LiFePO4 reference diagram

[s/v Jedi]

Originally Posted by WE9V

I don't see this path. As far as I know, the 24/12-30 can't take 12V and charge a 24V system. Or does one of your 24/12-30 DC-DC converters need to be changed to a 12/24-15? Also, 15A is pretty meager charging for a 400AH/800AH system. Might need a few of those.

I never guessed people would try to read labels on the pictures I used in the diagram… yes, one unit is 12-24 and the other 24-12.

Charging from alternator is mostly for the 12V bank because primary charging is solar.

We actually have 2x Orion 12-24 for this, taking 60A from the alternator, but we upgraded the alternator to a Balmar XT170 so had plenty available. With the oem 50A or 80A alternators, a single unit taking 30A is all it can handle

[s/v Jedi]

Originally Posted by WE9V

People often don't view their solar controller as a DC-DC converter, but that's exactly what it is. It won't work as a 12>12V, or 12>24V DC-DC converter, but they would work fine as a 24V>12V DC-DC converter. So, one option would be to use a MPPT Solar controller as your 24V>12V converter. The benefit here is that if your 'real' solar controller dies, you have an identical (or similar) one you could repurpose.

You are correct. For us, we have a total of 4 dc-dc converters and two mppt controllers so we have plenty redundancy already. When you scale up the house bank to 48V you need to use mppt controllers because these Orions don’t go up to 48V (yet).

[s/v Jedi]

Originally Posted by vpbarkley

S/V Jedi,

Thank you for posting. Any way you could place a number beside each item in the diagram and include a list of what those items are?

Thank you.

I’m sorry, it is what it is. When you zoom in, you can even read the labels and identify the components.

[s/v Jedi]

Originally Posted by Timpvf

Forgive my ignorance but what are the items between the house batteries and the negative bus?

When you zoom in, you can read the labels. There are “BMV’s” which are Victron battery monitors (see the gray wires to the BMV displays?) and there is one “SmartShunt” which is also a battery monitor but without display unit, which is used by the Cerbo GX to get a total readout of the complete house bank, with voltages from both 24V and 12V busbars.

[nwn]

This is a nice drawing. But I wonder how you will manage the two 24V batteries. The problem is that each of them need to have their own BMS. If they do not have the same capacity and voltage then current may run from one to the other. I have tried the same set up and even tried to use ideal diodes to prevent the backflow of current from one to the other when charging. So far with little luck. But I need more testing

[s/v Jedi]

Originally Posted by nwn

This is a nice drawing. But I wonder how you will manage the two 24V batteries. The problem is that each of them need to have their own BMS. If they do not have the same capacity and voltage then current may run from one to the other. I have tried the same set up and even tried to use ideal diodes to prevent the backflow of current from one to the other when charging. So far with little luck. But I need more testing

As you can see in the latest version of the diagram (attached here again) each battery has it’s own BMS, fuse, switch and monitor.

Normally current doesn’t run from one to the other; in theory this can happen when there is no charging and no discharging happening (batteries at rest) but with lfp the voltage difference is so low that this only happens in the lower and upper knee of the SOC graph. To get into those knees, you are charging or discharging and there will be no battery-to-battery current when that happens.

charging: the charge voltage is higher than the voltage of the highest SOC battery. The lowest SOC battery has the largest differential so takes the bulk of the charge current, while the highest SOC battery still gets some of the charge.

discharging: the voltage at the busbar is lower than the voltage of the highest SOC battery (voltage drop in fuse, switch and cabling due to the load) and the highest SOC battery throws the largest part of the current in.

This dude on the Youtube channel Offgrid Garage connected two batteries parallel: one was at 70% SOC and the other at 30% (something like that) and there was no current flowing to speak of.

So here is how to do this:

1. Switch one battery to the busbar and fully charge it.
2. Disconnect that battery from the busbar and now connect the other one, then fully charge it.
3. Now connect the other battery back so both are in parallel on the busbar. From here you can call it good as they will self-balance, even with different capacities. Of course the smallest battery will be at a lower SOC but as it’s voltage starts dropping, the other battery will be supplying all of the power and when charging starts, the low battery will take all the charge current.

There is no need for diodes. Ideally you can think of it as done in Star Trek: you can create power conduits running the length of the ship (positive and negative power cables) with power posts or busbars at regular intervals to make connections. At these connections you can hookup lfp batteries or loads like windlass, bow thruster, inverter/charger, alternator etc. and it will all work just fine. You can put a lfp battery at the hookups for the bow thruster and windlass to give them a boost there, even when there are no charge sources available at that power post or busbar.

The offgrid garage did all those experiments, check it out.

[Timpvf]

These items on the other side of the battery. Black boxes.

[Sailmonkey]

Originally Posted by Timpvf

These items on the other side of the battery. Black boxes.

Those are battery disconnect switches

https://www.bluesea.com/products/770..._-_12V_DC_500A

[s/v Jedi]

Originally Posted by Timpvf

These items on the other side of the battery. Black boxes.

That’s the positive side. First item left is the Class-T fuse, followed by a battery switch. These are Blue Sea System RBS (Remote Batter Switch) units. They can be operated with the yellow handle, or with a small electric switch remotely, like from a control panel.

