Jump to content

Cheap LiFePO4 BMS?


jetzi

Featured Posts

  

On 30/11/2021 at 16:45, nicknorman said:

one requirement for cruising boats is to charge the batteries and then stop charging and go to float whereby the batteries think they have been disconnected from the charging source, eg 14.4v to charge and 13.4v to float so that the alternator supplies any loads whilst not subjecting the batteries to any stress.

Does "going to float to avoid stressing batteries" simply mean to reduce the alternator output voltage to roughly equal to the voltage that you stop charging at, such that and volt drop from loads would be compensated for by the alternator?

 

 

On 01/12/2021 at 15:08, nicknorman said:

Shut off is 2.5v

Wow, really that low?? The data sheet for Winston Thundersky talks about 2.5V being the rock bottom (beyond which the cells are damaged) but I have set all my shutoffs (even the emergency one) much higher than that. Is that because your loads are at a high C value (similar to how you use a higher voltage for your top cutoff due to charging at up to C/2)

I think my system is unusual in that I have a big LFP battery (640Ah) relative to my charge/load capacity. My maximum semi-continuous load is about 220A or so (washing machine on heat - I'm limited by my use of Tyco BDS-A relays in my BMS which are rated at 190A, so I dare not load them more than that) which is C/3, and my maximum charge is probably only about C/5 or so (if I'm full sun I can get about 80A and I only get a maximum of 50A from my alternators). So I'm probably justified in my taking an extremely conservative approach with my cutoffs.

Link to comment
Share on other sites

54 minutes ago, jetzi said:

  

Does "going to float to avoid stressing batteries" simply mean to reduce the alternator output voltage to roughly equal to the voltage that you stop charging at, such that and volt drop from loads would be compensated for by the alternator?

 

 

Wow, really that low?? The data sheet for Winston Thundersky talks about 2.5V being the rock bottom (beyond which the cells are damaged) but I have set all my shutoffs (even the emergency one) much higher than that. Is that because your loads are at a high C value (similar to how you use a higher voltage for your top cutoff due to charging at up to C/2)

I think my system is unusual in that I have a big LFP battery (640Ah) relative to my charge/load capacity. My maximum semi-continuous load is about 220A or so (washing machine on heat - I'm limited by my use of Tyco BDS-A relays in my BMS which are rated at 190A, so I dare not load them more than that) which is C/3, and my maximum charge is probably only about C/5 or so (if I'm full sun I can get about 80A and I only get a maximum of 50A from my alternators). So I'm probably justified in my taking an extremely conservative approach with my cutoffs.


No, going to float means reducing the voltage to that which the battery would settle at if left open circuit for several hours, which is quite a lot less than the charging voltage. So if I stop charging at 80% SoC the voltage was probably 13.6v but I need to reduce the voltage to about 13.25v so that no current flows in or out of the battery long term, but when I put a heavy load on the alternator supplies it, not the batteries. If I left the voltage at 13.6v the SoC would creep up to perhaps 95% given enough time.

 

Actually one issue with the regulator chip I’m using is that I can only set the regulated voltage to 0.1v resolution, so I have to set it to 13.2v. 13.3v would mean the SoC crept up. This means that the batteries discharge slightly but the alternator controller kicks back in to charging mode if it gets down to 79% SoC - in other words during a long cruise the SoC will wander between 79% and 80%

 

2.5v is rock bottom but then the purpose of the emergency disconnect is to stop the batteries getting significantly damaged in an extreme event. It is certainly not a target and I very much doubt it will ever be activated. This is why I stopped my trial discharge at 2.9v. In 10 years of LA battery ownership we never got close to flat batteries so I can’t see why it would happen with Li and greater capacity. But if it did, it won’t totally trash the batteries. The BMS has a loud beeper that starts to chirp at 2.9v and gradually gives a longer beep until the final emergency disconnect at 2.5v.

 

But obviously the disconnect cell voltage is just one line of code, a #define, so I can easily change it.


