Two scenarios: Alternator to LFP House Bank or Alternator to Starter battery?

[jc1409]

Two scenarios:


1. Alternator connected to Lithium house bank, with DC-DC charger used to charge the start batteries


2. Alternator connected to start batteries, with DC-DC charger used to charge Lithium house bank


Assuming proper setups, charge configurations, sufficient wiring, and all other things being 'equal', which is better?

[Jammer]

#1. On cruising boats, the house bank needs vastly more charging, like 10x, than the start battery. Dc-Dc converters are fine things but you do lose about 10% and they are expensive in large sizes.

[s/v Jedi]

Definitely #2. The engine mounted alternators primary function is to charge the start battery. If the alternator regulator can do a lithium charge profile and you don’t care about the starter battery shorter lifespan because of undercharging then you can use an Automatic Charge Relay. If you want it to be 100% then use a suitable DC-DC charger.

I even have a DC-DC charger from solar/house bank to starter battery so that solar keeps the AGM start battery happy in float.

Losses during charging are huge no matter what you do. Solar is the worst, at 20% efficiency but alternator is pretty bad as well with a diesel firing and big heavy steel rotating like crazy, followed by belts and red hot alternator even before we get to diodes and regulators. I would not worry about DC-DC converter losses at all.

[Captain Bill]

I am currently getting ready to install my new lithiums using scenario 2. My logic is that I have a genset with a 150A battery charger and a 640W solar system to use for bulk charging of the lithiums. Having the alternators connected to my AGM start bank protects them from being overloaded by being exposed to a depleted lithium bank. It also protects them from a sudden cutoff of lithium charging by the BMS. It also protects the lithiums from being overcharged by the dumb regulators in the alternators during prolonged motoring. The DC-DC Charger is a much cheaper solution than installing smart regulators in the alternators. My primary use of the alternators will be to carry house and instrument loads while underway on engines at night.



If you need to use the alternators for bulk charging then you may want to make a different decision. I would think that if you go for Scenario 1 that you would want to make sure you have smart external regulators on your alternators.

[jc1409]

Originally Posted by Captain Bill

I am currently getting ready to install my new lithiums using scenario 2.

Captain Bill, will you use any supplemental charging for start batteries, to keep them topped up?

[Captain Bill]

Originally Posted by jc1409

Captain Bill, will you use any supplemental charging for start batteries, to keep them topped up?

I don't currently plan to have any. I don't have any loads running off my start bank besides chargers so the AGMs will not self discharge fast enough to make a difference. I run the engines and the genset once every three weeks even if I'm at the dock and even the genset has a 40 amp alternator connected to the start bank. In an emergency I have a manual battery combiner switch that I can use to put some charge from the lithium bank into the start batteries if I have to. I was advised once, by a battery specialist, to actually let my start batteries get a little bit depleted so that they will accept a high amperage charge. Normal start loads don't deplete them enough and they need a high amperage charge once in a while to clean the sulfation off the plates. I could always add another DC-DC charger running in the other direction if my scheme doesn't work.

[jc1409]

Thanks all - the dilemma of which way to pass alternator charging in a split lithium / AGM setup has haunted my drop-in Lithium upgrade plans for a long time, and from web searching it's pretty clear there is no 'right' way, and no industry standard or consensus.



Option 1 (alternator to Lithium, and then DCDC to start battery) gets complicated with regulator upgrades/alternator diode concerns/questionable protection modules/no BMS access/coordination (dropins)/etc. In this setup, DCDC would keep AGM start batteries topped up all the time. Lithiums are charged by all 'primary' charging methods (solar, shore power, alternator).



Option 2 (alternator to AGM start battery, and then DCDC to Lithium bank) seems like inefficient pass through of alternator to charge lithiums, with question of how to keep AGMs 'topped up' when engine not running. Complications of having competing DCDC chargers in opposite directions (if I try to top up AGMS back from Lithiums)/heat from DCDC charger running full blast when engine on/etc.





