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Battery Capacity for Solar


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If I were to install 1050 watts of solar panels what would be the ideal size for a battery bank? Given that I'm intending that the batteries will be Lithium, what's the minimum sensible size of the bank. As you will no doubt guess, it's the initial cost of lithium batteries that prompts this second question!

 

Is there a Victron MPPT controller that will handle this size of bank and, if not can, it be split into two? I assume that it would have to be wired in parallel. Early planning stages for a brand new installation.

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The question should be, how many amp.hours of charge do you use per day? Battery size can then be related to how deeply discharged you are happy to take them before recharging versus cost. Solar capacity related to how much of the year you want solar to make a significant contribution to your charging?

Do you have the boat yet? Is this a solar installation on an existing boat? Are you in a position to do a power audit to work out your typical daily charge consumption?

Not an answer to the question you asked, but perhaps the questions it would be better to ask...

 

Jen

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4 hours ago, Up-Side-Down said:

If I were to install 1050 watts of solar panels what would be the ideal size for a battery bank? Given that I'm intending that the batteries will be Lithium, what's the minimum sensible size of the bank. As you will no doubt guess, it's the initial cost of lithium batteries that prompts this second question!

 

Is there a Victron MPPT controller that will handle this size of bank and, if not can, it be split into two? I assume that it would have to be wired in parallel. Early planning stages for a brand new installation.

Firstly, I would go for one MPPT. We have 500W in 4 panels. 2 panels were on the boat with a 30A mppt and I added 2 panels with a further 15A MPPT. The MPPTs fought against each other so to stop that I wired them all into the 30A one. Works great and I get just about 30A on the best sunny days.

 As above, your bank size should be based on your usage but I would think a minimum of 320Ahrs if you use over 100Ahrs per day. I have 480Ahrs which works fine with the solar.

How much are you paying for your lithiums? If you buy second hand EV batteries then you should get 480Ahrs for £1000 which is half the price of new.

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10 hours ago, Dr Bob said:

Firstly, I would go for one MPPT. We have 500W in 4 panels. 2 panels were on the boat with a 30A mppt and I added 2 panels with a further 15A MPPT. The MPPTs fought against each other so to stop that I wired them all into the 30A one. Works great and I get just about 30A on the best sunny days.

 As above, your bank size should be based on your usage but I would think a minimum of 320Ahrs if you use over 100Ahrs per day. I have 480Ahrs which works fine with the solar.

How much are you paying for your lithiums? If you buy second hand EV batteries then you should get 480Ahrs for £1000 which is half the price of new.

You are on the right numbers Bob, I have 450 ah @  24 volts and its working well on slightly more solar than the OP [summer use] in winter it takes all of my 4.6 kw of solar 600 watt wind turbine and occasional whispergen to survive

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17 hours ago, Jen-in-Wellies said:

The question should be, how many amp.hours of charge do you use per day? Battery size can then be related to how deeply discharged you are happy to take them before recharging versus cost. Solar capacity related to how much of the year you want solar to make a significant contribution to your charging?

Do you have the boat yet? Is this a solar installation on an existing boat? Are you in a position to do a power audit to work out your typical daily charge consumption?

Not an answer to the question you asked, but perhaps the questions it would be better to ask...

 

Jen

Thanks Jen. I totally take your point that I appear to be coming at this from the wrong direction. What I'm trying to do is get an understanding of any relationship between potential panel output and it's storage.

 

To explain: I fitted my boat out 21 years ago before solar pv was a viable proposition. I am now half way through building a replica showman's living van which has space on its gently curving roof for a total of 1050W of flexible panels. I already know (based on the past 10 years or so of leccy bills – I'm on a landline most of the time) that I consume just over a unit per day so if my maths is correct, about 100 amps. 

 

This doesn't vary summer to winter as my cooking/heating/hot water is taken over by a Dickinson diesel stove October – April. In fact it can go down! I'll be using the living van to reach those places the boat can't go so anticipate that there'll be little change as I'll adopt the same cooking/lighting/etc regime.

 

I guess one of the critical factors is how much of that daily 100 amp demand my 1050W solar will meet in the dead of winter. I'll clearly be looking at other sources of charge in the dark months!

