On Grid

[Psychalist]

The notion of switching off MPPTs at 80% SOC intrigues me. Surely most installations leave the controller permanently connected in float charge? Is there some logic I'm not aware of or is it something to do with using a solar dump once your batteries are full?

[Tony Brooks]

3 minutes ago, Psychalist said:

The notion of switching off MPPTs at 80% SOC intrigues me. Surely most installations leave the controller permanently connected in float charge? Is there some logic I'm not aware of or is it something to do with using a solar dump once your batteries are full?

 

Me too. I could understand if it is a lithium bank, but not for lead acid. He might be dumping the excess to the immersion heater but if so he won't be turning the panels off. There might be some is laving the boat for a long period so the regular equalization charge does not dry out sealed batteries but I would deal with that by resetting the MPPT so it did not do an equalisation charge. In any case it would make more sense to charge lead acids to 100% rather than wasting solar output.

[Psychalist]

14 minutes ago, Tony Brooks said:

 

Me too. I could understand if it is a lithium bank, but not for lead acid. He might be dumping the excess to the immersion heater but if so he won't be turning the panels off. There might be some is laving the boat for a long period so the regular equalization charge does not dry out sealed batteries but I would deal with that by resetting the MPPT so it did not do an equalisation charge. In any case it would make more sense to charge lead acids to 100% rather than wasting solar output.

Thanks. I'm glad I'm not going mad. So mine sitting there at 13.9V taking 0.2A during the day is better than when I'm not on the boat and leaving them for a month or two disconnected where they drop down to about 12.48V?

 

(Actually that's going off topic of not on-grid. My bad.)

Edited August 27, 2022 by Psychalist
acknowledgement of straying off topic.

[Tony1]

36 minutes ago, Psychalist said:

The notion of switching off MPPTs at 80% SOC intrigues me. Surely most installations leave the controller permanently connected in float charge? Is there some logic I'm not aware of or is it something to do with using a solar dump once your batteries are full?

 

This unusual feature arose from my own lack of knowledge and common sense, to be honest.

(And just to clarify, mine are lithium batteries)

Initially I was using the victron solar charge controller to decide when charging stopped, and it would go into float, exactly as you say.  

I initially set my absorption voltage to 13.75 volts. My thinking was once the batteries went significantly above that, the MPPTs would go into float (which I set at 13.15v). 

My reasoning was that my absorption setting of 13.75 volts, when exceeded, would be the point when the batteries got to 80-85% full.

But I found that the end SoC that was reached at the point when the batteries went above 13.75v-  and the MPPTs went into float- was quite variable. 

For example, if it was very sunny and charging at say 60-90 amps, the battery voltage seemed to increase more, and faster. Sometimes with this setting, the batteries were only getting to 70-75% full before they went above 13.75v, and the MPPTs would go into into float at a time when I wanted more charging to be done.

 

Conversely, if the MPPTs were putting in say 15 amps, the voltage would stay quite low for much of the time, and a higher SoC would be reached by the time the battery voltage got up to 13.75v. 

Its a very first world problem, and most people just ignore it as a non-issue, but I was basically seized by a fit of lithium madness, so I shelled out on a victron BMV-712 to monitor the SoC, and with Nick's guidance I used the relay inside BMV712 to trigger the MPPTs to switch off when the monitor measured them at 80% full, using a tiny wire connected into the 'remote BMS' ports on the MPPTs.

When the SoC falls below a set value (currently 55%), the MPPTs are then switched back on by the BMV712, and charging starts again. 

This method helps to ensure that the batteries get cycled a bit on a daily basis, although I still have to give them a full cycle every couple of weeks. 

 

So that (somewhat overkill) method has become my method for managing the day to day charging, and I set the absorption voltage at a slightly higher value of 13.80volts. So the charge settings on the MPPT now act as a sort of backup method for stopping the charging, rather than being the front line method, as it is with normal people. 

A backup method to stop the charging is handy because the SoC values can drift in terms of accuracy over time, and one always has to look at both voltage and SoC together when trying to determine what the SoC is. 

This is what lithium batteries can do to you.

Edited August 27, 2022 by Tony1

[Psychalist]

Wow, thanks for the explanation. The world of lithium batteries is something I've yet to explore. It seems on the face of it to have an extra level of complexity. I'm still getting the hang of looking after LAs.

