Battery internal resistance

[drwormy]

Has anyone measured / have data on LiFePO4 battery "internal resistance"*.

By this i mean cells + wiring + BMS.

In building my first battery, i was very worried about overcurrent, having taken onboard anecdotes like "a Li battery will suck up all the charge you can throw at it". Also looking at individual cell spec sheets showing very low mOhms.

If i do a simple test such as apply 1V more than the battery rest voltage and measure the induced current one could think of this as an ohmic resistance.

To my surprise the resistance i measured was around 26 mOhm. Finding a spec sheet for my BMS (Daly Smart 60A), the resistance is quoted as < 20 mOhm, so this seemed about right. The 100A JBD BMS thats on its way to me claims < 10 mOhm. Has anyone measured similar for their assembled battery?

My application is direct alternator charging a secondary battery, and so if i limit the low voltage cutoff to 13V, and consider the max voltage i observe from my alt, and use 5m of 16mm2 cable (purposely enabling a voltage drop). the BMS is limiting the maximum current flow to around 1/3 my alternator capacity (which im comfortable with).

* I realize im blurring the term 'internal resistance'.

 

[Steve_S]

The individual cells will have their own Internal Resistance which will vary between the different cells as these are unmatched/unbatched commodity cells. The resistance of busbars and attachments is another issue.

ALTERNATORS ARE BAD CHARGERS FOR LITHIUM !
There are High Quality alternators that can handle it, alternately there are intermediary devices to protect from over/under voltages and high power shifts. The Marine folks have nailed this down to a simple science. Many people have Murdered the Lithium Battery Systems with Alternators ! It's no joke and not to be taken lightly.

Extensive coverage is here - Marine focussed but 100% applicable to ANY mobile system being charged by alternators etc.
Electrical Design For a Marine Lithium Battery Bank | | Nordkyn Design
LiFePO4 Batteries On Boats – Marine How To

 

[drwormy]

The individual cells will have their own Internal Resistance which will vary between the different cells as these are unmatched/unbatched commodity cells. The resistance of busbars and attachments is another issue.

ALTERNATORS ARE BAD CHARGERS FOR LITHIUM !
There are High Quality alternators that can handle it, alternately there are intermediary devices to protect from over/under voltages and high power shifts. The Marine folks have nailed this down to a simple science. Many people have Murdered the Lithium Battery Systems with Alternators ! It's no joke and not to be taken lightly.

Extensive coverage is here - Marine focussed but 100% applicable to ANY mobile system being charged by alternators etc.
Electrical Design For a Marine Lithium Battery Bank | | Nordkyn Design
LiFePO4 Batteries On Boats – Marine How To

Yes, Im aware of the considerations of the marine charging folks. My application ive mitigated for those risks by limiting charging current, knowing my alternator properties (ie 13.9V max), using a FET based isolator and still using a hybrid AGM as my main battery. Heck, my typical charge current will be similar to turning on my 'offroad' lights.