As you say, in part I decided to go with 2.5v for the case when there is a heavy load on (200A) that pulls the cell voltages down. I didn’t want a premature emergency disconnection! Although having experimented, at very low SoC the voltage dip under load is not much greater than at high SoC - ie not as much as one might expect.

 

Bear in mind that although the Tyco relay is rated at a nominal 190A, if you read the spec it says maximum current is 260A at 23 degC. The 190A applies at 85 degC. I presume this is ambient temperature, so for boaty use we are in the 260A range.

 

 

Edited by nicknorman
  • Greenie 1
Link to comment
Share on other sites

After a couple of cold nights on the boat with canal frozen and batteries down to +5 degC (which is ok, but uncomfortably cold) I thought I’d better install some battery heating. Outside temperature was only just below freezing so in an exceptionally cold spell, charging might be an issue.

So thanks to Craig (I think) for the motor home heating film idea. It arrived today and seems just the thing. I will connect it to a spare output on the aft Empirbus node. Main reasons being that: 1: it’s dimmable (by PWM) in case the full output is too much and 2: I can effortlessly add an auto-off after x minutes, to cater for the inevitable switching it on and then forgetting to switch it off!

 

So next time on the boat I’ll need to lift the batteries out and slot the film in underneath and run the wires to the Empirbus node.

Edited by nicknorman
Link to comment
Share on other sites

39 minutes ago, nicknorman said:

After a couple of cold nights on the boat with canal frozen and batteries down to +5 degC (which is ok, but uncomfortably cold) I thought I’d better install some battery heating. Outside temperature was only just below freezing so in an exceptionally cold spell, charging might be an issue.

So thanks to Craig (I think) for the motor home heating film idea. It arrived today and seems just the thing. I will connect it to a spare output on the aft Empirbus node. Main reasons being that: 1: it’s dimmable (by PWM) in case the full output is too much and 2: I can effortlessly add an auto-off after x minutes, to cater for the inevitable switching it on and then forgetting to switch it off!

 

So next time on the boat I’ll need to lift the batteries out and slot the film in underneath and run the wires to the Empirbus node.

 

Oddly enough, the other day it was not much above zero degrees when I was getting up in the morning, the canal had a thin ice covering, and the first thing I did was to take the battery out of the ebike- which lives on the stern- so that I could fully charge it for a 10 mile round trip planned for later. 

I was about to plug it in and get charging when I remembered the advice here about not charging LiFeP04 batteries when around zero degrees (I dont know what specific type of lithium battery it is, so I didnt want to run any risks).

So I let it stand in the lounge for a couple of hours before charging it, which might just have saved it from being damaged. 

 

Ed's advice is generally that lithium batteries are OK in the engine bay, perhaps with some heating pads or similar for really bitter weather, but mine were placed in an interior cupboard many months before I heard that.

 

But that said, I think I would still prefer the batteries indoors, tbh.

As a CCer with no shore power, who occasionally leaves the boat for a few days, I do have some nagging doubts that drawing power from the batteries to keep them above zero in the depths of winter might end up draining their charge to the point that they disconnect the loads anyway, so the heating will be stopped at some point.

Perhaps it would be fine for a few days, and perhaps not fine if it went on for two weeks? I guess it depends on the draw for the heating pads, and the size of the battery bank. 

 

 

 

Link to comment
Share on other sites

3 minutes ago, Tony1 said:

Perhaps it would be fine for a few days, and perhaps not fine if it went on for two weeks? I guess it depends on the draw for the heating pads, and the size of the battery bank. 

 

 

LFPs don't mind being stored below zero, or a discharge being drawn whilst below zero. Its charging whilst below zero that wrecks them.

 

Your BMS should low-voltage disconnect them if to protect them from being over-discharged by heater pads. At that point, charging becomes a problem as you'll have to wait for the weather to warm up. This is another reason hybrid systems can be a Good Idea. 

