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

[CaptainRivet]

Originally Posted by donradcliffe

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.

Well i didn't ruin anything till now. I have low resistance because my system till main load bus bar is designed for constant 600A current, so everything is beefy and i am normally in the range of 250A...
I just demonstrated what currents a shortend LFP bank can produce.

[T1 Terry]

Originally Posted by CaptainRivet

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.

The opening post starts like this "My Torqeedo has a 24v 50ah lifepo4 with built in bms. Can I simply parallel this to the lifepo4 house bank. "
If the 3 v per cell min is adhered to, the empty 50Ah 8 cell battery will be at a min of 24v, the max charged 24v house battery would be at 28v, that's only a 4v differential. The current would be high in lead acid terms, but even at 200 amps it couldn't continue for 15 mins, the 50ah pack would increase in voltage to 26v after a very short period, now the differential is only 2v and the charge rate will gradually reduce till around the 90% SOC mark, then the two packs will quickly balance to the same voltage, or close enough that maybe 10 amps max flows till they are close to the same capacity .... during this recharging process, the house battery would need to maintain that 28v or the voltage differential would be even less.
This was the reason I needed to build a 5 cell battery for the demonstration, I needed that voltage differential to remain high enough to continue the high current charging ..... this was to demonstrate just how fast an LFP battery could be recharged compared to a lead acid battery, and all the way to 100% SOC, not 70% SOC when high current charging a lead acid battery.

Doing the same demo on a drop in battery might be enlightening .... hopefully not in bright explosion kind either, more in the BMS either shutting down or failing completely, I have a few of those in the shop now .....

T1 Terry

[Fuss]

Originally Posted by T1 Terry

The opening post starts like this "My Torqeedo has a 24v 50ah lifepo4 with built in bms. Can I simply parallel this to the lifepo4 house bank. "

If the 3 v per cell min is adhered to, the empty 50Ah 8 cell battery will be at a min of 24v, the max charged 24v house battery would be at 28v, that's only a 4v differential. The current would be high in lead acid terms, but even at 200 amps it couldn't continue for 15 mins, the 50ah pack would increase in voltage to 26v after a very short period, now the differential is only 2v and the charge rate will gradually reduce till around the 90% SOC mark, then the two packs will quickly balance to the same voltage, or close enough that maybe 10 amps max flows till they are close to the same capacity .... during this recharging process, the house battery would need to maintain that 28v or the voltage differential would be even less.

This was the reason I needed to build a 5 cell battery for the demonstration, I needed that voltage differential to remain high enough to continue the high current charging ..... this was to demonstrate just how fast an LFP battery could be recharged compared to a lead acid battery, and all the way to 100% SOC, not 70% SOC when high current charging a lead acid battery.



Doing the same demo on a drop in battery might be enlightening .... hopefully not in bright explosion kind either, more in the BMS either shutting down or failing completely, I have a few of those in the shop now .....



T1 Terry

Thanks very much , this is roughly what I think happens. I haven’t done a sophisticated clinical test, but I am sure that what you wrote above is about right. Lifepo4 to Lifepo4.
Also the Torqeedo cable is quite long, so there will be some resistance in this area too.
There is never any sparks when using the Andersen connector.

[T1 Terry]

Originally Posted by Fuss

Thanks very much , this is roughly what I think happens. I haven’t done a sophisticated clinical test, but I am sure that what you wrote above is about right. Lifepo4 to Lifepo4.
Also the Torqeedo cable is quite long, so there will be some resistance in this area too.
There is never any sparks when using the Andersen connector.

There will be some spark, just not enough to be obvious. You could add a combiner switch or just a single contact DC on/off switch to ensure you don't damage the Anderson plug lugs over time. Be sure to keep the lug contact faces clean and oxide free, any resistance will generate heat and the spring tension clips under the lugs are the first things to suffer, then the lost tension just makes matters worse until the plug and or cable melts.
Don't be tricked into using non genuine Anderson plugs and lugs either, they might be able to copy them but they are never as good and prone to failure.

The young fella (76 I think) that helps out around the workshop has a house boat and regularly plugs his 4 cell 12v 100Ah LFP battery (he uses in his dingy for the little electric outboard) into his house battery to top it up and even uses it when it has been raining for a few days to increase his battery capacity ... well really to extend how long before he has to look for an alternate power source. During the Murray River floods he was without power for 4 mths, actually is still without power at the moment, but the solar was enough to keep his 400Ah LFP battery fully charged at least every 3 days when the solar was poor and never required to run his generator ... his houseboat, including the hot water 50 ltr storage is all electric.

T1 Terry

[s/v Jedi]

Like someone else wrote before: you don’t get those high currents in real life and that has nothing to do with proper installations.

A 24V house battery has eight cells in series, which means you get 8 times the internal resistance instead of just once. Also you have the resistance off all the jumpers, the shunt, the main switch/contactor and that all adds up to the values I presented in my math above.

[T1 Terry]

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....

