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

[OceanSeaSpray]

Originally Posted by SailRedemption

Regards to the 10a charge cutoff, what is the general rule of thumb here? I was under the impression that it was a percentage of charger capacity. Do I have that confused, is it percentage of battery capacity?

The termination condition is typically current down to C/30 - C/50. It is the battery capacity that matters indeed, not the charger capacity.

We alarm for any cell at 3.0V/cell and cut off if any reaches 2.8V (if it ever comes down to that) and this leaves just a few percent SOC behind. I have never found any valid reason for not using the available capacity, these cells are happier near the bottom end than at the top.

We still charge and absorb at 14.0V, which brings all the cells very close to 3.5V without issues. Things get quite touchy at 14.2V without balancing capability (and I am not talking about shunting cell boards here). HV alarming and tripping on a high cell is always around the corner, the slightest surge in voltage regulation becomes a problem.
However, the cells do need voltage to overcome increased internal resistance if they do a lot of partial cycles without a full charge at times (like we do in winter here). So far, 14.0V with absorption and termination appears to have been adequate. I have been looking into this in more depth for a while, but it is too early to comment.

[john61ct]

Originally Posted by OceanSeaSpray

We alarm for any cell at 3.0V/cell and cut off if any reaches 2.8V (if it ever comes down to that) and this leaves just a few percent SOC behind.

Too low for my comfort level, especially if discharge currents are very variable.

Maybe alarm at 3.2V, cutoff at 3 even?

I always assume I won't be able to see/hear the alarm.

[OceanSeaSpray]

Originally Posted by john61ct

Too low for my comfort level, especially if discharge currents are very variable.

Maybe alarm at 3.2V, cutoff at 3 even?

I always assume I won't be able to see/hear the alarm.

It makes no sense, precisely in the context of a sudden high discharge current. Crank up an inverter or windlass and it might alarm at 3.2V with a battery that isn't low.

Noticing the alarm or not is irrelevant. Protection doesn't rely on the alarm. The alarm is a warning to the user to stop discharging or face a disconnect, nothing else.

Nothing happens to the cells all the way down to 2.0V, which is the limit for the chemistry. The manufacturers conservatively recommends 2.5V, we use 2.8V for disconnect.
Many installations here have gone all the way down to tripping on LV at times in winter and recharged. It is a non event. We operate normally with 12.8V system voltage. Boats running fridges may or may not last through the night without alarming at that point. When it gets there, we start thinking about finding some power. That's how big a drama it is, actual installations, 365 days/year actively in service, broad range of users and habits. It has been working for everybody, no exception. The longer we can keep going, the higher the chances of getting some solar or wind power into the battery.

There is zero data supporting claims of reduced life and the like due to low SOC. There is plenty for high SOC, floating, overcharging and high ambient temperature. There is data showing least cell ageing at very low SOC. The chemistry is the most stable and inert at low SOC.

We use the whole battery, period. We cycle slowly and defer recharging to full. When we do (and can) recharge, we recharge properly. When we need capacity, we operate all the way down to 12V and then it gets a bit noisy and annoying to carry on deliberately, but there is a little reserve left.
If it does drop load, it recovers by itself thanks to the dual bus configuration and eventually reconnects the loads.

This also means we don't buy and install dead lithium. Explaining to people that they don't need to replace dead lead with dead lithium has been a recurring challenge, but it makes the projects cheaper. We sometimes settled on having a pathway to add extra cells: so far it has never happened. The performance is staying so consistent that the strategy hasn't seen any changes since absorption was set to 14.0V in the early days.

[senormechanico]

New Development in Lithium Ion cells !

https://www.allaboutcircuits.com/new...31c-266581441/

[witzgall]

What is "Dead Lithium??"

Originally Posted by OceanSeaSpray

This also means we don't buy and install dead lithium. Explaining to people that they don't need to replace dead lead with dead lithium has been a recurring challenge, but it makes the projects cheaper. We sometimes settled on having a pathway to add extra cells: so far it has never happened. The performance is staying so consistent that the strategy hasn't seen any changes since absorption was set to 14.0V in the early days.

