Lithium and dc to dc chargers

[Tin Tin]

I would have a start battery for each engine and a DC DC charger from each of them to the LifePo4 (internal BMS) house bank. This is for standard alternators. That is what we have and I just followed the Victron book recommendation on page 8.
https://www.victronenergy.com/upload...Systems-EN.pdf

[Emouchet]

There is something wrong with all those approaches: your are sacrificing one of the largest benefits of using lithium: it’s ability to absorb large charge currents right up to fully charged. Or to put it another way, bottle necking all your charging to 30 amps with the DC/DC charger is as pity when that lithium bank could easily absorb 100 amps.

2x 70/80 amps standard alternator can deliver a lot of charging current...

[OS2Dude]

What kind of regulator do your alternators have? If you have a smart programmable regulator on at least one engine, then that alternator can go directly to the LFP bank and if needed a DC-DC charger can charge the starter bank. This is the most efficient use of the alternator. If you don't have a smart regulator (WakeSpeed, Balmar, etc.) it may be worth it to get one for one or both engines. Depending on the BMS in the LFP bank, the regulator can receive a warning that the BMS is about to disable charging and can also throttle back the alternator if it is running hot.

[Tupaia]

Originally Posted by Emouchet

There is something wrong with all those approaches: your are sacrificing one of the largest benefits of using lithium: it’s ability to absorb large charge currents right up to fully charged. Or to put it another way, bottle necking all your charging to 30 amps with the DC/DC charger is as pity when that lithium bank could easily absorb 100 amps.

2x 70/80 amps standard alternator can deliver a lot of charging current...

It all depends on circumstances. In my case the solar 1200W with 2 x MPPT and 2 x Duo MPPT that charge both the Li and the start batteries is more than able to keep up except for voltage issue to raise anchor early in the morning. My engine hours are typically so low <50hours / year that fancy alternators are not worth the extra cost and I would still need a DC-DC to keep the start batteries charged.


I would prefer to start my engines on the Li like I did with my old AGM's and do away with the start batteries, but I would still need fancy "expensive" alternators or regulators.

[Cpt Mark]

Depending on the BMS in the LFP bank, the regulator can receive a warning that the BMS is about to disable charging and can also throttle back the alternator if it is running hot.[/QUOTE]

What BMS does this? I have not found one yet.

[Michigan_Eric]

Just to be clear, are we talking about Lithium-ion batteries or Lithium iron phosphate (LiFePo4) batteries?

[cram]

Originally Posted by Emouchet

There is something wrong with all those approaches: your are sacrificing one of the largest benefits of using lithium: it’s ability to absorb large charge currents right up to fully charged. Or to put it another way, bottle necking all your charging to 30 amps with the DC/DC charger is as pity when that lithium bank could easily absorb 100 amps.

2x 70/80 amps standard alternator can deliver a lot of charging current...

Agreed, but it comes down to cost. 2x 50 Amp DC-DC converters (working off the start battery) is one inexpensive way to do it, running the alternator continuously at 60-70% capacity. Another way is an external alternator regulator that is designed for lithium and communicating with the battery BMS to prevent an inadvertent disconnect. There are very few controllers and batteries that can do this.

I went with the DC-DC converter option and I do see an occasional BMS disconnect for some batteries as they reach their full capacities but others have not. My lithium batteries are "dumb" in that they don't talk to any other component as per the latest ABYC bulletin requirements.

At the end of the day, there are multiple ways of building a reliable and safe system with a lithium house bank. DC-DC charging off the start battery is one option. A fully integrated communication system between an external regulator and lithium batteries is a great alternative, although at a higher cost...

[hjohnson]

Originally Posted by Cpt Mark

What BMS does this? I have not found one yet.

My REC ABMS does this with aplomb. It’s connected via CANBus to the rest of my electrical system and is the one that controls the entire charge/discharge process. Basically it sends out a message containing the current battery voltage/current/temperature, and also what the current limit is on those values.

