LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

[Maine Sail]

Originally Posted by OceanSeaSpray

Bill,

Remember that the power you store in the capacitor(s) needs to go somewhere eventually and at 100A or so, it wouldn't take long to charge even a huge capacitor array.
Also - assuming that some kind of switch mode circuit could directly throttle the output - imagine the damage if it suddenly stopped switching. It would be highly stressed.
You can download Tina-TI or LTSpice free and try simulating a few circuits. I think you would be horrified by the wave forms.

10 paralleled MOSFETs like the one you mention could certainly handle very high currents. Carrying the current to and from them requires careful design and construction however. Also remember that in order to switch on the high side with N-channel FETs, you need to produce around 10V more than the battery voltage to drive their gates and if you start switching fast, then you need quite a bit of current at that voltage.

Now, thinking about it differently, your best bet could be using a linear circuit instead, forget about switching. If you can drop a few tenths of a volt in a controlled way between the alternator output and the battery, then you can effectively force the alternator output voltage up to its regulation limit: say it regulates at 14.5V and you drop 0.5V to keep LiFePO4s happy with 100A charging current, you need to get rid of 50W in heat. That is achievable with transistors on a good heatsink and maybe a fan.
That would be easier to do and a lot more reliable.

Best regards,

Eric

If takes 30 minutes for a competent tech to tap into the brushes on an alternator to control it externally.... This is a pittance in the whole scheme of LFP... Once you control the field you can safely stop charging any time you like....

[simat]

Originally Posted by OceanSeaSpray

I moved away from it because of the high power consumption and also because there is in fact quite a lot of hardware to add around it to get a truly hardened system with all the input channels and power outputs.

Thanks for your response, I value your feedback

One of my design assumptions was that battery would be connected to some charging source on a regular basis so power consumption is not such an issue, mind you, the Beaglebone only consumes around 1-2W. As well as the features I mentioned in my previous post I want the device to be easy to program, be easy to add new functionality to and be reasonably priced.

I am interested to know what you are aiming for with your 'truly hardened system'. This is always a complex problem.

[OceanSeaSpray]

Originally Posted by simat

Thanks for your response, I value your feedback

One of my design assumptions was that battery would be connected to some charging source on a regular basis so power consumption is not such an issue, mind you, the Beaglebone only consumes around 1-2W. As well as the features I mentioned in my previous post I want the device to be easy to program, be easy to add new functionality to and be reasonably priced.

I am interested to know what you are aiming for with your 'truly hardened system'. This is always a complex problem.

The aim is fairly simple: a LiFePO4 bank that can be charged and discharged without having to think about it and without the risk of losing it due to some complicated installation- or chemistry-related glitch. Just like a SLA bank.

Also I can think of better things to do than watching the news on the Lithium channel. I don't want news. If the management system is good, the end-user should be able to forget all about the cell voltages and shouldn't require any technical knowledge or interventions for successful ownership. It is just a battery.

So the idea is a black box that is straightforward to install, robust and takes the complexity of the protection side away.
In this instance it will also help with first balancing the battery pack.

It needs to have near-zero power consumption, because if the protection system runs the battery flat and cuts out, it has just created a new problem. Charge can't always be guaranteed, solar panels can be snow-covered for extended periods etc.
The consumption of many modern MCUs is in the microamp range, nearly all CPU boards use 80-200mA because of linear voltage regulators, interfaces, etc. Also, they are completely uneconomical in comparison with buying the chip.
A bare MCU is a bit more involved to program for sure, but it is largely a one-time effort and then it is integrated on the board, no headers, wiring etc.

In order to fully achieve the goal above, the charge regulators need to be sorted so the BMS doesn't complain about them, but that is a fairly simple secondary problem.
With a good BMS in place, you can actually afford to get charge control wrong and work on the issues without destroying anything. Worst case it will alarm and cut out, so it is the starting point.