Link to comment
Share on other sites

7 minutes ago, MtB said:

 

LFPs don't mind being stored below zero, or a discharge being drawn whilst below zero. Its charging whilst below zero that wrecks them.

 

Your BMS should low-voltage disconnect them if to protect them from being over-discharged by heater pads. At that point, charging becomes a problem as you'll have to wait for the weather to warm up. This is another reason hybrid systems can be a Good Idea. 

 

My BMS would indeed disconnect all loads (including the heating pads themselves) if the voltage or SoC got low enough to trigger the BMV712 relay.

 

Part of my reservations about having the batteries in the engine bay is that on a freezing morning, if the panels were tilted and the boat facing the right way, I might start getting some charge into them when they were still around zero. 

 

Only 2 weeks ago I was getting 2.8kw into the panels on a decent day, and it started around 8am- my worry would be that the engine bay would still be at or below zero on a winter morning, when the panels start doing their charging. 

 

My BMS is set up such that if the voltage or SoC get too low, only the loads are disconnected, not the chargers. 

Its not a terribly likely scenario, but I could see it happening sooner or later.  

 

Link to comment
Share on other sites

It's nice to have separate disconnect relays for load and charge for this exact scenario. Disconnecting the charge relay when below zero is a relatively easy thing to build into the BMS.

 

I don't have the battery heater on standby all the time (I'm also using the motorhome film to heat). I only put the heater on if I'm about to put charge into the battery and it's feeling chilly. Heating things with battery power is generally a bad idea, and if you are going to heat the battery to stay above zero 24/7 even when you aren't charging, you're going to reduce the usable capacity a lot.

 

My heating is currently a manual process. But it would be cool to detect when the voltage is higher on the charge side (and hence the BMS can detect that we're trying to charge) and turn on the heat, then reconnect the charge relay once up to temperature.

 

Seems like quite a lot of fuss for relatively minimal benefit in automation, but it would be one less thing to have to "know" about operating the battery.

54 minutes ago, Tony1 said:

2 weeks ago I was getting 2.8kw

2.8kW in November?? What's the total theoretical size of your array, I would have thought it would need to be well over 10kW of panels to get that much in winter.

Link to comment
Share on other sites

57 minutes ago, Tony1 said:

my worry would be that the engine bay would still be at or below zero on a winter morning, when the panels start doing their charging. 

I might be wrong about this, and you shouldn't do it, but I think that LiFePOs can tolerate really slow charging at low temperatures (which for the size of most people's PV array on a boat, on a day below freezing, it surely would be

Link to comment
Share on other sites

19 minutes ago, jetzi said:

It's nice to have separate disconnect relays for load and charge for this exact scenario. Disconnecting the charge relay when below zero is a relatively easy thing to build into the BMS.

 

I don't have the battery heater on standby all the time (I'm also using the motorhome film to heat). I only put the heater on if I'm about to put charge into the battery and it's feeling chilly. Heating things with battery power is generally a bad idea, and if you are going to heat the battery to stay above zero 24/7 even when you aren't charging, you're going to reduce the usable capacity a lot.

 

My heating is currently a manual process. But it would be cool to detect when the voltage is higher on the charge side (and hence the BMS can detect that we're trying to charge) and turn on the heat, then reconnect the charge relay once up to temperature.

 

Seems like quite a lot of fuss for relatively minimal benefit in automation, but it would be one less thing to have to "know" about operating the battery.

2.8kW in November?? What's the total theoretical size of your array, I would have thought it would need to be well over 10kW of panels to get that much in winter.

 

 

I wonder if perhaps I haven't expressed it in the right units. 

This is what the MPPT history page shows for two weeks ago- this is for the rear panels. The front will probably be fractionally less.

Bear in mind this was an almost ideal scenario. It was an open mooring, the boat was pointing slightly north of due east, so the sun was in a good place and it was worth titling the panels, and there was only an 8ft hedge on the northern bank (where I was), which didnt interfere in any significant way. 

