LiFePo vs PB (Lead acid) ......

[Richard10002]

LiFePo max voltage per cell is 3.6V. Nominal storage and shipping voltage is 3.2 ~ 3.3V. Maximum discharged state is often quoted at 2.5V - but I suggest that going below 3.0V is not advised if you wish to have long service life. If I was to use LiFePo - I would try to not go below 3.2V generally but with occasional drop to 3.0V acceptable.

Second that to get real advantage of Lithium based - you need to have balanced cells.

OK. So, subject to balance, a Low Voltage Cut Off at say, 12.5V, might be a reasonable compromise as a failsafe?

Plus a policy of starting to charge once the voltage has fallen to say, 12.9V?

This should result in never dropping as low as 3.0V per cell, and rarely dropping as low as even 3.2V per cell?

On the balanced cell side of things:

What would you suggest as a reasonable Delta in mV, for a 4 cell nominal 12V battery?

With separate cells, it shouldn't be too difficult to use a resistor, and a volt meter, to read and balance cells, top or bottom?
With 12V units, I suppose you have to hope that the internal BMS is up to the job.

 

[lw395]

Presumably, in the boating world, we would also do all possible to avoid LVC?

When you say "cut in too often", is it the cutting in that does the damage, i.e. the sudden stop of flow?

or is it due to the voltage falling to the LVC level. If the LVC was set to a high enough level, could this be avoided.... e.g. if the LVC were set at say 3.0V per cell, (3.2V?), or 12V, (12.8V?), for a nominal 12V drop in, would that be non fatal?

But it's rarely as simple as wanting a cut off at a certain voltage, because if you cut off at say 12.8V to reflect idle voltage at some arbitrary state of charge, it will cut off at a much higher state of charge if you draw a significant load like an inverter.
It's much easier to design a battery control system if you know how the pack is going to be used. Products like cars and laptops, the people who desing the electronics know how they will be used.

 

[Refueler]

I see we are getting various bits from other battery tech creeping in ...

ie : "What would you suggest as a reasonable Delta in mV, for a 4 cell nominal 12V battery? " ...... Delta is a detection used when charging NiXX based cells which have a slight drop when reaching full charge.
Lithium charging is not using delta - it uses detected voltage and once it hits the set value - charging shuts off.

The BMS should swap from CC to CV format in the charge routine - which is another reason Delta cannot be used.

My reason for saying I prefer not to go below 3.0V is that LiFe is designed as best working in the 3.3 to 3.5v range. It has a relatively shallow gradient to the discharge line and should provide good power for needs till then.

I fully realise that many will take sales blurb and allow discharge to a much lower value. Basically because at my 3.0 - 3.2V level - you are not using all available power. But that is part of my reasoning not to use LiFe in our boats. High price for what I consider small gain.

 

[Richard10002]

I see we are getting various bits from other battery tech creeping in ...

ie : "What would you suggest as a reasonable Delta in mV, for a 4 cell nominal 12V battery? " ...... Delta is a detection used when charging NiXX based cells which have a slight drop when reaching full charge.
Lithium charging is not using delta - it uses detected voltage and once it hits the set value - charging shuts off.

Sorry... I wasn't referring to charging, or discharging, I was referring to cell balancing. I have seen Delta used for this, but perhaps it's not correct.

What would you suggest is the acceptable mV difference between cell voltages in a 4 cell battery, nominal 12V?

 

[Richard10002]

But it's rarely as simple as wanting a cut off at a certain voltage, because if you cut off at say 12.8V to reflect idle voltage at some arbitrary state of charge, it will cut off at a much higher state of charge if you draw a significant load like an inverter.
It's much easier to design a battery control system if you know how the pack is going to be used. Products like cars and laptops, the people who desing the electronics know how they will be used.

I'm stabbing in the dark here, and much of what I post is in the form of an inquisitive question

Given where we are, it is fair to assume that the pack is going to be used in a boat, perhaps a seagoing yacht or motor boat, perhaps a narrowboat, (as in my case). There may be an inverter which could draw up to 2000W or so occasionally. What might you suggest as the LVC, in order to avoid the chance of damage.

Given that it has been suggested that 12.8V is an ideal lower voltage, but occasional drops to 12.0V is OK, perhaps 12V is a reasonable LVC, and perhaps it wouldnt be a bad thing if the pack were cut off if an inverter was used at low SOC, such that the voltage drop took it below the LVC. There could be a policy of not using the inverter below a certain SOC, or Voltage.

If we are going to build in some safety, it is going to be in a BMS, or an external switch, so there has to be a defined figure in mind, (doesn't there?).

