LiFePO4 bms/charger question

[ssurabus]

Hi,In reading about LifePO4 and issues with directly connecting a stock alternator, I think I learned:

a) the alternator could burn up due to low resistance of battery resulting in extended high output from alternator

b) there are problems if the battery's bms disconnects the battery resulting in voltage spike from alternator that destroys it (and potentially sends this voltage spike to throughout the dc system).

Hopefully I have the general gist of those two points. Point 'b' got me thinking that the possibility of the bms disconnecting the battery exists whether it's an alternator doing the charging or a charger. Let's assume that there are no alternators in the picture and just a battery charger (my existing which I think can be programmed with a charging profile suitable for the LiFePO4). If the battery's bms were, for some reason, to disconnect the battery while charging, what affect would that have on the charger? Would it be 'bad' as in the case of the alternator described above, or would the charger gracefully handle this situation? Basically, I don't want to risk destruction of my charger/inverter by the whims of a bms.

Also, since a house bank may be comprised of multiple batteries with each batttery having it's own bms, I guess it's conceivable that just one battery's bms could disconnect, leaving the bank at a fraction of its nominal size. This seems less problematic, but wondering what effect this situation may have on the charger.

Wasn't going to go down the LiFePO4 path, but considering now because I can't believe the price of AGM's.

Thanks for any info.

 

[Delfin]

Hi,In reading about LifePO4 and issues with directly connecting a stock alternator, I think I learned:

a) the alternator could burn up due to low resistance of battery resulting in extended high output from alternator

b) there are problems if the battery's bms disconnects the battery resulting in voltage spike from alternator that destroys it (and potentially sends this voltage spike to throughout the dc system).

Hopefully I have the general gist of those two points. Point 'b' got me thinking that the possibility of the bms disconnecting the battery exists whether it's an alternator doing the charging or a charger. Let's assume that there are no alternators in the picture and just a battery charger (my existing which I think can be programmed with a charging profile suitable for the LiFePO4). If the battery's bms were, for some reason, to disconnect the battery while charging, what affect would that have on the charger? Would it be 'bad' as in the case of the alternator described above, or would the charger gracefully handle this situation? Basically, I don't want to risk destruction of my charger/inverter by the whims of a bms.

Also, since a house bank may be comprised of multiple batteries with each batttery having it's own bms, I guess it's conceivable that just one battery's bms could disconnect, leaving the bank at a fraction of its nominal size. This seems less problematic, but wondering what effect this situation may have on the charger.

Wasn't going to go down the LiFePO4 path, but considering now because I can't believe the price of AGM's.

Thanks for any info.

Problem 'a' is advoided by using a smart regulator that allows de-tuning the output of the alternator. E.g. a Balmar 612/624. Problem 'b' is avoided by using an externally regulated alternator with the appropriate diode that prevents deadending the charging current. A battery charger would react to your scenario by zeroing out current, just as it does if you disconnect the battery while charging.

 

[ssurabus]

Thanks for that response. I have an alternate, though temporary solution to both 'a' and 'b' since I may be doing this change under duress. That is to just disconnect the alternator for the time being while I take some time to figure things out. Now that you say it, I do know that it doesn't hurt any charger I've seen to turn it on when it's not connected to anything so I believe that if I just disconnect the alternator for now and reprogram my charging profile I can just drop the LifePo4 in. I'll double check with my inverter/charger manufacturer just to be safe.

I guess an add-on question.....I've never had to do it, but like most boats I have the option of combining the starter bank with the house bank for an emergency starting situation. My starting battery will remain an AGM. How does this scenario playout? Will the bms disconnect the house batteries from the combined bank if it doesn't like the starting load? Is the common scenario of combining banks unavailable if the house is LifePo4?

 

[Delfin]

Thanks for that response. I have an alternate, though temporary solution to both 'a' and 'b' since I may be doing this change under duress. That is to just disconnect the alternator for the time being while I take some time to figure things out. Now that you say it, I do know that it doesn't hurt any charger I've seen to turn it on when it's not connected to anything so I believe that if I just disconnect the alternator for now and reprogram my charging profile I can just drop the LifePo4 in. I'll double check with my inverter/charger manufacturer just to be safe.