 

Having done my learning on the living van, it's more than likely I'll go for solar on the boat as at 21 years most of the electrical kit is getting towards its sell by date.

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17 hours ago, WotEver said:

And will the boat be static somewhere or will it cruise? Will it have shore power? If it’s going to cruise how will you control alternator charging to the Lithiums?

Thank you for your reply WotEver.

 

My response to Jen I think covers your first two points. I'm looking to BtoB charging for the lithiums and am eagerly awaiting Victron's BtoB device promised for the end of last year. This seems to be the most elegant solution to the charging demands of lithium but with my lack of experience I may well be missing something.

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14 hours ago, Dr Bob said:

Firstly, I would go for one MPPT. We have 500W in 4 panels. 2 panels were on the boat with a 30A mppt and I added 2 panels with a further 15A MPPT. The MPPTs fought against each other so to stop that I wired them all into the 30A one. Works great and I get just about 30A on the best sunny days.

 As above, your bank size should be based on your usage but I would think a minimum of 320Ahrs if you use over 100Ahrs per day. I have 480Ahrs which works fine with the solar.

How much are you paying for your lithiums? If you buy second hand EV batteries then you should get 480Ahrs for £1000 which is half the price of new.

Thanks for your reply Dr Bob.

 

I need to get an understanding of the relationship between total panel output (in this case 1050W) and the capacity of the MPPT required to handle this safely.

 

Is your 480 AH battery bank lithium? I'm assuming from your observation that it is and that you therefore have the ability to discharge by approx 90%.

 

What's the life expectancy of s/h EV batteries and where can they be sourced? I'd be expecting to pay around £2000 + VAT for a new 300 AH lithium – i.e. Victron or Relion

 

As you'll see from my response to Jen, I'm looking at a usage of around 100 amp hrs a day too – summer and winter. With a wee bit of lifestyle tweaking I can actually reduce this in the winter months.

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3 hours ago, peterboat said:

You are on the right numbers Bob, I have 450 ah @  24 volts and its working well on slightly more solar than the OP [summer use] in winter it takes all of my 4.6 kw of solar 600 watt wind turbine and occasional whispergen to survive

Thanks for this Mr Boat ........ real use/real results is what I want to tap into!

 

24 volt is probably the way I need to go for the living van set up I've outlined to Jen above as there will need to be at least one "electrical conversation" twixt the tug vehicle and the living van!

 

Looks like my 100W wind turbine will need upgrading.

 

For another project, I have serious interest in the Whispergen. From the research I've done so far it appears that production ceased in New Zealand when the earthquake struck. I've found reference to a Spanish connection but nothing to suggest that Whispergens are currently in production. Am I right? I'm one of the Rev Stirling's greatest fans and much disheartened that it would appear that I can't purchase a new Whispergen. Despite this I'd be very interested in your experience with one.

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11 minutes ago, Up-Side-Down said:

I need to get an understanding of the relationship between total panel output (in this case 1050W) and the capacity of the MPPT required to handle this safely.

1050 watts at a charging voltage of around 14 v is 1050/14 = 75 amps but in the UK you will be unlikely to get much more than half that so I would say maybe a 40 or 50 amp MPPT. However as Lithiums grab as much charge as the chargee source can supply it may well be that the charging voltage will be depressed but by how much the good doctor can probably advise based on experience.

Edited by Tony Brooks
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2 minutes ago, Tony Brooks said:

1050 watts at a charging voltage of around 14 v is 1050/14 = 75 amps but in the UK you will be unlikely to get much more than half that so I would say maybe a 40 or 50 amp MPPT. However as Lithiums grab as much charge as the chargee source can supply it may well be that the charging voltage will be depressed but by how much the good doctor can probably advise based on experience.

Thanks Tony. Could you elaborate please on the 'depressed charging voltage' bit for the benefit of this bear with a somewhat compromised electrical brain?

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2 hours ago, Up-Side-Down said:

I guess one of the critical factors is how much of that daily 100 amp demand my 1050W solar will meet in the dead of winter. 

 

Almost none. 

 

The energy produced by solar panels falls to near zero in mid winter in my personal experience.  So little as to be negligible, I'd say. My 560W will be pumping out 0.2A for a couple of hours in the middle of a day perhaps. 

