DC-DC charger - which direction for Lithium batteries?

[sailingunity]

Originally Posted by hjohnson

I agree that building your system so that a disconnect will never happen is the goal. However, there are several reasons why the BMS might go into alarm even when the voltage is otherwise within spec. A cell imbalance beyond a threshold, battery over/under temperature, C-rate exceeding limits, all are theoretical reasons it might disconnect while still being otherwise within voltage spec.

This is why there needs to be a certain level of integration between the BMS. Either a shutdown contact closure from the bms to kill charging sources, or something mor sophisticated to better govern the charging sources. The problem is that most of the “drop in” batteries do not have this capability.

On my boat, I have full integration, BMS controls and sets voltage/current limits for all my sources, but that’s because I was willing to spend the money and greenfield the thing, and did a full DIY battery.

I would have loved to have had a fully integrated system... I didn't want to build the cells from scratch though... and the batteries that have external BMS's (e.g. Victron) were 50% more expensive than what I chose.

[fxykty]

Originally Posted by hjohnson

…
On my boat, I have full integration, BMS controls and sets voltage/current limits for all my sources, but that’s because I was willing to spend the money and greenfield the thing, and did a full DIY battery.

We’ve got Balmar MC-614 regulators as the Wakespeed WS500 wasn’t available at the time. Our BMS turn offs the regulators when charge limit is reached (and in the case of battery disconnect) by cutting the regulator ignition wire, which performs a standard shutdown of the alternator and is recommended by Balmar. However, this is not ideal as there isn’t a way to provide trickle power to support loads and it does mess with the tachometers.

If the WS500 wasn’t so expensive (basically AUD1000 each) we’d consider replacing our Balmars in a heartbeat. Having connectivity via CANbus would be great for fine-grained alternator charge. But what we’ve got works well enough and we very rarely motor for extended periods.

[sailingharry]

Originally Posted by hjohnson

The problem is that most of the “drop in” batteries do not have this capability.



On my boat, I have full integration, BMS controls and sets voltage/current limits for all my sources, but that’s because I was willing to spend the money and greenfield the thing, and did a full DIY battery.

Indeed, that is the unresolvable problem with drop-ins. Most have very limited BMS functionality, and those with more sophistication lock you into a very rigid single provider system.

My AGM batteries are still quite functional - they have lost maybe 20% of their original capacity - so I'm not doing anything now. But I have been extensively learning and will be doing a complete integrated DIY system for just those reasons.

[hjohnson]

Originally Posted by fxykty

We’ve got Balmar MC-614 regulators as the Wakespeed WS500 wasn’t available at the time. Our BMS turn offs the regulators when charge limit is reached (and in the case of battery disconnect) by cutting the regulator ignition wire, which performs a standard shutdown of the alternator and is recommended by Balmar. However, this is not ideal as there isn’t a way to provide trickle power to support loads and it does mess with the tachometers.

If the WS500 wasn’t so expensive (basically AUD1000 each) we’d consider replacing our Balmars in a heartbeat. Having connectivity via CANbus would be great for fine-grained alternator charge. But what we’ve got works well enough and we very rarely motor for extended periods.

The 614 is a good regulator as well, and as long as you use the brown? wire to kill it before a disconnect, that will keep you safe. The WS5000 is a great unit, but I wouldn’t say it’s anywhere near AUD1000 better than the balmar unit. In my case, I was building from scratch, so the differential between the two units made sense for me (especially because I have mated a large alternator (85A) onto my small 40 year old 10hp engine.

[SOLAR SUPPORT]

I disassembled the rotor and regulator of my Mitzi 115A alternator and disconnected one of the rotor warning brush lead from the internal regulator. I took the heat resistant cable, which was soldered to each other at the cut point, out from the back of the alternator. When these cables are disconnected while the alternator is in operation, the alternator charge is terminated without any problem, no matter how many amperes the charge load is. When you bring the cables in contact with each other, charging resumes. Curious, I measured the current going to the rotor windings through this cable. I have seen that the current varies between 3-8 amps depending on the load and rotation speed of the alternator. The regulator tries to keep the charging capacity of the alternator at the highest level by controlling the intensity of this current. It is possible to disable the internal regulator in alternators produced in Mitzi and similar automotive standards used in boats. After this is done, there is a thought whether it is possible to make a simple circuit that transmits a constant and low current to the rotor, and the alternator can charge at a level that does not exceed 50-60% of the label capacity only when the rpm is max, but I do not have the electronic knowledge to do this..

