Lifepo4 house / lead starting bank interaction?

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Mac2 said:
I'm not sure how the dc2dc charger works. Will it protect the alternator from overheating and frying? Standard alternators are only designed to charge the starting battery. In this setup a large house bank will continue to demand power from the start battery (and alternator).
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@Mac2
I use the DC2DC charger to derate the ALT to about 75% of the ALT max charge rate, thus keeping it from toasting itself. So a 100A alt gets a 75A max DC2DC
 
SteveK said:
@Mac2
I use the DC2DC charger to derate the ALT to about 75% of the ALT max charge rate, thus keeping it from toasting itself. So a 100A alt gets a 75A max DC2DC
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I have a 24v 1200 amp. LifePo4 house bank. My start battery is 12v. I can't fit a larger alternator. I think my alternator is 100 amps. I have twins. I have no charge going to the house bank from my alternators now, so anything I can get is a improvement. I can parallel the dc2dc chargers (15 amp each). How did you come up with 75% derating? To be safe, I would have no issue going to 50% if that makes more sense.
TwistedTree: Please feel free to jump in here.
 
Mac2 said:
Thanks TT! Very helpful. Wasn't getting the fact that an alternator has a continuous rating. Wonder if the continuous rating is listed on alternator's, or if the distributor will tell you what it is? Also wasn't sure if the dc charger had heat sensing for the alternator (does not).
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I have NEVER seen a published continuous duty rating for an alternator. Never. It's therefore a question of figuring out yourself how hard you can run it continuously, and what people have found out the hard way is that it's MUCH lower than the nameplate rating. The only exception I'm aware of are the 4000 Series Prestolite "large frame" alternators, and I now have experience with Dixie/Delstar alternators. The Delstars in particular seem to be able to run indefinitely at full output and remain cool.
 
Thanks TT!
I'm looking forward to adding the dc2dc chargers. The 12v alternator and 24 house bank has been driving me nuts (not being able to send any charge to the house bank while running). This site always comes through.

Thanks ERTF for starting this thread!
 
This is a fine way to do it, and in many cases the easiest. Where it starts to fall apart is if you want a lot of alternator capacity which is one of the things motivated by larger LFP banks and large inverters. In that case you need a lot of DC/DC capacity to fully utilize the alternators, and an external regulator starts to be a much less complicated, and less expensive solution. I think the transition point is in the 100-150A range.

I'm pushing the envelope with my system with enough inverter power to run pretty much any AC loads (watermaker, electric dryer, electric oven, etc). While underway, the power comes from dual 250A alternators. Putting DC/DC converters between the alternators and the batteries/inverter would be inefficient, less reliable, and more costly. Instead the alternators go direct to the house batteries, and a relatively small DC/DC charger keeps the start batteries topped up.
 
TT: More great info! I have no room where my alternator is jammed into the engine. I was thinking about a 24 volt alternator to the house bank and dc2dc for start battery, but I don't think I could get a higher rated 24 volt alternator to make a difference based on efficiency of the chargers.
 
Mac2 said:
Thanks TT!
I'm looking forward to adding the dc2dc chargers. The 12v alternator and 24 house bank has been driving me nuts (not being able to send any charge to the house bank while running). This site always comes through.

Thanks ERTF for starting this thread!
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What converter have you planned from 12V DC to 24V DC. This is an area I will learn.
 
The new Victron Orion DC2DC can throttle DC amps from 1 to 50 amps in .1 increments and it supposedly 97% efficient.
 
As long as you don’t need more than 300 watts the tr will work for you, but they do get pretty hot. They are rated for 400 watts but once they get over 105 degrees ( which happens@80 ambient running full out) the output begins to drop. One of the big advantages of lithium is an almost limitless acceptance rate. The newer version mentioned by barking sands is more efficient but I believe still only available as a 12 2 12.
 
Mac2 said:

Victron Orion-Tr Smart DC-DC Charger.​

Bluetooth capable as well.
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@Mac2
After I asked the question I did some googling about the amps cut in half. 100A @ 12v becomes 50A @ 24V.
You have a 1200Ah 24 volt system. That is like 2400Ah 12 volt bank. What is your expected 12V amp input and then the 24V amp output. It seems like charging from the ALT will require many engine hours to top up the batteries.
If I am correct then it would take 12 hours to charge 600A.
Awaiting correction.
 
