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Dr Bob

Monitoring lithium batteries

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Daft question coming!

I've been reading all the threads on LiFePo batteries so reasonably up to speed on the technology.....but I have a question.

One of the benefits claimed is that the voltage of the lithiums doesnt vary "much" with SoC....but then here's me thinking you can still monitor SoC by looking at the 'at rest' voltage and I guess the 'smart' chargers like my victron IP22 Blue smart (when in lithium mode) are monitoring voltage when charging and deciding to switch off when a certain voltage is reached. Is the variation of lithium voltage vs SoC big enough for charge to be monitored and turned off at say 80% full.

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Just now, Dr Bob said:

Daft question coming!

I've been reading all the threads on LiFePo batteries so reasonably up to speed on the technology.....but I have a question.

One of the benefits claimed is that the voltage of the lithiums doesnt vary "much" with SoC....but then here's me thinking you can still monitor SoC by looking at the 'at rest' voltage and I guess the 'smart' chargers like my victron IP22 Blue smart (when in lithium mode) are monitoring voltage when charging and deciding to switch off when a certain voltage is reached. Is the variation of lithium voltage vs SoC big enough for charge to be monitored and turned off at say 80% full.

Short answer, yes. Second short answer, the BMS looks after all of that for you. 

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1 minute ago, WotEver said:

Short answer, yes. Second short answer, the BMS looks after all of that for you. 

Ok Tony, thanks. Yes, I have just had a google around and seen graphs that show voltage variation on slight discharge showing a drop of 1 volt (on a 14V battery) between 80% and 20% SoC. That does though raise another question. You are therefore looking at a smaller range of voltage to estimate the SoC (ie my Victron charger is seeing a smaller range so has to be 'more accurate' in knowing when to turn off .....or as you say, the BMS is going to be doing that job). What happens then if you compare batteries from different manufacturers? Are they likely to be at slightly different voltages so a BMS has to be exactly matched to the batteries in use.....so you have to use a BMS supplied by that battery manufacturer?

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4 minutes ago, Dr Bob said:

Ok Tony, thanks. Yes, I have just had a google around and seen graphs that show voltage variation on slight discharge showing a drop of 1 volt (on a 14V battery) between 80% and 20% SoC. That does though raise another question. You are therefore looking at a smaller range of voltage to estimate the SoC (ie my Victron charger is seeing a smaller range so has to be 'more accurate' in knowing when to turn off .....or as you say, the BMS is going to be doing that job). What happens then if you compare batteries from different manufacturers? Are they likely to be at slightly different voltages so a BMS has to be exactly matched to the batteries in use.....so you have to use a BMS supplied by that battery manufacturer?

If you DIY then you have to very carefully match the BMS settings to the Batteries. In particular the high voltage setting to keep them below say 70% SoC when charging.  

 

Victron stuff might be expensive but they’ve done all the hard work for you, effectively making it simply plug ‘n play. 

 

Note that for safety, Jono even added a LA battery in parallel to act as a dump load for the alternator when the BMS switches off. So even with Victron stuff it’s not necessarily dead straightforward. 

Edited by WotEver
Added a bit.
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2 minutes ago, WotEver said:

If you DIY then you have to very carefully match the BMS settings to the Batteries. In particular the high voltage setting to keep them below say 70% SoC when charging.  

 

Victron stuff might be expensive but they’ve done all the hard work for you making it simply plug ‘n play. 

yep, thanks for confirming all of that. Have a greenie.

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13 minutes ago, Dr Bob said:

yep, thanks for confirming all of that. Have a greenie.

Keep it simple Doc.......you will be buying an electric car at this rate 😎

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4 hours ago, mrsmelly said:

Keep it simple Doc.......you will be buying an electric car at this rate 😎

Me? I'm just a muppet with a duck.

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1 hour ago, roland elsdon said:

Let someone else do the development work thats my motto. Being stuck in 1937 is fine

Yep.my cheapo lead acids work 365 days a year without fault......simples.....

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4 minutes ago, mrsmelly said:

Yep.my cheapo lead acids work 365 days a year without fault......simples.....

And for how many periods of 365 days are they going to work for? 2? 3? 4??

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7 hours ago, WotEver said:

If you DIY then you have to very carefully match the BMS settings to the Batteries. In particular the high voltage setting to keep them below say 70% SoC when charging.  

 

Victron stuff might be expensive but they’ve done all the hard work for you, effectively making it simply plug ‘n play. 

 

Note that for safety, Jono even added a LA battery in parallel to act as a dump load for the alternator when the BMS switches off. So even with Victron stuff it’s not necessarily dead straightforward. 