My alternator regulator (wakespeed WS500) listens for those messages, and feeds them into its algorithm. Furthermore, if the BMS throws any kind of warning or alarm, or stops sending messages at all, the wakespeed will go into standby mode and kill the field current on the alternator. The effect is that it is virtually impossible to have an unexpected disconnect of the alternator. As the battery comes up to 100% SoC, it tapers back the voltage and current limits, preventing overshoot.

Those messages also are used by the Cerbo GX to control my inverter/charger, and solar MPPTs so that everything plays nice together. It’s a really great system.

[wholybee]

Originally Posted by PSGB

Just playing devils advocate because my knowledge level on this topic is low. “equipment on each side of the DCDC are connected to different power feeds”, how about the engine/alternator on one side and solar, shore power, wind on the other?

There is only one power feed in that scenario - shore power. Everything else has it's ground tied to shore power, or together in the absence of shore power.

The need for a DCDC converter comes into play when you have 2 power feeds. But on a boat, even if you had 2 power feeds, they are still coming from the same breaker panel and single feed from the utility.

The need arises because voltage is not an absolute, it is the difference between 2 potentials. So, if you have 2 power feeds, both 120V, but the grounds could be 50V apart. So by connecting the two grounds together, you have a direct short of 50V = huge dangerous current. That scenario does happen in some buildings or if connecting equipment between 2 buildings.

If you use an isolated DCDC on a boat, then the ground potential on each side is floating at whatever random potential, creating a voltage between the two separate grounds. You could receive a shock by touching a ground, or you could have stray current flowing on the ground eating away your underwater metals.

[wholybee]

Originally Posted by Michigan_Eric

Just to be clear, are we talking about Lithium-ion batteries or Lithium iron phosphate (LiFePo4) batteries?

Lithium Iron Phosphate. Lithium Ion refers to any battery that uses Lithium in it's cathode, so that term does include LFP, as well as others which are not as safe. But here we specifically are discussing LFP.

[Michigan_Eric]

Originally Posted by wholybee

Lithium Iron Phosphate. Lithium Ion refers to any battery that uses Lithium in it's cathode, so that term does include LFP, as well as others which are not as safe. But here we specifically are discussing LFP.

Good to know. I do have some portable Lithium Ion batteries on my boat (a Jackery 1500, Jackery 500, and a GoalZero Sherpa 100) that I charge with portable solar panels.

I'd like to discuss what the risks of these are, but that's probably best for another thread.

[Jef & Marin, Netherlands]

Just a few thoughts:
- With less than 50 engine hours per year, and enough solar, charging by the alternators is only relevant in case of emergency or failures. For instance your house battery or solar charging fails and you need to get home.
- So the main considerations are: kilo's, money, failure rate, simplicity.
- To have only one FLA start battery seems nice for the kilo's and the $$$, however the cable resistance for the start current (in our boat 400A) requires crazy thick and heavy wires.
- Also the length and size of the wires to the house battery need to be taken into account.



In our case I just use switches and fuses to connect the start batteries to the LFP bank. When the engines are just used briefly, they just charge the start batteries. For longer engine use, after 15 minutes of so I connect all the batteries together.
Note that if the start batteries and the LFP bank are connected all the time, the start batteries will not charge, and be depleted further every time you start

[Tupaia]

Originally Posted by Jef & Marin, Netherlands

Just a few thoughts:
- With less than 50 engine hours per year, and enough solar, charging by the alternators is only relevant in case of emergency or failures. For instance your house battery or solar charging fails and you need to get home.
- So the main considerations are: kilo's, money, failure rate, simplicity.
- To have only one FLA start battery seems nice for the kilo's and the $$$, however the cable resistance for the start current (in our boat 400A) requires crazy thick and heavy wires.
- Also the length and size of the wires to the house battery need to be taken into account.



In our case I just use switches and fuses to connect the start batteries to the LFP bank. When the engines are just used briefly, they just charge the start batteries. For longer engine use, after 15 minutes of so I connect all the batteries together.
Note that if the start batteries and the LFP bank are connected all the time, the start batteries will not charge, and be depleted further every time you start

This is exactly what a DC-DC charger does except it does it automatically. The charge to the Li will be less but you can't forget to disconnect them and flatten the start batteries.