Brace yourselves... It's ANOTHER 12V/230V fridge question

[captain flint]

So... I've been reading many threads here on this subject and if anything I'm just more confused than before. Although maybe a better quality of confusion. Maybe.

 

First off, I will own up right now to being immensely thick when it comes to electricity. Apparently I'm capable of not being entirely stupid about a few other things (then again maybe that's just people being "nice"). But electricity, my brain just freezes over. I expect this is the primary problem and I that many will sensibly point out getting to grips with the basics is really essential. But I'm going to blunder stupidly on anyway. Feel free to sling abuse. I can take it.

 

I have an old (15y) Coolmatic 12V fridge. It runs OK ish but somewhat less well as the seal isn't as good as the door doesn't sit quite straight after the hinge got damaged in, uh, a small boat fire (Long story. Not caused by onboard equipment). Plus it's noisy (sounds like the Tardis). And I'm keen to give it to a friend who is short of money (and a fridge). And I'd just like to get a better one.

 

Feel free to point and laugh, but the fact is I have a stupidly expensive OTT electric set up.

Inverter: New Victron Multiplus 2  - nominally - 3kW inverter charger (output drops to - nominally - 1.7kW at highest operating tempratures).  I'm not exactly clear on how much power it draws just by being on. I've looked in the manual and it says 13W, but, well, I think I said something about being stupid, no?

Battery: New LiFeP04 battery set up giving 560Ah.

I guess the obvious thing would be to get a very efficient 230V fridge and leave the inverter on.

 

But apart from force of habit, I guess it just feels more streamlined and efficient to go with a 12V fridge, plus less to go wrong. A few other reasons I'm thinking of forking out more on a 12V fridge:

- No new wiring needed (it wouldn't be all that easy running a new wire due to how things are set up)

- Although I have overkill on my electrics, I'm used to only switching the inverter on when I need it - plus I'm away for a few days at a time most weeks. I prefer to leave my fridge on the whole time, rather than switching it off at night/using it less in winter. I'm no stranger to living like that on board, and I get that it's an option). I want to look after my fancy battery as much as I failed to look after my last lead acid ones

- I suspect the chances of an inverter fire are small... But still, even the fire I had was quiet enough thank you. It's nice to come home and find it still there. 

 

On the other hand:

- I always leave the batteries charged up before I go away, so my new overkill set up could most likely cope without running them down even with the fridge left on (?)
- I do have 800+W solar (but not angled and in winter, only giving a little bit)Plus, any power saved feels like power I could use for something else, some time. I think my fridge and my laptop are the only regular big power draws, but there are plenty of other things I might like to get in future (hen I have saved up some more spending money - it's all going to be gone pretty soon...!)

All in all I don't mind forking out the extra for 12V (so long as the new fridge lasts several years). But if I can be persuaded that's stupid and I should just leave the inverter on I guess maybe I would go that way.

Am I being stubborn to think a decent 12V fridge is more elegantly efficient and with fewer possible issues? Should I bite the bullet and leave my inverter on the whole time, even when I'm away?

Go on. Do your worst

Thanks!

Edited November 1, 2023 by captain flint

[ditchcrawler]

I guess your inverter has a power save so if you buy a new, good240 volt fridge you need to ensure its not electronically controlled as it wont wake the inverter up when it warms up.

[Alan de Enfield]

33 minutes ago, captain flint said:

I guess the obvious thing would be to get a very efficient 230V fridge and leave the alternator on.

 

Inverter

 

34 minutes ago, captain flint said:

No new wiring needed (it wouldn't be all that easy running a new wire due to how things are set up)

 

 

Don't bank on that - I changed my 'old' 12v fridge for a 'new' 12v fridge and it wouldn't run. The new fridge was more sensitive to volt drop so I had to re-run the cables in a much larger size to get the new fridge to work.

 

37 minutes ago, captain flint said:

Battery: New LiFeP04 battery set up giving 560Ah.

 

It doesn't matter what battery technology you have - if you take 100Ah out then you need to put that (and more besides) back in  - how will you do that if you are away for 'days' and taking (probably) 40Ah - 50Ah per day out of the batteries, with either a 12v or a 230v fridge ?

I take it you are just moored up 'canal side' with no facilities ?

[ditchcrawler]

11 minutes ago, Alan de Enfield said:

  if you are away for 'days' and taking (probably) 40Ah - 50Ah per day out of the batteries, with either a 12v or a 230v fridge ?

