Lifepo4 tapping 12v from a 24v battery

[Fuss]

Has anyone done this successfully?
Normally you end up with an undercharged part that is drawing the load which then means that the other half is overcharged. That's why its not done.

However trucks often have this function with their lead batteries. To get around the problem they use a charge equaliser. This compares one half of the battery to the other and if there is a difference, it takes from one half and puts it into the other. They come in various sizes, Redarc Australia make 10,25 and 40ah versions for example.
I could imagine problems when you are drawing a large load, but I only need about 3ah.
I guess Lifepo4 presents the problem for the unit, of deciding which battery is discharged and by how much.

[s/v Jedi]

Originally Posted by Fuss

Has anyone done this successfully?
Normally you end up with an undercharged part that is drawing the load which then means that the other half is overcharged. That's why its not done.

However trucks often have this function with their lead batteries. To get around the problem they use a charge equaliser. This compares one half of the battery to the other and if there is a difference, it takes from one half and puts it into the other. They come in various sizes, Redarc Australia make 10,25 and 40ah versions for example.
I could imagine problems when you are drawing a large load, but I only need about 3ah.
I guess Lifepo4 presents the problem for the unit, of deciding which battery is discharged and by how much.

You only need this for emergency use. For everyday 12V use you simply use a 24->12V dc-dc converter which completely avoids the imbalance and the need for fixing it.

[Fuss]

Originally Posted by s/v Jedi

You only need this for emergency use. For everyday 12V use you simply use a 24->12V dc-dc converter which completely avoids the imbalance and the need for fixing it.

Yes, thats true. But lets say I'm at the Chagos Islands and I got a lightning strike and now I want to start my 12v motor from the 24v bank for the next 2 weeks until I can make repairs.

I could imagine that if I kept the 24v battery charging up by the knee that this Redarc lead solution would recognise the slight different voltages and transfer to the discharged side from the normal side.
The 24v BMS might not notice or at least not notice much and the next 2 weeks might go relatively smoothly.

[s/v Jedi]

Originally Posted by Fuss

Yes, thats true. But lets say I'm at the Chagos Islands and I got a lightning strike and now I want to start my 12v motor from the 24v bank for the next 2 weeks until I can make repairs.

I could imagine that if I kept the 24v battery charging up by the knee that this Redarc lead solution would recognise the slight different voltages and transfer to the discharged side from the normal side.
The 24v BMS might not notice or at least not notice much and the next 2 weeks might go relatively smoothly.

Simply carry a cheap bench power supply from Amazon or Ali Express. After a lightning strike it may be back to sailing only though… even batteries themselves can be taken out.

[CaptainRivet]

Originally Posted by s/v Jedi

Simply carry a cheap bench power supply from Amazon or Ali Express. After a lightning strike it may be back to sailing only though… even batteries themselves can be taken out.

they are all Li-ION so a fire hazard.
I assume if its drop ins an you combine them to 24V nothing happens if you tap into one of the 12V batteries.
If you build your own from cells you could do a 12V tap. You should be able to start easly and the imbalance should not be too big, especially if you have big cells (eg 280AH or 304 or 400).

for this I would get the biggest active balancer you can find, that should cure the imbalance if you not starting it hourly...for 8S thats the 8s 4A neey. heltec has a 10A but that was never tested above 4,8A but with 0,6V deviation needed to get 4,8A. THe neey does 4A independently from deviation.

Was thinking about that too when moving now to 24V as starter is 12V and I loose the possibility to start from the house.

lightening strike if then it mostly takes out the BMS, not the cells and I carry 2 BMS spares watertide packed and wrapped in aluminum folie, plug&play. if its a direct hit and cells are damaged then its all black means back to sailing only and cooking gas anyhow...