If I'm reading correctly its saying the rear panels collected 1.4kwh for the day- the rear panels are nominally rated at 750 watts.

 

On your other point of setting up a charger disconnect at low temps, the thing to bear in mind is that I personally don't have the knowledge to do that, so there would be a cost. 

The disadvantage of it might be that on a day when the engine bay stays around zero, my engine charging will be hampered by the low temp charger disconnect- at least until some heat from the running engine started to soak into the batteries. It doesnt sound at first glance like a snag-free solution. 

 

Before considering moving the batteries into the engine bay, I'd like to see more certainty around a solution, and one that will work when I leave the boat unattended for up to 2 weeks at a time, without flattening the batteries,

 

 

284891642_IMG_20211205_000727(1).jpg.a26bf6839ab68fa9e3c1aaf30bc07fd4.jpg

Edited by Tony1
Link to comment
Share on other sites

9 hours ago, Tony1 said:

 

My BMS would indeed disconnect all loads (including the heating pads themselves) if the voltage or SoC got low enough to trigger the BMV712 relay.

 

Part of my reservations about having the batteries in the engine bay is that on a freezing morning, if the panels were tilted and the boat facing the right way, I might start getting some charge into them when they were still around zero. 

 

Only 2 weeks ago I was getting 2.8kw into the panels on a decent day, and it started around 8am- my worry would be that the engine bay would still be at or below zero on a winter morning, when the panels start doing their charging. 

 

My BMS is set up such that if the voltage or SoC get too low, only the loads are disconnected, not the chargers. 

Its not a terribly likely scenario, but I could see it happening sooner or later.  

 

 

A two channel BMS is good, and I would have expected low (and high) temperature disconnects to be built into such a beast.

 

Which BMS do you have?

 

 

Link to comment
Share on other sites

8 hours ago, Tony1 said:

 

 

I wonder if perhaps I haven't expressed it in the right units. 

This is what the MPPT history page shows for two weeks ago- this is for the rear panels. The front will probably be fractionally less.

Bear in mind this was an almost ideal scenario. It was an open mooring, the boat was pointing slightly north of due east, so the sun was in a good place and it was worth titling the panels, and there was only an 8ft hedge on the northern bank (where I was), which didnt interfere in any significant way. 

If I'm reading correctly its saying the rear panels collected 1.4kwh for the day- the rear panels are nominally rated at 750 watts.

 

On your other point of setting up a charger disconnect at low temps, the thing to bear in mind is that I personally don't have the knowledge to do that, so there would be a cost. 

The disadvantage of it might be that on a day when the engine bay stays around zero, my engine charging will be hampered by the low temp charger disconnect- at least until some heat from the running engine started to soak into the batteries. It doesnt sound at first glance like a snag-free solution. 

 

Before considering moving the batteries into the engine bay, I'd like to see more certainty around a solution, and one that will work when I leave the boat unattended for up to 2 weeks at a time, without flattening the batteries,

 

 


 

 

Yes your units are a bit confused. Earlier you said 2.8kw which is an instantaneous power corresponding to over 200A. Whereas the display is showing KWh, the amount of energy over the whole day. It’s like the difference between miles per hour, and miles.

 

Anyway back to low temperature batteries, the idea of the heater is to manually switch on prior to charging. I only have a single disconnect for everything on the BMS, but the charging sources (alternator controller and Combi via BMS control) will not attempt to charge the batteries if below 1 degC. If you start the engine the alternator controller will stay in float. It might put a few amps in but that is ok.
 

We don’t have solar, but if we did that might be problematic unless the controller is set up for Li and knows not to charge if below zero. I think disconnecting the solar controller from the batteries is a fairly inelegant way of dealing with a routine situation.

Edited by nicknorman
Link to comment
Share on other sites

2 minutes ago, MtB said:

 

A two channel BMS is good, and I would have expected low (and high) temperature disconnects to be built into such a beast.