 

The thing to grasp is that in high summer your 1050W of solar will be massively more than you need to generate 100ah in a day. As the year progresses and the days get shorter, a day will arrive when the 1050W *just about* produces your 100ah in a day. Similarly as the Spring approaches and the days lengthen, a day will arrive when your <inadequate-in-mid-winter> 1050W of solar becomes useful again. 

 

The thing is, each time you add nominal solar capacity, you increase the number of days/weeks/months in the year the installation will be sufficient for your needs. Bottom line is fit as much solar you have space or money for, then use it and learn its characteristics.  In particular, note when it stops working sufficiently in autumn, and when it starts up again in Spring. Work out an alternative for when it does very little in the middle-of-winter period.

 

Whatever you fit, it will be not enough in midwinter and too much in midsummer.

 

The main thing is to recognise solar will never be enough for your needs in mid-winter unless you fit truly massive amounts of panel. The hard bit is working out how to cope with this 'dead' period. 

 

 

 

Edited by Mike the Boilerman
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44 minutes ago, Up-Side-Down said:

Thanks for this Mr Boat ........ real use/real results is what I want to tap into!

 

24 volt is probably the way I need to go for the living van set up I've outlined to Jen above as there will need to be at least one "electrical conversation" twixt the tug vehicle and the living van!

 

Looks like my 100W wind turbine will need upgrading.

 

For another project, I have serious interest in the Whispergen. From the research I've done so far it appears that production ceased in New Zealand when the earthquake struck. I've found reference to a Spanish connection but nothing to suggest that Whispergens are currently in production. Am I right? I'm one of the Rev Stirling's greatest fans and much disheartened that it would appear that I can't purchase a new Whispergen. Despite this I'd be very interested in your experience with one.

If you can fit rigid panels do so, they are cheaper and more output for a given size, I have a contact for LifePo4 batteries secondhand at decent money they have a BMS in them, alternatively he has batteries that you can build a 24 volt bank with brand new and very well priced. This is my second whispergen and I have a spare for another project I love them others hate them, Mike the Boilerman has one that he purchased from me brand new ex MOD, and it has made his winters easy fro him

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34 minutes ago, Up-Side-Down said:

Thanks for your reply Dr Bob.

 

I need to get an understanding of the relationship between total panel output (in this case 1050W) and the capacity of the MPPT required to handle this safely.

 

Is your 480 AH battery bank lithium? I'm assuming from your observation that it is and that you therefore have the ability to discharge by approx 90%.

 

What's the life expectancy of s/h EV batteries and where can they be sourced? I'd be expecting to pay around £2000 + VAT for a new 300 AH lithium – i.e. Victron or Relion

 

As you'll see from my response to Jen, I'm looking at a usage of around 100 amp hrs a day too – summer and winter. With a wee bit of lifestyle tweaking I can actually reduce this in the winter months.

My 500W of solar gives me up to 30A and no more. The 4 panels, when wired in via the two MPPTs never got to more than 30A - probably 18A on one (300W of panels) and 12A on the other (200W of panels). The problem with the 2 MPPTs was that when one was in Absorption, it would put the other into float. When wired into one MPPT we saw 30A max last summer (only just). Most of June/July we only saw 25A max - I guess a lot of humidity in the atmosphere was degrading the sun?- but in late august with the sun slightly lower we just got to 30A on the best sunny days. Our panels are tiltable.

I would think you should therefore just about make 60A from your panels.

 

On Lithiums. Mine are LiFePO4s (Thundersky bare cells 12* 160Ahrs) 2nd hand. I got them from Jeremy at sales@ev-support.co.uk

The use on a canal boat (or your application) around 100-150Ahr per day is very sedate so I think any secondhand battery that meets the EV spec (Jeremy tests them) will work for 10 years. Mine have been going a year now and I see no loss in performance when looking at the rested voltage vs amphrs used graph. I got  480Ahrs for £1000 which is less than 30% of what you will be paying. That has to be a better option. Dont be afraid of 2nd hand ones - there are at least 5 of us on here with different makes. I think mine would last easily 3 years if not 10. Why pay £3600 for 480Ahrs of new ones?

