Why can’t I parallel an empty lifepo4 with the full lifepo4 house bank

[hzcruiser]

Originally Posted by sailingchiro

I hooked my cells together in order to top balance them with 2 gauge wire and it went up in smoke and set off my smoke detector. My wife was not happy.
As many have said, you will need to limit the current (amps). This you will either need to increase the resistance or use a dc to dc charger.

Try using a power resistor such as a 200 watt but you will need to determine the resistance to reduce your current.

My guess here would be you tried to top balance them while they were still hooked up in series within the boat? That would be a dead short indeed and not a top balance. Or can you rule that out 100%?

[hzcruiser]

Originally Posted by CapnCookinOz

No, sorry you are wrong.
Batteries in series - you ADD the voltages. ie 2 x 12v batteries in series gives 24v at the same available current (amps).
Batteries in parallel - the voltage stays the same, but the current available doubles (in theory). ie 2 x 12v batteries in parallel still give you a 12v supply, but more current is available

He's already been corrected 2 months ago, mate...

[boatpoker]

There are a couple of good posts here and one dangerous member.
Read Rod Collins (Compass Marine) terrific article which covers a few of these issues.
https://marinehowto.com/drop-in-life...Qahr-TPKyHE_yA

[zavman]

Take a look at this it might help

https://youtu.be/fQz7vZEYLfY

[Fuss]

Originally Posted by boatpoker

There are a couple of good posts here and one dangerous member.
Read Rod Collins (Compass Marine) terrific article which covers a few of these issues.
https://marinehowto.com/drop-in-life...Qahr-TPKyHE_yA

Some feedback.
I don't get any sparks or huge current draw and I often connected the round about 80% discharged 60ah 24v Lifepo4 up to the Lifepo4 house bank through an anderson connector and a longish 20mm2 cable.
I guess it works due to Lifepo4 similar voltages between the knees and thats where it gets paralleled. I never tried it at opposite ends of the knees, maybe then sparks would fly, but sofar not for me.

[RaymondR]

E=IR: where E is the difference in voltage, I is the current, and R is the conductor resistance.

You have the same problem as connecting an alternator to a heavily discharged house battery - the high current flow is going to burn something out. The solution is the adjust R so that I is limited to the current capacity of the circuit components. I generally use a stainless steel push bike spoke as a resister since I always have a few of them lying about. The resister can get pretty hot so precautions are required.

[Fuss]

Does anyone have any real world experience of this being a problem as I now have been paralleling the batteries at completely the opposite ends of the scale?
I read everywhere how careful you have to be that the voltages are the same when paralleling two batteries.
I just plug in the andersen connector.
It appears to be a complete non event..

But how can this be when the whole world goes on about it….just look at some of the comments above.

Do the nay sayers above really not know what they are talking about or am I missing something.?

[Recy]

Originally Posted by Fuss

Does anyone have any real world experience of this being a problem as I now have been paralleling the batteries at completely the opposite ends of the scale?
I read everywhere how careful you have to be that the voltages are the same when paralleling two batteries.
I just plug in the andersen connector.
It appears to be a complete non event..

But how can this be when the whole world goes on about it….just look at some of the comments above.

Do the nay sayers above really not know what they are talking about or am I missing something.?

How much voltage difference have you had when connecting the batteries?

[wholybee]

Originally Posted by Fuss

Does anyone have any real world experience of this being a problem as I now have been paralleling the batteries at completely the opposite ends of the scale?
I read everywhere how careful you have to be that the voltages are the same when paralleling two batteries.
I just plug in the andersen connector.
It appears to be a complete non event..

But how can this be when the whole world goes on about it….just look at some of the comments above.

Do the nay sayers above really not know what they are talking about or am I missing something.?

A fully discharged battery is 10V (2.5 x 4) while a fully charged battery is 13.8V. That is a difference of only 3.8V. How bad connecting a fully charged LFP with an fully empty LFP is going to depend on the resistance in the wiring. It might be an issue, but isn't as likely as it is often made out to be because the voltage is low. Much worse is (for example) connecting an LFP to an inverter when the LFP is fully charged and the capacitors in the inverter are at 0V.

Since you have been doing it anyway. Is there a spark when you connect the Anderson connector? After you connect it, can you measure the current with a clamp on meter? There will obviously be some current flow. And probably a small spark. But if the spark is small and the current flow is low, then there is no problem.