 

[lw395]

I'm stabbing in the dark here, and much of what I post is in the form of an inquisitive question

Given where we are, it is fair to assume that the pack is going to be used in a boat, perhaps a seagoing yacht or motor boat, perhaps a narrowboat, (as in my case). There may be an inverter which could draw up to 2000W or so occasionally. What might you suggest as the LVC, in order to avoid the chance of damage.

Given that it has been suggested that 12.8V is an ideal lower voltage, but occasional drops to 12.0V is OK, perhaps 12V is a reasonable LVC, and perhaps it wouldnt be a bad thing if the pack were cut off if an inverter was used at low SOC, such that the voltage drop took it below the LVC. There could be a policy of not using the inverter below a certain SOC, or Voltage.

If we are going to build in some safety, it is going to be in a BMS, or an external switch, so there has to be a defined figure in mind, (doesn't there?).

I think the defined figure might vary a lot with the application.
The application might sense the current and know the capacity, so be able to make an estimate of SoC.
The designer might bear in mind the consequences of suddenly switching off, and use warnings to the operator or low-power modes, instead of leaving the user totally in the dark or whatever.
Designers who work at system level have so much more information that those trying to design a general purpose 'drop in' module, or worse still designers might be working with a specific application in mind, then the 'aftermarket' sells the module into a different role entirely.

For instance in one battery system I know a little about, the monitoring system won't shut the batteries down completely unless they are going to catch fire. Instead, it raises alarms, sheds unimportant loads and fires up a bloody great generator. Not so sure about Li batteries, but you can discharge Lead-Acid batteries further with less consequence if they are recharged immediately.
But sometimes the imperative is to keep something working and if it abuses the battery, tough. Other times, it's important for the battery to last through the warranty period...

 

[Richard10002]

I think the defined figure might vary a lot with the application.
The application might sense the current and know the capacity, so be able to make an estimate of SoC.
The designer might bear in mind the consequences of suddenly switching off, and use warnings to the operator or low-power modes, instead of leaving the user totally in the dark or whatever.
Designers who work at system level have so much more information that those trying to design a general purpose 'drop in' module, or worse still designers might be working with a specific application in mind, then the 'aftermarket' sells the module into a different role entirely.

For instance in one battery system I know a little about, the monitoring system won't shut the batteries down completely unless they are going to catch fire. Instead, it raises alarms, sheds unimportant loads and fires up a bloody great generator. Not so sure about Li batteries, but you can discharge Lead-Acid batteries further with less consequence if they are recharged immediately.
But sometimes the imperative is to keep something working and if it abuses the battery, tough. Other times, it's important for the battery to last through the warranty period...

This is a boat forum. They will be fitted on boats, maybe with an inverter up to around 2000W. Unless in gin palace territory, usage will be around 70-140Ah per day.

A BMV 712 can give an alarm at say, 12.8V, and a Victron Battery Protect could cut the power at say, 12V. There are also one or two relay facilities in the BMV which can relate to Temp, SOC, voltage.

As I have said, the above don't get to cell levels, but.......

You seem to have a built in personal policy of it not being worth having LiFePo4 batteries on boats, based mostly on money, and partly on an inability to protect them.

Yet plenty of people have destroyed their LA banks in short order, so there is an inability to protect built into LAs, just like Li's.

 

[Refueler]

Cell variation ? "acceptable mV difference between cell voltages in a 4 cell battery, nominal 12V? "

If we see more than 0.1V cell to cell variation - we start to wonder why.

OK - lets get onto the shut off part - the Low Voltage part is basically taken care off by a BMS shutting down the load and protecting the battery. That would occur whatever battery system we have ... first indication is lights dimming or something not working. Its not end of day .... start engine or instigate charging and all's well.
My biggest worry is actually at the other end of the charge scale ... at full charge. Lithium cells do not appreciate having charge hammered at them when full, no matter how low a rate it is ... but a Lead Acid will suffer it. Imagine your alternator .... nicely charging away ... amps dropping off as charge level rises .. and then BHAM - battery shuts off ..... So much for your regulator diodes. So we now need a system to avoid that embarrassing event ....

I know some alternators have built in protection to try avoid this - but I know through own experience that it is not always successful.

 

[Richard10002]

100mV then.... thanks.

I've seen a few suggestions that a Lead Acid battery/bank in the system can prevent the alternator problem. Either the lead acid is charged by the alternator, and some kind of Battery to Battery charger is fitted between the Lead Acid battery and the lithium bank. Alternatively, some kind of switch or relay stops the charge to the lithiums at a certain voltage, and diverts it to the Lead Acid. In either case, the alternator is free to run with gay abandon, without fear of BHAM

 

[lw395]

This is a boat forum. They will be fitted on boats, maybe with an inverter up to around 2000W. Unless in gin palace territory, usage will be around 70-140Ah per day.