Best regards,

Eric

[roetter]

Originally Posted by OceanSeaSpray

Bill,

Remember that the power you store in the capacitor(s) needs to go somewhere eventually and at 100A or so, it wouldn't take long to charge even a huge capacitor array.
Also - assuming that some kind of switch mode circuit could directly throttle the output - imagine the damage if it suddenly stopped switching. It would be highly stressed.
You can download Tina-TI or LTSpice free and try simulating a few circuits. I think you would be horrified by the wave forms.

10 paralleled MOSFETs like the one you mention could certainly handle very high currents. Carrying the current to and from them requires careful design and construction however. Also remember that in order to switch on the high side with N-channel FETs, you need to produce around 10V more than the battery voltage to drive their gates and if you start switching fast, then you need quite a bit of current at that voltage.

Now, thinking about it differently, your best bet could be using a linear circuit instead, forget about switching. If you can drop a few tenths of a volt in a controlled way between the alternator output and the battery, then you can effectively force the alternator output voltage up to its regulation limit: say it regulates at 14.5V and you drop 0.5V to keep LiFePO4s happy with 100A charging current, you need to get rid of 50W in heat. That is achievable with transistors on a good heatsink and maybe a fan.
That would be easier to do and a lot more reliable.

Best regards,

Eric

I do something similar on my mono. It came with a diode-based battery isolator. Not the best but worked like a charm for me. The alternator (Mitsubishi) has a remote voltage sensing connection. Using a $5 automotive relay I route the sense from the house terminal (I.e. behind the diode which drops 0.5-0.7V,) to before the diode. Now the alternator is regulated down by 0.5-0.7V which works great. The alternator is factory set to sense 14.2V. While sensing at the terminal it pushes until 14.2 would be reached there. Once I see 14V at the terminal I switch it to sense the 14.2V before the diode so I get 13.5-13.7V at the terminal. The house voltage drops to 13.5 and is then maintained by the alternator.

The starter battery is also behind a diode.



Rolf

[roetter]

Originally Posted by OceanSeaSpray

...
In this instance it will also help with first balancing the battery
...

Eric

I just checked my mono LFPs while charging with the alternator. They were about 90% full. The cell voltage was 3.45v on all cells while charging 360Ah with 45A.

These cells were balanced with a bench top power supply to 3.65V last year in May.
I used them during the summer for a several multi-day trips and some day trips. While on the dock I have a small solar panel that keeps them between 13.3 and 13.8V. If I am planning to go out I put them on a shore charger at 13.3-13.8V and have the fridges going.
From October to May they were disconnected. The starter battery was on shore power and connected to the house system.
I have since used them for a few short trips, but not too much.

No voltage drift at all between the cells after 18 months. Cell balancing for our use is a none issue. I believe the balancing boards just add another point of failure. If one goes it kills the cell it was supposed to protect.

Just check the cells with a voltage meter every few month. If out of balance, clamp an automotive light bulb on the high cell or cells and drain until balanced again.


Rolf

[evm1024]

Good to hear that you are using a relay to switch a diode in and out on the sense lead. I have plans for doing that as well. Using schottky diodes in the isolator (as I'm sure you know) gives the lowest voltage drop. Using "standard" diodes (or even 2) in the sense gives 0.6 to 1.2 voltage drop.

I'll be moving my cells to the boat this weekend and getting things set up and dialed in will be a high priority.

[Maine Sail]

Originally Posted by roetter

No voltage drift at all between the cells after 18 months. Cell balancing for our use is a none issue. I believe the balancing boards just add another point of failure. If one goes it kills the cell it was supposed to protect.

This is why I prefer a BMS that requires manual intervention in order to cell balance. On the HPBMS, if properly wired, it requires you to disable the warning level HVC for charge sources. In a properly wired HPBMS you can not balance automatically. I like this a lot...

In 498 cycles my cells have not been balanced and they seem to have converged rather than diverged.. In two more cycles I will again run a full capacity test. Yesterday morning there was a 6mV spread between the 4 cells while burning about 12A. Not bad for close to 500 cycles most of them to 80% DOD. (a lot of work to get there BTW)

I am not a fan of "automatic balancing" but do not mind a BMS that has the capability for you to do it manually and attended... Balancing to me is like equalizing a flooded battery only required less (if the LFP bank is charged at realistic & safe voltages).