[goboatingnow]

Originally Posted by PaulCrawhorn

The alternator does not push current, the loads/batteries pull the amps needed.

So long as a small lead batt is hardwired to the alt output, the LFP bank can be isolated from all charge sources for whatever reason.

In that scenario, nothing needs to tell the alt to stop, and of course there may be dozens of loads still feeding from its output after the LI bank is taken offline.

The "load dump" problem arises from a poor design, where the LFP bank is the only buffering load. Opening the circuit while huge amps are flowing then causes huge spikes / surges that can fry any sensitive circuitry connected.

Yes but since you cannot parallel chemistries , alternator charging the small starter battery and deploying a medium to big dc dc converter is less then optimum

Fix the alternator regulator so it can be controlled. Connect that to the li bank. Use a small dc dc to trickle charge the fla bank

[goboatingnow]

Originally Posted by SOLAR SUPPORT

I disassembled the rotor and regulator of my Mitzi 115A alternator and disconnected one of the rotor warning brush lead from the internal regulator. I took the heat resistant cable, which was soldered to each other at the cut point, out from the back of the alternator. When these cables are disconnected while the alternator is in operation, the alternator charge is terminated without any problem, no matter how many amperes the charge load is. When you bring the cables in contact with each other, charging resumes. Curious, I measured the current going to the rotor windings through this cable. I have seen that the current varies between 3-8 amps depending on the load and rotation speed of the alternator. The regulator tries to keep the charging capacity of the alternator at the highest level by controlling the intensity of this current. It is possible to disable the internal regulator in alternators produced in Mitzi and similar automotive standards used in boats. After this is done, there is a thought whether it is possible to make a simple circuit that transmits a constant and low current to the rotor, and the alternator can charge at a level that does not exceed 50-60% of the label capacity only when the rpm is max, but I do not have the electronic knowledge to do this..

A suitable power resistor is all that’s needed.

[PaulCrawhorn]

A small lead battery is a cheap load dump buffer.

It does not need to be Starter, nor have any other function.

Complexity reduces reliability.

[goboatingnow]

Originally Posted by PaulCrawhorn

A small lead battery is a cheap load dump buffer.

It does not need to be Starter, nor have any other function.

Complexity reduces reliability.

Yes but if it fails the Li will dump piles of a amps into it.

It’s a “ solution “
It’s not a good one. And the higher charge voltages of lithium are not great on a lead acid already fully charged.

[tanglewood]

Originally Posted by goboatingnow

Yes but if it fails the Li will dump piles of a amps into it.

It’s a “ solution “
It’s not a good one. And the higher charge voltages of lithium are not great on a lead acid already fully charged.

Agreed. And lead batteries DO fail, on a regular basis.

[PaulCrawhorn]

Of course all batteries fail.

Just replace on a schedule, they are very cheap.

LFP should terminate charge at 13.8V CC only holding any CV is not even needed.

That is lower than most lead specs.

[goboatingnow]

Originally Posted by PaulCrawhorn

Of course all batteries fail.

Just replace on a schedule, they are very cheap.

LFP should terminate charge at 13.8V CC only holding any CV is not even needed.

That is lower than most lead specs.

That’s very low for LFP lot of capacity potentially being sacrificed

[sailingunity]

Originally Posted by goboatingnow

That’s very low for LFP lot of capacity potentially being sacrificed

Ours are considered charged at 13.6 - but we keep charging to 14.1. The time it takes to go from 13.6 to 14.1 is very fast.

[PaulCrawhorn]

There is very little actual usable Ah capacity past that point.

At rest isolated for 24-48 hours 3.33-3.35V is absolute 100% Full

any voltage pumped in past that point is unnecessary stress, just causes reduced lifespan, balancing headaches, no positive result served.

If you really think you need those last few mAh capacity utilisation, then you should have built a bigger pack.

Of course there are extreme use cases like racing where the owner wants maximum energy density, replacing the bank after a few hundred (or even a few dozen) cycles is no problem

but then they might as well have gone with 3.7Vnom li-ion chemistries or even airplane LiPo's.

[CaptainRivet]

your 120A alt will longterm eg motorsailing produce 70A max, so 2x 30A Orions are perfect also for lifespan of it.
if that's a 115A Mitsubishi alt alternative would be the VMR 200 regulator from Nordkyn design