[s/v Jedi]

Originally Posted by wholybee

A fully discharged battery is 10V (2.5 x 4) while a fully charged battery is 13.8V. That is a difference of only 3.8V. How bad connecting a fully charged LFP with an fully empty LFP is going to depend on the resistance in the wiring. It might be an issue, but isn't as likely as it is often made out to be because the voltage is low. Much worse is (for example) connecting an LFP to an inverter when the LFP is fully charged and the capacitors in the inverter are at 0V.

Since you have been doing it anyway. Is there a spark when you connect the Anderson connector? After you connect it, can you measure the current with a clamp on meter? There will obviously be some current flow. And probably a small spark. But if the spark is small and the current flow is low, then there is no problem.

The current will be that 2.5V divided by the resistance. If the total circuit resistance is 0.5Ohm then we have 5A. At 0.1Ohm we have 25A. All not very exciting.

[CaptainRivet]

Originally Posted by s/v Jedi

The current will be that 2.5V divided by the resistance. If the total circuit resistance is 0.5Ohm then we have 5A. At 0.1Ohm we have 25A. All not very exciting.

yes a lot of underdimensioned and /or oxidated/coroded cabeling unintentionally saved a lot of peoples boat and asses...
but if you have a proper install we talk about 2,5/0,14mohm=2.5/0.0014ohm = 2083A...upsi...with LFP and lead...
now worst case one empty, the other full = 3,8V/0,0014mOhm=2714A...thats more then an inverter spark....

[wholybee]

Originally Posted by CaptainRivet

yes a lot of underdimensioned and /or oxidated/coroded cabeling unintentionally saved a lot of peoples boat and asses...
but if you have a proper install we talk about 2,5/0,14mohm=2.5/0.0014ohm = 2083A...upsi...with LFP and lead...
now worst case one empty, the other full = 3,8V/0,0014mOhm=2714A...thats more then an inverter spark....

Yeah, but even in a good installation, resistances add up. A class T fuse has a resistance of .25 to 1 mOhm. There is the resistance in the BMS cutoff device (either the FETs or the contactor) and the internal resistance of the cells will be some tens of mOhm, times 4 cells. And there is probably a shunt in there which is another .1 mOhm. None of that includes excessive resistance from a poor installation.

A total circuit resistance of .14 mOhm isn't a reasonable guess of the circuit resistance. It will be much higher than that even in a good installation.

[T1 Terry]

Originally Posted by wholybee

A fully discharged battery is 10V (2.5 x 4) while a fully charged battery is 13.8V. That is a difference of only 3.8V. How bad connecting a fully charged LFP with an fully empty LFP is going to depend on the resistance in the wiring. It might be an issue, but isn't as likely as it is often made out to be because the voltage is low. Much worse is (for example) connecting an LFP to an inverter when the LFP is fully charged and the capacitors in the inverter are at 0V.

Since you have been doing it anyway. Is there a spark when you connect the Anderson connector? After you connect it, can you measure the current with a clamp on meter? There will obviously be some current flow. And probably a small spark. But if the spark is small and the current flow is low, then there is no problem.

A critical problem has not been addressed here, what voltage is fully discharged? 100% of the stated capacity is fully discharged, any more than that is draining an LFP battery into the damage zone.
If all 100Ah are drawn from the fully charged 100Ah LFP battery, when the load is disconnected, the terminal voltage should recover to around 12.6v or higher, if it doesn't, you have drained more than the batteries genuine capacity. If the recovery is only 12v, you have actually drained more than the genuine capacity. If you can't get a read on the individual cell voltages (1 or more cells in parallel) make sure the terminal voltage is better than 12.2v and you have given it at least an hr for the balancer to shift capacity to the lowest cells.

The issue is safe charging cell voltages, if it remains under 3 volts, it needs to be trickle charged till it holds better than 3v, 2 amps per 100Ah capacity is the rule of thumb. If you charged faster than that on a deeply discharged cell, you will damage it if not kill it because the material from the LFP plate will start to separate and coat the graphite plate and at the same time, the electrolyte will heat up savagely boiling off the lighter elements causing gassing and pressure expansion of the cell.

Many early days EV owners discovered this, yet didn't understand the cause and blamed the cell chemistry.