 

Well that should last 10 days on your numbers

[captain flint]

17 minutes ago, Alan de Enfield said:

 

Inverter

 

 

Doh! Yes indeed. Thanks for pointing out, now edited 

Edited November 1, 2023 by captain flint

[Alan de Enfield]

4 minutes ago, ditchcrawler said:

Well that should last 10 days on your numbers

 

In theory - but I was led to believe that the Lithiums only had a 60% capacity usage (only charge to 80% and only take then down to 20%) so 6 days and they'd be 'flat'*. He is looking to do this every week - he needs some way of recharging for the odd days he will be there.

 

** I know its not flat flat like an FLA

Edited November 1, 2023 by Alan de Enfield

[IanD]

22 minutes ago, Alan de Enfield said:

 

Inverter

 

Don't bank on that - I changed my 'old' 12v fridge for a 'new' 12v fridge and it wouldn't run. The new fridge was more sensitive to volt drop so I had to re-run the cables in a much larger size to get the new fridge to work.

 

It doesn't matter what battery technology you have - if you take 100Ah out then you need to put that (and more besides) back in  - how will you do that if you are away for 'days' and taking (probably) 40Ah - 50Ah per day out of the batteries, with either a 12v or a 230v fridge ?

I take it you are just moored up 'canal side' with no facilities ?

 

800W of flat-mounted solar will give you something like 3kWh/day average in summer and maybe 0.5kWh/day in winter -- these are averages, some days will be better and some worse. An efficient 230V fridge consumes about 0.3kWh/day, many 12V ones are less efficient and consume somewhat more than this (0.4-0.5kWh/day?). Leaving the inverter on (13W zero-load power) consumes another 0.3kWh/day, meaning 0.6kWh/day to run a 230V fridge. Either way, available solar power in winter is borderline or not enough to run a fridge, so you'll need to run the engine to keep the batteries charged -- no problem in summer whichever way you go.

 

Of course if you want to run any other 230V gear you'll need to run the inverter anyway, in which case an AC fridge will be lower power and cheaper to buy... 😉

Edited November 1, 2023 by IanD

[captain flint]

1 minute ago, Alan de Enfield said:

 

In theory - but I was led to believe that the Lithiums only had a 60% capacity usage (only charge to 80% and only take then down to 20%) so  days and they'd be 'flat'*. He is looking to do this every week - he needs some way of recharging for the odd days he will be there.

 

** I know its not flat flat like an FLA

hmmm. I knew about the taking them down to 20% but thanks for telling me about only charging to 80% - can't believe I didn't come across this before, I spent days reading up on things before making such a big decision, but wasn't aware of that. I'll make an enquiry with the installer.

[IanD]

6 minutes ago, captain flint said:

hmmm. I knew about the taking them down to 20% but thanks for telling me about only charging to 80% - can't believe I didn't come across this before, I spent days reading up on things before making such a big decision, but wasn't aware of that. I'll make an enquiry with the installer.

If the LFP batteries have a decent BMS there's no issue running them above 80% SoC or below 20% in a boat, this is really a "lithium myth" coming from fast-charging of non-LFP lithium batteries. The battery itself will be fine even going up to 100% or down to 0% but then you risk disconnection if you go a tiny bit further, so you normally want to leave at least 5% margin.

Edited November 1, 2023 by IanD

[captain flint]

26 minutes ago, Alan de Enfield said:

 

I take it you are just moored up 'canal side' with no facilities ?

correct

 

[Tracy D'arth]

Why in winter do you need to leave the fridge on when you are away from the boat?

[captain flint]

11 minutes ago, Alan de Enfield said:

 

In theory - but I was led to believe that the Lithiums only had a 60% capacity usage (only charge to 80% and only take then down to 20%) so  days and they'd be 'flat'*. He is looking to do this every week - he needs some way of recharging for the odd days he will be there.

 

** I know its not flat flat like an FLA

What I'm about to type won't please anyone less technically illiterate than me, but Phil Owen who installed my system claims the set up he's designed (utilising a Victron Cerbo CX) is the only on board system that is absolutely reliable and accurate when it comes to battery usage, and claims to have tested it (for more info about it follow the link to his page in my original post). Today, I've not had the engine on, not had much sunshine, fridge has been on all day, and the readout is down from 87% to 85%. Presumably overnight it will take a few more percent off. It's only just been installed or I'd have a clearer idea. 

I should have said how much I'm away - normally one or two nights, sometimes 3. If I'm going away longer than that I would empty the fridge and turn it off, at least until I have ascertained that I can afford to leave it longer.

I appreciate that I'm just taking the word of the magic box and the reassurances of the installer that it's reliably accurate, but, um, er. Um.

I'm feeling right now that it should be OK?