I am using these batteries right now I have also spoken to Jono the other day when I met him at west stockworth. He is restricting the charge on these batteries like I am, he like me is charging to 13.8 volts and in float at 13.6 volts, this restriction of charging to approx 80% prolongs the life of the battery in my case to 10000 cycles down to 80% of life. My BMS does nothing as the cells are staying in balance by restricting the upper voltage. Now this is practical experience gained by James, JohnV and myself, James has been using these batteries for over a year John and myself months. I would not go back to lead acid or any of the other batteries available as their performance is so poor in comparison to the LifePo4s I have

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8 hours ago, peterboat said:

I would not go back to lead acid or any of the other batteries available as their performance is so poor in comparison to the LifePo4s I have

Certainly if it suits your lifestyle, that’s the way to go. I’m also aware of live-aboards who do not struggle with their LAs coupled with plenty of solar. They get in excess of 4 years from their batteries, at which time they can replace the entire set for less than half the price of a single lithium. Let alone worrying about matching a complete system of components. 

 

Horses for courses, as they say. 

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9 hours ago, peterboat said:

I am using these batteries right now I have also spoken to Jono the other day when I met him at west stockworth. He is restricting the charge on these batteries like I am, he like me is charging to 13.8 volts and in float at 13.6 volts, this restriction of charging to approx 80% prolongs the life of the battery in my case to 10000 cycles down to 80% of life. My BMS does nothing as the cells are staying in balance by restricting the upper voltage. Now this is practical experience gained by James, JohnV and myself, James has been using these batteries for over a year John and myself months. I would not go back to lead acid or any of the other batteries available as their performance is so poor in comparison to the LifePo4s I have

Thanks Peter. What are your voltages at rest at say 50% and 20% SoC? Are you still able to look at your BMV type gauge and estimate SoC from voltage at rest and AHrs used?

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1 hour ago, WotEver said:

Certainly if it suits your lifestyle, that’s the way to go. I’m also aware of live-aboards who do not struggle with their LAs coupled with plenty of solar. They get in excess of 4 years from their batteries, at which time they can replace the entire set for less than half the price of a single lithium. Let alone worrying about matching a complete system of components. 

 

Horses for courses, as they say. 

Precisely. My lead acids take zero maintenance. I never revisit them after fitting until replacement at two plus years and 700 or more cycles. No rewiring and charging systems and monitoring that are cheap, simple and known to be safe. The picture on my telly and whirling of my washing machine work perfectly a prime example of its not broken so I ain't gonna fix it. Why the hell fork out thousands to work my telly it's as daft at present as buying an electric car then wanting to do my usual 500 mile run without parking up to recharge whilst a petrol car would have me at the destination. Let companies and the MOD etc spend their money on development then in around twenty years time they may well be worth looking at.

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8 minutes ago, mrsmelly said:

Precisely. My lead acids take zero maintenance. I never revisit them after fitting until replacement at two plus years and 700 or more cycles. No rewiring and charging systems and monitoring that are cheap, simple and known to be safe. 

LiFePO4's are safe.    The reason to go LiFePO4's are because it's cheaper long term if your off-grid and charge via generator/engine.   This isn't just due to the amount of cycles they can do, but due to the reduced time in charging them, which equals less generator/engine time. 

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My question in the original post was to try and get an understanding of how to monitor Lithiums given some sites say that voltage doesn't vary much with SoC. The post was not to open up the debate on which is the best battery system to use which is a simple decision for the majority who are not on the Bleeding edge. As the use of Lithiums grow, then price will drop and they will become more attractive.

 

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10 minutes ago, Dr Bob said:

My question in the original post was to try and get an understanding of how to monitor Lithiums given some sites say that voltage doesn't vary much with SoC. The post was not to open up the debate on which is the best battery system to use which is a simple decision for the majority who are not on the Bleeding edge. As the use of Lithiums grow, then price will drop and they will become more attractive.

I don't think the prices will drop dramatically (if it all),  once they start using the batteries for cars on a serious scale you'll probably get a Lithium demand causing the price to go up (they expect it to quadruple by 2025).

 

Lithium Price from 2002-2018; https://www.metalary.com/lithium-price/

Edited by Robbo

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1 hour ago, Dr Bob said:

Thanks Peter. What are your voltages at rest at say 50% and 20% SoC? Are you still able to look at your BMV type gauge and estimate SoC from voltage at rest and AHrs used?

 

14 minutes ago, Dr Bob said:

My question in the original post was to try and get an understanding of how to monitor Lithiums given some sites say that voltage doesn't vary much with SoC. The post was not to open up the debate on which is the best battery system to use which is a simple decision for the majority who are not on the Bleeding edge. As the use of Lithiums grow, then price will drop and they will become more attractive.

 

Its difficult Bob because they are normally at 13.3 by the end of the day, which as you know is still to high for normal LA equipment [my BMS is at 105% all the time] For me its as others have said is the speed of charging that is my draw to use them.

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31 minutes ago, Robbo said:

The reason to go LiFePO4's are because it's cheaper long term if your off-grid and charge via generator/engine.

Which is exactly why it doesn’t make sense for CCers who cruise a lot and have Solar to boot. Their LA batteries get fully charged anyway. 

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1 hour ago, Dr Bob said:

Thanks Peter. What are your voltages at rest at say 50% and 20% SoC? Are you still able to look at your BMV type gauge and estimate SoC from voltage at rest and AHrs used?

 

 

Dr Bob has raised a question with this thread that has troubled me too with lithiums.