 

Which BMS do you have?

 

 

 

I'm sure you're right about the high temp disconnect bit, and my system doesn't currently include that, which means I am running some risks - and especially with lithiums being a bit prone to overheating when stressed. 

The B2B chargers have high battery-temp sensors that I could use, but I'm not (yet) aware of such a thing on the victron MPPT 100/50 units that I'm using.

Ideally I would want a single temp control unit - it would want a temp sensor on each of the three batteries, and ideally it would do its disconnecting via the existing BEP switch that I have in place for the high voltage disconnect

In fairness this is probably a timely reminder to put some provision in place for temp monitoring, and I shall see if something can't be done in the next couple of months.

Expect a long painful discussion thread to be opened on the issue soon...

 

The thing is that I dont have a single unit that acts as a BMS- I use three BMV712s that control the charge and load disconnects. You may remember from a discussion some weeks ago that I cant manage my batteries at cell level (not unless I break into the outer plastic casing and install a BMS)- so my system, such as it is, is focused on monitoring things at battery level- and maybe for a temp sensor that would be ok.  

 

I dont know how common it is to have a low-temp disconnect in an off the shelf BMS unit. Before using one of those, I'd need to be sure to what extent it might impede my engine charging on the freezing winter mornings, and that it would all work if the boat was moored without shore power and left unattended for a couple of weeks- in fact that's turned out to be one of my criteria for most of the electrickery that's currently in place. 

 

Link to comment
Share on other sites

20 minutes ago, nicknorman said:

 

Anyway back to low temperature batteries, the idea of the heater is to manually switch on prior to charging. I only have a single disconnect for everything on the BMS, but the charging sources (alternator controller and Combi via BMS control) will not attempt to charge the batteries if below 1 degC. If you start the engine the alternator controller will stay in float. It might put a few amps in but that is ok.
 

We don’t have solar, but if we did that might be problematic unless the controller is set up for Li and knows not to charge if below zero. I think disconnecting the solar controller from the batteries is a fairly inelegant way of dealing with a routine situation.

 

I think my reservations around charging at low temps arise from sheer ignorance, to be fair. 

I don't know whether/how much my LiFeP04s would be damaged if I start the engine at 8am on a bright but freezing morning, and started throwing 90+ amps into them, when the batteries were at 2 degrees, say, or 4 degrees. Its above zero, but not by very much, and I wonder if there might be some level of deterioration caused?

(Or if I was away on holiday, say- and my panels did some charging on a freezing morning).

 

I've never had a low temp thermometer in the engine bay, and I have no real idea how low the temp gets in below-zero weather, so I'd want to get soem more info about that environment before I used the engine bay to store the batteries. There are quite a few different scenarios I'd need to take into account.

 

That said, I bet plenty of CCers use their engine bays to house lithiums, so its clearly not too difficult an issue to resolve, but I just dont know enough about the whole issue as yet, so it seems wiser to use a cupboard which 99% of the time is around 15-20 degrees, where these batteries seem to work well. 

Again though, even the boat interior temp is going to sink to near-zero if I left it alone for a week or two in a harsh winter, so some thought needs to be given to that scenario. 

Lets hope it doesnt get that bad this winter.

 

Famous last words....

 

 

Edited by Tony1
Link to comment
Share on other sites

Ah I didn't realise your B2Bs were your ONLY method of control, I imagined you would have a proper BMS underneath it all. But I lost the will to live with that thread, months ago. 

 

Also despite all the money your evolved system cost. I think not having cell monitoring is asking for trouble. How do you know how well matched and balanced the cells are?

 

And finally not many BMSs have low temp cut-off mainly because most are designed in warm climates in the far east, where the critical need for it here in the colder climates simply has not penetrated the consciousness of the designers. Someone earlier said it is ok to charge at 'low rates' below 0c but from what I've read, no it isn't. Its not ok to charge below 0c AT ALL unless you're happy to play Russian roulette with your cells. If you watch a few of Will Prowse's videos you'll notice whenever he reviews a BMS or a battery, he always looks for and comments on the presence or otherwise of a low temp cut-off, and then he tests it to see if it works.