 

I use mine between 20 and 80%. Below 20% you are getting into the bottom voltage knee so then you have to start being careful about cell balancing. If you keep out of the voltage knee then you can ignore balancing to a large extent. You can charge to 100% but again you will be in the top knee so need to take more care. I suggest you read up on Li's on the various other threads where there is great debate on charging and charging limits. For someone not familiar, I would start figuring on 20-80% or maybe 20-90%. That might push you to 480Ahrs rather than 300Ahrs.....but if you charge each day then maybe 300Ahrs is enough.

If you are relying on solar then watch out for cloudy days as you may not get 100Ahrs in a day and certainly for Oct-Mar you will be struggling with almost none in Dec/Jan. Also think about trees. In the summer it is often hot and you park under trees. Solar is not good under trees.

 

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40 minutes ago, Up-Side-Down said:

Thanks Tony. Could you elaborate please on the 'depressed charging voltage' bit for the benefit of this bear with a somewhat compromised electrical brain?

I am not 100% sure with MPPTs but in general the higher the current a charge device provides the lower the voltage. So as an example using lead acid batteries that gradually reduce the current they will accept as the become more and more fully charged. T first the batteries will accept far more than anything the charge source can supply so the source delivers its maximum output current. The magnetic fields inside the device would be very   strong because of the high current and these also try to produce voltage but in the opposite direction to the main charging voltage so that depresses the charging voltage. As the batteries accept ;less and less current the magnetic field reduce so the charging voltage rise until eventually the regulated voltage is reached so the charge device starts regulating or holding the voltage at a fixed value. Say 14.4 volts. Now, if I understand things properly, Lithium batteries tend to to reduce the current they will accept so  it is always high (until they are disconnected from the charge source) and this the charge source voltage is lower than one would expect with lead acid batteries. The reason this is important is that a lower charging voltage (I hear some run lithium at well under 14 volts so the wattage divided bu the charging voltage would give a higher current flow. I can't see an MPPT controller acting any differently to then other charge sources but will be happy to be told I am wrong.

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8 minutes ago, Tony Brooks said:

I am not 100% sure with MPPTs but in general the higher the current a charge device provides the lower the voltage. So as an example using lead acid batteries that gradually reduce the current they will accept as the become more and more fully charged. T first the batteries will accept far more than anything the charge source can supply so the source delivers its maximum output current. The magnetic fields inside the device would be very   strong because of the high current and these also try to produce voltage but in the opposite direction to the main charging voltage so that depresses the charging voltage. As the batteries accept ;less and less current the magnetic field reduce so the charging voltage rise until eventually the regulated voltage is reached so the charge device starts regulating or holding the voltage at a fixed value. Say 14.4 volts. Now, if I understand things properly, Lithium batteries tend to to reduce the current they will accept so  it is always high (until they are disconnected from the charge source) and this the charge source voltage is lower than one would expect with lead acid batteries. The reason this is important is that a lower charging voltage (I hear some run lithium at well under 14 volts so the wattage divided bu the charging voltage would give a higher current flow. I can't see an MPPT controller acting any differently to then other charge sources but will be happy to be told I am wrong.

My LiFePo4s just seem to take all the power the MPPT will give them if below the voltage knee - wotever that is. Need to get an MPPT that can be set to an absorbtion voltage and a float voltage. If you are putting in 30-60A you may find setting the absorption voltage to 14.0 to 14.2V works so it stops charging at 80-90% full....but Peter will have a better idea here. I am a bit concerned though that if it charges at 10-20A then you can get to 100% SoC without the voltage reaching 14.0V so it can overcharge. Again, Peter is the expert here.

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4 minutes ago, Dr Bob said:

My LiFePo4s just seem to take all the power the MPPT will give them if below the voltage knee - wotever that is. Need to get an MPPT that can be set to an absorbtion voltage and a float voltage. If you are putting in 30-60A you may find setting the absorption voltage to 14.0 to 14.2V works so it stops charging at 80-90% full....but Peter will have a better idea here. I am a bit concerned though that if it charges at 10-20A then you can get to 100% SoC without the voltage reaching 14.0V so it can overcharge. Again, Peter is the expert here.

 

Have you got any idea about the voltage while the MPPT is in bulk - that is not regulating. It seem to me that with Lithium batteries it shoudl be in bulk for a very long time, in fact I would not be surprised if it was still in bulk when charging has to be cut off. The point is that if the load does depress the charging voltage then the amps calculation will give a higher figure but I may not be understanding exactly how MPPT controllers work.

