Monitoring lithium batteries

[WotEver]

16 minutes ago, cuthound said:

These components are most likely to be external to the battery.

As part of the external BMS. Messy, innit?

[Dr Bob]

6 minutes ago, WotEver said:

As part of the external BMS. Messy, innit?

Bristol Motor Speedway is far from messy. It is quite a tidy place.

[peterboat]

The internal BMS must do something because when connected to the puter it balances the cells not that they are far out anyway but you can see the cells going up and down to achieve perfik balance

[nicknorman]

50 minutes ago, peterboat said:

The internal BMS must do something because when connected to the puter it balances the cells not that they are far out anyway but you can see the cells going up and down to achieve perfik balance

Yes obviously cell balancing has to be done internally as you only have the 0v and 12v terminals accessible outside the box.

[peterboat]

I think I will post a video of me plugging in and show you what happens with valence batteries when you do it may clear up misconceptions on here about them

[Dr Bob]

35 minutes ago, peterboat said:

I think I will post a video of me plugging in and show you what happens with valence batteries when you do it may clear up misconceptions on here about them

Peter, that would be very interesting.

[nicknorman]

1 hour ago, peterboat said:

I think I will post a video of me plugging in and show you what happens with valence batteries when you do it may clear up misconceptions on here about them

Yes please.

[Robbo]

11 hours ago, peterboat said:

I think I will post a video of me plugging in and show you what happens with valence batteries when you do it may clear up misconceptions on here about them

Interested to see as well.

[WotEver]

11 hours ago, peterboat said:

I think I will post a video of me plugging in and show you what happens with valence batteries when you do it may clear up misconceptions on here about them

Make that 4 of us

[Dr Bob]

13 minutes ago, WotEver said:

Make that 4 of us

My God. Agreement!

[peterboat]

Right will do I will get my mate to film it so its clearer

[MoominPapa]

OK, you naughty people. You've got me interested in this and I've wasted hours down the internet rabbit-hole reading stuff. 

 

Did find this, which is the best primer for marine 12V systems I've seen.

 

http://nordkyndesign.com/category/marine-engineering/electrical/lithium-battery-systems/

 

MP.

 

[nicknorman]

Unfortunately my Trojans are still perfect after 5 years so I have no excuse for trying out Lithiums, but if I’ve been thinking about how I’d do it. We already have a Mastervolt Masterbus system comprising Combi, Mastershunt (AH-counting battery monitor) and Masterview Easy control and monitoring panel. I’d get Mastervolt’s external alternator regulator Alpha Pro III which is Masterbus compatible. It’s a 3-stage type regulator. Then I’d configure an event generated by the Mastershunt reaching 80% SoC to force the alternator to float voltage. Float voltage would be set to the open circuit battery voltage at 80% SoC, so it would supply any boat loads without taking the SoC over 80%.

 

Ditto for the Combi, the same trigger of 80% SoC would be used to force the Combi to float mode. But curses, our current Combi doesn’t have such an event command! So I’d need the Combi to blow up as well so I could get one of the new ones with event commands to, amongst other things, switch to float mode!

 

Oh well, the Trojans and the Combi can’t last for ever!

[MtB]

2 minutes ago, nicknorman said:

Unfortunately my Trojans are still perfect after 5 years so I have no excuse for trying out Lithiums, but if I’ve been thinking about how I’d do it. We already have a Mastervolt Masterbus system comprising Combi, Mastershunt (AH-counting battery monitor) and Masterview Easy control and monitoring panel. I’d get Mastervolt’s external alternator regulator Alpha Pro III which is Masterbus compatible. It’s a 3-stage type regulator. Then I’d configure an event generated by the Mastershunt reaching 80% SoC to force the alternator to float voltage. Float voltage would be set to the open circuit battery voltage at 80% SoC, so it would supply any boat loads without taking the SoC over 80%.

 

Ditto for the Combi, the same trigger of 80% SoC would be used to force the Combi to float mode. But curses, our current Combi doesn’t have such an event command! So I’d need the Combi to blow up as well so I could get one of the new ones with event commands to, amongst other things, switch to float mode!

 

Oh well, the Trojans and the Combi can’t last for ever!

 

I too am considering lithium. I however am thinking of doing it low tech. 