Link to comment
Share on other sites

8 hours ago, Tony1 said:

perhaps I haven't expressed it in the right units. 

This is what the MPPT history page shows for two weeks ago- this is for the rear panels. The front will probably be fractionally less.

Bear in mind this was an almost ideal scenario. It was an open mooring, the boat was pointing slightly north of due east, so the sun was in a good place and it was worth titling the panels, and there was only an 8ft hedge on the northern bank (where I was), which didnt interfere in any significant way. 

If I'm reading correctly its saying the rear panels collected 1.4kwh for the day- the rear panels are nominally rated at 750 watts

 

Ah yeah, you just left off the "h" before, as Nick said.

 

Still, 1.4kWh for a day is really really good (2.4kWh is a good day for me in summer!). The most I have got recently is 0.46kWh, that was middle of last week. I reckon your tilt is probably the thing that make the difference. have 1500W of panels but they are not on a tilt. I decided to rather just add more panels and leave them flat, my controller and roof still has capacity for 2 more panels which I think I might add since they are so inexpensive. But really I think to make a difference I should add at least a one axis lateral tilt.

Link to comment
Share on other sites

2 minutes ago, MtB said:

Ah I didn't realise your B2Bs were your ONLY method of control, I imagined you would have a proper BMS underneath it all. But I lost the will to live with that thread, months ago. 

 

Also despite all the money your evolved system cost. I think not having cell monitoring is asking for trouble. How do you know how well matched and balanced the cells are?

 

And finally not many BMSs have low temp cut-off mainly because most are designed in warm climates in the far east, where the critical need for it here in the colder climates simply has not penetrated the consciousness of the designers. Someone earlier said it is ok to charge at 'low rates' below 0c but from what I've read, no it isn't. Its not ok to charge below 0c AT ALL unless you're happy to play Russian roulette with your cells. If you watch a few of Will Prowse's videos you'll notice whenever he reviews a BMS or a battery, he always looks for and comments on the presence or otherwise of a low temp cut-off, and then he tests it to see if it works.

 

Its the BMV712s that do the controlling bit, in truth. The B2Bs and and MPPTs have their own built in disconnects, and I've set those to trigger at 85% SoC, so that's a routine day-to-day way of ensuring the SoC stays within set limits without manual interference- more aimed at visitors than for myself, if I'm being honest. 

 

There is another BMV712 that triggers physical disconnection of the actual charging cables via a BEP switch- that's more the emergency disconnect for high voltages. 

 

When I first bought the batteries I had no idea at all of what might be suitable monitoring methods, and I agree that in not monitoring each cell I am running a risk that one cell might go out of balance and be ruined by overcharging or undercharging. 

 

I wont say that I'm over the moon about it, and I would like to monitor each cell, but it would be a swine of a job to break into the casing and get at the cells. In teh longer term, I might well have a go at taking out the cells and installing a BMS, but I've got no plans at the moment.  

 

My way of addressing that is to make sure I dont push the batteries too hard in terms of high a low voltages, and I hope that by giving them an easy life (at battery level), there will be less chance of one of the cells getting stressed/damaged. 

 

We went over this a few pages ago in this thread as I recall, and I have definitely given it some thought since then, but no action (yet!).

 

 

Link to comment
Share on other sites

9 minutes ago, Tony1 said:

I don't know whether/how much my LiFeP04s would be damaged if I start the engine at 8am on a bright but freezing morning, and started throwing 90+ amps into them, when the batteries were at 2 degrees, say, or 4 degrees. Its above zero, but not by very much, and I wonder if there might be some level of deterioration caused?

