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Old 03-31-2023, 12:16 PM   #1
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New LiFePO4, all else same?

I have 3x 8D AGM 200Ah each. They are about 10 years old now and are no longer holding a strong charge (engine struggles to turn over on start). Plan is to replace them with LiFePO4 of same specs (200Ah, same dimensions). Charger is Xantrax Trucharge 40+, doesnít have Li setting, but does have AGM which should work.

My question: Is there any reason I canít just do a one-for-one swap of these batteries and leave ALL else the same (cabling, fusing, charging, etc)? I hope!
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Old 03-31-2023, 12:37 PM   #2
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I would use a LiFePO4 charger. Compared to the cost of those batteries, the charger is cheap. Not to mention cheap insurance for not having to worry about your boat.
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Old 03-31-2023, 01:01 PM   #3
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I have 3x 8D AGM 200Ah each. They are about 10 years old now and are no longer holding a strong charge (engine struggles to turn over on start). Plan is to replace them with LiFePO4 of same specs (200Ah, same dimensions). Charger is Xantrax Trucharge 40+, doesnít have Li setting, but does have AGM which should work.

My question: Is there any reason I canít just do a one-for-one swap of these batteries and leave ALL else the same (cabling, fusing, charging, etc)? I hope!
I changed my lead acid batteries 4 months ago on my sailboat for LFP and made no physical changes to my boat. All I did was change my settings for my solar controller and alternator/regulator and change the battery charger to a Gel setting that the upper limit was well below a problem to the LFP (I have a bigger charger than you).

Be sure you check that the LFP you chose is OK for use as a starting battery if it is going to be used as such.
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Old 03-31-2023, 01:11 PM   #4
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I would use a LiFePO4 charger. Compared to the cost of those batteries, the charger is cheap. Not to mention cheap insurance for not having to worry about your boat.
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Old 03-31-2023, 01:31 PM   #5
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If the charger allows custom absorption and float settings you should go to what the battery manufacturer recommended voltages are. Perhaps on absorption, you can go 0.2V lower than the recommended as this will increase the charging/discharging cycles dramatically. No need to go all the way up. LiiFePo4 prefer not being pushed all the way up to the allowable voltage. For prismatic Cell Batteries I always use 3.5V/Cell or 14V as absorption setting and 3.4V/cell as float. That puts the battery at about 95-98% charged
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Old 03-31-2023, 01:31 PM   #6
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If you change one for one, and do not updrade your charging capability to utilize the higher charge currents LIFEP04 can accept, what are you really gaining, by making the change???

My opinion very little gain for your dollars spent.
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Old 03-31-2023, 01:34 PM   #7
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If you change one for one, and do not updrade your charging capability to utilize the higher charge currents LIFEP04 can accept, what are you really gaining, by making the change???

My opinion very little gain for your dollars spent.
While I agree with you in general, he is still gaining charge at the max current that his charger can provide to the the very end of the charging cycle without tapering off and the last 20% to require 80% of the time to fully change as on lead acid or AGM batteries.
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Old 03-31-2023, 02:03 PM   #8
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Besides getting the full output of a charging source to go into the batteries longer instead of the tapering down like lead acid, the next big plus to the LFP is that you don't have to worry about them fully charged all the time.

If LFP sits around at 50% state of charge for weeks it matters nothing. If lead acid does that there will be an issue down the road.
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Old 03-31-2023, 02:09 PM   #9
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While I agree with you in general, he is still gaining charge at the max current that his charger can provide to the the very end of the charging cycle without tapering off and the last 20% to require 80% of the time to fully change as on lead acid or AGM batteries.
He only has a 40 amp charger on a 46 foot trawler.

My educated guess is that he is only putting 15 amps into his batteries anyway much of the time when you count boat loads at anchor

Seems like very little benefit for thousands of dollars in outlay.
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Old 03-31-2023, 02:17 PM   #10
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If you change one for one, and do not updrade your charging capability to utilize the higher charge currents LIFEP04 can accept, what are you really gaining, by making the change???



My opinion very little gain for your dollars spent.


Agree with optimizing charging as much as possible. But main gains are:

1. Significantly lighter weight batteries for tight quarters in ER (47lbs ea vs 140lbs ea).
2. Keeping same form factor and wiring, but with much less weight, means I can DIY, saving significant cost.
3. I have found that LiFePO4 are no longer more expensive from many vendors. In fact, like for like, they were actually half the price it would have cost for new AGMs at West Marine.
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Old 03-31-2023, 02:23 PM   #11
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He only has a 40 amp charger on a 46 foot trawler.

My educated guess is that he is only putting 15 amps into his batteries anyway much of the time when you count boat loads at anchor

Seems like very little benefit for thousands of dollars in outlay.
Not $1000s, can get a drop in 200ah LFP off Amazon for a little over $600.
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Old 03-31-2023, 02:56 PM   #12
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New LiFePO4, all else same?