[john61ct]

Secondhand? often come up leftover from EV projects

[Maine Sail]

Originally Posted by witzgall

What is "Dead Lithium??"

Dead Li just means buying more LFP than you need and not effectively using the full range of capacity. I coined the term dead lead, meaning non-usable battery capacity, many years ago for seminars and I use the term pretty regularly..

For example with lead acid you will generally use only 30-35% of the batteries actual capacity, 50% SOC to 80-85% SOC, and the rest of the lead is "dead lead" that you just lug around for the sake of cycle life. The concept of dead lead gets even worse when we get into bow banks where 90% of the capacity is dead lead and carrying around starting batteries too where 99% of the battery is dead lead......

With Li there is no sense shallow cycling or buying more than you need to, or purchasing an equivalent AH capacity to lead acid. because you can easily utilize 80% - 98% of the actual capacity meaning almost none of it is lugged around as dead lithium/dead weight..

[roostermt]

Hi guys...

Getting my batteries in a week or so. 500AH. BMS ideas and contactor? Now that the one everybody was using is unavailable. I really need to get one coming.

Help!

Rooster

[OceanSeaSpray]

Originally Posted by Maine Sail

Dead Li just means buying more LFP than you need and not effectively using the full range of capacity.

Thanks Maine Sail... been away, busy with projects.

The majority of systems I have commissioned are operating with just 200Ah in 4S or 2P4S. The boats seem to use 40-70Ah overnight due to refrigeration and TV. At this point, investing in solar capacity (or energy efficiency!) makes so much more sense than increasing storage.

For all practical intents and purposes, the battery performance doesn't degrade over time and it doesn't care about not getting fully recharged for long periods, so... any more than that would have been dead lithium each time - which is quite expensive. A justification really needs to exist before building a large bank.

People always started along the lines of "oh, this is not going to be enough for me"... and yet it has been. What they don't realise is that charging efficiency is also near 100% all the way to full. No gassing and making heat for hours while the SOC is at best creeping up.

[jram]

Originally Posted by roostermt

Hi guys...

Getting my batteries in a week or so. 500AH. BMS ideas and contactor? Now that the one everybody was using is unavailable. I really need to get one coming.

Help!

Rooster

I was unable to identify a BMS that met my needs so I rolled my own. However, its not a quick process. The thread above has lots of detail of the contactors to use and the different approaches to 'BMS' - You'll have to pick the one that suits you and then find a 'BMS' that fits your approach. Personally I chose a Battery Protection System, choosing to protect the cells from under/overvoltage/charging and the like rather than one that focusses on shunt balancing.

HTH

[john61ct]

Links please?

[senormechanico]

Over Spray described our system to a "T".
The only difference is our overnight use is usually less than that.

[200MPH]

So here we go, first post and it's about changing our house batteries from Lead AGM over to LiFePO4.

Why did we go Li? Fact is, Li makes economic sense, right here and now.

We are replacing 2 x 260Ah quality AGM batteries, which in theory give a safe long life usable capacity of about 210 Ah, assuming you can get them charged to 90% capacity and then drain them to 50% capacity (12.2V). Bear in mind you generally can't get the last 10 - 15% of charge into a lead battery easily, it takes hours of absorbtion time, easy on a shore charger, but not gonna happen at sea, unless you have massive solar or love listening to engines running (I'm talking about our sailing catamran here, I don't want to hear engines running!).

1 x 300 Ah Li battery gives around 210 Ah of very conservative usage, based on 100% - 30% capacity discharge. The great thing about Li is that they have over 95% charge efficiency. So nearly all of the power you present to them, they absorb, right up to basically full charge. So you have every chance of quickly (generator) or efficiently (wind/solar) getting charge into a Li battery. Lead on the other hand, well figure on more like 85% charge efficiency when new and getting worse all the time as it ages.


So, bottom line?

A$1400 for 2x 260Ah quality AGM battries (SSB) - will last maybe several hundred cycles if looked after and get worse from day one.
A$2200 for 4x3.2V Winston 300Ah cells. - Will last 2000 plus cycles and give full performance essentially if looked after.