In principle, though I'm right with you when you say it doesn't matter which type of battery you have, you need to put in what you take out, of course!

 

6 minutes ago, Tracy D'arth said:

Why in winter do you need to leave the fridge on when you are away from the boat?

Goddammit I don't know. I always used to turn it off when I went away. Why change? Maybe milk. Um. Christ. Glad you asked.

I will just turn it off. Duh.


 

[Tracy D'arth]

Beware that battery capacity monitors tell lies unless you regularly  re-calibrate them.

[captain flint]

17 minutes ago, IanD said:

If the LFP batteries have a decent BMS there's no issue running them above 80% SoC or below 20% in a boat, this is really a "lithium myth". The battery itself will be fine even going up to 100% or down to 0% but then you risk disconnection if you go a tiny bit further, so you normally want to leave at least 5% margin.

They have a very decent BMS.The installer just got back to me and said that keeping them between 30% and 70% is the sweet spot for the longest life but that they are very tolerant and not to overthink it every day. IE a good general rule that's basically fine to break now and then, like giving them a good charge before going away for a few days

3 minutes ago, Tracy D'arth said:

Beware that battery capacity monitors tell lies unless you regularly  re-calibrate them.

Thank you

[IanD]

16 minutes ago, captain flint said:

What I'm about to type won't please anyone less technically illiterate than me, but Phil Owen who installed my system claims the set up he's designed (utilising a Victron Cerbo CX) is the only on board system that is absolutely reliable and accurate when it comes to battery usage, and claims to have tested it (for more info about it follow the link to his page in my original post). Today, I've not had the engine on, not had much sunshine, fridge has been on all day, and the readout is down from 87% to 85%. Presumably overnight it will take a few more percent off. It's only just been installed or I'd have a clearer idea. 

I should have said how much I'm away - normally one or two nights, sometimes 3. If I'm going away longer than that I would empty the fridge and turn it off, at least until I have ascertained that I can afford to leave it longer.

I appreciate that I'm just taking the word of the magic box and the reassurances of the installer that it's reliably accurate, but, um, er. Um.

I'm feeling right now that it should be OK?

In principle, though I'm right with you when you say it doesn't matter which type of battery you have, you need to put in what you take out, of course!

 

 

The Cerbo is just a system controller (my boat uses one too), it's the battery BMS that determines SoC sensing/accuracy, and some are better than others at doing this whether inbuilt to the battery or external. With a good BMS which also controls the charging sources via the Cerbo (MPPT, inverter/charger, alternator) there's no problem going all the way up to 100% SoC since the BMS will stop charging at this point. Going down to very low SoC becomes a problem if the BMS does a disconnect and you lose all power, but won't cause any damage. This all assumes that the cells are properly balanced with small voltage differences between them...

 

In fractional-C applications like boats LFP batteries don't really care how you use them, for example if they're rated at 3000 cycles from 0% to 100% SoC you'd expect 6000 cycles at 10-60% or 40%-90%, and excursions outside this range aren't an issue for the battery.

8 minutes ago, captain flint said:

They have a very decent BMS.The installer just got back to me and said that keeping them between 30% and 70% is the sweet spot for the longest life but that they are very tolerant and not to overthink it every day. IE a good general rule that's basically fine to break now and then, like giving them a good charge before going away for a few days

Thank you

The last comment from @Tracy D'arth is not correct for decent LFP BMS, these track the charge/discharge curves over the life of the battery and adapt the SoC reading to suit (self-calibration). They will often report "battery health" which is the actual capacity compared to what it was when new, but the drop in this will be negligible.

Edited November 1, 2023 by IanD

[captain flint]

28 minutes ago, IanD said:

 

800W of flat-mounted solar will give you something like 3kWh/day average in summer and maybe 0.5kWh/day in winter -- these are averages, some days will be better and some worse. An efficient 230V fridge consumes about 0.3kWh/day, many 12V ones are less efficient and consume somewhat more than this (0.4-0.5kWh/day?). Leaving the inverter on (13W zero-load power) consumes another 0.3kWh/day, meaning 0.6kWh/day to run a 230V fridge. Either way, available solar power in winter is borderline or not enough to run a fridge, so you'll need to run the engine to keep the batteries charged -- no problem in summer whichever way you go.

 

Of course if you want to run any other 230V gear you'll need to run the inverter anyway, in which case an AC fridge will be lower power and cheaper to buy... 😉

I've usually not run 230V applicances all the time, and only turned the invertor on when I need to. But with my new batteries I can see I may get into the habit of leaving it on when I'm on board as I've plenty more wriggle room on power and the new invertor is more efficient than the old in terms of draw. 

