Charging 2 lifepo4 batteries of different SOC..what happens??

[Barking Sands]

So this is broken out of the other thread and I decided to try to conduct a semi-proper experiment. Its actually not that easy due to things like cabling, soc drift etc.

The question at hand:
If you combine two batteries of vastly different soc how much current flows between them to equalize? In addition, if you then charge from that point how will the 2 batteries of very different SOC respond.

I first charged both batteries to 14.5v and let them absorb until current was 0. This also reset soc to 100% so soc should be somewhat accurate from here for this test. Batteries actually full and soc set to 100%.

I then used some rigs to discharge each battery. Batt 1 soc of 20%. Batt 2 soc of 81%.

I used 4/0 cables with good lugs to connect batt 1 and batt 2.

Power up the batteries and measure initial surge from B2B.
20.4 amps measured in clamp meter with each battery registering similar in app.

Apply approximately 73 amp charge on opposing post on batt 1 positive and batt 2 negative.

Record at roughly 10% soc intervals what the SOC% , voltage and amperage is and jot it into a terrible hand written chart.

 

[Barking Sands]

Keep in mind...on the chart..the fact that batt 2 eventually reads an soc of 100% is not an actual representation of capacity...it just means the soc counter hit the parameters to set 100%. Current tells the story.

The time from the last to entries to each battery current falling to roughly 0 and both soc reset to 100% was maybe 10 minutes.

 

[Barking Sands]

Here are a few pics

 

[SteveK]

Looks like my writing so I can read it easily. That has proven the theory that the lower SOC will accept a higher charge.

 

[Delta Riverat]

Pretty much what I was expecting with my experience. Thanks for the effort!

People worry too much about this stuff. Real world tests reduce the drama.

 

[Barking Sands]

Looks like my writing so I can read it easily. That has proven the theory that the lower SOC will accept a higher charge.

LOL...my old mechanic hands sometime work at half speed.

 

[Barking Sands]

Pretty much what I was expecting with my experience. Thanks for the effort!

People worry too much about this stuff. Real world tests reduce the drama.

I have done this a million times...like you and many others. But I can say I never really paid attention closely.

Definitely learned a bit.

 

[Jeff F]

Record at roughly 10% soc intervals what the SOC% , voltage and amperage is and jot it into a terrible hand written chart.

Thanks!

What do you suppose is happening inside the battery that stops accepting current? Do you have insight into cell voltages?

 

[Barking Sands]

Thanks!

What do you suppose is happening inside the battery that stops accepting current? Do you have insight into cell voltages?

Im sure an EE could put it all into an equation. I cant. But I would say its mainly about voltage differential between the charger source 14.4 volts and pack voltage of each battery and internal resistance of each battery that changes over the capacity that governs where the current goes and in what ratio to each battery.

 

[Jeff F]

Im sure an EE could put it all into an equation. I cant. But I would say its mainly about voltage differential between the charger source 14.4 volts and pack voltage of each battery and internal resistance of each battery that changes over the capacity that governs where the current goes and in what ratio to each battery.

There is a major change in state in the full battery that causes it to stop accepting further current. Again, a lithium thing. Do you have insights into cell voltages for that battery?

What's the criteria for stopping the 14.4 charge?

 

[Barking Sands]

There is a major change in state in the full battery that causes it to stop accepting further current. Again, a lithium thing. Do you have insights into cell voltages for that battery?

What's the criteria for stopping the 14.4 charge?

because that was the most convenient charger I had with any real current that I could wheel to the batteries without further ripping apart my other test bench...lol. It was an old school automotive charger with AGM settings that could not be user defined. So 14.4 it was.

BTW I didnt stop the charge. I let it charge till amps dropped to near 0 and the charger went to float. I couldnt really jot that part down

 

[Delta Riverat]

The charger does that based on voltage and current flow. A lithium charger will peg the output current until the absorb (voltage) point is met. Then it holds voltage until the float (current) point is met and then just sits at float.

 

[SteveK]

When battery voltage equals charge voltage the current stops. This has been occurring since internal regulated alternators and flooded batteries.

 

[Jeff F]

When battery voltage equals charge voltage the current stops. This has been occurring since internal regulated alternators and flooded batteries.

It doesn't occur with lithium. You can't carry LA thinking over. That's what I've been trying to show.

How do you explain the current stopping on battery 2? And what implications would that have on the rated delivery for the pack? 100% of current goes to one battery.

 

[Jeff F]

What happens if you hold a battery at 14.4 with zero current? Does it harm the battery? Can you keep it there long?

 

[Barking Sands]

It doesn't occur with lithium. You can't carry LA thinking over. That's what I've been trying to show.

How do you explain the current stopping on battery 2? And what implications would that have on the rated delivery for the pack? 100% of current goes to one battery.

There was still current present on those two entries. I know my handwriting is terrible, but it just says "less than" 0.2 amps because that is the current detection threshold of the internal shunt. But there was likley current flowing...just very very low.

