Automatic low voltage disconnect for house batteries

[dbostrom]

It's clear to me already that JONF is a great place to think aloud and benefit from other people seeing gaps in reasoning, etc. Please bear with me as I do a little thinking aloud here regarding battery management and charter operations. As well, somebody might go down the same path so why not share?

We're purchasing a boat that is an established and successful fleet member of a very excellent charter outfit, ours to use as much or as little as we want subject to our own tolerance for lower revenue, and ours to maintain in face of the various challenges thrown at it by customers (customers being required to have certification of training in cruising operations and a local practicum test).

The 39i we're adopting has a nice almost-fresh set of 3 group 31 NorthStar AGM batteries, one of many sweet features of our choice of boat.

Up until this point I've been exclusively a charter customer when it comes to operating a cruising boat. Most of us here are probably familiar with common quirks of charter boats, in particular knackered batteries. A frequent complaint I've heard from charter owner-operators is failure of customers to understand battery management and the dire impact on operations and economics of running batteries beyond their design discharge level. There's a bit of a vicious circle here; owners become demoralized by dealing with the aftermath of the 1 out of 10 customers who can't manage batteries while the other 9 of 10 competent customers are stuck trying to use abused and shaky batteries in the normal way, hastening the electrochemical death spiral.

I think of the happy batteries in our boat and then cringe when I imagine them facing reality.

It's puzzled me that none of the charters I've been on have included a low voltage disconnect (LVD) for house batteries. Having been in the business of operating remote telecomms sites running on solar power, to me a battery system without an LVD is just as incomplete as a battery system without fuses or circuit breakers. The expense of excessive discharge is brutal, while the expense of adding an LVD is trivial. Modern LVDs manage themselves, with hysteresis, instantaneous load dead bands and all the rest accounted for.

Battery LVD needs to account for its context:

    Clearly in the case of a boat facing possible emergency situations and mandatory minimum function we don't want to shed all     loads when batteries go slack. At minimum, VHF and GPS input to DSC should be available until batteries simply are not delivering     minimum voltage for these devices. Similarly for navigation lights, bilge pumps. What else?

    LVD should not be sensitive to transient loads leading to false trip.

    LVD should allow an override function that is easily available, easily understood by operators not intimately acquainted with the     vessel. 

    LVD should not be prone to failure or an inherent reliability problem itself. Means for bypass beyond built-in function would be     good.

    LVD should be programmable for disconnect voltage, offering a range of tolerance for discharge.



This Blue Sea Systems device seems to tick most boxes, assuming it's inserted into the electrical system at the correct juncture:

    assets.bluesea.com/files/resources/instructions/980016770.pdf

Unlike many LVD devices the Blue Seas unit utilizes a contactor instead of semiconductor MOSFETs, meaning there's essentially no voltage drop across it. 

At ~820W sustaind capacity and ~1,500W for 5 minutes peak load the Blue Seas device is adequate for powering domestic systems on a typical mid-size cruiser, with the conspicuous exception of handling large inverters*. At less than ~$100 US, it's not hard to imagine deploying more than one of these in a boat. 

A 200A LVD would handle a substantial inverter along with other loads. Victron makes a suitable device, the BP-200-i:

    www.victronenergy.com/upload/documents/Manual-Battery-Protect-EN-NL-DE-FR.pdf

The Victron device is ~$175 US. As it uses MOSFETs for conduction it may not be quite as robust to lightning and will necessarily impose a small voltage drop. It's also not clear from the documentation whether it has an override function. 

Has anybody else on board here installed an LVD for their vessel? If so, what model/type and what's your experience with it?

_{*Better inverters won't attempt to operate when battery voltage is low.}

[rxc]

I don't have an LVD, but I do have three battery combiners that close when they sense that a charging source is operating. I use one combiner to parallel the house batteries and the main engine starting battery, and another to parallel the house batteries with the windlass/bow thruster battery. The third one is used to run a fan in the battery compartment - it senses when the voltage is high enough to charge the batteries, and runs a tiny fan to exhaust the battery compartment. The main combiner is a Blue Seas model, the one for the windlass battery is made by Cyrix, and the fan "combiner" is a very inexpensive one bought from a UK camping store for use in caravans.

It makes sense to disconnect loads when the voltage is too low. I think the trick is to figure out what loads to shed, and how to avoid losing those loads without knowing that they have been shed. Your list of requirements is good - I would only add an alarm to tell you when it has activated. A conspicuous light might be sufficient, but an audible alarm would be best. Hooking one of these to an inverter feed sounds like a good idea, but I have noticed that the battery voltage drops down into the high 11s range when I run my microwave (the largest load), so you would need something that can be set pretty low if you want to protect from things like this.