 

I suppose the question of warm but overcast summer days when I'm away for a couple of nights and would like to leave the fridge on might be pertinent here and make me lean towards a 12V fridge

53 minutes ago, Alan de Enfield said:

1 hour ago, captain flint said:

No new wiring needed (it wouldn't be all that easy running a new wire due to how things are set up)

 

 

Don't bank on that - I changed my 'old' 12v fridge for a 'new' 12v fridge and it wouldn't run. The new fridge was more sensitive to volt drop so I had to re-run the cables in a much larger size to get the new fridge to work.

Hmmmm. Thank you for sharing this. I've been looking at the wiring and am typically very confused. But I have a feeling this might be the single biggest obstacle and might even make me just stick with the devil I know.

 

The cables are 3mm* and the travel is 6 or 7** metres. So - I think? - the cables are maybe half what they should be?

*Nothing printed on the sheath, but, well, they're 3mm.

**This is a sort of measured estimate - cables come out of the back of the fuse box, disappear behind the cabin lining, and reappear behind the fridge. So, hard to say exactly how long they are but I measured the travel plus the bits sticking out behind the fridge.

It's is on the other side of the boat from the fuse box. My uneducated guess is that while in theory you might be able to use the thin cable to pull a fatter cable from fusebox to fridge, in reality it's unlikely to work on such a long and potentially tricky travel (?). In which case I'd have to think about a whole new travel for new cables. Not impossible, I'm sure, but not the same as simply disconnecting one fridge and putting a new one onto the same connection. 

[nicknorman]

1 hour ago, Alan de Enfield said:

 

In theory - but I was led to believe that the Lithiums only had a 60% capacity usage (only charge to 80% and only take then down to 20%) so 6 days and they'd be 'flat'*. He is looking to do this every week - he needs some way of recharging for the odd days he will be there.

 

** I know its not flat flat like an FLA

You were led wrongly. Lots of “20-80%” has been bandied about by people who just repeat what other people say. It is inadvisable to take the batteries “up and down the knee” a lot but that still gives at least 80% if not 90% of badged capacity

59 minutes ago, captain flint said:

hmmm. I knew about the taking them down to 20% but thanks for telling me about only charging to 80% - can't believe I didn't come across this before, I spent days reading up on things before making such a big decision, but wasn't aware of that. I'll make an enquiry with the installer.

Don’t believe everything someone tells you on the internet, especially when they have no experience of what they are talking about.

Edited November 1, 2023 by nicknorman

[captain flint]

1 hour ago, Alan de Enfield said:

Don't bank on that - I changed my 'old' 12v fridge for a 'new' 12v fridge and it wouldn't run. The new fridge was more sensitive to volt drop so I had to re-run the cables in a much larger size to get the new fridge to work.

Question. Very very hopeful and probably silly question. To which I can guess the answer:

Looking on the data sheet for the danvoss compressor that is in my fridge, it says 6mm cable for 6m travel on a 12V system. This does mean two cables of 6mm each, rather than 6mm in total (ie 2 x 3mm), right? I'm sure it does but I would hate to make that assumption if in fact my 2x3mm cables are sufficient... Chance would be a fine thing 

 

 

8 minutes ago, nicknorman said:

Don’t believe everything someone tells you on the internet, especially when they have no experience of what they are talking about.

Every statement on this forum comes with an advisory. As do most statements about most things in life, really! Nevertheless it is a useful resource and there's as much wheat as chaff, in general, I reckon. As it happens my installer kind of agreed. He said not to be worried about charging or depleting it more and that the cells will last for years and years and year even with regular heavy-handed treatment, but that the sweet spot for the longest life possible for them is 30-70%. It's like you're both right - it doesn't matter that much, but it does make some difference (according to Phil Owen, anyway)

Edited November 1, 2023 by captain flint

[Alan de Enfield]

27 minutes ago, captain flint said:

The cables are 3mm* and the travel is 6 or 7** metres. So - I think? - the cables are maybe half what they should be?

 

They are well undersize.

 

A cable is measured by the CSA (Cross sectional area) of the copper and NOT the overall diameter of the wires. a 3mm diameter wire will be less than 3mm2 (CSA)

 

The MINIMUM CSA should be 1mm2 (1mm square) per metre of length of run.

2 minutes ago, captain flint said:

Looking on the data sheet for the danvoss compressor that is in my fridge, it says 6mm cable for 6m travel on a 12V system. This does mean two cables of 6mm each, rather than 6mm in total (ie 2 x 3mm), right? I'm sure it does but I would hate to make that assumption if in fact my 2x3mm cables are sufficient... Chance would be a fine thing 

 

 

EACH cable should be 6mm2 (bigger would be better - if it is a 7mt run then it will be undersized.

