Simulating a BMS

[zboss]

With the new ABYC recommendation of the BMS signaling when current is cut off I was wondering if there were a way to simulate this behavior for "drop-in" lithiums that do not have canbus and support custom high and low cutoffs.

Solar charge controllers, alternator controllers, shore power chargers, etc... all individually have voltage settings but few actually send out a signal indicating their state.

I was thinking I could use my cerbogx as sort of an external BMS where we could have it cut off charging below the BMS high and low cutoffs and then set a light to off (or on or maybe make a sound or something) indicating the state.

Personally, I think that recommendation was super rotten of ABYC. Basically saying that if you can''t afford $8000 for 400 Ah of lithium batteries then you shouldn't have them. They didn't even offer up an alternative solution for drop-ins.

[mimijane]

I have not read the new technical brief, just heard online of this requirement. It's odd as there was never a requirement for other battery types with user installed low voltage disconnects. Is it actually written as a "should" vs a "shall" ?

[Philtao]

Originally Posted by zboss

Personally, I think that recommendation was super rotten of ABYC. Basically saying that if you can''t afford $8000 for 400 Ah of lithium batteries then you shouldn't have them. They didn't even offer up an alternative solution for drop-ins.

I think this recommendation makes sense for us sailors. Imagine sailing by a stormy night as you approach the coast... when suddenly you loose power (no VHF, no auto pilot, no chart plotter... and no possibility to leave the helm as the auto pilot is out...). Not a situation I want to experience.

An advanced warning of at least 10 minutes would give me time to either fix the root cause or take actions to avoid endangering the boat and the crew.

It does not have to cost $8000 for 400 Ah. For 400 Ah in 12 V, all you need is:

• $1000-2000 for quality cells (can find for less, but quality uncertain?)
• $500-1000 for a good BMS with multiple thresholds and adjustable advanced visual and audible warning at the chart table
• $200-400 for a couple of contactors

That is a total between $2000 and $3500 for a safe energy storage system that should last 10 years or more without any maintenance or worries (between $17 and $29 per month). I installed mine 5 years ago with Winston cells, never had to do any maintenance and no noticeable loss of capacity.

[goboatingnow]

While I don’t disagree with a need for disconnect warnings , the point is these are events that should never happen in normal operation.

So it’s somewhat moot if you get a warning and the headroom between charged and HVC can be small and HVC fault conditions can arrive suddenly.

So it’s not like in the middle of a storm a warning will be a lot of use.

[wholybee]

I just wish such a simple and inexpensive to implement feature was available on other than the highest priced BMSs. Not all of us want or need contactors, CANBUS etc. But we all need an alarm if we are to be ABYC compliant.

The overkill BMS has an unused output pin, which in theory could drive a relay or alarm with a simple firmware change.

I am contemplating an arduino that would connect to a BMS to monitor it, and trigger an alarm or relays for certain events. Probably only $20 to implement. Overkill has sample code on their website. The arduino could on power up read the BMS configuration, then monitor voltages and currents, compare them the the configured limits, and alarm as they get close.

[LeighC]

I this where a manual accessible override switch for the BMS cut off solenoid (some are built in) would be a good idea to get you out of danger & have its procedure rehearsed for all on board.
Leigh

[tanglewood]

It's worth reiterating that TE-13 is a technical note, not a standard, and that all the language is "should", not "shall". But it is likely a pre-cursor to a standard, and I'm sure there will be an interesting debate over which "shoulds" turn in to "shalls".


I personally think it would be a great addition for internal BMSes to provide a couple of simple warning signals in accordance with the technical spec. I don't think it would be difficult, and would close the gap quite a bit between internal and external BMS solutions. Perhaps TE-13 will further motivate such changes.


Again, my own personal opinion, I think these warnings are super important. I definitely want to know as far in advance as possible if things are heading towards a disconnect so I have an opportunity to correct the problem before the boat goes dark and silent.

[CaptnBry]

I designed and installed my own LiFePO4 system


The "cut-out" is a BMS feature to protect from over charging by cell voltage,bank voltage, or over current during discharge. These are programmable in most BMSs.


Measuring between the B- and Ship's negative ground will indicate near zero in normal operation. If the BMS 'disconnects', there will be a voltage difference of about 0.5 volts, which can be used to trigger an alarm to meet the "should" of ABYC


In practicality, if you change to LiFePO4 from SLA, you should be well versed in the system, it's needs, and all your charging/discharging systems technicalities. Rigging a 0.5v alarm is well within those skills. Personally, I don't recommend Lithium batteries for boaters who do not do their own electrical work.