[Tony Brooks]

As lithiums seem not to sulphate or similar that is a sensible precaution to take, I think, and I suspected you had lithiums. For others reading this, stopping charging at 80% is NOT very sensible for lead acid batteries.

 

The MPPT (as will many mains chargers) is probably doing adaptive charging where (I think) it measures voltage rise over time to guess when to drop from abortion to float, so a high panel charge may well cause it to drop to float early and a low solar panel output late.

 

Out of interest why do you not divert the solar to a 12V immersion heater once battery charging is done. During this summer I suspect charging was completed by about11 am.

[Tony1]

 

10 minutes ago, Tony Brooks said:

 

Out of interest why do you not divert the solar to a 12V immersion heater once battery charging is done. During this summer I suspect charging was completed by about11 am.

 

You are spot on about the timings Tony- the batteries are usually back up to 80 or 85% by 11am on a really sunny day. 

I'm not clever enough to wire in a automatic switch to divert the solar to the immersion heater, but I did get Ed Shiers to connect my immersion heater up properly so that it runs from the inverter, and it has a manual on/off switch. 

If I know its going to be sunny all day I actually run the immersion early in the morning, which helps to run down the SoC a bit more, and I then have hot water earlier in the day. It only takes about 30 mins to get the water to about 40 degrees (hot enough to wash in), and that seems to use about 40Ah, but it doesn take too long for that to be replaced on a sunny day. 

Of course the solar hot water will be no more once we get halfway into September, but I'm enjoying it while its here, and in any case the stove will be a decent source after that. 

I'm hoping the fridge and other things will continue to run on solar (at least on most days), until mid October, and on some sunny days even into November. 

 

When I'm feeling brave and clever enough, I'm going to fit a knob that will allow me to control the amount of current going to the immersion heater. I think reducing its load from the flat-out 80 amps down to say 40 amps will reduce the stress it places on the batteries, and hopefully maintain their 10-year plus longevity. 

 

 

 

Edited August 27, 2022 by Tony1

[Tony1]

16 minutes ago, Psychalist said:

Wow, thanks for the explanation. The world of lithium batteries is something I've yet to explore. It seems on the face of it to have an extra level of complexity. I'm still getting the hang of looking after LAs.

 

I do fear I'm making it sound more complex than it needs to be, with my extra backup method for charging control. 

I dont think any other boaters use this sort of approach based on SoC, they just control their charging based on the voltage, and it works absolutely fine for them. 

And when I'm the only boater who is doing a particular thing, its usually a safe bet that the thing I'm doing is either unnecessary, or downright dangerous. 

I probably caught some form of lithium fever (its definitely a thing), but I left the system in place even after the fever wore off. 

 

[MtB]

7 hours ago, Psychalist said:

Wow, thanks for the explanation. The world of lithium batteries is something I've yet to explore. It seems on the face of it to have an extra level of complexity. I'm still getting the hang of looking after LAs.

 

 

Understatement of the decade, I would suggest!!

 

 

[Jen-in-Wellies]

17 hours ago, Tony1 said:

When I'm feeling brave and clever enough, I'm going to fit a knob that will allow me to control the amount of current going to the immersion heater. I think reducing its load from the flat-out 80 amps down to say 40 amps will reduce the stress it places on the batteries, and hopefully maintain their 10-year plus longevity. 

Alternatively, a simpler way would be to fit a lower power immersion element in the cauliflower. Available in 12V, or 240V. Pick as you see fit. 240V is easier to switch, either manually, or with a timer. 12V needs a substantial switch, or relay to break the high current without getting burnt out.

Some 12V ones here. They take some searching, but are available. Some dual voltage ones from Surecal.

Edited August 28, 2022 by Jen-in-Wellies

[Tony1]

15 minutes ago, Jen-in-Wellies said:

Alternatively, fit a lower power immersion element in the cauliflower. Available in 12V, or 240V. Pick as you see fit. 240V is easier to switch, either manually, or with a timer. 12V needs a substantial switch, or relay to break the high current without getting burnt out.

Some 12V ones here. They take some searching, but are available. Some dual voltage ones from Surecal.

