Merged LiFeYPO4 1000Ah Winston prismatic cells and all electric galley...

[goboatingnow]

This is interesting as I’m designing a BMS

I think it’s useful to review what’s needed as a minimum

Cell balancing can be left out of this as it’s a “nice to have “

The two key protections are HVC and LVC , the former needs to disconnect any charge source and latter needs to disconnect any load

That’s the key protections to ensure safe Lithium operation.

For charge source disconnects the only “ problem “ is the alternator disconnect , either an alternative load is substituted or the field needs to be disconnected

For LVC , there are several issues

Firstly you need to ensure further current draw is stopped as you have no idea how long the LVC event will continue for.

This means that any device remains connected to the battery ( BMS , devices still connected electrically but nominally “ off”) must be carefully evaluated or they will discharge the battery into the danger /damage zone.

The second issue is some means to retain minimum electrical feed and to ensure any control electronics retains the ability to reconnect charge sources is needed.

Hence you need to be careful where devices are “ software “ commanded off. There can be considerable residual current still flowing . Hard disconnects are better in my view.

In my case the Li LVC event actually kills the bms itself. A coin cell retains background microprocessor activity to facilitate charge source connection. I also think any system should be capable of manually being overridden and the Li battery connected to a charge source or reconnected to loads even if at LVC.

I agree various alarms need to be instituted to warn of impending LVC.

[Philtao]

Originally Posted by goboatingnow

Cell balancing can be left out of this as it’s a “nice to have “

Having Winston cells, that is what I thought initially. But some (cheaper) cells with large variance in internal resistance and other parameters will need it.

Originally Posted by goboatingnow

For charge source disconnects the only “ problem “ is the alternator disconnect , either an alternative load is substituted or the field needs to be disconnected

Yes. Field disconnect is the best, but not always possible. For alternate load I have two options:

• have the starter battery paralleled with lithium when engine operates (commanded by contact key)
• have the BMS high voltage disconnect command two relays sequentially with time delay between them. first one to parallel the starter battery, the second one to disconnect lithium.

There is also the option to connect the alternator to the starter battery and charge lithium with DC/DC charger - no good if alternator is main charge source and current is high. Maybe more options?

Originally Posted by goboatingnow

Firstly you need to ensure further current draw is stopped as you have no idea how long the LVC event will continue for.

Yes, this is a real issue that not many people think of (the BMS, a battery monitor - like Victron BMV - or even a few led - will eventually drain the battery). Most important in current (covid) time when people cannot go back to their boat for a long time.
This issue is of concern mostly when you are not on board for an extended period. I see two situations:

• boat is on the hard: power is not needed and it is better to disconnect the battery entirely (after it has been charged to 50% SOC) - that should be fine for a year or two.
• boat is in the water and power must be available for safety reasons (bilge pumps, anchor light, AIS, remote Cloud monitoring...): keep a solar panel operational and use the BMS to control charge between lets say 40 and 60% SOC - and a full charge every 30-60 days to re calibrate SOC (this is one of the purpose of the charge cycle management function I have in my BMS). Could also use cell voltage as current draw is minimum (turn solar charge on when cell voltage dips below 3.2v and turn it off when voltage reaches 3.35 or 3.4v?)

The approach with a small battery to keep the BMS alive still require to disconnect all other small consumers (battery monitor, leds, voltage sense wires, SSR that have a leakage current...)

Originally Posted by goboatingnow

I also think any system should be capable of manually being overridden and the Li battery connected to a charge source or reconnected to loads even if at LVC.

Absolutely - all electronic system will fail one day! on my boat I wire all relay outputs from the BMS through 3 way switches (ON - OFF - BMS)... but I know someone with similar setup who inadvertently left the charge bus switch in the OFF position and left the boat with the fridges on... a week later the battery was at 7v (or <2v per cell)! I am thinking of installing the kind of switch with a red cap that sticks out when not in the BMS position and returns to the BMS position by just pushing on the cap.
You also need to think in what position you want the relay outputs when the system fails (that is a philosophical discussion with no right or wrong). On my side I leave the choice to the user, so it is not my responsibility if they lose power at the wrong time and the boat ends up on a rock.

[CatNewBee]

Over-engineering and over-nanny-ing causes more harm then good. Most people kill their batteries by good intentions to increase longevity in storage with discharged cells.

[goboatingnow]

Originally Posted by CatNewBee

Over-engineering and over-nanny-ing causes more harm then good. Most people kill their batteries by good intentions to increase longevity in storage with discharged cells.

Could you give an example of either case ( over engineering or over nannying )

Otherwise it’s just a unsupported viewpoint

[goboatingnow]

Cell balancing is never “ needed “ ie cells are not damaged by being out of balance , it’s just that you can’t extract the optimum energy from a battery where some cells are out of balance.