My setup goal is to make it as 'hands off' as possible, and so far Option 2 seems like right direction, possibly with a supplemental small amp shore power charger to keep my AGM start batteries topped up if needed.



Maybe two DCDC chargers (from AGM start battery to Lithiums) in parallel to get as much current over to the LFP bank if I needed it in a pinch.

[Captain Bill]

Just an FYI, in my case I plan on setting the dc-dc chargers to float voltage of the other battery bank. That way the batteries will be fully charged up by the primary charging systems for each bank, but held at float voltage for keeping them topped up or driving house loads. Given that they'll only be passing float voltages and currents, I don't think than they will be producing significant heat. My pretty much worst case DC house load is 22 amps with refrigeration and all lights and electronics on, with an intermittent pulse to about 30 amps from an electric toilet flush. I'm not sure I see a scenario where I'll be trying to put a major charge on the AGMs from the lithiums while the engines are running. The alternators on the engines will be doing that in scenario 2. Of course if you don't have a genset you will have a different set of issues.

[Pete7]

Same here, we used the DC>DC charger twice this year whilst out cruising as solar did the majority of the work. Used it a couple of times since in power supply mode when working on the boat during the winter to just keep the lithium tickling along with a light discharge and charge back up, so option 2 for us.

Originally Posted by Captain Bill

I am currently getting ready to install my new lithiums using scenario 2. My logic is that I have a genset with a 150A battery charger and a 640W solar system to use for bulk charging of the lithiums. Having the alternators connected to my AGM start bank protects them from being overloaded by being exposed to a depleted lithium bank. It also protects them from a sudden cutoff of lithium charging by the BMS. It also protects the lithiums from being overcharged by the dumb regulators in the alternators during prolonged motoring. The DC-DC Charger is a much cheaper solution than installing smart regulators in the alternators. My primary use of the alternators will be to carry house and instrument loads while underway on engines at night.

[PaulCrawhorn]

You need current limiting and precise regulation matching the optimal charge profile

Therefore #1 requires a sophisticated VR like Wakespeed in order to do without a DC-DC charger. Neither are cheap.

I think #2 gives more flexibility, your LFP bank can be easily repurposed for other use cases

with #1 the investment is more hardwired to that boat only.

[alctel]

When I installed my lifepo4 pack on my old boat I ditched the start battery entirely, a lifepo4 pack will start a motor even when almost completely dead (and it should never get to that point, because you have a BMS, right?) and I wanted to free up the space/weight.

There is a really good way to integrate a lead acid start battery into a lifepo4 in a way that it also acts as a buffer and protects the lifepo4 pack, it's well described here:

https://nordkyndesign.com/electrical...-battery-bank/

Worth taking a look at if you want to keep a lead acid battery on the boat

[Jammer]

Originally Posted by jc1409

Option 1 (alternator to Lithium, and then DCDC to start battery) gets complicated with regulator upgrades/alternator diode concerns/questionable protection modules/no BMS access/coordination (dropins)/etc. In this setup, DCDC would keep AGM start batteries topped up all the time. Lithiums are charged by all 'primary' charging methods (solar, shore power, alternator).

It does have to be done correctly, i.e. with a suitable regulator and with an appropriate mechanism for the BMS to shut off the alternator without damaging it. You can set up the DC/DC to charge all the time or just when the engine is running, depending on your goals.


The advantage of this method is that you will be able to get significant charging of your house bank from the alternator. Over 100 amps is realistic in most installations. This allows the engine alternator to make a significant contribution to house charging, possibly eliminating the need for a separate genset.

Option 2 (alternator to AGM start battery, and then DCDC to Lithium bank) seems like inefficient pass through of alternator to charge lithiums, with question of how to keep AGMs 'topped up' when engine not running. Complications of having competing DCDC chargers in opposite directions (if I try to top up AGMS back from Lithiums)/heat from DCDC charger running full blast when engine on/etc.

My setup goal is to make it as 'hands off' as possible, and so far Option 2 seems like right direction, possibly with a supplemental small amp shore power charger to keep my AGM start batteries topped up if needed.

