Upgrade from AGM > LifePo4 help

[Capt Ben]

I'm interested in converting my AGM system to a hybrid, AGM (start) & LifePo4 (house). My current setup includes a 120A Iskra alternator (internal regulation) and a Zantrex TrueCharge30+ for shore power charging. I also have a Link20 battery monitor system (2 banks).
One method seems to be with a DC>DC charging system that piggys off the starter battery to charge the House bank when the starting battery is being charged (alternator or shore power).

If I have the Xantrex only charge the starter battery vs both banks as is now, would the DC>DC charger continue to charge when at the dock and on shore power?

What size DC>DC charger is suggested with the 120A alternator? BTW, current house bank is (2) 105ah AGM's.

My current racing/cruising power needs has worked pretty well with this size house bank. I am considering just (1) 100AH LifePo4 battery for now and a Renogy 12V ??A DC to DC On-Board Battery. At sea, I'd like to re-charge the new house bank with about 45min>1hr of alternator use (~1800rpm out of gear). Any suggestions, insights, or advice would be most appreciated as I've learned already from reading this groups postings....
TIA

[wholybee]

Usually, if you use a DCDC charger, it would only be for the Alternator. Your shore charger, solar, etc, would still connect to the LFP bank.

If you want to charge a 100Ah battery via the alternator and charge it quickly, you should look at an alternator upgrade instead of a DCDC charger. If you can convert your alternator to external regulation, then you don't need to replace it. And buy a Wakespeed or Balmar regulator with a temperature probe. This will run your alternator at max power (120A) for a short period, and then dial it back to keep the temperature under control.

If you go the DCDC route, then a 60A DCDC charger is the max you should consider with a 120A alternator, and that might be pushing it. It would take about an hour and a half to charge a 100Ah LFP at 60A.

[Capt Ben]

Thank you. I do have instructions on how to convert the alternator to be externally regulated. I have owned and liked the Balmar regulators before and would certainly consider the external regulator.

If there was no DC>DC charger, and an external regulator is employed, how does one charge the different battery types properly? Without the DC>DC would my 120A alternator would charge the LifePo4 battery much quicker than 1.5hrs?

How would you recommend charging the LifePo4 bank when on shore power?

TIA, Ben

Originally Posted by wholybee

Usually, if you use a DCDC charger, it would only be for the Alternator. Your shore charger, solar, etc, would still connect to the LFP bank.

If you want to charge a 100Ah battery via the alternator and charge it quickly, you should look at an alternator upgrade instead of a DCDC charger. If you can convert your alternator to external regulation, then you don't need to replace it. And buy a Wakespeed or Balmar regulator with a temperature probe. This will run your alternator at max power (120A) for a short period, and then dial it back to keep the temperature under control.

If you go the DCDC route, then a 60A DCDC charger is the max you should consider with a 120A alternator, and that might be pushing it. It would take about an hour and a half to charge a 100Ah LFP at 60A.

[Cwens]

When thinking through "how long will it take to charge?", bear in mind that the alternator / charging system's job is to replace the energy you've used since you last charged. To a first approximation, the size of the battery bank doesn't matter, what matters is how much energy you use. If you use 75 amp hours per day, then 75 AH is what your alternator and charging system needs to produce, irrespective of the size of your battery bank. In calculating alternator run time, it doesn't (again, to a first approximation) matter if you have a 150 AH battery bank (which you drew down to 50% SOC) or a 300 AH battery bank (which you drew down to 75% SOC). "approximation" because the batteries' rate of charge acceptance is going to be slightly different in these two profiles, but in either case you alternator and charging setup is going to be the limiting factor.

[Capt Ben]

Chris, I get the amp/hrs info but my understanding is it isn't quite that simple.
That the actual SOC is affected by a batteries ability to take in the energy 'pressed' into it. Supposedly, the LifePo4 batteries can absorb current faster than an AGM for example. So, my question has more to do with given a technology, what might the charge duration required be? IE, if a 100ah LifePo4 battery is depleted to say 20ah remaining, with a dc>dc charger and a 120amp alternator (let's assume an output at 1800rpm of 80ah), would I need a 80ah DC>DC charger to re-charge the battery in 1 hour? If I had a 40ah dc>dc charger bulked it then take 2 hours or can these batteries absorb the current quicker?