 

At 2c or 4c they would be fine. The threshold for damage really is exactly 0c from what I've read. Consequently the recommended temp for low temp cutoff is often stated to be 4c or 5c, to give a safety margin. This however leads to endless confusion such as you express above. 

 

Another thing to bear in mind is canals can freeze over at above 0c. The evaporation of water from wind blowing over it requires the latent heat of evaporation and it gets this from the water itself, artificially and locally lowering the surface temperature. The air temp in your engine bay is quite likely to be a degree or three above zero despite the ice on the water. Also, the canal water that hasn't frozen against the sides of your hull is by definition above 0c which helps keep the air temp in there, up. 

Link to comment
Share on other sites

16 minutes ago, MtB said:

Ah I didn't realise your B2Bs were your ONLY method of control, I imagined you would have a proper BMS underneath it all. But I lost the will to live with that thread, months ago. 

 

Also despite all the money your evolved system cost. I think not having cell monitoring is asking for trouble. How do you know how well matched and balanced the cells are?

 

And finally not many BMSs have low temp cut-off mainly because most are designed in warm climates in the far east, where the critical need for it here in the colder climates simply has not penetrated the consciousness of the designers. Someone earlier said it is ok to charge at 'low rates' below 0c but from what I've read, no it isn't. Its not ok to charge below 0c AT ALL unless you're happy to play Russian roulette with your cells. If you watch a few of Will Prowse's videos you'll notice whenever he reviews a BMS or a battery, he always looks for and comments on the presence or otherwise of a low temp cut-off, and then he tests it to see if it works.

Tony doesn’t have visible cells, he has batteries. They do have internal electronics that deals with balancing, but not disconnection.

Link to comment
Share on other sites

1 minute ago, MtB said:

 

At 2c or 4c they would be fine. The threshold for damage really is exactly 0c from what I've read. Consequently the recommended temp for low temp cutoff is often stated to be 4c or 5c, to give a safety margin. This however leads to endless confusion such as you express above. 

 

No I think this is incorrect. There is nothing special about 0C except that it’s the freezing point of water. There is not water in an Li battery and it’s contents freezing is not the issue. The issue is the slowing of the reaction rate which means that rather than the Li ions migrating as usual, some of them end up plating the electrode, which is an irreversible process. The reaction rate slows progressively with reducing temperature, albeit not linearly.

 

My cells quote minimum charging temperature of zero, and the maximum charge current is 1C (600A in my case). It is clearly not the case that one can charge at 600A at 0 and not charge at all at -1

Link to comment
Share on other sites

26 minutes ago, nicknorman said:

No I think this is incorrect. There is nothing special about 0C except that it’s the freezing point of water. There is not water in an Li battery and it’s contents freezing is not the issue. The issue is the slowing of the reaction rate which means that rather than the Li ions migrating as usual, some of them end up plating the electrode, which is an irreversible process. The reaction rate slows progressively with reducing temperature, albeit not linearly.

 

My cells quote minimum charging temperature of zero, and the maximum charge current is 1C (600A in my case). It is clearly not the case that one can charge at 600A at 0 and not charge at all at -1

 

I was wondering whether low temp damage was a 'sliding scale' sort of issue, where you would see gradual deterioration by charging at near-zero temps over a long period- as you might see if you keep the batteries above 90% SoC for most of their life. 

 

 

31 minutes ago, nicknorman said:

Tony doesn’t have visible cells, he has batteries. They do have internal electronics that deals with balancing, but not disconnection.

 

 

Just on this- I found a video a while ago of a chap who extracted his cells from a valence and installed a BMS:

It seems like its a feasible job, but I've not long completed a major phase of upgrades on my electrics, and I haven't got the stomach to do this sort of thing!

 

 

 

 

Edited by Tony1
Link to comment
Share on other sites

46 minutes ago, nicknorman said:

We don’t have solar, but if we did that might be problematic unless the controller is set up for Li and knows not to charge if below zero. I think disconnecting the solar controller from the batteries is a fairly inelegant way of dealing with a routine situation.