Edit: also, some BMS’s, like the REC BMS, can control these switches too. This means they can control up to 500A current and do not have the losses of MOSFET-based BMS’s nor the reduced reliability of those.

[s/v Jedi]

Originally Posted by Sailmonkey

Those are battery disconnect switches

https://www.bluesea.com/products/770..._-_12V_DC_500A

Yes, thanks for posting the link! You’re quick

[fxykty]

Originally Posted by s/v Jedi

As you can see in the latest version of the diagram (attached here again) each battery has it’s own BMS, fuse, switch and monitor.



Normally current doesn’t run from one to the other; in theory this can happen when there is no charging and no discharging happening (batteries at rest) but with lfp the voltage difference is so low that this only happens in the lower and upper knee of the SOC graph. To get into those knees, you are charging or discharging and there will be no battery-to-battery current when that happens.



charging: the charge voltage is higher than the voltage of the highest SOC battery. The lowest SOC battery has the largest differential so takes the bulk of the charge current, while the highest SOC battery still gets some of the charge.



discharging: the voltage at the busbar is lower than the voltage of the highest SOC battery (voltage drop in fuse, switch and cabling due to the load) and the highest SOC battery throws the largest part of the current in.



This dude on the Youtube channel Offgrid Garage connected two batteries parallel: one was at 70% SOC and the other at 30% (something like that) and there was no current flowing to speak of.



So here is how to do this:



1. Switch one battery to the busbar and fully charge it.

2. Disconnect that battery from the busbar and now connect the other one, then fully charge it.

3. Now connect the other battery back so both are in parallel on the busbar. From here you can call it good as they will self-balance, even with different capacities. Of course the smallest battery will be at a lower SOC but as it’s voltage starts dropping, the other battery will be supplying all of the power and when charging starts, the low battery will take all the charge current.



There is no need for diodes. Ideally you can think of it as done in Star Trek: you can create power conduits running the length of the ship (positive and negative power cables) with power posts or busbars at regular intervals to make connections. At these connections you can hookup lfp batteries or loads like windlass, bow thruster, inverter/charger, alternator etc. and it will all work just fine. You can put a lfp battery at the hookups for the bow thruster and windlass to give them a boost there, even when there are no charge sources available at that power post or busbar.



The offgrid garage did all those experiments, check it out.

This is very interesting and I hope works in real life! We have a 12 month old 4S 12V 700Ah battery using Winston cells. We want to double our battery capacity and the options are either:

1) create a second 4S 12V battery and run it in parallel as your diagram shows. Great for complete redundancy, but extra cost of another BMS, contactors, fuse, switch, etc. And of course the worry of the two batteries getting wildly out of balance, but hopefully a moot point based on your information.

2) take the current battery apart and create a 2P4S 12V battery, with one old cell and one new cell in each 2P pair. Our cells are slightly above their original internal resistance but the Winston factory says they can match new cells to the existing cells. This setup means no changes are needed except for a new tie down system to handle the bigger battery. But we do lose redundancy.

Thoughts on which option we should choose?

[s/v Jedi]

Originally Posted by fxykty

This is very interesting and I hope works in real life! We have a 12 month old 4S 12V 700Ah battery using Winston cells. We want to double our battery capacity and the options are either:

1) create a second 4S 12V battery and run it in parallel as your diagram shows. Great for complete redundancy, but extra cost of another BMS, contactors, fuse, switch, etc. And of course the worry of the two batteries getting wildly out of balance, but hopefully a moot point based on your information.

2) take the current battery apart and create a 2P4S 12V battery, with one old cell and one new cell in each 2P pair. Our cells are slightly above their original internal resistance but the Winston factory says they can match new cells to the existing cells. This setup means no changes are needed except for a new tie down system to handle the bigger battery. But we do lose redundancy.

Thoughts on which option we should choose?

Definitely option #1. The only disadvantage is the higher cost but this adds much more advantages than just a better working battery:

- you can’t parallel old with new cells. You could parallel the old ones and add new pairs from new cells in series, but the old cells will reduce battery performance for sure.

- once the old cells reach end of life, your battery stops working and you lost all power, even though half the cells are newer and in working condition.

- the 2-battery setup adds full redundancy. Extra cost for bms, switch, monitor etc. also mean you have two of each and can shut down one while staying operational.

- I have another thread with this setup for 12V, which will better fit your situation. It actually recommends the Winston 700Ah cells

[Baronkrak]

Originally Posted by s/v Jedi

I show a REC BMS image, which can control the Blue Sea Systems battery switch in case it needs to take the battery offline. It also shows a warning signal buzzer that warns a HVC or LVC event is imminent. You can connect relays to shutdown an alternator or other charger when HVC warning is given or a load in case of LVC.

Having some free time.

Ni.. what happens if one or both BMS detect over heating or over charging?

[Emouchet]

Originally Posted by Baronkrak

What happens if one or both BMS detect over heating or over charging?

My understanding is that if both BMS disconnect the batteries, you go into a "Load dump state": total power blackout on the boat, unless the 12v batteries via the 24v DC/DC converter wakes up...

And this will happen in the middle of a foggy night, in a busy channel. Murphy's law

[Baronkrak]

Originally Posted by Emouchet

And this will happen in the middle of a foggy night, in a busy channel. Murphy's law

Good guess.