[s/v Jedi]

Originally Posted by CaptainRivet

they are all Li-ION so a fire hazard.
I assume if its drop ins an you combine them to 24V nothing happens if you tap into one of the 12V batteries.
If you build your own from cells you could do a 12V tap. You should be able to start easly and the imbalance should not be too big, especially if you have big cells (eg 280AH or 304 or 400).

for this I would get the biggest active balancer you can find, that should cure the imbalance if you not starting it hourly...for 8S thats the 8s 4A neey. heltec has a 10A but that was never tested above 4,8A but with 0,6V deviation needed to get 4,8A. THe neey does 4A independently from deviation.

Was thinking about that too when moving now to 24V as starter is 12V and I loose the possibility to start from the house.

lightening strike if then it mostly takes out the BMS, not the cells and I carry 2 BMS spares watertide packed and wrapped in aluminum folie, plug&play. if its a direct hit and cells are damaged then its all black means back to sailing only and cooking gas anyhow...

It’s much easier to just have a 12V battery for starting the engine. Even a spare 12V battery. The real problem is that you are trying to make 12V alternators charge a 24V LFP bank without doing the right thing which is either use solar or replace the alternators with 24V models

[Fuss]

The reason I suggested the Redarc solution is that it tries to balance 1 half of a 24v battery against the other, whichever is lower takes from the other side. It’s a simple solution in a potentially stressful situation as it only has 3 wires. If I simply have a spare battery, then it has to be maintained somehow or when needed 5 years later possibly does not work.

[Nicholson58]

Caution!

Good advice above, totally.

If as a last resort you need to tap 12 on a 24 volt boat from two 12 volts in series use the battery whose negative terminal is tied to the boat ground.

[CaptainRivet]

Originally Posted by s/v Jedi

It’s much easier to just have a 12V battery for starting the engine. Even a spare 12V battery. The real problem is that you are trying to make 12V alternators charge a 24V LFP bank without doing the right thing which is either use solar or replace the alternators with 24V models

You have the same 12V alternator, 12V Starter, 24V house and DC2DC 12 to24V (follow your diagram wirh small adaptions due to my BMS, planed solution to swap to 24V, needed as my 2 new inverter/charger beasts are 2x24V/3500W Studer Xtender, peak power to 9kw each)....so not wrong.
I just plan a emergency backup by "flip a switch solution" in case my 4 lead starter (2 per side, so backups) are all dead or discharged due to any reason (offgrid offshore). It works, i tested it already and the balancer fixed it in approx 6h.
Could be i replace the lead with LTOs if i find a possibility to get them shipped here.

[s/v Jedi]

Originally Posted by CaptainRivet

You have the same 12V alternator, 12V Starter, 24V house and DC2DC 12 to24V (follow your diagram wirh small adaptions due to my BMS, planed solution to swap to 24V, needed as my 2 new inverter/charger beasts are 2x24V/3500W Studer Xtender, peak power to 9kw each)....so not wrong.
I just plan a emergency backup by "flip a switch solution" in case my 4 lead starter (2 per side, so backups) are all dead or discharged due to any reason (offgrid offshore). It works, i tested it already and the balancer fixed it in approx 6h.
Could be i replace the lead with LTOs if i find a possibility to get them shipped here.

Why do you need two 70A converters for backup?

Yes, I do this too but I wouldn’t do it twice with two engines.

[CaptainRivet]

Originally Posted by s/v Jedi

Why do you need two 70A converters for backup?

Yes, I do this too but I wouldn’t do it twice with two engines.

Who said i buy 2x70A converter?
Very seldomly i have 2 engines running over longer periode, so i optimize for one.
I will have smalish 2x72AH lead per hull/engine as starter. These are combined with a Victron battery combiner when one or two alternator works, so doesn't matter if STB or BB engine running all 4 lead will be joined together. Then on STB lead goes the 80A 12-24V DC2DC but it will run when either of the engines running and max one alternator out. If both running i waste 80A from 2nd one and both running at 50% load. That's how it actually is already installed with a STB lead starter/house bank and BB lead starter bank, i just replace the STB house/starter old 4x100AH with 2x72AH lead starter and add the DC2DC to it and connect the output to the 24V LFP bank. Job done.
If i find a 70A cheap in the 200Euro range i am tempted to get a 2nd one when 2 engines running and as backup. For that amount of money you cannot get two 24V alternator and starter and no 24V windlass motor as this is 12V and new and will run of 4x72AH lead starter. Will be the first i swap to 24V as first improvement but first swap from 12V to 24V house bank with as liitle effort as possible and get all running asap. Will have some surprises for sure where to place 24V-12V step down converters for all the 12V equipment, enough work here waiting so alternators i make my life easy as possible so all is running asap.