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1 minute ago, Dr Bob said:

My LiFePo4s just seem to take all the power the MPPT will give them if below the voltage knee - wotever that is. Need to get an MPPT that can be set to an absorbtion voltage and a float voltage. If you are putting in 30-60A you may find setting the absorption voltage to 14.0 to 14.2V works so it stops charging at 80-90% full....but Peter will have a better idea here. I am a bit concerned though that if it charges at 10-20A then you can get to 100% SoC without the voltage reaching 14.0V so it can overcharge. Again, Peter is the expert here.

Would you believe Bob that I am reconditioning some LA batteries at the moment via my solar panels and inverter! very inefficient I know. But before I started my batteries were in absorb and down to 4 amps going in, float was around the corner for them, the voltage was at 13.9  volts so in a few minutes it would have gone into float at 13.6 volts and zero amps going in, until it had dropped down to the afore mentioned 13.6 volts anyway. So yes Bob splash out on a good MPPT controller I recommend Midnite and Prism solar is the best company I have found for the job, metion me to Callum and he should charge you double!! or might give you a discount?

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46 minutes ago, Mike the Boilerman said:

 

Whatever you fit, it will be not enough in midwinter and too much in midsummer

Choose your MPPT controller to provide the current you need for your charging plus a margin in reserve. It doesn't matter if the panels, in the height of summer, can produce more than that.  Good MPPTs are designed to cope with the higher input (although the output is limited).  And having tge extra panel capacity means you will get more power in the spring and autumn, extending the period you can survive on solar alone.

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33 minutes ago, David Mack said:

Choose your MPPT controller to provide the current you need for your charging plus a margin in reserve. It doesn't matter if the panels, in the height of summer, can produce more than that.  Good MPPTs are designed to cope with the higher input (although the output is limited).  And having tge extra panel capacity means you will get more power in the spring and autumn, extending the period you can survive on solar alone.

Spot on.

 

In practice the actual solar limits on boats are roof space and budget in that order.  You always need more than will fit, and in high summer (if we get such a thing!) the excess panels at least provide some shading on the roof.  That's when running an immersion heater helps a bit, those 2 days in July.

 

 

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33 minutes ago, Tony Brooks said:

 

Have you got any idea about the voltage while the MPPT is in bulk - that is not regulating. It seem to me that with Lithium batteries it shoudl be in bulk for a very long time, in fact I would not be surprised if it was still in bulk when charging has to be cut off. The point is that if the load does depress the charging voltage then the amps calculation will give a higher figure but I may not be understanding exactly how MPPT controllers work.

On my Li's, before dawn with no solar, the bank maybe at say 13.10V so 50% Soc. The solar cuts in and the voltage rises typically with 10-20A charge to around 13.40V. This voltage then slowly increases during the day to say 13.7V with that charge rate.....but I've never got the bank to 100%. If you had enough solar to charge at 40A+ then the voltage would rise faster and if the MPPT is set to 14.1v bulk, absorbtion and 13.5V float, then the voltage would rise to 14.1V (likely over 80% SoC) and then go to float. Li's dont want an absorption unless the charge rate is high enough which is when you may then need a constant voltage phase. It is a lot different to LA's where I find with my system, I never charge fast enough(high enough current) to see a contstant voltage phase. I think MP does.

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On 11/03/2020 at 13:45, Mike the Boilerman said:

 

Almost none. 

 

The energy produced by solar panels falls to near zero in mid winter in my personal experience.  So little as to be negligible, I'd say. My 560W will be pumping out 0.2A for a couple of hours in the middle of a day perhaps. 

 

The thing to grasp is that in high summer your 1050W of solar will be massively more than you need to generate 100ah in a day. As the year progresses and the days get shorter, a day will arrive when the 1050W *just about* produces your 100ah in a day. Similarly as the Spring approaches and the days lengthen, a day will arrive when your <inadequate-in-mid-winter> 1050W of solar becomes useful again. 

 

The thing is, each time you add nominal solar capacity, you increase the number of days/weeks/months in the year the installation will be sufficient for your needs. Bottom line is fit as much solar you have space or money for, then use it and learn its characteristics.  In particular, note when it stops working sufficiently in autumn, and when it starts up again in Spring. Work out an alternative for when it does very little in the middle-of-winter period.

