Charging at the dock & New LiFePo4 Setup

[Qismat]

Hi LiFePo4 Experts,

I have some questions after doing dozens and dozens of hours of research.

First, my set up:

Batteries: 600 Ah LiFePo4 bank at 12V - 6p4s 24 100Ah cells - Not yet ordered.
I haven't yet ordered my BMS
Inverter/Charger: Xantrex SW3012 Inverter/charger
Alternator: Small Case 90 Amp w 3/8" Vbelt, MC614 Regulator BLM down to 80amps.
Solar:610W solar with Victron 100/50 charger w Battery sense through VE.Smart with BMV-712.
Engine battery: 12V deep cycle AGM
Windlass Battery: 12V AGM Deep Cycle

I haven't yet devised my wiring plan, that will be another thread. I hope not to require a DC/DC charger, or any other equipment - Please let me know if there is anything you think I may be missing...



1) What happens when charging at the dock with a LiFePo4 house bank? These things don't like to sit at 100% - So do I adjust the settings in my inverter/charger to keep the voltage at a low enough voltage that the batteries sit at 60%? Do I have to flip my battery charger on/off all the time? I hope not.

2) When charging under the alternator, when the batteries are full, I know i need to keep my alternator from blowing up. Where does that current go? Does the stator from the alternator just shut off with the MC614? Thats what it sounds like - It just stops charging when instructed to by the MC614 based off of the battery voltage. So I would need to program the BMS to trigger the MC614 to stop charging before they cut the power to the cells? Or do I wire it somehow that the charging switches to the engine battery?

3) If I wired it so that the alternator fed the engine battery and the lithiums charged off the house battery, wouldn't that slow the charge rate of the lithiums as they would only charge as fast as the engine battery charged?

4) I've read that I shouldn't use the battery temp sensor on the SW3012 Inv/charger - Why is that? Wouldn't the inverter charger not want to charge the batteries if they were too hot/cold?


I've read the MarineHowTo probably three times and countless other articles and I can't seem to come up with answers to these questions.. Hopefully I'm not being dumb here.

[nebster]

I can try for two of your questions.

Originally Posted by Qismat

1) What happens when charging at the dock with a LiFePo4 house bank? These things don't like to sit at 100% - So do I adjust the settings in my inverter/charger to keep the voltage at a low enough voltage that the batteries sit at 60%? Do I have to flip my battery charger on/off all the time? I hope not.

It really depends on the capabilities of the charger and how programmable it is.

a) Some chargers you can force into float. Set a low float voltage that corresponds with the lower SOC you want to hold when on shore.

b) Some chargers are easy to reprogram. Configure a second charging algorithm that results in a lower SOC.

c) Some chargers can tolerate losing the battery when on shore. If you trust the shore feed, you could totally isolate the battery.

Alternatively, you could configure your charger to take the battery to a bit lower than 100% and accept the tradeoffs (simplified single charging scheme, slightly less usable energy, slight chance of holding the batteries high reducing lifespan).

The right answer might also depend on how long you spend on shore vs off grid, how reliable the shore supply is, and other things specific to your cruising.

4) I've read that I shouldn't use the battery temp sensor on the SW3012 Inv/charger - Why is that? Wouldn't the inverter charger not want to charge the batteries if they were too hot/cold?

Those are usually designed to let the charger alter its scheme for lead acid based on temperature. That doesn't apply for lithium.

[LoudMusic]

I leave my shore charger set to a lower absorb and float than my batteries' recommended state. I think each value is 0.2v lower? Possibly more. And when I'm shore connected I either disable solar chargers or reconfigure them to match the lower values of my shore charger. I also have Victron 100/50 (3x), but I'm using Relion batteries and a Magnum inverter/charger.

I like my batteries to stay between 20 and 90 % charge. I've only gotten down to 20% once, and most of the time they are between 60 and 80%. If we get two or three consecutive full sun days the batteries will top up. That has happened a few times so far this January.

[mbartosch]

Originally Posted by Qismat

Hi LiFePo4 Experts,
1) What happens when charging at the dock with a LiFePo4 house bank? These things don't like to sit at 100% - So do I adjust the settings in my inverter/charger to keep the voltage at a low enough voltage that the batteries sit at 60%? Do I have to flip my battery charger on/off all the time? I hope not.

What nebster said, plus: you wrote that you have a BMV7xx. This device has a programmable relay which you can set to state of charge controlled operation.
What I have done on Entropy is allowing to force/simulate a HVC-like event to disable all charging sources. This condition can be triggered either manually (via a manual switch labeled "Disable charging") or via the BMV 700 relay (also controllable via switch labeled "Lazy charging".
Program the SoC relay to open at, say, 50 % and close at 75 % and your SoC will oscillate slowly between those limits.

2) When charging under the alternator, when the batteries are full, I know i need to keep my alternator from blowing up. Where does that current go? Does the stator from the alternator just shut off with the MC614? Thats what it sounds like - It just stops charging when instructed to by the MC614 based off of the battery voltage. So I would need to program the BMS to trigger the MC614 to stop charging before they cut the power to the cells? Or do I wire it somehow that the charging switches to the engine battery?

When the MC614 detects end of absorption phase it slowly reduces field current to a point where almost no current flows through the field coil. If the magnetic field is zero the alternator does not produce electric energy.
This is also a solution to stop charging via the alternator: connect B+ of the alternator directly to the battery (not to the charge bus) and have a HVC disable field current e. g. by switching off the alternator regulator. (The MC614 has an ignition input which can be used for this purpose, or you can cut power supply to the MC614. But do not cut field directly.)

3) If I wired it so that the alternator fed the engine battery and the lithiums charged off the house battery, wouldn't that slow the charge rate of the lithiums as they would only charge as fast as the engine battery charged?

What you describe sounds like you want to parallel LA and LFP batteries.
I advise against paralleling different cell chemistries. This does not really work and will be detrimental to one of the cell chemistries (in this case will result in chronically undercharging and thus quickly destroying the LA batteries).
Invest in a B2B charger and adopt one of the common approaches (either charge LFP from LA or vice versa, while putting the alternator on the charge side of the B2B).

[Avionics_Joe]

You cannot parallel a lead-acid & Lithium battery but instead of an expensive DC to DC charger you might want to consider a "Victron Cyrix-Li - Intelligent Microprocessor Controlled Combiner/Isolators for Lithium Batteries"
$75.65 on Amazon.