Final goal is:
STB: replace the starter lead STB with the 24V house (so new 24V starter and alternator) and
BB: add 2nd 24V/min 70A alternator to BB engine (if i find a cost effective alternator and can fabricate the needed brackets myself) and a 12V LTO 80AH starter/backup bank with the 80A Dc2DC charging 24V house too.

[s/v Jedi]

Originally Posted by CaptainRivet

Who said i buy 2x70A converter?
Very seldomly i have 2 engines running over longer periode, so i optimize for one.
I will have smalish 2x72AH lead per hull/engine as starter. These are combined with a Victron battery combiner when one or two alternator works, so doesn't matter if STB or BB engine running all 4 lead will be joined together. Then on STB lead goes the 80A 12-24V DC2DC but it will run when either of the engines running and max one alternator out. If both running i waste 80A from 2nd one and both running at 50% load. That's how it actually is already installed with a STB lead starter/house bank and BB lead starter bank, i just replace the STB house/starter old 4x100AH with 2x72AH lead starter and add the DC2DC to it and connect the output to the 24V LFP bank. Job done.
If i find a 70A cheap in the 200Euro range i am tempted to get a 2nd one when 2 engines running and as backup. For that amount of money you cannot get two 24V alternator and starter and no 24V windlass motor as this is 12V and new and will run of 4x72AH lead starter. Will be the first i swap to 24V as first improvement but first swap from 12V to 24V house bank with as liitle effort as possible.

Final goal is:
STB: replace the starter lead STB with the 24V house (so new 24V starter and alternator) and
BB: add 2nd 24V/min 70A alternator to BB engine (if i find a cost effective alternator and can fabricate the needed brackets myself) and a 12V LTO 80AH starter/backup bank with the 80A Dc2DC charging 24V house too.

Okay, I like the setup with the combiner, make sure it is FET based to minimize losses. Also, plan for how to deal with equipment failures, i.e. what to do if the combiner fails. Maybe a switch or two can bypass it for manual operation, preventing the need for rewiring while underway etc.

Also, with your setup I would install a 360W Orion dc-dc converter near each engine. This gives you 60A rather than 70A when either or both engines run but also it still gives 30A when one Orion would fail
(I have two as well)

[CaptainRivet]

Originally Posted by s/v Jedi

Okay, I like the setup with the combiner, make sure it is FET based to minimize losses. Also, plan for how to deal with equipment failures, i.e. what to do if the combiner fails. Maybe a switch or two can bypass it for manual operation, preventing the need for rewiring while underway etc.

It a Victron Cytrix battery combiner with combiner+isolater switches, means I can combine both side or isolate Start STB from STB and BB from BB.

Thats how it is already installed by former owner (without fuses which i will add too) so 2x72AH lead in STB and DC2DC eg 70A Sterling or 2x30A Victrons or? and job done.

