LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

[mitiempo]

With an external BMS it shuts off the field wire to the regulator.

Batteries with internal BMS have no place on a boat in my opinion.

[newhaul]

Originally Posted by mitiempo

With an external BMS it shuts off the field wire to the regulator.

Batteries with internal BMS have no place on a boat in my opinion.

I actually put in a manual switch. There may bs times when I need all my hp for moving the boat . Alternators under heavy load can require as much as 4 hp for a fully loaded 100 zmp alternator. My 75 amp takes 2hp and with a total of 12 well 2 is significant.

[CharlieJ]

@mitiempo #7591

With an external BMS it shuts off the field wire to the regulator.

It is actually considered better to shut off B+ to the regulator rather than the field. Same result but gentler on the regulator.

Batteries with internal BMS have no place on a boat in my opinion.

I absolutely agree. I made a business decision not to install internal BMS LFP batteries on boats unless they communicate their status outside of their box.

I will, and have, installed internal BMS LFP batteries in RV's. My position is if the LFP system in an RV has a problem that opens a contactor, the operator can pull over to the side of the road and sort it out. On a boat, in the middle of the Gulf Stream, not so much.

[Na Mara]

Charlie J. Many thanks for this reply. You have confirmed my suspitions against internal BMS’ for house banks aboard. Having already had a near miss with a electrical fire aboard in the middle of the North Sea due to an alternator pumping amps into a dead battery I’m a safety first kind of guy with electrical these days.

[sparrowhawk1]

The way I've heard most people charging with an alternator is to keep your starting battery lead and have a battery to battery charger for your lifepo4 house. By choosing a charger that has a lower output than your alternator you will also extend the life of the alternator so it doesn't run full blast the whole time.

[newhaul]

Originally Posted by sparrowhawk1

The way I've heard most people charging with an alternator is to keep your starting battery lead and have a battery to battery charger for your lifepo4 house. By choosing a charger that has a lower output than your alternator you will also extend the life of the alternator so it doesn't run full blast the whole time.

Yep seems to be general consensus. Also 20% seems to be the best derating of alternator output. For high output but also alternator longevity.

[Na Mara]

Trouble with doing it that way is that it’s really inefficient. I’m planing 800Ah house bank charged off a 250A genmaxx alternator limited to 190A. This will make best use of my limited use of my overpowered engine. But the starter AGM is 110Ah so it can only charge at 50A continuous assuming standard acceptance rates. That would mean I could at most have one 30A DC-DC charger. It would take over a day to charge the house bank like that. I could double up on starter batteries to get 100A charging capacity on them and then I could run 3 parallel 30A DC DC chargers but then it would still take 7-8 hrs to charge from 80% discharge. Moreover those chargers run very hot meaning significant inefficiencies over and above excessive fuel burn.

It’s much more efficient to charge the LFP batteries off a high output alt and DCDC charge the engine battery off the LFPs. But to do that safely requires an external BMS that can talk to the regulator which is a more complex and expensive install.

[newhaul]

Originally Posted by Na Mara

Trouble with doing it that way is that it’s really inefficient. I’m planing 800Ah house bank charged off a 250A genmaxx alternator limited to 190A. This will make best use of my limited use of my overpowered engine. But the starter AGM is 110Ah so it can only charge at 50A continuous assuming standard acceptance rates. That would mean I could at most have one 30A DC-DC charger. It would take over a day to charge the house bank like that. I could double up on starter batteries to get 100A charging capacity on them and then I could run 3 parallel 30A DC DC chargers but then it would still take 7-8 hrs to charge from 80% discharge. Moreover those chargers run very hot meaning significant inefficiencies over and above excessive fuel burn.

It’s much more efficient to charge the LFP batteries off a high output alt and DCDC charge the engine battery off the LFPs. But to do that safely requires an external BMS that can talk to the regulator which is a more complex and expensive install.

Ok but your numbers don't make any sense to me a start battery will be recharged from the start of the iron wind within a few minutes But sounds what you need is an external programmable regulator to the lfp and a simple 10 or 20 amp b2b to recharge the 5 to 7 amps required to start your engine .

[CharlieJ]

@Na_Mara #7597

requires an external BMS that can talk to the regulator which is a more complex and expensive install.

Also, much, much safer. All that is needed is the BMS ATC signal to a relay in the B+ supply to the regulator. When ATC is rescinded by the BMS, the relay opens and alternator safely stops alternating. The Victron SolidSwitch 104 is purpose built for this application.

[CharlieJ]

Regarding starting an engine. Here is a thought experiment:
Assume:
> It takes 1000A to start your engine.
> The engine must spin for 3.6 seconds before it starts

The amp-hr used to start this mythical engine is: 1,000A x 3.6 seconds = 3,600Asec/3,600sec/hr = 1Ahr.

A Group 31 is about 110Ahrs+/-; 50% rule yields 55Ahrs usable. Point of this is that, yes we want to start our engines when needed, but I submit most boats could go an entire boating season with a good starting battery and never have to charge it.

[CharlieJ]

Anybody that is installing an LFP bank had best check their alternator temperature carefully with an IR gun at a minimum to ensure that when it is charging a depleted bank its highest temperature is <100C. I am in the business and I use a FLIR IR camera for this purpose and to document the alternator's thermal performance under load.

Some YouTube videos that are germane:
>>

https://www.youtube.com/watch?v=ShtGB07fCSs

[newhaul]

Originally Posted by CharlieJ

Regarding starting an engine. Here is a thought experiment:
Assume:
> It takes 1000A to start your engine.
> The engine must spin for 3.6 seconds before it starts

The amp-hr used to start this mythical engine is: 1,000A x 3.6 seconds = 3,600Asec/3,600sec/hr = 1Ahr.

A Group 31 is about 110Ahrs+/-; 50% rule yields 55Ahrs usable. Point of this is that, yes we want to start our engines when needed, but I submit most boats could go an entire boating season with a good starting battery and never have to charge it.

You forgot the electric lift pump and glow plug draws

[CharlieJ]

newhaul #7602

[Simi 60]

Originally Posted by newhaul

You forgot the electric lift pump and glow plug draws

And for some, anchor windlass

[newhaul]

Originally Posted by Simi 60

And for some, anchor windlass

Most don't run the windlass off of the starting battery. Mine is manual. Most have a dedicated battery and hoist after the engines are already running .