DC-DC back to start alternatives?

[themorb]

I have read as many threads here as I can absorb (pun intended) but would appreciate if anyone has other ideas I haven't thought of.

Our boat came with:

• 100A Balmar alternator wired to house bank
• Balmar 614 external regulator
• Magnasine 2812 inverter/charger
• Victron MPPT 100/50 solar controller
• 800 ah lead acid house bank old needs replacement.
• lead acid starter battery with combiner switch to house bank

I have ordered 600 ah of LiFePO batteries. ("LiTime" - hopefully decent quality Amazon.)

I am planning to program the Balmar regulator, Magnasine, and Victron MPPT for lithium charge profiles. And use the Balmar belt regulator feature to limit to 70 amps or so to avoid burnout. Also temp probe on alternator.

I may add an alternator protection device from Balmar or Sterling to be safe from BMS disconnect.

Question is what is the simplest way to charge the start battery? Just to replace a few amps lost each time starting... A $300 Victron DC-DC seems a bit overkill maybe? Is there a cheaper decent DC-DC trickle charger? What about a 120v trickle charger from my inverter? Not efficient but does it matter?

[Statistical]

House bank and starter are both 12V?

Yes a $300 victron would be overkill use a smaller one ($160).

https://www.amazon.com/Victron-Orion.../dp/B0851TPKV7

There are other smaller DC chargers from other brands maybe a bit cheaper (i.e. renogy) but big fan of the smart victrons as they are highly configurable and have bluetooth. A 120V trickle AC charger would also work as long as you normally have inverter on. If you don't then that is just setting yourself up for failure.

In either case you don't need a crazy amount of amps but I would avoid trying to go too cheap buying off brand junk or skimping to the bare minimum on amps. Keep in mind you might have a hard starting engine and need to crank it multiple times and then it dies shortly after starting and you need a second crack at it. Don't think of the ideal scenario think of the worst scenario. Personally I don't want even the slightest doubt that the engine will start when I turn the key.

[themorb]

Ah yes good point, both house and starter are 12v. Higher would be nice but the boat is all set up with 12v so changing would be expensive.

Ok, that smaller Orion is still $225 in Canada. But yes, probably shouldn't cheap out. Starting is important.

The whole concept of a lead acid starting battery seems like it should be obsolete by now -- the LiFePO batteries I ordered said they can do 800 amps for 5 seconds and 400 amps continuous. That should be enough to start my engine. And many boats install much more.

I found a little LiFePO "outboard starting battery" online for my dinghy -- the size and weight of a binocular case and works flawlessly. No idea what the BMS specs are but it was specifically marketed for starting small outboards. No spilling, light weight, should last a decade or more.

[sailorboy1]

Originally Posted by themorb

Question is what is the simplest way to charge the start battery?

The same you have been charging it. I changed nothing except setpoint 6 months of full time cruising. The lead acid battery is fine charging at the lower LFP voltages.

[Statistical]

Originally Posted by themorb

Ah yes good point, both house and starter are 12v. Higher would be nice but the boat is all set up with 12v so changing would be expensive.

Ok, that smaller Orion is still $225 in Canada. But yes, probably shouldn't cheap out. Starting is important.

The whole concept of a lead acid starting battery seems like it should be obsolete by now -- the LiFePO batteries I ordered said they can do 800 amps for 5 seconds and 400 amps continuous. That should be enough to start my engine. And many boats install much more.

I found a little LiFePO "outboard starting battery" online for my dinghy -- the size and weight of a binocular case and works flawlessly. No idea what the BMS specs are but it was specifically marketed for starting small outboards. No spilling, light weight, should last a decade or more.

Lead acid is just realllllllllllly cheap and for applications like starter or bow thruster or windlass (high power minimal energy) it is still hard to beat.

Sure you could just go with house bank only but then you lose the redundancy of a deadicated starter bank. Maybe that is an acceptable compromise to reduce cost. Likewise you could use LFP for the starter too but you would still need a DC to DC charger. Hell super capacitors are even an option. For starter batteries though lead acid just works and is so cheap it has stuck around and probably will continue to do so.

[themorb]

Originally Posted by sailorboy1

The lead acid battery is fine charging at the lower LFP voltages.

I was wondering about that too..... But wouldn't it actually be subject to higher voltages while the LiFePOs are filling up at 14.6v and the starter should be in float 13.5v, resulting in over charge to the starter battery?

And then of course if I forget and leave the battery switch connected when I start the engine, that exposes the LiFePOs to starting which is apparently bad but as I said earlier, don't really see why if they can deliver 400-800 amps.

[Statistical]

Originally Posted by themorb

I was wondering about that too..... But wouldn't it actually be subject to higher voltages while the LiFePOs are filling up at 14.6v and the starter should be in float 13.5v, resulting in over charge to the starter battery?