Originally Posted by s/v Jedi

Also, with your setup I would install a 360W Orion dc-dc converter near each engine. This gives you 60A rather than 70A when either or both engines run but also it still gives 30A when one Orion would fail
(I have two as well)

well thats a business case in the end...
I am massivly overpowered but have no daggerboards means sailing close AWA >55 degrees means i am motorsailng because i get +2kn/can keep heading at 1l/h motoring with 1 engine @1300RPM is a noBrainer. And seperate well isolated engine rooms so you nearly don't hear the engine at all. means that happens more often then I like running at 1300RPM for 10h or more...
thats why i care about maxing one alternator out, i have it and extra AH in at passage is always good. An overhauled replacment 115A original costs 200Euro so maxing it out&stressing&reducing lifespan is ok, i have a spare.
1x Stering is 450Euro and delivering 70A, so 10h motoring is 700AH and net 550AH in the bank
2x Victron is 600Euro and really delivering 2x25-27A=50-54A, doing eg 10h motorsailing I have 500-540AH, around 350A net in the bank
so using Victron is 150Euro more expensive for 200AH less in the bank plus the sterling is no cooking brick (at least my buddies 12V 60A one) compared to Victrons.

But I hope to find maybe cheaper 12-24V DC2DC eg 2oA ones for around 100Euro maybe..so i get several as backup. or good deal on Victrons...

I will test in parallel a 70Euro 80A chinese boost converter variable input 9-17V and 28,0V output controlled by the Electrodacus BMS, thats backup. My buddy has the same model/manufacturer in the other direction 24V to 12V in 120A version for his anchor whinch (will use that too) which is hard on it being 900W and going strong for 2 years now. Will be in med this season so can test it out and easy replace if needed. Charged LFP for 20 years with a fix voltage power supply, no problem.

2nd I am working on a mod kit (which i also plan to sell for optimising the rum kitty, have already 10 that want such a kit) for the 115A Mitsubishi on Volvos to gear it higher+Nordkyn VSR 200 regulator and will get around 100A from it instead of 80A constant charging. Main reason I am overpowered with huge 3-blade 18x13 FOFF (instead tiny 12x11 2 blade) so engines run on low rpm frequent so gearing up alternator makes sense by default, the 115A Mitsubshi has some reserves hidden adding to that too (i have construction plans and Mitsubshi intel so I know the 100A goal is realistic) and there is even a 200A aftermarket version where the kit could fit too, so realistic 140A could be possible for a budget you won't even get just the 24V/70A alternator that will deliver 40A@24V. Lets see what works...
Depending on the results here I might even go dual voltage house banks and erase the lead completly and start from 12V house LFP or LTO.

So one road/final solution in maybe a year or 2 could be keep both 12V 115A delivering 2x100A and max them out with each a 70A+20A DC2DC (or eg 2x40A/4x20A when avaliable) and LTO starters.

or the other way have 24V/min 70A alternators with nordkyn VSR regulator on both engines charging house+24V starters

or maybe both (modified 12V and 24V alts with 12V starters and DC2DC), depends on the costs and what works in the end.

First get running what I have with the 24V house, then I will try to modifiy/adapt what I have for max output and 2nd add new if make sense from costs/output gain.

[poiu]

I think this is being overly anxious. How many potential issues to we need a perfect planned solution for?

Another solution is to change the genset starter motor for 24V. But it does cost money.

[CaptainRivet]

Originally Posted by poiu

I think this is being overly anxious. How many potential issues to we need a perfect planned solution for?

Another solution is to change the genset starter motor for 24V. But it does cost money.

My genset is 230V output and uses the shore power charger of the inverter/chargers, so by swapping them out to 24V i have automatically 2x90A/24V chargers.

Well its a question where you wanna go and how much risk you wanna take...
Offgrid and remote area in eg Pacific every solution you can think off now and makes economically sense.
I am just replanning my installation to swap to 24V house and can easier integrate now the fallback solutions when half the interior is torn apart anyhow and i have good access to parts. If thats a matter of rooting an additional short cable and add a 20Euro switch i have easy access too now, I prefer to do it now....
And that also means get 2 or 3 small units in parallel instead 1 big, if the big fails you have nothing but if small fails still 2/3 or half...and thats what if done now with inverter/charger, solar controllers, 2 smaller lithium bank instead 1 big and will do now with DC2DC chargers.
You will still have an issue you don't plan for but less and creating a solution if you have several fallbacks is easier.