Oof. Well, further rsearch indicates that LiFePO4 might be fine for house batteries, but will NOT work as my starter battery, due to discharge limits placed by the BMS. I guess I still need AGM for that one. Hope I can mix types on the same charging system.
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Old 03-31-2023, 03:13 PM   #13
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Oof. Well, further rsearch indicates that LiFePO4 might be fine for house batteries, but will NOT work as my starter battery, due to discharge limits placed by the BMS. I guess I still need AGM for that one. Hope I can mix types on the same charging system.
Yes. Read the article on LFP batteries on marinehowto.com before you proceed. Rod knows his sh*t on electrical work.
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Old 03-31-2023, 03:19 PM   #14
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While I agree with you in general, he is still gaining charge at the max current that his charger can provide to the the very end of the charging cycle without tapering off and the last 20% to require 80% of the time to fully change as on lead acid or AGM batteries.
Is that really true when using a FLA pr AGM setting?
As V increases A is decreased
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Old 03-31-2023, 03:24 PM   #15
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Oof. Well, further rsearch indicates that LiFePO4 might be fine for house batteries, but will NOT work as my starter battery, due to discharge limits placed by the BMS. I guess I still need AGM for that one. Hope I can mix types on the same charging system.
Mixing not a good idea.
Read Rods / CMS article. You NEED to consider the whole system.
So called "Drop Ins" are not air what the marketing implies.
Just my opinion, but unless you separate start & house charging you will regret it.
How does your alt charging work now while underway?
If you aren't sure where & how house & start are connected its worth figuring out... B4 less expensive than after/ the hard way
Also understand any hi amp demands on house... thrusters, windlass, inverter, etc and be sure they can be handled from house.
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Old 03-31-2023, 03:33 PM   #16
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You can separate with FET. Lots of people using LFP house with a lead acid start battery
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Old 03-31-2023, 05:34 PM   #17
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What does the battery manufacturer say the charger settings need to be, and will all your charging sources be within that range?
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Old 03-31-2023, 09:21 PM   #18
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Oof. Well, further rsearch indicates that LiFePO4 might be fine for house batteries, but will NOT work as my starter battery, due to discharge limits placed by the BMS. I guess I still need AGM for that one. Hope I can mix types on the same charging system.
Not necessarily true. Check out:

https://dakotalithium.com/product/da...ttery-1000cca/

They sell several sized and voltage, and are designed with BMS that they claim will work as start batteries. No affiliation, but we're considering them.
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Old 04-01-2023, 10:09 AM   #19
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Mixing not a good idea.
Read Rods / CMS article. You NEED to consider the whole system.
So called "Drop Ins" are not air what the marketing implies.
Just my opinion, but unless you separate start & house charging you will regret it.
How do I find the article you mentioned? My house and start batteries combine for 24v to power the thruster -- would that require separation (from an electrical load perspective, I don't see how one bank would know about the other)? And is there any reason not to replace the start batteries with the fancy ones, other than max current draw? How big a bank would I need to be able to handle 500 amps (just guessing, probably less, but that is the fuse size in my current setup for a pair of QSM11 engines with 12 v starters). These are the questions I am trying to research, among many others, but I think the other issues can be handled. For example, if my charger/inverter can't properly charge the fancy batteries, it would be an easy matter for me to add a new charger, since the boat has lots of redundancy, including two separate charging systems. If I were to do that, I would keep the current charger/inverter in place, but disable its charging function. Similarly, since my genset is necessary to handle the boat's electrical demands when not connected to shore power, I don't really need the alternators at all. If their charging profile creates problems, I can remove them.
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Old 04-01-2023, 10:47 AM   #20
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How do I find the article you mentioned? My house and start batteries combine for 24v to power the thruster -- would that require separation (from an electrical load perspective, I don't see how one bank would know about the other)? And is there any reason not to replace the start batteries with the fancy ones, other than max current draw? How big a bank would I need to be able to handle 500 amps (just guessing, probably less, but that is the fuse size in my current setup for a pair of QSM11 engines with 12 v starters). These are the questions I am trying to research, among many others, but I think the other issues can be handled. For example, if my charger/inverter can't properly charge the fancy batteries, it would be an easy matter for me to add a new charger, since the boat has lots of redundancy, including two separate charging systems. If I were to do that, I would keep the current charger/inverter in place, but disable its charging function. Similarly, since my genset is necessary to handle the boat's electrical demands when not connected to shore power, I don't really need the alternators at all. If their charging profile creates problems, I can remove them.
Sorry... meant to include the link.
https://marinehowto.com/drop-in-life...ated-consumer/
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