 

[Delta Riverat]

It's not going to like that much (life cycle wise) if held there forever. Shouldn't be a safety concern but not good for battery health. LiFePO4 likes to be cycled. Charge in discharge out.

Charge 'em up and run 'em down, rinse and repeat. Lead likes to sit at 13.5. Lithium likes to swing!

 

[Jeff F]

There was still current present on those two entries. I know my handwriting is terrible, but it just says "less than" 0.2 amps because that is the current detection threshold of the internal shunt. But there was likley current flowing...just very very low.

Right. Essentially zero. Battery is full.

 

[Barking Sands]

Right. Essentially zero. Battery is full.

Not quite full...look at the battery voltage when the current was less than 0.2 amps. Still a little on the low side. In addition, the current actually picks up after that point as the other battery closes in.

The target absorption voltage needs to be met by both batteries. 1) to pack the last bit of capacity in and 2) to allow for balancing conditions and balancing time. 13.6 is really too low for this to be effective.

If we were charging a pair of batteries that were balanced with capacities at the same level and my preferred charge profile it would be 14.2 absorption with an absorption time of around 15 or 20 minutes for this pair, followed by a drop to float voltage of 13.5 where it would remain until needed. I would want both to hit those marks. Charging with the initial imbalance makes hitting those marks a bit funky.

 

[Jeff F]

Not quite full...look at the battery voltage when the current was less than 0.2 amps. Still a little on the low side. In addition, the current actually picks up after that point as the other battery closes in.

At this point tying the two has caused a disconnect from reality for each individual battery.

When I say full I mean it in a real-world way, rather than a battery spec. Pushing more electrons in at this point should be setting off BMS alarms.

This is a danger zone for lithium, one of the few.

 

[Jeff F]

Just did some quick googling. Think there was a sudden spike in IR in batt 2 that stopped current. That's serious stress on the battery.

Normally voltage-based measures would shut the battery down before it reaches that state.

Maybe DDR can confirm my IR hypothesis.

 

[Barking Sands]

At this point tying the two has caused a disconnect from reality for each individual battery.

When I say full I mean it in a real-world way, rather than a battery spec. Pushing more electrons in at this point should be setting off BMS alarms.

This is a danger zone for lithium, one of the few.

Danger zone? Not at all?! What safe operating envelope parameter has been exceeded here?
Is the voltage too high?
Is the temp too high?
Is the current too high?
What parameter contained in any BMS might be exceeded?

You just saying it should be setting off BMS alarms is not enough. At least give me a working theory? There is nothing here that is too unusual or out of bounds.

Keep in mind the drop in current to below 0.2 amps for those two check points followed by the slight rise in current might seem impossible...but in reality, it is just an anomaly of the conditions of the trio of participants (charge source, batt1 and batt2) and at what phase they find themselves in at any given time.

The BMS has no effect on anything here. It's just 4 cells in one pack at a particular capacity, 4 cells in another pack at a different capacity and a charge source and a few wires in between. No BMS electronics are in play. There is no disconnect from reality. It is reality in action.

 

[Barking Sands]

Just did some quick googling. Think there was a sudden spike in IR in batt 2 that stopped current. That's serious stress on the battery.

Normally voltage-based measures would shut the battery down before it reaches that state.

Maybe DDR can confirm my IR hypothesis.

The voltage was well within limits. The battery that slowed to a crawl did so because of the voltage applied at the terminals vs its internal SOC/capacity and the attributes that come from that state. It has no inkling of anything else.

 

[Jeff F]

Ok. Why did the current split change so suddenly? What caused current on battery 2 to flatline?

 

[Jeff F]

Voltage measurements can really lead you astray with lithium. I think this is IR related.

 

[Barking Sands]

Voltage measurements can really lead you astray with lithium. I think this is IR related.

IR is but one facet in this beautiful arrangement.

 

[Barking Sands]

When battery voltage equals charge voltage the current stops. This has been occurring since internal regulated alternators and flooded batteries.

Especially when you have another "reservoir" of different electrical state in parallel... it can change the full equation.

 

[Jeff F]

Why does a battery suddenly go from accepting 30% of the current on a common bus to 0%?

Help me out, guys. It's not normal. I won't jump the gun here, but we can't ignore it.

 

[DDW]

Why do you think it isn't normal? This is exactly what I expected to see. Even a single LFP battery goes from 30% current to near zero very quickly at top of charge. There is no time scale on Barking's data, maybe he can characterize. How quickly did battery 1 go from 30A to very low? A few minutes would be textbook.

You have some rather unusual theories about this with no backing for them at all.

What I see there is completely normal behavior. These things really aren't that mysterious.

 

[Delta Riverat]

Yup, nothing spooky here. When the lithium has no more places to go current stops flowing.

You have to consider the construction, it's not like lead with heavy thick plates that take time to fully react. Lithium in like 2 sheets of foil a hundred feet long rolled up on itself. Huge area, no thickness. Stuff happens quick.