[captain flint]

2 minutes ago, Alan de Enfield said:

 

They are well undersize.

 

A cable is measured by the CSA (Cross sectional area) of the copper and NOT the overall diameter of the wires. a 3mm diameter wire will be less than 3mm2 (CSA)

 

The MINIMUM CSA should be 1mm2 (1mm square) per metre of length of run.

 

EACH cable should be 6mm2 (bigger would be better - if it is a 7mt run then it will be undersized.

Thanks, Alan. Kind of what I was expecting to hear but with some extra info, I was ignorant about CSA vs diameter, so that's another tiny (and very basic) chip off the enormous rock of my ignorance. 

Ah well. It was nice to dream of a new fridge. But I guess now I need to focus on dreaming of new cables, first. I wish I could be advised online about the best way to replace my fridge cables, but I think it's the kind of thing you really need to be on board in person to be able to say anything much sensible, right.

[nicknorman]

5 minutes ago, captain flint said:

Question. Very very hopeful and probably silly question. To which I can guess the answer:

Looking on the data sheet for the danvoss compressor that is in my fridge, it says 6mm cable for 6m travel on a 12V system. This does mean two cables of 6mm each, rather than 6mm in total (ie 2 x 3mm), right? I'm sure it does but I would hate to make that assumption if in fact my 2x3mm cables are sufficient... Chance would be a fine thing 

 

 

Every statement on this forum comes with an advisory. As do most statements about most things in life, really! Nevertheless it is a useful resource and there's as much wheat as chaff, in general, I reckon. As it happens my installer kind of agreed. He said not to be worried about charging or depleting it more and that the cells will last for years and years and year even with regular heavy-handed treatment, but that the sweet spot for the longest life possible for them is 30-70%. It's like you're both right - it doesn't matter that much, but it does make some difference (according to Phil Owen, anyway)

As IanD pointed out, it is more about the total Ah taken out than the depth of discharge. Ie you can have 6000 cycles of between 30% and 70% (40% range) or 3000 cycles between 10% and 90% (80% range). The same total Ah will have been taken out. I think what Phil told you is basically right but he, like me, is just repeating what he’s heard from someone else! He won’t have actually tested this hypothesis.

 

The only thing that LiFePO4 seems to dislike is being held up the knee for a long time (held at a charging voltage at say 14v or more after the charge current has fallen below 5%) or left fully discharged eg below 12v no load voltage) for a long period.

[captain flint]

14 minutes ago, Alan de Enfield said:

A cable is measured by the CSA (Cross sectional area) of the copper and NOT the overall diameter of the wires. a 3mm diameter wire will be less than 3mm2 (CSA)

'Ere, Alan. I'm not doubting you may be right about this and that I may have the wrong end of the stick in some way, but that's not what it seems to say here

[Alan de Enfield]

7 minutes ago, captain flint said:

Thanks, Alan. Kind of what I was expecting to hear but with some extra info, I was ignorant about CSA vs diameter, so that's another tiny (and very basic) chip off the enormous rock of my ignorance. 

Ah well. It was nice to dream of a new fridge. But I guess now I need to focus on dreaming of new cables, first. I wish I could be advised online about the best way to replace my fridge cables, but I think it's the kind of thing you really need to be on board in person to be able to say anything much sensible, right.

 

 

You have got what you have got, but any boat wiring should be installed so it can be changed, repaired, added to etc. This is generally done by running trunking under the Gunwales, or behind a 'centre board' (easily unclipped) running down the centre of the roof.

 

The problem is that folks who build their boats are ignorant of the needs of boaters and do not have the skills or knowledge to make the boat easily maintained.

[magnetman]

It is all about the knees. 

 

Keep away from these areas and the batteries will last for ages. 

 

Someone who knows did explain it to me once. Its to do with the ions. 

 

They don't like being too excited or too dormant. 

 

Something like that anyway. 

 

I like LTO batteries as they are abuse tolerant. 

 

[captain flint]

3 minutes ago, nicknorman said:

The only thing that LiFePO4 seems to dislike is being held up the knee for a long time (held at a charging voltage at say 14v or more after the charge current has fallen below 5%)

Thanks for your post, your points all sound sensible to me, and I even understood most of them. Help me out here, though, can you say this one again in a slightly different way as I kind of think I know what that means but I'm not sure  (I'm guessing in practice you mean continuing to charge them when they're getting nearly full... ?)