If you're worried about a dead starter battery, just get a sufficiently sized jump pack. I have a NOCO GB150 and it easily starts my Cat 3208. No need for the old school "combine" banks function.
 
SteveK said:
I also charge the start from ALT, then DC2DC to house
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Still learning a lot on this thread.
From what I've read, the DC to DC charger is similar to an ACR that it wont forward any power until it reaches past its its lockout point (12.6 volts). So it won't be charging you house bank until it surpasses that set point . Right or wrong? Not sure.
Cheers J.T.
 
Not sure about other DC-DC chargers, but the Victron can be configured to sense engine start (voltage above a certain level) so it only charges when the alternator is active. Unlike an aCR, you can also define a different battery chemistry/charge profile on the output side.
 
J.T kearney said:
Still learning a lot on this thread.
From what I've read, the DC to DC charger is similar to an ACR that it wont forward any power until it reaches past its its lockout point (12.6 volts). So it won't be charging you house bank until it surpasses that set point . Right or wrong? Not sure.
Cheers J.T.
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It depends on the DC2DC charger, some do, others do not. The more expensive ones even have reverse charging feature. You have to read the specs of each and know what you buy.
I chose the cheaper one with a on/off trigger from the run circuit of the ignition key.
 
My personal preference with DC-DC chargers and ACRs is (if possible) to wire the "allow to charge" or "ignition sense" wire to the engine key switch. That way charging stays independent on shore power or other sources (assuming each set of batteries has its own charger) and the DC-DC or ACR cuts off immediately at engine shutdown rather than pulling some power out of the start battery until the voltage drops a bit.
 
SteveK said:
@Mac2
After I asked the question I did some googling about the amps cut in half. 100A @ 12v becomes 50A @ 24V.
You have a 1200Ah 24 volt system. That is like 2400Ah 12 volt bank. What is your expected 12V amp input and then the 24V amp output. It seems like charging from the ALT will require many engine hours to top up the batteries.
If I am correct then it would take 12 hours to charge 600A.
Awaiting correction.
Click to expand...
Darn good question. My alternator has no markings orher then 12v. It’s between 50-100 amps-still researching. Also looking into 24v alternators (if mine are only 50 amp) that can fit into the ridiculously small hole of the existing alternator. I have 2800w of solar to help top the house bank.
 
This the way I set up my LifePo4 house batteries. 3 years ago. I would say get as many charging amps as you can to start. I have added more batteries since the original. I also added an A/C charger (60 amp) from gen set to batteries.
 

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twistedtree said:
As others have already stated, there are lots of way to do this with varying levels of work/cost and various levels of performance.

What is your current setup? Single or twin engine? Single alternator on each engine, or dual? What charging sources exist for start batteries and for house bank? How do things charge underway, and how do things charge while on shore or gen power?

Once we know what your starting point is, we can offer some alternatives for how to proceed.
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Wow this thread really took off without me. I'm different than most you guys in that my boat moves very infrequently, but spends its entire life at anchor. I have 1700w of solar. I don't need to run a generator too often but i do have a diesel, as well as a portable gas unit with a built in battery + pull cord. I have twin 6bt engines, no idea on the alternators, obviously not very consequential with this much solar and so little engine time.

I have a mini dc aircon that i run at about 180 watts at night in the summer (crank it up to 275w if i wanna chill during the day). Have an old 12v marine "apartment sized fridge/freezer" that runs continuous (unless its winter), as well as a regular 120v 5cu ft chest freezer. Add in a few fans, and i'm basically pulling 20-30amps (12v) continuous overnight in the summer.

I put in 1075ah duracell gc2's 3 years ago, but it doesn't look like they can do another summer. I currently have no dedicated start batteries, i wanted the biggest house bank i could fit. So as you can see several things need to change to accomodate a lifepo4 house, but i obviously need lithium.

Not sure if I could keep 4 of the gc2's for starting. They're not made for it and are probably beat, but at the very least I would want a way to keep them topped.

I also have an antique (apparently installed in 2003!) xantrex freedom 10 inverter/charger. It's been on continuously for 5 years with no issues. But can it be used to charge lithium? A guy at xantrex said no. I also have 1 old solar controller (C40 PWM), which he also said can't do lithium. But it has manual knobs for bulk and float voltages. Do I really gotta junk that too?
 