A BMV 712 can give an alarm at say, 12.8V, and a Victron Battery Protect could cut the power at say, 12V. There are also one or two relay facilities in the BMV which can relate to Temp, SOC, voltage.

As I have said, the above don't get to cell levels, but.......

You seem to have a built in personal policy of it not being worth having LiFePo4 batteries on boats, based mostly on money, and partly on an inability to protect them.

Yet plenty of people have destroyed their LA banks in short order, so there is an inability to protect built into LAs, just like Li's.

I don't think Li batteries are current cost effective for any boat I'm involved with, but boats and the way people use them are quite diverse.
I wouldn't be advising anyone to buy a BEV with lead-acid batteries, or a laptop with NiCads.

In general discharging any battery any amount takes something from its life. In general you need to strike a balance between 'abusing' the battery and the fact that the whole point of it is that it's there to be discharged.
The goal is not to make your battery last as long as possible, it is to go sailing, the battery is only a tool to achieve that aim. I don't want my battery to switch off when I'm sailing just because I've reached an arbitrary threshold of most economic battery management. I think you need to be clear about what exactly you want a BMS to do for you in every likely scenario.
With lead acids, if I discharge £200 worth of batteries further than optimum, that might knock 10% off their life. So be it. 20 quid to keep sailing, or not sit in the dark.
If you've got £800 worth of batteries to look after, which you've not had the value out of yet, you have to worry more in my book.

In my world, if I want to sell a more expensive design to a customer, I have to persuade him it delivers better value.
Yes, a lot of people have expensive LA based systems which they don't get great value from before killing the batteries. Li is the same with higher stakes.

 

[lw395]

100mV then.... thanks.

I've seen a few suggestions that a Lead Acid battery/bank in the system can prevent the alternator problem. Either the lead acid is charged by the alternator, and some kind of Battery to Battery charger is fitted between the Lead Acid battery and the lithium bank. Alternatively, some kind of switch or relay stops the charge to the lithiums at a certain voltage, and diverts it to the Lead Acid. In either case, the alternator is free to run with gay abandon, without fear of BHAM

This is the lee bow effect in a different dimension....

 

[Refueler]

I class using Lithium batterys on a boat as part of the 'throw money into a hole in the water' quote ... I can think of much better things to spend money on.

Lead Acid is relatively, cheap, doesn't need any fancy setups, easy to maintain - easy to replace from any garage / marina / car shop ..... or as I do - Car Breakers Yard !

 

[noelex]

I have fitted my new boat with lead acid batteries rather than lithium. These were a better choice in my case. However, lithium batteries certainly have a place and in some yachts they are the better choice.

There are several advantages and drawbacks to lithium batteries. The biggest advantages are the very high charge acceptance, high battery efficiency, larger range of acceptable SOC values, lighter weight, low voltage sag, and longer service life in deep cycle applications.

The drawbacks are a significantly more complicated electrical system, greater up front costs, lack of a float option, the more significant risk of complete battery failure, and still some doubts about the safety.

At the moment the most successful implementations involve DIY installations, but unfortunately to go down this path you need to do considerable research and have some technical knowledge. The upfront costs are higher, but providing you keep the boat long enough, the cost per year is not high. Only boats with reasonably complex, high demand electrical systems (especially without solar) or other more specialty requirements (such as lightweight racing boats) are the best candidates.

 

[geem]

A lot of people seem to shell out big sums of money for lots of Trojans and so forth, one could question the actual value they get from those too.
Back last century, we made do with quite modest battery capacity, despite using filament lightbulbs.
OK, these days we use a heater more often, and get value from that in terms of more sailing in a longer season.
But 20 years ago, most sailing boat owners would have questioned the 'need' for more than a couple of hundred quid's worth of batteries at today's prices.

My new Trojan T105REs were £120 each. We use four. They have a design life of 8 years. As full time liveaboards they are a great choice. For weekend sailors there are cheaper solutions.

 

[Refueler]

The argument about Trojans and similar are the non spill advantages and maintenance free. But those arguments are now old hat as other manufacturers have cottoned on to this ... now you can buy maintenance free .... non spill ... AGM format ... in so many other and cheaper brands.
There was a distinct difference in that evolution - no changes to circuitry design and charging ... price differential was not so great ... Basically there was and still is tangible advantage.

But Lithium is in a different game and needs serious consideration before spending out.