My take on balancing:

#1 You should be there when this is happening and know it is happening
#2 You should limit the current (the absolute lowest that will do the job)
#3 You should be able to control the voltage accurately
#4 You should monitor cell temps

Initially top balanced well and then charged correctly I have seen little to no need for balancing with fractional C use.

[OceanSeaSpray]

Originally Posted by Maine Sail

On the HPBMS, if properly wired, it requires you to disable the warning level HVC for charge sources. In a properly wired HPBMS you can not balance automatically. I like this a lot...

In 498 cycles my cells have not been balanced and they seem to have converged rather than diverged.. In two more cycles I will again run a full capacity test. Yesterday morning there was a 6mV spread between the 4 cells while burning about 12A. Not bad for close to 500 cycles most of them to 80% DOD. (a lot of work to get there BTW)

I am not a fan of "automatic balancing" but do not mind a BMS that has the capability for you to do it manually and attended... Balancing to me is like equalizing a flooded battery only required less (if the LFP bank is charged at realistic & safe voltages).

My take on balancing:

#1 You should be there when this is happening and know it is happening
#2 You should limit the current (the absolute lowest that will do the job)
#3 You should be able to control the voltage accurately
#4 You should monitor cell temps

Initially top balanced well and then charged correctly I have seen little to no need for balancing with fractional C use.

You are certainly not going to see cell imbalance if you are looking for it on a partly charged bank under load... for the simple reason that the voltage curve is flat and doesn't reflect SOC other than at the upper or lower voltage knee.

Every cell manufactured is slightly different, with a slightly different self-discharge rate and, given enough time, it is a certainty that they will go out of balance. The time to look for imbalance is at the end of charge, just before cut-off, under no load.

Your laptop doesn't ask for permission to balance its cells when you charge, it does it. In the end, it is a matter of choice: do you want to keep going back, check and intervene, or do you want it to look after itself? Nothing wrong with draining a bit of juice here and there as Rolf does if this is your preferred option.
A cell balancer that "fails" should essentially go open-circuit, blowing a fuse if needed, so I don't think the reliability argument goes very far. It just needs to be designed and built to industrial standards.

Also it is a fallacy to think that you have to push the cell voltage above the normal limit to balance. At 3.5V/cell you are already half-way up the upper knee and at low C-rates there is very little capacity left to go, so all your cells should be reading the same there already. You can perfectly balance up to the 3.55V/cell limit and then you can automate this when needed, because it is no different than normal charging. Balancing should just be selective normal charging (or discharging if done manually) close to the top end, not some hazardous process involving disabled alarms and IR temperature guns.

Even when preparing a new pack, under no circumstances should you push it above 3.6V when parallel charging. It is just heating and damaging the cells and it makes no sense.

I will be building a balancing module to go with the BMS because it is the only way of ensuring a truly long-term no-maintenance no-issues system. I haven't integrated balancing because the current capacity needs to relate somewhat to the size of the pack, but I have integrated the control of the module. Balancing wastes energy, so it is something you want to happen quite rarely, when needed only.

[OceanSeaSpray]

Originally Posted by roetter

I do something similar on my mono. It came with a diode-based battery isolator. Not the best but worked like a charm for me. The alternator (Mitsubishi) has a remote voltage sensing connection. Using a $5 automotive relay I route the sense from the house terminal (I.e. behind the diode which drops 0.5-0.7V,) to before the diode. Now the alternator is regulated down by 0.5-0.7V which works great. The alternator is factory set to sense 14.2V. While sensing at the terminal it pushes until 14.2 would be reached there. Once I see 14V at the terminal I switch it to sense the 14.2V before the diode so I get 13.5-13.7V at the terminal. The house voltage drops to 13.5 and is then maintained by the alternator.

The starter battery is also behind a diode.