I don't use an LVD because I am a bit obsessed with knowing my battery condition, so I check it at least once/day, and am very sensitive to the operation of charging supplies(solar and wind). I also check battery water levels quite often, and electrolyte specific gravity, as well.

[Trevor]

Hi Guys,

I could imagine this could work for a charter fleet but I am a bit like rxc, a little obsessive when it comes to the state of my batteries. It is not something that I would ever be caught 'unawares' on because I take a great interest in the state of the batteries and what charge is coming from the solar panels and the alternator.

Regards,

[dbostrom]

Yes, definitely something I'm thinking of as a means of dealing with chartering quirks. I completely understand and sympathize with obsessing over batteries; many of them die long before their time because of a lack of attention to some fairly simple details, after all. I've developed heavy PDBSD (Premature Dead Battery Stress Disorder) from handling the logistics of replacing GC2 batteries in various extremely inconvenient locations featuring mud, mosquitoes and no road access so I'm fully on board with the emotions of this. 

I'm trying to find an accurate wiring diagram for this boat because whether or not it's possible to shed noncritical loads without making a proper mess of things is down to details of how things pass by the breaker panel. Failing having a diagram I'll just noodle it out. 

The Blue Seas unit linked above features audible and visual alarms for imminent disconnect and a handy 10 minute override feature (good if somebody hasn't finished drying their hair). Fully adjustable too; should allow a configuration where a battery sadist will become so sick of pushing the override button they'll give up and start the engine before actual battery damage happens. 

[rene460]

Returning to a not so old thread that has slipped back a few pages, I would hazard a guess that the load which brings us all to grief is the fridge.  I have just collected some data after installing a data logger on my fridge, and over a 5 hour period it averaged 4.0 amps when running and ran for very close to 50% of the time.  It is equivalent to 53 Amp.hours over a 24 hour period, and did not include adding warm items to the fridge.  The temperature in the cupboard was a consistent 26-28 deg C.  (No, it was not a good sleeping night!).  Short term loads generally do not come close.  The other major drain on batteries is overnight sailing with Nav lights, instruments, radio and perhaps an autopilot all working, though this is not normally available to charters.

A low voltage disconnect on the fridge would protect the batteries in most circumstances, but at the risk of spoiling food, so the indication of disconnect would be very important.  I would suggest a red light of the type used for cabins when night sailing, so as not to be confused with the many power on LED's, would be suitable.  The fridge will generally stay cold for a few hours, so people could be allowed to sleep, or an audible alarm with a silence button would give the option.

I suspect rather than allowing overriding of the disconnect, it is enough that the engine must be started to restore voltage, as this will focus attention on the issue.  Possibly a little draconian, and may not sit well with the charters, but they do not have to pay to replace the prematurely failed battery.  Keep it simple.

It would need to be supported by more information on battery management in the briefing, so crews could understand the issue and action required.  My experience is that they tell you very little on this, and some operators do not keep a good check on battery condition.  One operator assured me the engine only needed to run for around 2 hours per day at idle for battery charging.  He was very surprised when I found the voltage dropped to under 10 V within 30 minutes after 3 hours motoring, and that with the fridge off!  I had brought my own voltmeter as, like others above, I try and take care of the batteries when on my own boat, and also when chartering.  So a little more attention to batteries may be needed on both sides.

rene460

[dbostrom]

Good thoughts Rene, and your experience with charter bases pretty much matches mine. It's odd how many charter firms are also very sad about their batteries lasting such a short time, passively and without doing much to help themselves or their customers.

For those interested, do take a look at the Blue Seas disconnect linked from the first post. These are quite inexpensive (compared to even a single decent group 31 battery), so much so that I could see deploying more than one, with different dropout settings for different loads. Alarm, override etc. are all there.  They seem simple enough to be charter-friendly.

[pbunning]

As an aside (I do not charter my boat), main power usage on my boat is the fridge - I started thinking about installing a low voltage cut out solely for that purpose. I then had look at Danfoss instructions - it seems that Danfoss do provide low voltage cut out - defaults 10.4V cut out, 11.7V cut in. Settings can be changed by swopping out a resistor. Maximum cut out (for example) 11.3V with 12.5 cut in. Voltage (measured at compressor) will be lower than at battery when compressor is running.

Interested whether anyone has tried this and whether such an arrangement really does provide some protection for the batteries.