Well regardless of what you do you may not want to charge the LFP at 14.6V. Yes LFP can be charged at up to 3.65V/cell (14.6V for 4 cell 12V battery) but you don't need to and a lower charging voltage can be a lot more benign on the battery.

And then of course if I forget and leave the battery switch connected when I start the engine, that exposes the LiFePOs to starting which is apparently bad but as I said earlier, don't really see why if they can deliver 400-800 amps.

If your LFP can deliver the required current that is not a concern. The issue would be trying to start an engine with a relatively small LFP bank where the inrush current exceeds the BMS trip threshold and you litterally can't start it.

The advantage of DC Charger for the starter bat is it is off on its own isolated circuit. No energy is used for anything but starting the motor. Nothing charges the starter bat except the DC charger set to exactly what you want for the start bat. It is simple and isolated and doesn't ever require flipping switches (or forgetting to flip switches).

Note in addition to this you could still have an A B (but not combine) switch to temporarily allow using the LFP house bank as an emergency start option.

[ABJ87]

Just as another option Balmar also makes a “battery to battery” charger. Not as sophisticated as the victron but works fine fine for charging a start battey.

https://balmar.net/products/digital-duo-charge/

[hjohnson]

We use the victron 18A Dc-DC charger. Keeps the starter battery in its happy place (13.8v) no matter what our house bank is doing. We typically let the house lithium bank sit at 13.2 or 13.3v when the boat isn’t in use, and then it’s all over the place while it is in use.

[donradcliffe]

I'd wire the alternator to a lead acid or AGM start battery and use a battery combiner (voltage sensitive relay) to charge the house bank. I'd also dial down the bulk and absorption voltage on the Balmar to about 14.2 volts and minimize the absorption time.

[hjohnson]

Originally Posted by donradcliffe

I'd wire the alternator to a lead acid or AGM start battery and use a battery combiner (voltage sensitive relay) to charge the house bank. I'd also dial down the bulk and absorption voltage on the Balmar to about 14.2 volts and minimize the absorption time.

That would be a terrible idea. OP has a balmar 614 advanced regulator, which can be used to safely charge LFP directly. What you’re suggesting is crazy inefficient, and would seriously lengthen charging times.

Stock alternator with internal regulator and not enough budget to go with a 614 or a wakespeed? Then sure that will work. But if you’ve got the good regulators, always charge the LFP directly.

[donradcliffe]

Originally Posted by hjohnson

That would be a terrible idea. OP has a balmar 614 advanced regulator, which can be used to safely charge LFP directly. What you’re suggesting is crazy inefficient, and would seriously lengthen charging times.

Stock alternator with internal regulator and not enough budget to go with a 614 or a wakespeed? Then sure that will work. But if you’ve got the good regulators, always charge the LFP directly.

Suit yourself, but don't come here crying when your BMS has a high voltage cutoff and fries the alternator diodes. From the 614 manual

Many LiFePO4 batteries have a Battery Management System (BMS) that may disconnect the battery from the alternator as a protective action or when charging is complete. The regulator must be shut down before the battery is disconnected .Running an alternator without a battery will damage the alternator and may damage any attached system. This is doubly true if the battery can be disconnected during high current charging, causing a load dump. The load dump can easily cause a high voltage spike which will destroy the alternator’s rectifier, at minimum. This is not a warrantable failure. To reiterate: THE ALTERNATOR MUST BE SHUT DOWN BEFORE DISCONNECTING THE BATTERY. THE ONLY SAFE WAY TO SHUT DOWN THE ALTERNATOR IS TO TURN OFF THE REGULATOR.

Explain what you think is crazy inefficient.

[bstreep]

Watching…. What Don said is exactly why we use a Victron 12 volt 30-30 to charge our house from the alternator via our start battery.

[Statistical]

Originally Posted by bstreep

Watching…. What Don said is exactly why we use a Victron 12 volt 30-30 to charge our house from the alternator via our start battery.

Don did not say to use a DC to DC charger he said to parallel an AGM and LFP battery together. That is what an ACR does. When the charging voltage is hit it closes and puts the two batteries in parallel.

It is the exact opposite of using a DC to DC charger which keeps the two isolated using the DC to charger to feed power from one to the other and that can happen at completely different voltages and amps.

[s/v Jedi]

The proper way is to charge the LA start battery with the engine, just like the manufacturer shows you in the installation instructions.

When you switch to a higher amp alternator and really want to charge the house batteries with it, then you take a 30A dc-dc charger like explained above.

Yes, it’s more expensive. Lithium batteries are also more expensive.

The above method is the only proper way for different chemistry batteries. You can use an automatic charge relay or a battery isolator if you don’t care that the start battery is charged with a lithium algorithm.

Edit: this is for the cheap drop in batteries that the OP mentioned. When you have more expensive batteries you may get a BMS that can shut down the alternator before a HVC event.