So, it makes simple economic sense to go Li, not to mention all the other benfits you get.

Then after purchasing the batteries, I learnt about the elephant in the room! To get the super long and happy (plus safe) life out of Li, you have to completely reprogram (where able) or change to programmable battery charge/discharge management. Thanks to information here and elsewhere I've been on a steep learning curve!

With great power comes great responsibility I guess.

Had the Li only a few weeks and only in dock testing so far, but got it pretty close to where I think it should be. It's off for a few days trip with one of the other owners as of today, so should get some real-world feedback shortly.

Plan is not to push them too hard, but to use them well withing limits, like sailing really. Sure, you can run full sails and sheet them in hard in 20kn plus wind and reach the hull speed of the yacht. Or you can take in a reef or two and still go at 95% of hull speed without stressing the gear and rig. What would you do?

So our boat is a Seawind 1160 with more options than you can poke a stick at. But electrically it has two fridges and a double freezer. Normal sailing electrics, AIS transmitter, radar etc plus electric windlass and main halyard. All these things don't have to be on, except perhaps for one fridge and the sailing nav instruments. It also has a 3000W Victron inverter/charger unit and many 240V ac powerpoints, so you can go pretty hard with big current draws if you are not paying attention.

Charging options are via the 240V AC input to the Victron 12/3000/120 unit. It can charge at 120A if you let it, though I'm limiting it to 90A for now. Either shore power or the Honda 2kW portable genny power that up.
250W solar. 350W wind generator. And of course each diesel has an 80A Hitachi dumb alternator.

So there are many ways to kill a battery under uncontrolled discharge or charge!

So far as I can work out, a LiFePO4 battery really only cares about two things. When charging, keep voltage conservative. 13.8V will fill it up, so no need to go above 14V. When charged full, stop charging. Don't let it float at 13.8V or whatever like a normal Lead battery. They don't like it long term. When discharging, try to keep it around 70-80% discharged, though there are published tests under controlled conditions of 2000 cycles life at 100% discharge. Note the controlled conditions caveat there! Sure there are other things to consider like too hot or too cold issues, but they're not so much of an issue in Australia on the water if you keep the thing out of the engine bay. It's located on the bridgedeck under the main saloon, so should be happy there.

So, luckily for me the Victron is very progarmmable now I have the adaptor to let it talk to the configure program on my computer, though I should update the firmware in it to give the a tiny bit more control. Presently it only lets me specify absorbtion based on time, not tailing current. So I've set it to charge at 90A continuous up to 13.8V (bulk) and then hold 13.8V for an hour under diminishing current until it's basically full. Last test I did it got down to 16A charge at the end of 1 hour when it finished the fixed absorbtion time. That was about 98% full according to BEP battery monitor, so will leave it at that for now and be a little conservative. Then let it discharge down to 13.0V for 'float' mode. This will hold it at a mid point state of charge, while left in dock with fridges/freezers running in summer time so it's ready to go out with just an hour or two of 90A charge again to top it up. If just out for a day sail no need to charge up before going I think.

Dingo 20A solar controller has a custom program setting that is also fully controllable, so lucky there as well. Been set similarly, not that it will get up battery fully charged every day.

Wind generator controller is a bit harder to deal with, needs to be customised as not programmable and wants to get batteries up to 14.2 - 14.4 V, like any normal Lead acid battery charger. Because it can deliver 30A at 24Knots and upward wind speed, that's a real possibility if on a windy mooring.

Alternators are dumb, not smart battery charger or adjustable regulators. Deliver about 100A when both diesels running at around 1500 RPM, which is good. If they tried to put out 80A each they would burn themselves out after a while, and that's a real risk with Li batteries. They get warm in the first couple of minutes coming off a 25% SOC battery, but amps settle down to around 50A each pretty quckly, so looks like won't need to special regulators or battery chargers etc, though I know this is not optimal. The diesels charge into the starter battery (lead acid) first, then into the Li house bank if you want.