Rolf

I have the same alternator as Rolf and splitting diodes. Manipulating the sensing input of the built-in regulator is exactly what I am going to do. It is a far better way than trying to do something at the output. Don't produce the power in the first place if you don't want it. Worst case, it means a trip to a car wrecker to find an alternator that is externally sensed.

This leaves the built-in alternator temperature protection intact and everything else as designed by the manufacturer for a little reliability...

Eric

[roetter]

I check the cell balance:
- while charging
AND
- at or above 3.5V


It is a little bit like a marathon race. Only close to the finish line ill you know who will get there first.

[Maine Sail]

Originally Posted by OceanSeaSpray

Every cell manufactured is slightly different, with a slightly different self-discharge rate and, given enough time, it is a certainty that they will go out of balance. The time to look for imbalance is at the end of charge, just before cut-off, under no load.

I agree they will eventually need balancing but I am already at 25% of the total 2000 rated cycles and in need of zero balancing. 12mV to 16mV at voltage and 10A net accepted current on a 400Ah bank. This is where I deem it "full". I glance at the cell balance regularly and even at 120A of charge current the spread is next to nothing even after nearly 500 cycles.

Originally Posted by OceanSeaSpray

Your laptop doesn't ask for permission to balance its cells when you charge, it does it. In the end, it is a matter of choice: do you want to keep going back, check and intervene, or do you want it to look after itself? Nothing wrong with draining a bit of juice here and there as Rolf does if this is your preferred option.

My laptop has one charger & BMS built specifically for that battery and it can maximize the crap out of it I suppose.. A boat has multiple charge sources some with quite high "C" rates and while similar it is really different ball game. We are using much larger batteries and drawing them at substantially lower rates then EV or even computer use. I am also charging them at lower voltage limits than an EV guy or a computer may to try and maximize every last bit of energy.

Originally Posted by OceanSeaSpray

A cell balancer that "fails" should essentially go open-circuit, blowing a fuse if needed, so I don't think the reliability argument goes very far. It just needs to be designed and built to industrial standards.

And most do but some don't. At my last count there are over 50 BMS systems out there some good, some bad, some very expensive but few geared towards fractional "C" use with multiple charge sources. Genasun was the last one offering what I consider a a well thought out and engineered BMS for marine use but they stopped selling only the BMS...

On my own bank I still see no need to force cell balancing when it is simply not needed. When it is needed I will simply do it, and perhaps get another 200 or 500 cycles before I need it again. Some folks will want this, I don't, but I am a hands on guy. This sort of system is likely best for the average Joe and his family but not something I personally want.

Originally Posted by OceanSeaSpray

Also it is a fallacy to think that you have to push the cell voltage above the normal limit to balance.

I am referring to the voltage at which balancing begins on many BMS systems. Try to find one that will balance at sub 14.0V or sub 3.5VPC, that does not cost as much as the cells... On my own system cell balancing begins at a far higher a voltage than I would ever charge to on a regular basis.

Originally Posted by OceanSeaSpray

At 3.5V/cell you are already half-way up the upper knee and at low C-rates there is very little capacity left to go, so all your cells should be reading the same there already. You can perfectly balance up to the 3.55V/cell limit and then you can automate this when needed, because it is no different than normal charging. Balancing should just be selective normal charging (or discharging if done manually) close to the top end, not some hazardous process involving disabled alarms and IR temperature guns.

I really don't even get to 3.5 VPC during normal charging and my peak voltage is 3.475VPC. The bank was top balanced to 3.8VPC and it has remained in as perfect a balance as could ever be asked for.

Why do I need a system that balances when it is not needed? I don't mind having it, but if I only need it, when I need it, then I don't want it running when I don't want it to. Just my own personal preference. I also don't like solar controllers that auto equalize....

Originally Posted by OceanSeaSpray

Even when preparing a new pack, under no circumstances should you push it above 3.6V when parallel charging. It is just heating and damaging the cells and it makes no sense.