Originally Posted by ChrisOwens

When thinking through "how long will it take to charge?", bear in mind that the alternator / charging system's job is to replace the energy you've used since you last charged. To a first approximation, the size of the battery bank doesn't matter, what matters is how much energy you use. If you use 75 amp hours per day, then 75 AH is what your alternator and charging system needs to produce, irrespective of the size of your battery bank. In calculating alternator run time, it doesn't (again, to a first approximation) matter if you have a 150 AH battery bank (which you drew down to 50% SOC) or a 300 AH battery bank (which you drew down to 75% SOC). "approximation" because the batteries' rate of charge acceptance is going to be slightly different in these two profiles, but in either case you alternator and charging setup is going to be the limiting factor.

[Cwens]

Originally Posted by Capt Ben

Chris, I get the amp/hrs info but my understanding is it isn't quite that simple.
That the actual SOC is affected by a batteries ability to take in the energy 'pressed' into it. Supposedly, the LifePo4 batteries can absorb current faster than an AGM for example. So, my question has more to do with given a technology, what might the charge duration required be? IE, if a 100ah LifePo4 battery is depleted to say 20ah remaining, with a dc>dc charger and a 120amp alternator (let's assume an output at 1800rpm of 80ah), would I need a 80ah DC>DC charger to re-charge the battery in 1 hour? If I had a 40ah dc>dc charger bulked it then take 2 hours or can these batteries absorb the current quicker?

be careful with the units. Amps is a rate, amp hours is a quantity. So alternator output would be amps, not amp hours.


In general, LiFePO4 battery chemistry can accept charge astoundingly fast, fast enough that in a typical installation on a typical boat, the rate is going to be limited by the alternator or the charger and not by the battery. But

the specifics of any given installation are going to vary. You need, from your battery manufacturer, information about how fast the battery can accept charge. This will be stated as something like "0.3C", "0.8C" etc. where the charging rate is in amps and C is the battery capacity is in amp hours. So a 100 AH battery rated to accept charge at 0.3C can accept charge at 30 amps. Amp Hours is simply amps * hours, so at a charging rate of 30 amps it would take 2 hours and 40 minutes to put 80 AH of charge into the battery. In theory, with a little off for thermal losses, internal chemical losses in the battery, etc.

You can't charge faster than the slowest link in the chain, which will be either the alternator, the dc-to-dc charger, or the battery itself. Real-world continuous performance of most alternators is way, way, way below maximum rated output. Real-world performance of a DC-to-DC charger is probably pretty close to what the label says, but check the data sheet. So if you have a "160 amp" alternator, I would (guessing wildly here) think that it's probably a waste to buy a DC-to-DC charger of much more than 80 amps, irrespective of how fast your battery will accept charge, but, if your battery will accept charge at, say, 50 or 60 amps, it might be worth upgrading from the 40amp charger to the 80 amp one. But I'm not a qualified electrical engineer or marine electrician and you should without doubt consult one.

[wholybee]

A Lithium battery, withing some limitations, can accept as charge as fast as you can do it. It would be a uniquely poorly built LFP that could only be charged at a .3C rate. Cheaper Drop-ins might be a .5C rate, or 50A for a 100Ah battery. It shouldn't be difficult to find an LFP that will accept 1C, or 100A for a 100Ah battery.

So with that in mind, some basic math. A 60A charger will replace 60Ah of charge in 60 minutes. A 120A charger will replace 120Ah in 60 minutes. This is for LFP, for AGM the amount the battery can accept will be much lower.

A 120A alternator can provided 120A for only a short time. Then it is either damaged by heat, or needs to reduce it's output. That is the job of the external regulator. You get max power for a few minutes, then the power is reduced to whatever can be delivered without overheating the alternator. What that is is unknown, but probably higher than whatever save sized DCDC charger you use. If total charge time is only an hour, then getting 15 minutes of 120A before dropping to 50A, is a whole lot better than a constant 30A.

To answer your question about how to charge both banks without a DCDC charger, often it is done with an isolator, like a victron argofet device. Sometimes a DCDC charger, but to charge the start battery can be a very small one.

The charge voltages of LFP and AGM are similar (although the profiles are different) so it isn't generally harmful to charge an AGM at the same time as LFP with the charger set to an LFP profile. The reverse is not true. Charging an LFP with an AGM profile can damage the LFP.