I guess an add-on question.....I've never had to do it, but like most boats I have the option of combining the starter bank with the house bank for an emergency starting situation. My starting battery will remain an AGM. How does this scenario playout? Will the bms disconnect the house batteries from the combined bank if it doesn't like the starting load? Is the common scenario of combining banks unavailable if the house is LifePo4?

I have sealed Lead Acid for the starter and LiFePO4 for the house bank. I can combine them if need be, but never have. And yes, the BMS will disconnect a combined bank if current exceeds BMS settings because it is between the LFP battery and the combiner switch. Worth noting that not all BMSs are equal. Rod Collins at Marinehowto.com does a great job of demonstrating that some manufacturers do not have sufficient cable sizing within the case to handle high loads. I have Lithionics LFP batteries and BMS, and it can handle starting loads with no issue. Renology - apparently and definitely not. Expensive vs. cheap, I suppose.



The scenarios and battery bank used are as follows:


1. Underway, LFP BMS off, isolating the house bank from charging sources, so alternator current just goes to the starter. I've found the alternator and charger settings for LFP work just fine for LA.

2. At anchor, LFP BMS on, starter bank switched off.
3. At the dock on shore power, LFP BMS off, starter bank on.


I have the alternator de-tuned from 120 amps to around 90 through a Balmar 624, and even though the alternator has the necessary diode to prevent it from deadheading if the destination for charge current is lost I still make sure I have the LFP online before disconnecting the starter bank, and vice versa.


Hope that helps.

 

[ssurabus]

For context, I have twin motors that share one start bank. One alternator charges the start bank and one charges the house (although I don't think very impactfully...it's just stock Yanmar and the house bank is currently 3 8d's).

Let me see if I understand this. You say:

"1. Underway, LFP BMS off, isolating the house bank from charging sources, so alternator current just goes to the starter. I've found the alternator and charger settings for LFP work just fine for LA. "

​

So when underway, you do no charging of the house bank from the alternator and your, I assume 1 alternator, charges the start bank. This would be equivalent to my temporary solution of disconnecting the alternator from the house bank.

then you say

"2. At anchor, LFP BMS on, starter bank switched off."

​

Your start bank is isolated at anchor (no charging, no load). What is charging your house bank in this case; solar, generator? If that's the case, when is your alternator ever being used to charge the house?

then you say

"3. At the dock on shore power, LFP BMS off, starter bank on.
I have the alternator de-tuned from 120 amps to around 90 through a Balmar 624, and even though the alternator has the necessary diode to prevent it from deadheading if the destination for charge current is lost I still make sure I have the LFP online before disconnecting the starter bank, and vice versa."

​

I'm a little lost here. Are you not charging/float charging the house at the dock? I guess from your cases, I'm missing when and how the house gets charged.

and you say "Hope that helps.[/QUOTE]"

​

Yes it does a lot. Your way ahead of me and just typing has clarified some things for me. So thanks.

 

[SteveK]

What I learned (stand to be corrected).
If you have an internal regulated Alternator then charging LFP should be done through DC2DC charger while the ALT is connected to a lead acid start battery. This avoids a load dump should BMS shuts down. The DC2DC also avoids overcharging LFP and triggering BMS shut down.
De rating an Alternator also helps prevent it from running amok.
Spend an arm and a leg on add on devices to solve these pesky issues.

Fair enough, while I do not understand, I would follow the suggested process.

 

[Delfin]

For context, I have twin motors that share one start bank. One alternator charges the start bank and one charges the house (although I don't think very impactfully...it's just stock Yanmar and the house bank is currently 3 8d's).

Let me see if I understand this. You say:

"1. Underway, LFP BMS off, isolating the house bank from charging sources, so alternator current just goes to the starter. I've found the alternator and charger settings for LFP work just fine for LA. "
So when underway, you do no charging of the house bank from the alternator and your, I assume 1 alternator, charges the start bank. This would be equivalent to my temporary solution of disconnecting the alternator from the house bank.