 

Whatever you fit, it will be not enough in midwinter and too much in midsummer.

 

The main thing is to recognise solar will never be enough for your needs in mid-winter unless you fit truly massive amounts of panel. The hard bit is working out how to cope with this 'dead' period. 

 

 

 

I find that a most helpful way of looking at it. The Whispergen seems like the most elegant solution for the alternative for the middle-of-winter period. Mr Boat didn't confirm (or otherwise) my understanding that they are no longer in production following the New Zealand earthquake. Can I still source a new one anywhere?

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On 11/03/2020 at 13:59, Dr Bob said:

My 500W of solar gives me up to 30A and no more. The 4 panels, when wired in via the two MPPTs never got to more than 30A - probably 18A on one (300W of panels) and 12A on the other (200W of panels). The problem with the 2 MPPTs was that when one was in Absorption, it would put the other into float. When wired into one MPPT we saw 30A max last summer (only just). Most of June/July we only saw 25A max - I guess a lot of humidity in the atmosphere was degrading the sun?- but in late august with the sun slightly lower we just got to 30A on the best sunny days. Our panels are tiltable.

I would think you should therefore just about make 60A from your panels.

 

On Lithiums. Mine are LiFePO4s (Thundersky bare cells 12* 160Ahrs) 2nd hand. I got them from Jeremy at sales@ev-support.co.uk

The use on a canal boat (or your application) around 100-150Ahr per day is very sedate so I think any secondhand battery that meets the EV spec (Jeremy tests them) will work for 10 years. Mine have been going a year now and I see no loss in performance when looking at the rested voltage vs amphrs used graph. I got  480Ahrs for £1000 which is less than 30% of what you will be paying. That has to be a better option. Dont be afraid of 2nd hand ones - there are at least 5 of us on here with different makes. I think mine would last easily 3 years if not 10. Why pay £3600 for 480Ahrs of new ones?

 

I use mine between 20 and 80%. Below 20% you are getting into the bottom voltage knee so then you have to start being careful about cell balancing. If you keep out of the voltage knee then you can ignore balancing to a large extent. You can charge to 100% but again you will be in the top knee so need to take more care. I suggest you read up on Li's on the various other threads where there is great debate on charging and charging limits. For someone not familiar, I would start figuring on 20-80% or maybe 20-90%. That might push you to 480Ahrs rather than 300Ahrs.....but if you charge each day then maybe 300Ahrs is enough.

If you are relying on solar then watch out for cloudy days as you may not get 100Ahrs in a day and certainly for Oct-Mar you will be struggling with almost none in Dec/Jan. Also think about trees. In the summer it is often hot and you park under trees. Solar is not good under trees.

 

Thanks Dr Bob. That's really helpful stuff which I'll take to heart. I take it when you quote an output of 30 amp that's per hour? I've not met the concept of the two voltage knees before so, yes, more reading required! Do Jeremy's lithiums come with built in BM?

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3 minutes ago, Up-Side-Down said:

Mr Boat didn't confirm (or otherwise) my understanding that they are no longer in production following the New Zealand earthquake. Can I still source a new one anywhere?

 

Sadly they went out of production when the earthquake in NZ destroyed the factory.

 

No, new ones make hens' teeth seem commonplace. 

 

I found my new one on eBay having had a search running for a couple of years. A bod listed three all together near Peterboat, who nipped over and bought all of them, and passed one on to me. 

 

 

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16 minutes ago, Up-Side-Down said:

I take it when you quote an output of 30 amp that's per hour?

An 'amp' is an instantaneous  unit.

 

It is not a amp per minute or per hour or per day.

When you multiply it by time the unit becomes an Ah, so if you are generating 30amps for 5 hours the figure becomes 150Ah. (30amps for 30 minutes becomes 15Ah, 30 amps for 1 hour becomes 30Ah)

 

You will find that Solar will rarely give a constant figure over time.

You may get an instantaneous 30amps but as the Sun moves across the sky and the angles change the output will change, as a cloud crosses the sky the output will drop.

 

So "5 hours at 30 amps" will probably not produce 150Ah, more likely 120Ah. You can only tell by looking at your controller display at the end of the day.

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