu), Aluminum (Al), Iron (Fe), and Tin (Sn), wire melting amperage

www.powerstream.com

 

[Solarbake]

Hi Solarbake. Have you opened a Swytch Max battery pack? I’m guessing the the front panel prises out of the hard rubber case surround?

No I have a Bosch pack with faulty Bms which I repaired with new Bms a new Samsung pack and a old Xiaomi pro scooter pack for the Brompton so left the Swytch max intact as I have two Bromptons with Swytch kits