 

Until this problem of prevention of charging below 0c is resolved, the "LFP for boats" industry cannot be regarded as mature. As you say, disconnecting the cells from the charging source as the temp approaches 0c is crude and inelegant. What really needs to happen is all the different charging sources on the market need to routinely monitor temp at the cells and turn themselves off at (just above) 0c. The emergency low temp disconnect in the BMS should be only a back-up for if/when the charge source fails to disconnect due to a fault.

 

We need to start seeing Solar controllers, alternators and generators which could perhaps be described by marketing departments as "Lithium ready". I.e. having cell temperature monitors built in. The obvious way is with a long wire with a thermocouple on the end to attach to one of the cells, but maybe a better way would be to have a single bluetooth type monitor on the cell(s) which can communicate wirelessly with all the charge devices on the boat. 

Link to comment
Share on other sites

3 minutes ago, Tony1 said:

 

I was wondering whether low temp damage was a 'sliding scale' sort of issue, where you would see gradual deterioration by charging at near-zero temps over a long period- as you might see if you keep the batteries above 90% SoC for most of their life. 

Not exactly. The issue is that once you push current into the battery faster than it can be absorbed by the normal reaction process, the excess current goes to lithium metal plating which is irreversible and damaging.

 

One charge cycle of say charging at 1C at -20 degC will likely completely trash the battery.

 

So it is a matter of exceeding this threshold (or not). You can still charge below zero provided the rate is sufficiently low not to exceed the threshold where Li plating occurs. 

Link to comment
Share on other sites

19 minutes ago, nicknorman said:

No I think this is incorrect. There is nothing special about 0C except that it’s the freezing point of water. There is not water in an Li battery and it’s contents freezing is not the issue. The issue is the slowing of the reaction rate which means that rather than the Li ions migrating as usual, some of them end up plating the electrode, which is an irreversible process. The reaction rate slows progressively with reducing temperature, albeit not linearly.

 

My cells quote minimum charging temperature of zero, and the maximum charge current is 1C (600A in my case). It is clearly not the case that one can charge at 600A at 0 and not charge at all at -1

 

I'd long ago picked up this impression from reading the Nordkyn Design website. Nothing to do with water in the cells freezing as there is none, just that 0c happened to be the temperature ISTR him saying was the threshold to be avoided. The site had a major re-write a few months or a year ago (and all the previous stuff he published on lithiums was dumped) at which point IIRC he introduced the concept of electrode plating when charge rates exceed the rate the reaction can absorb it, so I think he has developed the point about low temp charging. Yes I can see it is unlikely there would be a sudden threshold and claiming 0c must be an oversimplification. 

 

I'll have a read of it again. 

Link to comment
Share on other sites

8 minutes ago, MtB said:

 

Until this problem of prevention of charging below 0c is resolved, the "LFP for boats" industry cannot be regarded as mature. As you say, disconnecting the cells from the charging source as the temp approaches 0c is crude and inelegant. What really needs to happen is all the different charging sources on the market need to routinely monitor temp at the cells and turn themselves off at (just above) 0c. The emergency low temp disconnect in the BMS should be only a back-up for if/when the charge source fails to disconnect due to a fault.

 

We need to start seeing Solar controllers, alternators and generators which could perhaps be described by marketing departments as "Lithium ready". I.e. having cell temperature monitors built in. The obvious way is with a long wire with a thermocouple on the end to attach to one of the cells, but maybe a better way would be to have a single bluetooth type monitor on the cell(s) which can communicate wirelessly with all the charge devices on the boat. 

As to the Bluetooth idea, that requires compatibility between different manufacturer’s devices. Not going to happen! If you stick with Victron, they seem pretty good at sharing data so 1 temperature sensor (not a thermocouple, perleese!) would send data to be shared by all the kit. Well except for the alternator I suppose, until Victron start selling an alternator controller!

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.