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.

[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.

[SaylorMade]

Originally Posted by BAD ORCA

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.

Why would you set this on the charge sources?
The BMS will monitor each cell, and shut off charging when the first cell reach the max voltage you have set in the BMS parameters.

[s/v Jedi]

Originally Posted by SaylorMade

Why would you set this on the charge sources?
The BMS will monitor each cell, and shut off charging when the first cell reach the max voltage you have set in the BMS parameters.

Because it is not the task of the BMS to control the charging process, which is done by the charger.

The BMS must monitor cells and send out warnings if limits are near and protect the battery with HVC or LVC events when limits are crossed.

If a warning is issued and used to shutdown a charge source, something is seriously wrong.

[fxykty]

Originally Posted by s/v Jedi

Because it is not the task of the BMS to control the charging process, which is done by the charger.



The BMS must monitor cells and send out warnings if limits are near and protect the battery with HVC or LVC events when limits are crossed.



If a warning is issued and used to shutdown a charge source, something is seriously wrong.

While not a core function of a BMS, having the BMS manage all the different charge sources present on a boat is certainly a nice set of functionality. Each individual charger does it’s thing and should be set up correctly, so that if the BMS goes offline each charger behaves correctly. But having the BMS control when charging happens and what voltage and current values to use is very handy. Any charger that understands CanBUS (such as Victron equipment via a Cerbo GX) can be controlled by a BMS that uses CanBUS as a communication method.

As an example, if it’s very cold or very hot, you can still charge, but at lower current values than in nominal temperatures. Do many (any?) chargers have temperature settings for charge parameters?

[tanglewood]

Originally Posted by s/v Jedi

Because it is not the task of the BMS to control the charging process, which is done by the charger.

The BMS must monitor cells and send out warnings if limits are near and protect the battery with HVC or LVC events when limits are crossed.

If a warning is issued and used to shutdown a charge source, something is seriously wrong.

The exception would be if you are using DVCC or similar charger control.

[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.

[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.

[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?

[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.

[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.

[s/v Jedi]

Even if there are no transient surges during a HVC (there will always be some when this is the only battery going offline) then you still end up loosing power to all electronics which can be trouble when you’re surrounded by reefs, sandbanks etc.

So for electronics, you create a hardened power supply, which is the diagram I show. It is for 24V house and 12V start batteries but you can simply change that to 12V house in which case you use 12-12 dc chargers for both house and start bank.

The trick is that you put surge suppression on both house and start banks, then create a third battery bank for electronics (can be a small, cheap motorcycle battery) around an AGM battery, which is always kept at float charge by a dc-dc charger that is isolated (i.e. DC negative and positive are both galvanically isolated from house and start batteries). This electronics bank also has surge suppression.
When there is lightning in the air, you simply turn off the charger and disconnect using the double pole breaker and continue on battery.

When I made this diagram I still had the VHF on this hardened power supply but I decided the risk is too high with many antennas defeating isolation via the coax shield and the antenna being a target for lightning. I now recommend during lightning storms, to turn the VHF off, disconnect antenna and use a handheld VHF instead.

Attachment shows the panel that has these breakers and on/off switches for the dc-dc chargers in the lower left corner.