Pulled the trigger on the cheapest LiFePO4 batteries anywhere. And I have a question.

[SaylorMade]

Originally Posted by wholybee

Most cheap drop-ins do not allow user configurable settings, and yes, most of them actually have the cut-off at 3.7V per cell (14.8 pack) or higher. The reason is because the BMS is NEVER supposed to stop charging. It is a last ditch effort to save the battery if the charger is set at too high a voltage. So, the BMS will allow a charge to 14.6V without disconnecting, to give the charge a chance to do it's job.

Also, if a BMS disconnect at full charge while charging from an alternator, you risk blowing up the alternator, and all other electronics on board. A BMS disconnect should never actually happen. If you are relying on a BMS to stop charging, you are asking for all kinds of problems.

I have never ever seen a LiFePo4 BMS set to 3.7V / 14.8 V from factory.
Why would they do that?

They are set to 3.65V / 14.6V as per specifications.

Personally I think that charging should be stopped when the first cell reaches the upper limit (3.65), not pack voltage.
If you have a rogue cell you want that to be the determining factor.

The plug-in batteries I have seen and bought myself have had Bluetooth (only thing I would buy) and gives me the opportunity to set all parameters.
And they are what you would consider "cheap".

I usually back off a bit on the OVP to 3.4 ish Volts. There is close to zero extra power put into the battery after that, but 3.65V (as per spec) is fine as well. It will drop down below 3.4V very quickly after charging stops anyway.

Totally agree with the BMS disconnect / Alternator issue. That must be dealt with.
I have no alternator on my boat (Electric Propulsion), so there is one more thing not to worry about

[Rohan]

Originally Posted by Stu Jackson

No one suggested you rewire anything.

Did you miss that part? It was the very first link I provided.

No, I didn't miss it. The first link you provided was this one. And the title is, all in caps and bolded: "HERE'S HOW THE OEM WIRING ORIGINALLY WORKED AND WHY YOU SHOULD CHANGE IT".

And insulting people who are honestly trying to help you is not a wise move.

I haven't insulted anyone, but thanks.

[AKA-None]

Originally Posted by Rohan

No, I didn't miss it. The first link you provided was this one. And the title is, all in caps and bolded: "HERE'S HOW THE OEM WIRING ORIGINALLY WORKED AND WHY YOU SHOULD CHANGE IT".









I haven't insulted anyone, but thanks.

Hmm a quick glance that is a a discussion that talks about subjects mentioned in this conversation and includes why certain things are suggested along with why

Why would you not want more background on why people are posting the things they are writing

It’s to be considered educational

[sailorboy1]

I don't even understand what is going on anymore in this thread. Other than battling of course.

[Tortuga's Lie]

Originally Posted by Rohan

And getting any boat professional out here is very difficult. If I always waited for a pro I never would have left the dock. For years I have had to do almost everything myself. So I'll figure this one out too. I'm not in a hurry.

I have absolutely no affiliation with Steve D'Antonio but he is an expert in marine systems and consultant. He offers such a service:

https://stevedmarineconsulting.com/b...ms-inspection/

I believe he has services in the Chesapeake region.

[Rohan]

Originally Posted by Tortuga's Lie

I have absolutely no affiliation with Steve D'Antonio but he is an expert in marine systems and consultant. He offers such a service:

https://stevedmarineconsulting.com/b...ms-inspection/

I believe he has services in the Chesapeake region.

Thanks, I'll check it out! Problem is, "Chesapeake region" is a big place. If I lived in Annapolis, I couldn't swing a cat without hitting a boating service. But here in Quantico, it feels like we are in the middle of nowhere, and have to make appointments months in advance.

[Num1fireman]

https://www.crownbattery.com/news/cranking-battery-vs-deep-cycle-marine-battery?hs_amp=true

[Tupaia]

Originally Posted by krid2000

What chemistry are they?

Dirk

https://www.mlperformance.co/product...tery-26ah-600a

[senormechanico]

AGM is Absorbed Glass Mat. It's a gelatin version of Lead Acid.

[Starbuck]

Scoobert's little brother, apparently.

