BMS disconnect question?

[BAD ORCA]

I'm trying to learn as much as i can about lithium systems, but im still currently confused about one thing, and i cant seem to find a concise explanation or answer.

If im out sailing with all my DC navigation equipment turned on, (engine off) and for some unexpected reason my BMS disconnects the batteries from these loads, will it create a damaging voltage spike that will ruin the equipment, autopilot etc connected to the charge bus, or is this really only a problem for an alternator if it disconnects while running the engine or an AC charger when plugged into shore power?

[Greg Brannon]

A possibly dumb question from the peanut gallery: Does the answer to this question vary with battery chemistry?

General question: At what voltage is your battery management system (BMS) set to disconnect the battery bank from the DC loads?

Greg.

[wholybee]

It is only a problem if the engine and alternator is running. When it happens the voltage spike from the alternator can damage other equipment as well. There are several easy fixes. The fear is overstated.

[s/v Jedi]

Surge suppression.

[Ballsnall]

Originally Posted by BAD ORCA

If im out sailing with all my DC navigation equipment turned on, (engine off) and for some unexpected reason my BMS disconnects the batteries from these loads, will it create a damaging voltage spike that will ruin the equipment, autopilot etc connected to the charge bus, or is this really only a problem for an alternator if it disconnects while running the engine or an AC charger when plugged into shore power?

No, no voltage spike if the alternator is not charging providing there are no other charging sources although other sources like wind and solar would not produce a significant spike likely to cause damage.

[BAD ORCA]

Originally Posted by Greg Brannon

A possibly dumb question from the peanut gallery: Does the answer to this question vary with battery chemistry?

General question: At what voltage is your battery management system (BMS) set to disconnect the battery bank from the DC loads?

Greg.

Im not sure if the questiion was was directed at me. It sounds like its a non issue if charge sources are turned off, but since lead acid batteries dont have an BMS im going to guess no.

I just ordered my first lithium battery (dont have it yet) to learn on over the next year or two before upgrading my entire house bank. I dont have all the user manuals and tech data sheets yet either but im guessing it will be 14.6 volts. The BMS is built in and im not sure those settings can be changed. I will use Victron chargers, shunt etc....so i will set my charge sources to end at 14.2 volts or lower or 3.55ish volts per cell if thats possible.

The bluetooth app shows individual cell voltage so it will be possible to get a cell level view of the pack.

[BAD ORCA]

Originally Posted by wholybee

It is only a problem if the engine and alternator is running. When it happens the voltage spike from the alternator can damage other equipment as well. There are several easy fixes. The fear is overstated.

Ok then that will be a non-issue for me because i intend to send my alternator charging to the lead start bank. The lithium bank will be isolated from the start bank and will only be charged via solar or shore power for now. Once i get a lot more hands on experience i'll start thinking about a DC-DC charger.

[BAD ORCA]

Originally Posted by s/v Jedi

Surge suppression.

ok, so many questions. I need a day or two to absorb this one :P

first obvious question though. what is that bronze/black doohickey between the battery and switch panel?

[BAD ORCA]

Originally Posted by Ballsnall

No, no voltage spike if the alternator is not charging providing there are no other charging sources although other sources like wind and solar would not produce a significant spike likely to cause damage.

what about a shore power charger? If im charging at .5C and i get a disconnect, will it send a voltage spike back into the AC charger or inverter? I know chargers have built in fuses and i have a fuse between my charger and busbar, but those protect from high amps not voltage, correct?

I feel like i need to go to electrical engineering school to manage my sailboat now...

[Jammer]

Originally Posted by BAD ORCA

If im out sailing with all my DC navigation equipment turned on, (engine off) and for some unexpected reason my BMS disconnects the batteries from these loads, will it create a damaging voltage spike that will ruin the equipment, autopilot etc connected to the charge bus

No.


Your DC navigation equipment is not an inductive load.

[Jammer]

Originally Posted by Greg Brannon

A possibly dumb question from the peanut gallery: Does the answer to this question vary with battery chemistry?

No, because batteries do not have significant inductive properties regardless of chemistry.

[Jammer]

Originally Posted by BAD ORCA

first obvious question though. what is that bronze/black doohickey between the battery and switch panel?

Those are terminal strips. They just connect the groups of wires together. There are two groups, the bronze group, and the black group. The DIN rail terminal components are common in industrial process automation applications. Typically they are not used on yachts, though there are exceptions.

[Jammer]

Originally Posted by BAD ORCA

what about a shore power charger? If im charging at .5C and i get a disconnect, will it send a voltage spike back into the AC charger or inverter?

AC chargers and inverters typically show a very large capacitance on the DC terminals. Getting a voltage spike into that would be a neat trick.


More broadly, the "voltage spike during BMS disconnect problem" is entirely about alternators, because of their high inductance and the high current that passes through them. Voltage in an inductor is proportional to inductance times the change in current. There are additional electromechanical effects in alternators that make this worse.

[BAD ORCA]

Originally Posted by Jammer

Those are terminal strips. They just connect the groups of wires together. There are two groups, the bronze group, and the black group. The DIN rail terminal components are common in industrial process automation applications. Typically they are not used on yachts, though there are exceptions.

Oh got it, thanks Jammer

[s/v Jedi]

Originally Posted by BAD ORCA

ok, so many questions. I need a day or two to absorb this one :P

first obvious question though. what is that bronze/black doohickey between the battery and switch panel?

It’s red and black DIN rail mounted busbars for positive and negative.

The surge suppression is shown on the left, with the ground wires connected to a bonding plate.