 

GIven they accept (effectively) unlimited current during charging, and it is critically important to stop charging before they get to 100%, how is the state of charge determined during charging?

 

Peterboat is the only only so far to give an answer, which to summarise is "don't worry about it, the BMS handles all that". Now that just doesn't cut it for me. Dunno about Dr Bob!

 

So the question I have, and perhaps Dr Bob too, is "how does the BMS know the SoC during charging"? I think the only way for it to know is to count the AH out and back in repeatedly from a known starting SoC. 

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1 hour ago, Dr Bob said:

Are you still able to look at your BMV type gauge and estimate SoC from voltage at rest and AHrs used?

You can go from Ah used, or if you go Victron then it gives an accurate ‘fullness’ reading. A ‘standard’ battery monitor is useless although all the manufacturers are aware of that and are bringing (or have brought) solutions to market. Gibbo for instance already has lithium monitoring devices, although for military use as per all of his current products. 

1 minute ago, Mike the Boilerman said:

how does the BMS know the SoC during charging"?

Voltage. It’s very cut and dried. Cut off the charge at the correct voltage and you’ll limit charging to 70%, or 80%, or whatever you choose. 

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2 minutes ago, WotEver said:

 

Voltage. It’s very cut and dried. Cut off the charge at the correct voltage and you’ll limit charging to 70%, or 80%, or whatever you choose. 

 

So the charge voltage rises proportionally with SoC regardless of charge current? 

 

 

 

ISTR the internal resistance of a lithium is discountably small so the corollary to this must be that during discharge, the terminal voltage does not change with load either, so SoC during discharge is really easy for the BMS to know too. Yes?

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13 minutes ago, WotEver said:

Which is exactly why it doesn’t make sense for CCers who cruise a lot and have Solar to boot. Their LA batteries get fully charged anyway. 

I've always argued that they make sense if your off-grid in winter.   Solar only really provides 100% for a short period of the year every day (unless your boat looks like a solar farm), Lithium still has a advantage even when charging via solar as you can do the 20%-80% DoD without damage on the months where you don't get as much sun daily.

20 minutes ago, WotEver said:

or if you go Victron then it gives an accurate ‘fullness’ reading

If you going Victron, the Smart LiFoPO4's don't need a external monitor as the batteries have it built in.

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1 hour ago, mrsmelly said:

Precisely. My lead acids take zero maintenance. I never revisit them after fitting until replacement at two plus years and 700 or more cycles. No rewiring and charging systems and monitoring that are cheap, simple and known to be safe.

However, let’s not forget the £5000 per year you spend on candles!

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1 hour ago, Mike the Boilerman said:

 

 

Dr Bob has raised a question with this thread that has troubled me too with lithiums.

 

GIven they accept (effectively) unlimited current during charging, and it is critically important to stop charging before they get to 100%, how is the state of charge determined during charging?

 

Peterboat is the only only so far to give an answer, which to summarise is "don't worry about it, the BMS handles all that". Now that just doesn't cut it for me. Dunno about Dr Bob!

 

So the question I have, and perhaps Dr Bob too, is "how does the BMS know the SoC during charging"? I think the only way for it to know is to count the AH out and back in repeatedly from a known starting SoC. 

 

1 hour ago, WotEver said:

You can go from Ah used, or if you go Victron then it gives an accurate ‘fullness’ reading. A ‘standard’ battery monitor is useless although all the manufacturers are aware of that and are bringing (or have brought) solutions to market. Gibbo for instance already has lithium monitoring devices, although for military use as per all of his current products. 

Voltage. It’s very cut and dried. Cut off the charge at the correct voltage and you’ll limit charging to 70%, or 80%, or whatever you choose. 

 

1 hour ago, Mike the Boilerman said:

 

So the charge voltage rises proportionally with SoC regardless of charge current? 

 

 

 

ISTR the internal resistance of a lithium is discountably small so the corollary to this must be that during discharge, the terminal voltage does not change with load either, so SoC during discharge is really easy for the BMS to know too. Yes?

 

1 hour ago, Robbo said:

I've always argued that they make sense if your off-grid in winter.   Solar only really provides 100% for a short period of the year every day (unless your boat looks like a solar farm), Lithium still has a advantage even when charging via solar as you can do the 20%-80% DoD without damage on the months where you don't get as much sun daily.

If you going Victron, the Smart LiFoPO4's don't need a external monitor as the batteries have it built in.

To answer as many questions as possible my alternator is not connected I charge from solar [midnite] it only charges at 13.8 volts for about 50 mins after that it goes to float at 13.6 volts this protects the batteries from over voltage [I also have a disconnect if midnite fails] I check the batteries on the puter every now and then the BMS gives me the state of charge of the battery ie 85% the BMS in internal in Valence batteries it also gives lots of other info. My batteries accept all the amps in the world until the voltage hits a certain point then thats it they are charged and the external charge has to be turned off, if it isnt the batteries will be destroyed the BMS in my case does internal cell balancing, it cant disconnect charging. so thats why I have an over voltage disconnect to protect the batteries. anymore questions and if I can I will

Edited by peterboat

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