 

Four bare cells on  bench, wired up to my alternator. To charge, I first measure the terminal voltage and look up the SoC on my chart on the wall. Then I start the engine and set my alarm clock when to expect 80% SoC to be reached when I stop the engine again. 

 

Should the batteries over-charge and overheat, my smoke alarm ought to alert me.  My low voltage 'alarm' will be when the lights go dim. 

 

Can anyone foresee any problems with this  simple and elegant solution? I can't....

[WotEver]

9 hours ago, Dr Bob said:

My God. Agreement!

Only so we can argue over what we see...

Just now, Mike the Boilerman said:

 

I too am considering lithium. I however am thinking of doing it low tech. 

 

Four bare cells on  bench, wired up to my alternator. To charge, I first measure the terminal voltage and look up the SoC on my chart on the wall. Then I start the engine and set my alarm clock when to expect 80% SoC to be reached when I stop the engine again. 

 

Should the batteries over-charge and overheat, my smoke alarm ought to alert me.  My low voltage 'alarm' will be when the lights go dim. 

 

Can anyone foresee any problems with this  simple and elegant solution? I can't....

Sounds great. Don’t forget to nail the cells to the bench.

[MtB]

5 minutes ago, WotEver said:

 

Sounds great. Don’t forget to nail the cells to the bench.

 

Oh yes. Same as gas bottles. I forgot narrowboats tend to turn upside down periodically...

[MtB]

On 07/08/2018 at 14:31, 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.

 

After 240-odd posts, did you ever get an answer to your question, Dr Bob?

 

Plenty of comments about AH counting in and out so presumably the answer to your Q is 'no'. 

[WotEver]

6 minutes ago, Mike the Boilerman said:

 

After 240-odd posts, did you ever get an answer to your question, Dr Bob?

 

Plenty of comments about AH counting in and out so presumably the answer to your Q is 'no'. 

It’s been answered several times I believe. If the maximum charge voltage on a constant current charger is limited to the 80% value then the battery will only be charged to 80%. 

41 minutes ago, MoominPapa said:

OK, you naughty people. You've got me interested in this and I've wasted hours down the internet rabbit-hole reading stuff. 

 

Did find this, which is the best primer for marine 12V systems I've seen.

 

http://nordkyndesign.com/category/marine-engineering/electrical/lithium-battery-systems/

 

MP.

 

An excellent link sir

[MtB]

1 minute ago, WotEver said:

It’s been answered several times I believe. If the maximum charge voltage on a constant current charger is limited to the 80% value then the battery will only be charged to 80%. 

 

Thanks. I thought that was the case but couldn't find it. 

 

However if the charge voltage is EXACTLY the same as the 80% SoC terminal voltage, surely 80% will be approached but never reached? Once there is no potential difference between charger output voltage and battery terminal voltage, no current will flow Shirley. 

[WotEver]

4 minutes ago, Mike the Boilerman said:

 

Thanks. I thought that was the case but couldn't find it. 

 

However if the charge voltage is EXACTLY the same as the 80% SoC terminal voltage, surely 80% will be approached but never reached? Once there is no potential difference between charger output voltage and battery terminal voltage, no current will flow Shirley. 

As the voltage reaches the 80% value the current will drop to zero so yes, the value will be reached but only for a short while until all 4 cells stop drawing current. Note that to safely charge a 12V Lithium it MUST contain cell-level internal BMS circuitry to protect against cell imbalance (where one cell could be at 100% and the other three at 73%). That’s one of the reasons why a BMS is essential. 

[MoominPapa]

18 minutes ago, WotEver said:

As the voltage reaches the 80% value the current will drop to zero so yes, the value will be reached but only for a short while until all 4 cells stop drawing current. Note that to safely charge a 12V Lithium it MUST contain cell-level internal BMS circuitry to protect against cell imbalance (where one cell could be at 100% and the other three at 73%). That’s one of the reasons why a BMS is essential. 

But doesn't the balancing process require a charge to ~100%, which needs Ah counting? If you only ever charge to 80% your cells will never get balanced?

 

MP.

 

[WotEver]

1 hour ago, MoominPapa said:

But doesn't the balancing process require a charge to ~100%, which needs Ah counting? If you only ever charge to 80% your cells will never get balanced?

 

MP.