 

Thanks very much Jen, that'll go on my 'to consider' list. 

I would rather take power directly from the panels, instead of having the batteries do the work of taking it in and then passing it out to the inverter.

At the moment it feels like a lot of Ah for the batteries to be processing on a daily basis, and it will probably be a factor in shortening the batteries' lifespan by at least some degree. 

 

I don't have the electrical nous to consider doing this job, so it will be one for a professional. 

 

The 12v elements are only 300 watt, but at full chat on a midsummer day the two MPPTS can put out almost 100 amps combined for short periods (at around 13.5 volts), which if I understand it right is about 1350 watts- more than four times the power rating of the 12volt element.

So I think I need some way of limiting the amount of power that is sent to the 12v immersion element. 

 

The other concern is a thermostat. At the moment it uses a 240v thermostat that switches off the 240v supply once the element gets up to the correct temp, so everything seems fairly safe. 

My worry would be this- even if I can get hold of a thermostat switch or system that will disconnect the thick 12v cable from the MPPTs once the water is hot, wouldn't a sudden disconnect  damage the MPPTs or panels? 

The MPPTs do have a tiny port for a control wire, and that will act to switch them off safely (which is what I currently use to switch them off when the batteries get to 80% SoC).

But I would need a way for the thermostat to send a 12v signal to the MPPTs, via the tiny control cable into the BMS port.

 

At the moment, with my very limited understanding of electrics and the potential issues to manage, it seems like its not worth getting into the cost and hassle of converting the immersion to 12v (and whatever I put in would have to get past a BSS inspector next year),  

But never say never.....

[LadyG]

On 25/08/2022 at 07:27, rusty69 said:

To clarify, I was referring to solar thermal, not using PV and immersion as a dump load. 

 

Though, as you say, with the relatively low cost of PV, maybe solar thermal is no longer worth considering. 

 

When I dabbled with solar thermal, I got all the bits off ebay for less than 100 quid. It was a moderate success. I just wish I had paid 5 quid for some antifreeze,otherwise, I might still have a working system. 

 

Doh. 

 

Eta. I seem to recall @blackrose has a Morco gas water heater 

 

[Jen-in-Wellies]

2 minutes ago, Tony1 said:

The 12v elements are only 300 watt, but at full chat on a midsummer day the two MPPTS can put out almost 100 amps combined for short periods (at around 13.5 volts), which if I understand it right is about 1350 watts- more than four times the power rating of the 12volt element.

So I think I need some way of limiting the amount of power that is sent to the 12v immersion element. 

Such a 12V element would be self limiting. At 12V in, the power they will consume is 300W, irespective of what the panels are capable of putting out.

Can't answer your other questions yet.

 

[rusty69]

What happens if you put a 12V immersion on the Mppt load terminal,does it  share the load with the battery charging at the same rate, a lower rate, or not at all ? 

 

I have 2 mppt controllers. Never really looked into what the load function does, but appears to have a timer function on it. 

[Tony1]

6 minutes ago, Jen-in-Wellies said:

Such a 12V element would be self limiting. At 12V in, the power they will consume is 300W, irespective of what the panels are capable of putting out.

 

 

Thanks again Jen, that's one thing less to have to think about. 

With the growth in solar usage and off grid tech in recent years, the gear to do this sort of job will be around by now.

 

To be honest I'm surprised there aren't more threads here about solar hot water. My guess is that its because there are so few CCing liveaboards (what is it- maybe 6,000 of the total 35,000 boats?)- and of those, not many have enough solar panels to make it a subject of direct interest.

 

Boaters who dont live aboard full time tend to cruise a lot more, so they dont need solar hot water, because they have the engine running most days anyway.

To a degree, the same issue deters them from investing a lot of money in lithium batteries, and you can see why. 

 

 

[George and Dragon]

On 27/08/2022 at 12:24, Tony1 said:

 

I think it will be a very long time before panels and MPPTs would pay for themselves, but it might be possible to give a very, very approximate idea of the cost saving over a year. 

My 1400watts of panels have recorded generating about 640kW of electricity in the almost 12 months since I finished the setup. 

I must add that I have a switch that disables the MPPTs when the battery SoC reaches its target value (currently set at 80% SoC), and that means during the summer they could have generated more, but on many days they were switched off some time in the early afternoon because by then the water was hot and the batteries were as full as I wanted them to be.  