Attachments:

Danfoss_compressor.pdf (144.85 KB)

[dbostrom]

That's a great tip about the Danfoss. Goes to show the value of reading documentation.

[vasko]

I use Victron BP-60i 12v Low Voltage Disconnect with combination with zoomer and NASA battery monitor and I'm quite happy...

[keithwatson]

I have a 42DS and had a new ice box to the fridge fitted in March. Sailing in September the fridge has not worked all week until I connected to shore power for a day. I suspect this is due to the battery cut out although there has been little wind all week and therefore motoring and charging batteries.

I have a victron system. How do I find out the cut out and cut in levels for the fridge and could the new ice box have changed these settings in any way?

Any my help appreciated.
SY Jay Jay
Currently in Elba

[vasko]

Nov 28, 2014 22:08:11 GMT dbostrom said:

It's clear to me already that JONF is a great place to think aloud and benefit from other people seeing gaps in reasoning, etc. Please bear with me as I do a little thinking aloud here regarding battery management and charter operations. As well, somebody might go down the same path so why not share?

We're purchasing a boat that is an established and successful fleet member of a very excellent charter outfit, ours to use as much or as little as we want subject to our own tolerance for lower revenue, and ours to maintain in face of the various challenges thrown at it by customers (customers being required to have certification of training in cruising operations and a local practicum test).

The 39i we're adopting has a nice almost-fresh set of 3 group 31 NorthStar AGM batteries, one of many sweet features of our choice of boat.

Up until this point I've been exclusively a charter customer when it comes to operating a cruising boat. Most of us here are probably familiar with common quirks of charter boats, in particular knackered batteries. A frequent complaint I've heard from charter owner-operators is failure of customers to understand battery management and the dire impact on operations and economics of running batteries beyond their design discharge level. There's a bit of a vicious circle here; owners become demoralized by dealing with the aftermath of the 1 out of 10 customers who can't manage batteries while the other 9 of 10 competent customers are stuck trying to use abused and shaky batteries in the normal way, hastening the electrochemical death spiral.

I think of the happy batteries in our boat and then cringe when I imagine them facing reality.

It's puzzled me that none of the charters I've been on have included a low voltage disconnect (LVD) for house batteries. Having been in the business of operating remote telecomms sites running on solar power, to me a battery system without an LVD is just as incomplete as a battery system without fuses or circuit breakers. The expense of excessive discharge is brutal, while the expense of adding an LVD is trivial. Modern LVDs manage themselves, with hysteresis, instantaneous load dead bands and all the rest accounted for.

Battery LVD needs to account for its context:

    Clearly in the case of a boat facing possible emergency situations and mandatory minimum function we don't want to shed all     loads when batteries go slack. At minimum, VHF and GPS input to DSC should be available until batteries simply are not delivering     minimum voltage for these devices. Similarly for navigation lights, bilge pumps. What else?

    LVD should not be sensitive to transient loads leading to false trip.

    LVD should allow an override function that is easily available, easily understood by operators not intimately acquainted with the     vessel. 

    LVD should not be prone to failure or an inherent reliability problem itself. Means for bypass beyond built-in function would be     good.

    LVD should be programmable for disconnect voltage, offering a range of tolerance for discharge.



This Blue Sea Systems device seems to tick most boxes, assuming it's inserted into the electrical system at the correct juncture:

    assets.bluesea.com/files/resources/instructions/980016770.pdf

Unlike many LVD devices the Blue Seas unit utilizes a contactor instead of semiconductor MOSFETs, meaning there's essentially no voltage drop across it. 

At ~820W sustaind capacity and ~1,500W for 5 minutes peak load the Blue Seas device is adequate for powering domestic systems on a typical mid-size cruiser, with the conspicuous exception of handling large inverters*. At less than ~$100 US, it's not hard to imagine deploying more than one of these in a boat. 

A 200A LVD would handle a substantial inverter along with other loads. Victron makes a suitable device, the BP-200-i:

    www.victronenergy.com/upload/documents/Manual-Battery-Protect-EN-NL-DE-FR.pdf

The Victron device is ~$175 US. As it uses MOSFETs for conduction it may not be quite as robust to lightning and will necessarily impose a small voltage drop. It's also not clear from the documentation whether it has an override function. 

Has anybody else on board here installed an LVD for their vessel? If so, what model/type and what's your experience with it?

_{*Better inverters won't attempt to operate when battery voltage is low.}

The latest boat-vitals has a LVD connector to which a external relay switch can be attached - the LVD logic is quite sophisticated and handles all intermittent loads etc