As far as linking an LFP battery to a battery with a higher voltage and the ability to deliver a lot of current ..... I did just that in the early days of demonstrating the advantages of the LFP chemistry.
I would solar charge a 5 cell battery to get a 17.5v to 18v fully charged LFP battery, while demonstrating the capability of a 4 cell LFP battery with a nom. 12.8, fully charged to hold 14v, by connecting it to an inverter and plugging a bread maker. While I was talking about the LFP battery and showing the cell voltages and current draw for different cycles as the cooking progressed, the smell of freshly cooking fruit loaf would capture the audience :lol:
When the loaf was cooked, under an hr, I'd disconnect the inverter and connect the 18v battery through the same Victron 700 BMV shunt, so they could watch the current flow back into the battery and the used Ah quickly count back to zero. By that time the loaf had cooled enough to remove it from the bread maker and start slicing it up while the BMS disconnected the link between the batteries ..... then I'd ask the audience if anyone wanted to bring over their lead acid battery to take on the same test, if they equalled or bettered it, I would give them the whole rig, batteries, inverter and bread maker ....... never had anyone take me up on it

It did required the 5 cell battery charging into the 4 cell battery to get the big current flow and of course, have enough capacity to fully charged the 4 cell battery because the 5 cell battery would maintain a higher voltage throughout the recharge.

I might modify that demo to comparing a drop in 100Ah lithium to one of our built up 100Ah lithium batteries, both the load test and recharging test .... it might open a few eyes

T1 Terry

[donradcliffe]

Originally Posted by CaptainRivet

yes a lot of underdimensioned and /or oxidated/coroded cabeling unintentionally saved a lot of peoples boat and asses...
but if you have a proper install we talk about 2,5/0,14mohm=2.5/0.0014ohm = 2083A...upsi...with LFP and lead...
now worst case one empty, the other full = 3,8V/0,0014mOhm=2714A...thats more then an inverter spark....

So, your "proper" install is going to ruin alternators and batteries and cost lots more in fuses and DC to DC chargers.

My installs are engineered to passively limit peak currents. It may take a few minutes more to charge your batteries, but they will be more reliable and a lot simpler.

[CaptainRivet]

Originally Posted by T1 Terry

A critical problem has not been addressed here, what voltage is fully discharged? 100% of the stated capacity is fully discharged, any more than that is draining an LFP battery into the damage zone.
If all 100Ah are drawn from the fully charged 100Ah LFP battery, when the load is disconnected, the terminal voltage should recover to around 12.6v or higher, if it doesn't, you have drained more than the batteries genuine capacity. If the recovery is only 12v, you have actually drained more than the genuine capacity. If you can't get a read on the individual cell voltages (1 or more cells in parallel) make sure the terminal voltage is better than 12.2v and you have given it at least an hr for the balancer to shift capacity to the lowest cells.

The issue is safe charging cell voltages, if it remains under 3 volts, it needs to be trickle charged till it holds better than 3v, 2 amps per 100Ah capacity is the rule of thumb. If you charged faster than that on a deeply discharged cell, you will damage it if not kill it because the material from the LFP plate will start to separate and coat the graphite plate and at the same time, the electrolyte will heat up savagely boiling off the lighter elements causing gassing and pressure expansion of the cell.

Many early days EV owners discovered this, yet didn't understand the cause and blamed the cell chemistry.

As far as linking an LFP battery to a battery with a higher voltage and the ability to deliver a lot of current ..... I did just that in the early days of demonstrating the advantages of the LFP chemistry.
I would solar charge a 5 cell battery to get a 17.5v to 18v fully charged LFP battery, while demonstrating the capability of a 4 cell LFP battery with a nom. 12.8, fully charged to hold 14v, by connecting it to an inverter and plugging a bread maker. While I was talking about the LFP battery and showing the cell voltages and current draw for different cycles as the cooking progressed, the smell of freshly cooking fruit loaf would capture the audience :lol:
When the loaf was cooked, under an hr, I'd disconnect the inverter and connect the 18v battery through the same Victron 700 BMV shunt, so they could watch the current flow back into the battery and the used Ah quickly count back to zero. By that time the loaf had cooled enough to remove it from the bread maker and start slicing it up while the BMS disconnected the link between the batteries ..... then I'd ask the audience if anyone wanted to bring over their lead acid battery to take on the same test, if they equalled or bettered it, I would give them the whole rig, batteries, inverter and bread maker ....... never had anyone take me up on it

It did required the 5 cell battery charging into the 4 cell battery to get the big current flow and of course, have enough capacity to fully charged the 4 cell battery because the 5 cell battery would maintain a higher voltage throughout the recharge.

I might modify that demo to comparing a drop in 100Ah lithium to one of our built up 100Ah lithium batteries, both the load test and recharging test .... it might open a few eyes

T1 Terry

Cooking class with benefits, like that Terry. Well done...
Correct better don't go below 3V on LFP cell.
If you read the thread poster carefully he wanna charge a empty L-ION from a Lifepo4, thats a different game as they have higher energy density and current that will flow is much higher even with small capacity cells.