But it’s not damaging anything to be out of balance per se.

My experience is that besides an initial top balance to establish maximum energy storage. Fractional C discharge , fractional C charging and conservative stop voltages mean that cells don’t need cell balancing and if they do it’s about once a year.

[newhaul]

Originally Posted by CatNewBee

Over-engineering and over-nanny-ing causes more harm then good. Most people kill their batteries by good intentions to increase longevity in storage with discharged cells.

Best thing I found for me is leave refer on lower setting and charge controllers to 13.2.
That's when I'm not on the boat for extended periods.

[CatNewBee]

Originally Posted by goboatingnow

Could you give an example of either case ( over engineering or over nannying )

Otherwise it’s just a unsupported viewpoint

Sure, like the one guy, that discharged his battery to 50% and turned off solar and all other chargers, but forgot, that the relays he used draw power to hold (no bi-stable), also the battery monitor and the BMS draw power. He was thinking the BMS would protect his pack, and it did disconnect, but the cells also self discharge further, and there is nothing that can prevent this except a charger. Covid came, he could not get to the boat in time, battery was toast (cells swollen, deep discharged) .

If he had left the battery fully charged, he may have been lucky to revive it, and if he had left solar on, the battery would be still alive, even if charged full daily, there would be less damage.

[goboatingnow]

Originally Posted by CatNewBee

Sure, like the one guy, that discharged his battery to 50% and turned off solar and all other chargers, but forgot, that the relays he used draw power to hold (no bi-stable), also the battery monitor and the BMS draw power. He was thinking the BMS would protect his pack, and it did disconnect, but the cells also self discharge further, and there is nothing that can prevent this except a charger. Covid came, he could not get to the boat in time, battery was toast (cells swollen, deep discharged) .

If he had left the battery fully charged, he may have been lucky to revive it, and if he had left solar on, the battery would be still alive, even if charged full daily, there would be less damage.

That’s an example of under-engineering, ie the system was not engineered properly to have zero draw. It’s entirely practical to engineer a system that at LVC draws zero current from the lithium pack.

[goboatingnow]

Originally Posted by newhaul

Best thing I found for me is leave refer on lower setting and charge controllers to 13.2.
That's when I'm not on the boat for extended periods.

A fault in anything will destroy your Li pack.

[newhaul]

Originally Posted by goboatingnow

A fault in anything will destroy your Li pack.

In my experience this works quite well for me not to mention I do have backups but that's not anything I need to share with anyone that is not genuinely interested beside a point to pick at. My boat my setup. Has been working just fine for several years.

[CatNewBee]

Originally Posted by goboatingnow

That’s an example of under-engineering, ie the system was not engineered properly to have zero draw. It’s entirely practical to engineer a system that at LVC draws zero current from the lithium pack.

You need to charge, to cover self discharge, zero draw is not enough.

[Philtao]

Originally Posted by newhaul

Best thing I found for me is leave refer on lower setting and charge controllers to 13.2.
That's when I'm not on the boat for extended periods.

Yes, I used to do that and worked well for me. I recommend that approach when no other way can do it.

[CaptainRivet]

Originally Posted by Philtao

Both TAO and REC BMS can remotely command external equipment either by wire or CAN bus (in addition to relays) - so no current limit.
I would add Electrodacus to the BMS comparison chart, but I have not found specifications in a clear format that correspond to the criteria in the chart (see previous post). If someone can post a link or send that information...

ElectroDacus and then click on user manual
Several parameters can be chosen eg voltage and current resolution depend on the shunt you are using, a 100A 100ohm shunt or a 1000A 50 ohm shunt determines what’s the max resolution and the Max current you can use.
Same with the switches or relays, your choice….
Not as flexible as yours or the rec bms but if you are using Victron equipment it’s great. Switch off the MPPT solar charger under load works only with Victron ones because it’s the only one that supports that…

[CatNewBee]

After 2 years, finally installed th DS18B20 sensors outside, in the cockpit fridge, galley fridge and the freezer (was a little hesitant to drill 3.5mm holes through the isolation to get the sensor cable trough, but worked well) and tweaked the Victron Firmware further...

Now it shows the temperatures instead of an empty box when there is no AC input, otherwise shows the AC data.

[newhaul]

Well I had a failure of my pwm controller so I bit the proverbial bullet and purchased a trace MPPT controller. Darn what a difference even this time of year I start producing power at 0700 about 45 min after sunrise. And am fully recharged by 10am with a sunny day . But I only use about 15ah overnight this time of year . Refrigerator doesn't run to much with the temps in the low 40°'s F at night as I shut my furnace off when I'm in bed . Don't need heat then . Did the calculations and I have a 20% increase in power out of controller vers the power in . And yes still just 200 watts at 12v .