That's all fine but, this being Cruiser's Forum, the question becomes what you're going to do to run the fridge on the third cloudy day in a row at anchor. If you have enough solar that you get plenty of electricity on a cloudy day, or you are using a separate genset for that, you're good.

Maybe two DCDC chargers (from AGM start battery to Lithiums) in parallel to get as much current over to the LFP bank if I needed it in a pinch.

You have to be a little careful because you could end up depleting the AGM to the point where it won't start the engine. Some people use ACRs or a MOSFET equivalent of them.

[s/v Jedi]

I have a total of four Victron smart Orion 360W DC-DC converters. Two are configured to detect an alternator spinning on the input and will start charging the lithium house bank in the output, taking up to 60A from the alternator. They actually do more than 60A when they’re not too hot yet and they work flawlessly. You can even lower their output if the alternator can’t handle full power setting.

Orion #3 is configured to keep the starter battery fully charged. It is set to the AGM charge profile and to automatically start when the solar controllers start pumping out power. It’s always in float because my normal (at anchor) 12V loads are never more than the 30A output of this Orion.

Orion #4 is configured to take the house bank input and give a stable 13.8V power supply output (so not a charger but a power supply). This power is used for all electronics aboard so that they never get surges or dips (engine starting, windlass, electric winch) etc.

This has worked flawlessly, without any user intervention for two years now. All programming is easy and done with a smartphone app that uses Bluetooth to communicate with the Orions.

My house bank is 24V; this only matters for the model converter you buy. I have two different models in use: 12->24 and 24->12 while for a 12V house bank they are all the same 12->12 model.

[jc1409]

Thank you S/V Jedi - your post was eye-opening for me and helped me think through this:


Alternator (internally regulated) 120a connected to AGM start battery bank. One or two Orions (12v smart 360w) to charge the lithium house bank from the AGM start bank when the engine is on. One Orion (12v smart 360w) to charge from the lithium house bank to the AGM start bank when Solar MPPT is charging.



Charging from the Orions is triggered only when ignition is on, or when the solar MPPT is charging - not triggered based on voltage sensing.



- I didn't realize the Orion could be set to lower output
- I wasn't thinking that I could trigger the Orion (to the start battery) from an MPPT trigger

- Your situation could result in scenario where solar is causing charge from one Orion from the house bank to the start battery, and the alternator is causing charge from the 2 Orions to the house bank, at the same time, but I guess that doesn't cause an issue.

[s/v Jedi]

Originally Posted by jc1409

Thank you S/V Jedi - your post was eye-opening for me and helped me think through this:


Alternator (internally regulated) 120a connected to AGM start battery bank. One or two Orions (12v smart 360w) to charge the lithium house bank from the AGM start bank when the engine is on. One Orion (12v smart 360w) to charge from the lithium house bank to the AGM start bank when Solar MPPT is charging.



Charging from the Orions is triggered only when ignition is on, or when the solar MPPT is charging - not triggered based on voltage sensing.



- I didn't realize the Orion could be set to lower output
- I wasn't thinking that I could trigger the Orion (to the start battery) from an MPPT trigger

- Your situation could result in scenario where solar is causing charge from one Orion from the house bank to the start battery, and the alternator is causing charge from the 2 Orions to the house bank, at the same time, but I guess that doesn't cause an issue.

If both of your banks are 12V and the starter battery is only used for starting… and the starting battery is AGM, then I don’t think you need a DC-DC converter for charging it. I believe the self discharge rate for AGM is 3% per month so even when you don’t run the engine for a year, it will still start. You probably also have an emergency switch to start from your house bank.

I have that Orion because my house bank is 24V and I still have large 12V loads during sailing (electric winches).

My Victron SmartSolar controllers have an output relay that can be programmed to switch on when solar power is detected. The Orion DC-DC converters have an input that you can connect to that to enable/disable them based on solar production.

For the alternator you don’t need this as the Orions have a very good alternator detection algorithm. I also think the voltage level detection works pretty good and that is how I currently trigger.

Just make sure you get the smart versions of this gear.

I attached a diagram of our DC.