tool to explore dead shorts of LFP cells and batteries

[John Frum]

The attached spreadsheet is in OpenDocument Spreadsheet format and is zip encapsulated.
Its intent is to explore the dead short ampacity of LFP cells and batteries.

The typical internal resistance for the big blue LFP cells is ~0.00015 ohms.
The resistance of a typical shunt is ~0.0001 ohms.
For the rest of the values I used order of magnitude guesses.

If you find any errors in my math or you have better guesses for the resistance values please share.

Hope you find this useful.

 

[John Frum]

Tagging @Hedges and @upnorthandpersonal as both have previously expressed interest in the topic.

 

[upnorthandpersonal]

I've not had time yet to short an actual cell and measure. I might not before the weekend, but I hope next week.

 

[RCinFLA]

Under small to moderate cell current, <0.5 C(A), cell overpotential voltage drop will significantly exceed cell resistance voltage drop. It has an exponential time decay slump that takes 1 to 3 minutes to reach equilibrium. The cell overpotential voltage drop depends on cell condition. Older cells will have 2x to 3x a new cell.

For example, a new 280AH EVA cell, with 0.2 C(A) current, 56 amps, will have about 50 millivolt cell terminal voltage drop, after equilibrium reached in a minute or two. Considering only cell resistance, 0.15 milliohm internal resistance X 56 amps says there should only be 8.4 millivolt cell voltage drop. The difference between 8.4 mV and 50 mV voltage slump is the cell overpotential voltage overhead required to move lithium ions through cell to meet demanded cell current.

Starting with 3.65v is a cell with post-charging surface charge buildup voltage. It will drop below 3.45v within 60-90 seconds with load of an amp or more. The surface charge only amounts to about 0.01% of cell capacity and is mostly due to capacitance of graphite negative anode layer of cell.

A 0.05 ohm shunt would be a low amperage shunt. A 100A/100mV shunt is 1 milliohm. A 500A/50mV shunt is 0.1 milliohms. Chinese shunts are often 75 mV at max current instead of 50 mV at max current.

I doubt you will have a fuse with 0.1 milliohms. More like 3-5 milliohms. Good battery terminal connection is about 0.05- 0.08 milliohms for each terminal.

 

[John Frum]

Going to edit my post to fix the shunt thinko.

A 0.05 ohm shunt would be a low amperage shunt. A 100A/100mV shunt is 1 milliohm. A 500A/50mV shunt is 0.1 milliohms. Chinese shunts are often 75 mV at max current instead of 50 mV at max current.

Going to edit my post to fix the shunt thinko.
Any ideas for a good plugin value for a fet based bms?

 

[John Frum]

I doubt you will have a fuse with 0.1 milliohms. More like 3-5 milliohms.

I found this.

https://www.victronenergy.com/upload/documents/Datasheet-Midi,-Mega-and-ANL-fuses,-and-fuse-holders-EN.pdf

The high and low are .0015 and .0001 ohms.
Not sure how representantive those numbers are though.

 

[RCinFLA]

Any ideas for a good plugin value for a fet based bms?

Depends on amperage rating of BMS. A good guess for Chinese BMS is assume 20 watts for heating at maximum current rating.

For 100A BMS, that would be 20w/ 100A^2 = 2 milliohm.

JK active balancer BMS actually lie less than most manufactures. They actually list BMS series resistance. They don't meet the number but give them credit for listing it.

For example, JK 200A with 2 amp active balancer lists 0.3 milliohms. Just the MOSFET's at 25 degs C is about 0.35 milliohms. Add in shunt resistors, PCB resistance and at 25 degC they are more like 0.4 milliohms, not including the small gauge stub connection leads. At 200 amps max rating, with the amount of heating that will occur, the MOSFET resistance rises and the net BMS will have a little over 0.5 milliohms. Still pretty good by Chinese spec standards. 200A^2 x 0.5 milliohms = 20 watts of BMS heating.

 

[John Frum]

I think its reasonable to conclude an MRBF fuse with a breaking capacity of 10,000 amps at 14 volts is sufficient protection for a 12 volt nominal battery made of the big blue LFP cells.

MRBF Terminal Fuse - 300A - Blue Sea Systems

Space-saving ignition protected fuse for 30 to 300 Amp loads. Must use with Terminal Fuse Block.

www.bluesea.com

 

[RCinFLA]

I found this.

https://www.victronenergy.com/upload/documents/Datasheet-Midi,-Mega-and-ANL-fuses,-and-fuse-holders-EN.pdf

The high and low are .0015 and .0001 ohms.
Not sure how representantive those numbers are though.

That is a bit of shady spec. Maybe they are spec'g just the fuse internals. With those type of interconnects there will be more resistance in the actual fuse terminal connection than actual fusing element. Between the fuse two terminal connections plus the two cable lugs connecting to holder, I would estimate about 0.2 to 0.5 milliohms just for the connections to fuse.

You could argue the cable lug connections to fuse holder are not part of their domain.

Just connections to a T-class fuse is about 0.1 milliohm and they are a lot better than ANL fuse.

 

[John Frum]

Maybe they are spec'g just the fuse internals.

That is my guess too.