Correct. We will charge the LFP from the alternator, or alternator + genset when we pull anchor and head out for the next stop. However, once I see from a Link 20 battery monitor that the LFP is "full", that is, has reached 29.2 vdc (24vdc system), then I switch the starter on and the BMS off. Charge current is then directed to the starter bank and the LFP just sits offline, now fully charged.

then you say

"2. At anchor, LFP BMS on, starter bank switched off."
Your start bank is isolated at anchor (no charging, no load). What is charging your house bank in this case; solar, generator? If that's the case, when is your alternator ever being used to charge the house?

It's a 600 Ah LFP house bank, and we use a max of 300 Ah per 24 hour day. I'll recharge at anchor using the genset plus 3 Sterling 30 Ah charger in parallel if we're in one spot for more than overnight. These put out around 180 amps, so 1 hr and 40 minutes of run time recharges the LFP. If we're leaving anchorage after using 300 Ah, then my response above applies - charge the LFP underway until full, then switch on starter bank and switch off the LFP.

then you say

"3. At the dock on shore power, LFP BMS off, starter bank on.
I have the alternator de-tuned from 120 amps to around 90 through a Balmar 624, and even though the alternator has the necessary diode to prevent it from deadheading if the destination for charge current is lost I still make sure I have the LFP online before disconnecting the starter bank, and vice versa."
I'm a little lost here. Are you not charging/float charging the house at the dock? I guess from your cases, I'm missing when and how the house gets charged.case, when is your alternator ever being used to charge the house?

No, I don't float the LFP dockside. I've noticed that many seem to spend a lot of time and money trying to design their system so they can manage their LFP bank just like it was a LA bank, meaning floating. I don't get it. I do get it that if your float voltage is set just at or below resting voltage of a charged LFP bank that you run little risk of continually pumping current into a charged LFP battery, which is a really good way to kill it, but this ignores the fact that LFP batteries prefer to be stored at 50% - 75% of "full". LFP lose about 1% of capacity per day, so if I disconnect the bank at 75% charge, its almost 2 months before the battery gets down to 20% charged, at which point I can top them up a bit. I believe this regime is the way you get maximum life span out of the lithium bank, as well as maintain peak capacity.

 

[Delfin]

What I learned (stand to be corrected).
If you have an internal regulated Alternator then charging LFP should be done through DC2DC charger while the ALT is connected to a lead acid start battery. This avoids a load dump should BMS shuts down. The DC2DC also avoids overcharging LFP and triggering BMS shut down.
De rating an Alternator also helps prevent it from running amok.
Spend an arm and a leg on add on devices to solve these pesky issues.

Fair enough, while I do not understand, I would follow the suggested process.

If you have an internally regulated alternator, a.k.a and automotive alternator, then you probably have the wrong kind of alternator for a vessel, since you have zero control over output voltage, absorption time, etc. But if you REALLY don't want a marine alternator, then buying a DC2DC charger instead of a decent alternator plus 3 stage regulator makes sense. Otherwise, not so much, IMHO.


https://www.fisheriessupply.com/battery-charging-101

 

[diver dave]

Modern car alternators are typically externally computer controlled.

 

[Delfin]

Modern car alternators are typically externally computer controlled.

As opposed to the internally regulated automotive alternators under discussion.

 

[Iggy]

I have not read every single post here. BUT there are many other treads that goes right into this. With an old or new ALT the price to upgrade with additional parts is not to bad.

 

[Iggy]

As opposed to the internally regulated automotive alternators under discussion.

My old ALT I would say has a external computerized regulator with a 2 digit readout. But it can not handle LFP.

 

[kabyak]

My LiFePo system is built with Victron components including an external BMS that has an output signal which changes state if charging current or voltage exceeds limits. This goes to a programable input on my Wakespeed 500 external regulator and it drops field voltage so alternator output falls safely. The WS is also programmed to limit output of my 160A alternator to 130A (80%). As a backup, I have a Sterling device that shunts any inductive surge to ground. So far so good.

 

[normandawn1]

Video worth a look

Installing LFP batteries
https://youtu.be/1nHX7F1N2P0