ERTF said:
Wow this thread really took off without me. I'm different than most you guys in that my boat moves very infrequently, but spends its entire life at anchor. I have 1700w of solar. I don't need to run a generator too often but i do have a diesel, as well as a portable gas unit with a built in battery + pull cord. I have twin 6bt engines, no idea on the alternators, obviously not very consequential with this much solar and so little engine time.

I have a mini dc aircon that i run at about 180 watts at night in the summer (crank it up to 275w if i wanna chill during the day). Have an old 12v marine "apartment sized fridge/freezer" that runs continuous (unless its winter), as well as a regular 120v 5cu ft chest freezer. Add in a few fans, and i'm basically pulling 20-30amps (12v) continuous overnight in the summer.

I put in 1075ah duracell gc2's 3 years ago, but it doesn't look like they can do another summer. I currently have no dedicated start batteries, i wanted the biggest house bank i could fit. So as you can see several things need to change to accomodate a lifepo4 house, but i obviously need lithium.

Not sure if I could keep 4 of the gc2's for starting. They're not made for it and are probably beat, but at the very least I would want a way to keep them topped.

I also have an antique (apparently installed in 2003!) xantrex freedom 10 inverter/charger. It's been on continuously for 5 years with no issues. But can it be used to charge lithium? A guy at xantrex said no. I also have 1 old solar controller (C40 PWM), which he also said can't do lithium. But it has manual knobs for bulk and float voltages. Do I really gotta junk that too?
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What ever charge source you have/get needs to match the charge profile of whatever LifePo4 you purchase. I was able to keep my xantrex inverter/charger because it had the digital panel to program to the tenths not just a position that said lithium, agm, etc. I was surprised at the difference in LifePo4 batteries. Even the batteries I purchased changed the float charge value over time, so you need the versatility to change the charge profile.
 
Okay so I have got conflicted info elsewhere regarding using a lifepo4 house bank to start engines. In that rare scenario is it ok to combine with a simple switch to the depleted lead start battery? Or what about simply starting directly from the house bank?

This will factor into how much space i save for starting batteries. If the lithium can be used in a pinch, then i see no need to waste money & space on independent start batteries for each engine. Just a single start battery would suffice.

I have twin 210hp cummins 6bt's. What is the minimum reasonable start battery size i can install?
 
@ERTF
Never mind that you can start an engine with the LFP. It is a bad idea to put all your eggs in one basket.
Start batteries are designed to start and house batteries are designed for house loads, simple as that. You can use start for house loads and LFP to start, but why swim upstream?
 
SteveK said:
@ERTF
Never mind that you can start an engine with the LFP. It is a bad idea to put all your eggs in one basket.
Start batteries are designed to start and house batteries are designed for house loads, simple as that. You can use start for house loads and LFP to start, but why swim upstream?
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All my eggs wouldn't be in 1 basket. The 1st basket would be the lead start battery. The backup basket would be the lithium house bank (if that is compatible for an emergency start).
 
Last edited:
You said
"If the lithium can be used in a pinch, then i see no need to waste money & space on independent start batteries for each engine. Just a single start battery would suffice."
One extra start battery is a waste of money and space? Emergency use anything goes. You do what you think best for you.
 
The question of starting and engine from LFP is really a question about the BMS, not about the battery cells or chemistry. Lots of BMSes, especially electronic vs contactor BMSes, will shut down over their rated current which is typically 100A or so. Multiple BMSed in parallel as you get with drop-in will increase the max current because it is spread across the paralleled batteries, but it’s still not a sure thing since the current division won’t be equal, and can’t really be controlled. On the other hand, a contractor BMS can often handle 1000A, and possibly more for a short time.
 
Thanks. So if a bms trips from over amperage, is it easily reset? Or is it a disaster that can cause damage and should not be risked?

For the house bank i'm thinking 8 x 100ah. Apparently starting amps for a 6bt are 450-700.

I can definitely fit 1 start battery + 800ah house. But 2 start batteries might require me to shorten house capacity.
 
I am waiting for somebody to come out with a 110volt A/C starter. That would solve the engine starting issue with LFP.
Bud
 
SteveK said:
I also charge the start from ALT, then DC2DC to house
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I have a switch so I charge the START BATTERY and house battery from the aux generator. Seems reasonable.
 
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