Will a DC-to-DC charger protect the alternator from lithium BMS cutoff?

[newhaul]

Originally Posted by NPCampbell

I'm curious as to your setup.

When an alternator is putting out high current, removing the load by the BMS disconnecting or the charge source disconnecting is still called a load dump.

The charging source will no longer see a load and the back EMF in the windings of the stator will generate a voltage spike. This spike can easily be 75-100v on a 12v system. If the charge source disconnects downstream of the regulator then the spike will burn the regulator out. If there are other electronics in the circuit outside of the disconnected load, they may be damaged as well.

If a load isn't presented to the alternator after a load dump, the time constant of the voltage spike will be incredibly long. A TVS diode like goboatingnow mentioned can clamp that spike at a preset voltage as long as the TVS diode is large enough to absorb the energy until the spike decays.

It sounds like you have a setup that clamps your output voltage at 14.8v?

My personal setup is 250ah lfp with a Daly bms with separate charge and discharge taps.
My alternator is a 75 amp small frame Chevy unit on a westerbeke 10 two running to a grp24 fla start battery and a 50 amp b2b charger set to my lfp bank.

My primary is 400 watts solar running via an mppt and have an airx 400 I'm planning to install for winter .

[CatNewBee]

Originally Posted by SOLAR SUPPORT

Hello Everyone, I am not a LFP user but I am planing to replace my four years old AGMs next year. I think LFP's self control by cuting off the circuit and alternator over heat problem is common both for RV and MV. Otomobiles do not have a house battery bank but have the same alternator to be protected against HVC. I started to think our genious inventors whom create a super battery made with Lithium Iron Phosphate are not enough genious to find a simple and cheap solution to common alternator HVC and over heat protection. I think those problems create a huge market for alternator producers to upgrade their existing Alternators working in those MVs an RVs. Replacing the Alternator's internal charge regulator with a LFP Friendly one for instance if possible technically. New regulator must have HVC & Over Heat Protection both.

Because this arguments a bogus.Alernator regulators are set to a 14.2V limit for wet LA batteries, 14.6V are considered an absorption voltage for LFP, HVP is considerd at 3.8V cell voltage and above for drop in BMS batteries, or 15.2V, LFP cells are stable up to 4V, (16V), so there IS NO disconnect on a battery from the alternator UNLESS you mess up the settings OR the battery is damaged / way out of balance (again messed up by user settings).

A disconnect is very unlikely.

[goboatingnow]

Originally Posted by CatNewBee

Because this arguments a bogus.Alernator regulators are set to a 14.2V limit for wet LA batteries, 14.6V are considered an absorption voltage for LFP, HVP is considerd at 3.8V cell voltage and above for drop in BMS batteries, or 15.2V, LFP cells are stable up to 4V, (16V), so there IS NO disconnect on a battery from the alternator UNLESS you mess up the settings OR the battery is damaged / way out of balance (again messed up by user settings).

A disconnect is very unlikely.

Absolutely agree. As opposed to charge control , ie turning off sources for good lithium health ( ie no float charges ) , a disconnect is a last ditch safety measure that should never occur in normal Lithium operation. It’s entirely overplayed by BMS manufacturers and the “scary lithium” brigade

[barryglewis]

Originally Posted by CatNewBee

Because this arguments a bogus.Alernator regulators are set to a 14.2V limit for wet LA batteries, 14.6V are considered an absorption voltage for LFP, HVP is considerd at 3.8V cell voltage and above for drop in BMS batteries, or 15.2V, LFP cells are stable up to 4V, (16V), so there IS NO disconnect on a battery from the alternator UNLESS you mess up the settings OR the battery is damaged / way out of balance (again messed up by user settings).

A disconnect is very unlikely.

What brand of battery are you using that are stable to 16V ?

[CatNewBee]

Originally Posted by barryglewis

What brand of battery are you using that are stable to 16V ?

Winston Thundersky LiFeYPO4 1000Ah
Look up the Data sheet. Attachment 247060

[barryglewis]

Originally Posted by CatNewBee

Winston Thundersky LiFeYPO4 1000Ah
Look up the Data sheet. Attachment 247060

Thanks, but the attachment link fails with this message:
"Invalid Attachment specified. If you followed a valid link, please notify the*administrator"

Any idea how this is notified to the administrator?

[lmxr]

Winston datasheet: https://files.gwl.eu/inc/_doc/attach...-Datasheet.pdf

[barryglewis]

Thanks, that worked

[sailingharry]

Originally Posted by goboatingnow

In a properly designed lithium system wide solution all charge sources should be controllable by the BMS. sadly marine dc electronics is a hodge lodge of incompatible , often user installed equipment of varying age, technology and often not interworked as a complete system. This is the opposite to modern car and truck systems.

Hence BMS introduced into this environment often have no means of controlling charge sources other then by a very crude battery disconnect

What’s a better idea of course is full control over each charging source. Failing that , the next best thing is charge source disconnects , rather then a battery disconnect , but this is often eschewed on cost grounds ( multiple disconnect devices ) . Of course charge source disconnects bring the issue of alternator control into play because it’s a load dump situation in many cases ( or can be)

Yes, what you say makes a great deal of sense.


To my mind, as mentioned, you should never reach a high voltage level to begin with (all charge sources should dial back long before that point).



But, more importantly, even if the BMS doesn't actually "control" the charge sources, it seems to me that most can be disabled by a small, low power relay, triggered by the "alarm" or "pre-alarm" and long before a disconnect event. Alternators can be disabled by opening the field or the regulator power. Shore power (inverter chargers) have a "disable switch" (for sure, the Victron I'm looking at does). Wind, solar, and hydrogeneration are harder. But with a proper BMS (not available on drop-in batteries from what I can tell), I would put the alternator and inverter/charger on the battery side of any disconnect (also a problem on drop-in solutions), and they would NEVER be disconnected, just "disabled."



Am I missing something?