Agreed. I think the estimated 2714 amps is more like a short circuit current than a transfer current between two 24v batteries as mentioned in the opening post.
as I mentioned before, if the LFP battery for been drained to 10v for a 12v system or 20v for a 24v system, charging at a rate below 2 amps per 100Ah of battery capacity is required to avoid destroying the battery until the voltage reaches a known 3v per cell rested, unknown as in a drop in battery, I'd be looking at 3.2v per cell average after a 12 hr rest to allow the balancer to at least attempt to get the cells at a similar voltage.
20v across a 24v battery where the actual cell voltage is unknown is really asking for a cell to end up with reverse current flow under load, that always results in cell failure and in a battery you can't replace a single cell on its own, the death of the battery.
As a way of expaining what I'm saying, a cell pulled down to 0V in an 8 cell in series battery (24v nom.) would still have 7 cells averaged at 2.85v and still show 20v at the battery terminals. Any one of those 7 remaining cells still have some capacity, it would only take 1 of those cell to be at 3v and the 0v cell drops to -0.15v, the current running backwards through that cell, yet still showing 20v at the terminals ......

T1 Terry

[CaptainRivet]

Originally Posted by T1 Terry

The opening post starts like this "My Torqeedo has a 24v 50ah lifepo4 with built in bms. Can I simply parallel this to the lifepo4 house bank. "
If the 3 v per cell min is adhered to, the empty 50Ah 8 cell battery will be at a min of 24v, the max charged 24v house battery would be at 28v, that's only a 4v differential. The current would be high in lead acid terms, but even at 200 amps it couldn't continue for 15 mins, the 50ah pack would increase in voltage to 26v after a very short period, now the differential is only 2v and the charge rate will gradually reduce till around the 90% SOC mark, then the two packs will quickly balance to the same voltage, or close enough that maybe 10 amps max flows till they are close to the same capacity .... during this recharging process, the house battery would need to maintain that 28v or the voltage differential would be even less.
This was the reason I needed to build a 5 cell battery for the demonstration, I needed that voltage differential to remain high enough to continue the high current charging ..... this was to demonstrate just how fast an LFP battery could be recharged compared to a lead acid battery, and all the way to 100% SOC, not 70% SOC when high current charging a lead acid battery.

Doing the same demo on a drop in battery might be enlightening .... hopefully not in bright explosion kind either, more in the BMS either shutting down or failing completely, I have a few of those in the shop now .....

T1 Terry

But Torqueedo uses LMO-NMC not Lifepo4, same game as LI-ion with eg thermal runaway and different charge curve. Thats exactly the problem, people think Lithium are all Lifepo4 but its not and startering to mix the different chemistry.
To charge that safe just use the 110/230V charger from torqueedo via your inverter, small loss but safe.

[Fuss]

Originally Posted by CaptainRivet

But Torqueedo uses LMO-NMC not Lifepo4, same game as LI-ion with eg thermal runaway and different charge curve. Thats exactly the problem, people think Lithium are all Lifepo4 but its not and startering to mix the different chemistry.
To charge that safe just use the 110/230V charger from torqueedo via your inverter, small loss but safe.

Torqeedo Cruise uses it own separate battery, i currently use a lifepo4 60ah

[CaptainRivet]

Originally Posted by Fuss

Torqeedo Cruise uses it own separate battery, i currently use a lifepo4 60ah

https://www.torqeedo.com/es/producto...0/2106-00.html

All torqueedo original batteries are LI-NCA (cruise) or LMO-NMC, not Lifepo4 as too small energy density=more weight and too slow charging...

Can be you use your own battery but then state that as the next unexperienced newbee torqueedo user reads that and connects the original to his/her Lifepo4 bank...

[Fuss]

Originally Posted by CaptainRivet

https://www.torqeedo.com/es/producto...0/2106-00.html

All torqueedo original batteries are LI-NCA (cruise) or LMO-NMC, not Lifepo4 as too small energy density=more weight and too slow charging...

Can be you use your own battery but then state that as the next unexperienced newbee torqueedo user reads that and connects the original to his/her Lifepo4 bank...

this is a torqeedo cruise and it does not come with a battery, it has only the 2, 3 meter cables which torqeedo recommend that you connect to a 100ah lifepo4 battery that they also supply if you wish for 2000€.

[Fuss]

Some feedback…
Well, we had it all in this thread, sparks flying, dangerous forumite, thousands of amps. But in the end, it’s clear that a few forum members know what they are talking about and said it’s not very interesting as not much going on with the low voltage differential. Well, you know what, they are right, but even worse than that, there is exactly the opposite happening as there is not enough amps flowing. Andersen plugging in the torqeedo lifepo4 battery and it’s still not recharged some hours later.
Thousands of amps flowing…really, more like not enough amps flowing.

[hzcruiser]

Originally Posted by Fuss

Some feedback…
Well, we had it all in this thread, sparks flying, dangerous forumite, thousands of amps. But in the end, it’s clear that a few forum members know what they are talking about and said it’s not very interesting as not much going on with the low voltage differential. Well, you know what, they are right, but even worse than that, there is exactly the opposite happening as there is not enough amps flowing. Andersen plugging in the torqeedo lifepo4 battery and it’s still not recharged some hours later.
Thousands of amps flowing…really, more like not enough amps flowing.

Not a big surprise, really. If you can, just measure the voltage at your house bank and compare it with the voltage of the outboard batt. The difference are your losses and can be more than 1 V.

Bigger and shorter cables will reduce this delta, but ultimately, you might be better off to put a DC-DC booster in between to speed things up.

[donradcliffe]

My solar regulator is set the life house battery with an acceptance charge of 13.7 volts, which it keeps for half an hour, then drops back to a 13.4 volt float usually by 1 pm. If you want to charge the torquedo, plug it in during the early morning, or temporarily tweak the regulator settings.

I am in the Yandina camp, and adjust my peak amps between batteries by the length and size of the cables. I agree that many posters on this thread have no real world eperience in paralleling batteries.