Charging on a mooring at sea, first option is to use the Honda 2kW generator and let the Victron charger sort it all out quickly and efficiently.

So, for now I've programmed the battery monitor unit to have a high and low voltage alarm, which I've hooked into an obnoxious external buzzer from Jaycar ($4!). This will go off as soon as battery goes above 13.9V or below 12.8V. Starter battery circuit will only be connected to the house battery circuit when manually switched over, not automatically like was previously the case using a Voltage Sensitive Relay. Though I may purchase a specially designed VSR intended to connect Li to lead starter batteries should the other owners not like manual intervention. Will see how it goes on the first few days away from dock I guess. Thanks to the power of ebay i have a programmable voltage switch for $18 that allows me to automatically control the external input of a VSR at my chosen voltage settings. So for under $150 can have an automated connection/disconnection of house battery from starter battery, allowing for automated charging whenever engines are running. Not too difficult, but may not be necessary after a bit of trial at sea.

In addition there are some automated last lines of defence. A victron BP-100 unit has been installed to prevent house battery bus bar loads going below 12V, which is about 95% discharged, but won't hurt the battery if it happens every now and then. Also, the victron inverter has a programmable cut-off voltage, which I've programmed to 12.65V for now, to give you a bit of time to turn off appliances should you hear the low battery buzzer annoy you.

Wind generator (Superwind 350) uses a special switch and dump load resisitors, so not an easy thing to change. Will leave it off unless someone is on board to hear the buzzer and turn it off if battery voltage is high (full).

Don't have any cell level monitoring. Local supplier of Winston cells (evworks.com.au) advises they are not a necessity for a 4S pack provided you keep it under 14.2 V maximum. They have many years of experience with these cells, though perhaps more in car conversions and some off grid house arrangments. Off grid house is just a bigger version of a yacht I guess however!

I have noted individual cell voltages very carefully during charging and discharging in dock a few times overnight. This was done with a calibrated high quality voltmeter (we have more electronic data logging and measurement toys at work than I care to remember! So i can run a full charge discharge test logging every parameter possible should I want.) I noted that there was a decent voltage drop between the Victron charger readings and the battery, despite big cables and a short cable run. At 90A charging, victron was reading 13.85V but was only actually 13.61V at the battery! So I stopped charging and hooked up the dedicated battery sense wires from the terminals to the Victron. Now it would lock on the programed voltage very steadily during absorbtion charging, within 40 mV. Much better!

Before you ask, I haven't forgotten the lead starter battery. With the arrangement described above, it would get not float charge when in dock and shore power is on, as there is not automatic connection between start and house batteries. And over time it would lose charge and, capacity and life. Luckily Victron have though of this and provided for small current termainals to connect to your dedicated starter battery. So it should keep it floated (albeit at 13V) and make it a bit happier when in dock on the charger and waiting for some sailing action.

So there you go...big learning curve, but it'll be worth it I hope! We've spent the money now, so have to make it work I guess! Thanks to all the good info posted here I have some good starting points, but happy to hear any thoughts about what can be done to keep it performing well, for a good time and safely. Fire away please, I'm all theory and very little practice as yet!

[Hoopla]

TWIN SERVICE BANK QUESTION

With a twin bank, twin BMS dual bus arrangement you can either charge/discharge together or on a bank by bank basis. This twin service bank approach obviously provides a backup where in the case of the former approach no manual switching over is required if one bank was to go offline.

However unlike LA LFP doesn't like to be left at a high SOC so it seems to me you are better off charging and discharging with both banks combined and not seperately. You also avoid the situation where if they have widely different SOC/Voltages a large transfer of current between the two banks won't occur they are inadvertantly paralleled or combined.

Does anyone have any views on this?

[OceanPlanet]

We typically run two or more independent banks, but in parallel. They essentially run as one bank and are monitored as one, however there is the added redundancy of separate banks.

The need for the redundancy is actually quite rare, however we know of at least one client that was hit by lightning which took out all of the instrumentation, and one of the battery banks BMS. However the 2nd BMS/bank was ok, so that most of the systems could still operate.