[wholybee]

Originally Posted by SaylorMade

I have never ever seen a LiFePo4 BMS set to 3.7V / 14.8 V from factory.
Why would they do that?

They are set to 3.65V / 14.6V as per specifications.

Personally I think that charging should be stopped when the first cell reaches the upper limit (3.65), not pack voltage.
If you have a rogue cell you want that to be the determining factor.

The plug-in batteries I have seen and bought myself have had Bluetooth (only thing I would buy) and gives me the opportunity to set all parameters.
And they are what you would consider "cheap".

I usually back off a bit on the OVP to 3.4 ish Volts. There is close to zero extra power put into the battery after that, but 3.65V (as per spec) is fine as well. It will drop down below 3.4V very quickly after charging stops anyway.

Totally agree with the BMS disconnect / Alternator issue. That must be dealt with.
I have no alternator on my boat (Electric Propulsion), so there is one more thing not to worry about

It is certainly ok to not charge your batteries all the way to 3.65Vpc. But, they can not be balanced at lower than 3.4Vpc. So, setting OVP to 3.4Vpc is a bad idea. Your cells will slowly drift out of balance, and at some point (maybe a couple years) you will find the BMS disconnecting when the total pack voltage isn't even 13 volts. You should at least once in a while charge to a minimum of 3.5Vpc, and hold that voltage while you wait for the cells to balance. When they get so bad that the pack won't get to 13V total, you might need to cut open the battery, and individually charge each cell the 3.65V, then reassemble to get it working right again. Several on this forum have had to do that from the same thinking, that it was ok to set them up to not fully charge.

Over voltage protection should NEVER EVER happen. If you plan to *ever* charge your batteries to 14.6V, then you must not set OVP to 14.6V. You can actually charge LFP to 3.75 on rare occasions, and not kill them. So there really isn't any harm in having OVP at 3.7V factoring in that you will never have an OVP event. And if you set your OVP low because you want it to regularly disconnect to keep the batteries from charging higher than 3.4Vpc (or whatever voltage) then you are thinking about the whole system backwards.

[GILow]

Originally Posted by Starbuck

Scoobert's little brother, apparently.

Now you are revealing how long you've been on CF. [emoji16]

[SaylorMade]

Originally Posted by wholybee

It is certainly ok to not charge your batteries all the way to 3.65Vpc. But, they can not be balanced at lower than 3.4Vpc. So, setting OVP to 3.4Vpc is a bad idea. Your cells will slowly drift out of balance, and at some point (maybe a couple years) you will find the BMS disconnecting when the total pack voltage isn't even 13 volts. .

This is simply not true.
You can set the minimum balance voltage yourself, but never ever have I seen it set as high as 3.4 V from factory.
I think the widely used JK BMS is set at 3V from factory, but there is conensus that it should be a tad higher though.
I can also tell you, that a LifePo4 cell will not stay above for 3.4V for very long after charging, and no, nowhere is it recommended that you keep charging in order to balance cells. Not a thing.

My batteries balance fine from around 3.3 and up.
They are kept within 1-3 millivolt.

I monitor cell voltages closely, and I can assure that they do not drift, or well they do if you do heavy discharging or charging, but balancing takes care of that in short order.

EDIT:
Just to clarify, when I say they do drift on heavy discharge/charge I mean like up to 10-15 mv, until rebalanced by the BMS.
So no, no drift to speak about at all.

[elliot_russo]

Rohan with your setup i'd recommend keeping one of the agm's in parallel with the lifepo4. Simplifies for cheap bms solutions. resolves the blackout possibility. Protects the alternator from disconnect, but not from burning up! Use ext. Regulator for that. Charge settings as for lithium. Not ideal for agm but, you already own them and are discarding.
Putting my hard hat on now in preparation...

[Rohan]

Originally Posted by elliot_russo

Rohan with your setup i'd recommend keeping one of the agm's in parallel with the lifepo4. Simplifies for cheap bms solutions. resolves the blackout possibility. Protects the alternator from disconnect, but not from burning up! Use ext. Regulator for that. Charge settings as for lithium.

I think that's what I will do for now, put an AGM in bank 1, then all the lifepo4s in series in bank 2 hooked to solar. Just keep the battery switch on bank 1 when I'm motoring.