I would agree. When I balanced two years ago Winston insisted I top balance to a bare minimum 3.8VPC (they really wanted 4.0V but I refused to do that). They even insisted on this when I told them my max charge voltage would be 3.45VPC - 3.5VPC.. There was no heat, it was measured and everything extremely closely monitored with NIST calibrated equipment... Each cell was charged to 3.750V individually then wired in parallel and allowed to sit. The voltage was then brought to 3.8V. The cells have not budged in almost 500 cycles. Funny thing is the call balance has actually become a little tighter/better over time and I have no good explanation for that.

Originally Posted by OceanSeaSpray

Balancing wastes energy, so it is something you want to happen quite rarely, when needed only.

I still don't need balancing even at 497 cycles. So yes I would agree with "rarely"...

If people choose to charge at higher voltages the game changes a lot and cell balancing will be more frequent. For frac "C" use at reasonable charge voltages my cells have stayed in balance well beyond what I had ever anticipated. Won't be the same for ever cell brand but these Winstons have done remarkably well.

[SV THIRD DAY]

The more I hear about all the complexity that folks want to build into their LiFePO4 battery management system (BMS), the happier I am with simply setting my charging sources at a "Can Do No Harm" voltage (13.5v) and then enjoying the benefits of my 400AH LiFePO4 battery bank. What ever happened to the KISS concept? Maybe the title of this thread should be changed to, "Ideas to make the most complicated/expensive BMS for a Cruising Yacht"?


Simplicity wins folks:
1. High and Low voltage alarms
2. Simple High and Low cut-outs for safety (if you want it)
3. Set your charge devices to 13.5v
4. Install a simple volt meter to monitor individual cells
5. Drink a Beer and Enjoy your boat.

My system now doesn't have the High/Low cut-outs because as a live aboard cruiser that knows how to pay attention, I'm happy with just audible alarms to warm me of a problem. I don't have a high temp engine automatic shut-down...I have a red fire bell that scares the **** out of everyone aboard when it goes off. So I tied the High/Low voltage alarms into the red fire bell. Simple and I didn't even have to buy a new alarm noise maker!

[Jd1]

Originally Posted by SV THIRD DAY

My system now doesn't have the High/Low cut-outs because as a live aboard cruiser that knows how to pay attention, I'm happy with just audible alarms to warm me of a problem. I don't have a high temp engine automatic shut-down...I have a red fire bell that scares the **** out of everyone aboard when it goes off. So I tied the High/Low voltage alarms into the red fire bell. Simple and I didn't even have to buy a new alarm noise maker!

I can just see the sequence here .... the alarm goes off, everybody lines up in front of the head to clean up, somebody hopefully remembered to shut off the engine in case it was a high temp alert, once cleaned up, crew scrambles for life jackets as the alarm could maybe be the high water alert, the dinghy is launched just in case, the red panic button on the radio might be pushed by confused crew, somebody drops the anchor in case the bell signifies proximity to an unidentified floating object or maybe a big honking rock, by this point surly somebody has ripped the alarm off the wall to quieten it down and then everybody looks around, finds nothing wrong, has a good laugh over the false alarm and starts cleaning up the boat to get under way again. At this point your batteries are at 10% and falling .... and nobody knows about it.

[SV THIRD DAY]

Originally Posted by Jd1

I can just see the sequence here .... the alarm goes off.....at this point your batteries are at 10% and falling .... and nobody knows about it.

Oh there's a Simple solution for that.
If the crew is that stupid...then they shouldn't be playing with a $1700 battery bank...Darwin wins again and the herd is thinned.

[goboatingnow]

I do a lot with Lithiums in smaller capacities,

The issue of cell balancing is often presented as "a problem" in practice cell balancing is merely an issue about extracting max efficiencies from a bank. It really doesn't matter a lot if you have small cell imbalances, all that results in a small loss of overall capacity , so what

With Lithiums, YOU MUST monitor cell voltages, monitoring systems that simply monitor the total charge voltage across a series string are very poor , liable to mislead and can result in individual cell damage. If you cell monitor then small cell imbalances can be ignored

In practice in fractional C situation, typical of boats, you get no imbalances over time.

I fly quadcopters, we discharge at upto to 50C! and recharge at up to 20 C!, now that generates imbalances !!!

dave