 

It depends. Mastervolt (and others) employ active balancing, which aims to keep all cells balanced all the time. Many others, with Victron being one of the leaders, employ passive balancing which can be done either high or low. 

 

However, what I was referring to above is a situation where the cells have become unbalanced after an extended period of time. In such an instance you can’t simply take the battery to say 85% by calculating the external charge voltage and assume that all 4 cells are at 85%. One of them might have got to 100% some time ago and so the internal BMS is essential to ensure that that cell doesn’t become overcharged. 

[MoominPapa]

2 minutes ago, WotEver said:

It depends. Mastervolt (and others) employ active balancing, which aims to keep all cells balanced all the time. Many others, with Victron being one of the leaders, employ passive balancing which can be done either high or low. 

 

However, what I was referring to above is a situation where the cells have become unbalanced after an extended period of time. In such an instance you can’t simply take the battery to say 85% by calculating the external charge voltage and assume that all 4 cells are at 85%. One of them might have got to 100% some time ago and so the internal BMS is essential to ensure that that cell doesn’t become overcharged. 

 

Your second para is the reason why you need a BMS that has access to the cell voltages, and the ability to terminate charge at any time.

 

My understanding is that top and bottom balancing actually achieve different states. Given that the cells will have slightly different capacities, top balancing tries to ensure that the cells all reach 100% charge at the same time: this implies that they won't all reach 0% charge at t he same time. Bottom balancing tries to ensure they reach 0% charge at the same but implies that they won't all reach 100% at the same time. The net effect is that on a bottom balanced battery it's possible to detect 0% charge accurately, but not 100% charge, and a top balance battery it's possible to find 100% more accurately then 0%.

 

Bottom balancing is useful eg in EV cars where you want to discharge are far as possible, and, more importantly, you need to know at any time how much charge is left with as much accuracy as possible. The car telling the driver it has 20 miles left then suddenly shutting down because the weakest cell hit minimum voltage is not a good look.

 

For off-grid systems, top balancing is better because it allows you to get closer to maximum charge without overcharge, at the expense of more uncertainty about where the cut off will be at maximium discharge, which is less likely to ever occur.

 

MP.

 

[peterboat]

2 minutes ago, MoominPapa said:

 

Your second para is the reason why you need a BMS that has access to the cell voltages, and the ability to terminate charge at any time.

 

My understanding is that top and bottom balancing actually achieve different states. Given that the cells will have slightly different capacities, top balancing tries to ensure that the cells all reach 100% charge at the same time: this implies that they won't all reach 0% charge at t he same time. Bottom balancing tries to ensure they reach 0% charge at the same but implies that they won't all reach 100% at the same time. The net effect is that on a bottom balanced battery it's possible to detect 0% charge accurately, but not 100% charge, and a top balance battery it's possible to find 100% more accurately then 0%.

 

Bottom balancing is useful eg in EV cars where you want to discharge are far as possible, and, more importantly, you need to know at any time how much charge is left with as much accuracy as possible. The car telling the driver it has 20 miles left then suddenly shutting down because the weakest cell hit minimum voltage is not a good look.

 

For off-grid systems, top balancing is better because it allows you to get closer to maximum charge without overcharge, at the expense of more uncertainty about where the cut off will be at maximium discharge, which is less likely to ever occur.

 

MP.

 

You are right but because in our boat systems we dont draw big current the batteries dont go out of balance, in cars huge current is drawn and a BMS is essential to balance both the batteries and cells that make up the pack. My drive batteries will hardly be working in my system because my plan is to move when the sun is shining and solar is producing a lot of amps, my batteries are for emergencies not day to day travel

[Dr Bob]

3 hours ago, Mike the Boilerman said:

 

After 240-odd posts, did you ever get an answer to your question, Dr Bob?

 

Plenty of comments about AH counting in and out so presumably the answer to your Q is 'no'. 

Yes, I am a happy bunny now. Wotever answers it in his posts in the last few hours. Also because there is little surface charge effect, then determining SoC from rested voltage is ok. Very happy that charging to say 13.8V will not overcharge.

The issues that still bug me are

- will it overload my  90A alternator although likely it will only run for an hour or hour and a half to replace my 100Ahr overnight usage

- are LiFePO4 really that stable to thermal runaway....I'm not totally convinced?

- Can I get some cheap 2nd hand Lithiums?