So that 650 Kw could have become 900 Kw or more, if I hadnt had them switched off for many hours on many summer days. 

But in terms of the electricity I've actually used, its been 640kW, which on hookup would cost me about £130. 

The panels, mounts and MPPTs cost about £600 and I fitted them myself (with some expert guidance from our resident gurus), so at the current rate it would take well over 4-5 years to pay for themselves, if was on hookup. 

 

But as I'm CCing, and only cruising about once a week, the big saving for me has been in diesel. Without the panels I'd have had to run my engine purely for electricity on 6 days of each week, and I'd be using about a litre of diesel each day to do that (and on some days more). 

If I was to price diesel at say £1.50 per litre since the start of April, the panels have saved me using diesel on about 100 days, so that's saved £150 so far this year.  

Over a full year I would imagine the panels will save me about £230, so they'll have paid for themselves in another two years. 

 

(That's not including the cost of an engine service every 200 hours, which could be from £30 if DIY up to £150 if done by a professional. Plus any costs/reduced boat value that might arise from the extra wear and tear on the engine itself from the increased running hours) 

 

Good points. How much does the PV system cost to add to the existing electrical system? I see Bimble's cheapest offering (350W) is £450 + VAT. Even at 64p/kWh (which is where commercial rates were heading before Ms Truss said she'd help businesses out)  that's going to take some time to pay back.

You're in a marina on a shore line so Tony's points about the cost of diesel and engine servicing don't apply.

 

On the basis that it's a quarter of the size of Tony's system and needing to drag every last milliamp out for LAs (do you have Lithiums?) you might get 225 kWh. That's going to take 45 months to break even if you do the install yourself. 

BBC News is now reporting that businesses will pay a wholesale rate capped at 21p/kWh. I think we can assume that excludes VAT @ 20% so about 25p. That takes the payback time out to about 15 years... it's your money - spend it as you choose.

[rusty69]

I think our 1000 w of solar and ancillaries cost around £1000. They were paid off in savings over 3 years. 

 

The lithium batteries will be paid off via savings on approx the same timescale. 

[IanD]

The payback time obviously depends on electricity prices, and how these will change in the future is a big unknown -- the current proposed price cap ("£2500 per year") for domestic users is 34p/kWh, and it's unlikely that this will drop for quite some time.

 

Looking at Bimble, the cheapest panel per kW is £195 inc. VAT for a 455W panel, which according to calculators like the Victron one would be expected to yield about 500kWh/year (worth £170) in the Midlands *assuming* that you can actually make use of all that energy -- which is probably true in a house (especially with feed-in tariffs) but may not be on a boat -- Nick's 1400W of panels would have given about 1500kWh/year if he could have used it all, but he only actually got 650kWh/year. For smaller panels where all the energy can be used this wouldn't be the case.

 

So just looking at the raw panel cost, if you can use all the energy (small panels, or feed-in, or enough onboard energy storage) the payback period would be 1.2 years today, or 2.7 years for Nick. Adding on the other costs like MPPT controller and cables would push this up to maybe 2 years with all the power used or maybe 4 years for Nick (added cost is proportionally less for bigger installations) -- which agrees with his figures 🙂

 

Either way, solar panels are a great idea for boats, not just for saving money but largely avoiding the need to plug-in or run the engine (at least in summer, assuming enough panels are fitted to meet energy needs).

[ditchcrawler]

On 28/08/2022 at 09:24, Tony1 said:

 

Thanks again Jen, that's one thing less to have to think about. 

With the growth in solar usage and off grid tech in recent years, the gear to do this sort of job will be around by now.

 

To be honest I'm surprised there aren't more threads here about solar hot water. My guess is that its because there are so few CCing liveaboards (what is it- maybe 6,000 of the total 35,000 boats?)- and of those, not many have enough solar panels to make it a subject of direct interest.

 

Boaters who dont live aboard full time tend to cruise a lot more, so they dont need solar hot water, because they have the engine running most days anyway.

To a degree, the same issue deters them from investing a lot of money in lithium batteries, and you can see why. 

 

 

It often comes up on Facebook using excess power to the immersion heater