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

[mbianka]

I don't know of any suitable chargers for the marine environment designed to charge 3.2V LiFePO4 cells... At home I use a 30V X 30A bench top power supply.. Top balancing is pretty easy and it seems from my very limited experience that once the initial balance is done the pack stays in-check pretty well. Mine have converged at about a 0.004A difference when compared to one another with a Calibrated Fluke 179.. Close enough for horse shoes as they say.

Yeah, the market is not yet caught up with the need at this point. But, I'm sure it won't be long before it does. In 2008 when I converted to EP Lith Ion were still fairly new with long lead times for orders. It was a little too new and $$ for my taste. With the AGM's I was able to get a lot of my charging devices i.e. 48 volt solar controller and 48 volt wind turbine off the shelf which made things easier to setup for charging. In addition to things like the Dual Pro Quad charger. I can tell at glance how the batteries are charging with the Dual Pro. Though I have custom built some instrumentation that also shows me battery voltage and current at the helm when underway too.





These are all little learning's that when working with lead acid you'd never know were even possible. I never knew you could drop the absorption or bulk voltages on the Balmar lower by working backwards and starting at float and I have custom programmed these for years but never needed to drop bulk or absorption low like we do with LiFePO4........


Indeed, Though with my EP system since I no longer have an engine there is no alternator on board anymore. Which makes charging the bank somewhat easier since it is either through an electronic chargers via a Honda 2000 generator/dock power and/or solar and wind. Though it is sometimes interesting when it comes to charging the bank. A month ago one battery in the string was lagging the other three when I plugged in the Dual Pro in one of my winter boat checks. A few days ago I went back on board and two different batteries lagged behind in charging the other two (including the one that had been previously lagging). I'm talking maybe 10 minutes or so nothing severe. Still have not figured why this happens. It's interesting that LifePO4 seem to hold their balance better and may not need to be kept an eye on as closely as LA at least as far as balancing goes. Still I think it would be a good idea to be able to keep an eye on each cells voltage. Especially when charging with both battery chemistries.

[Maine Sail]

Originally Posted by diugo

I keep harping on Peukert b/c it seems to go in one ear and out the other with a lot of the crowd here. Like it or not, Peukert is nowhere near unity (ranging from 1.04 in my case to 1.12 for MaineSail and 1.20 for DeckOfficer) and needs to be taken into account especially if accurate amp-hour counting is desired.

I agree with you that 100% SoC charging should be avoided for longevity. However, in practice, the limit will end up being pushed by many, mostly on account that a certain highly revered "someone" here keeps insisting that 100% is perfectly okay---since battery current is "zero" when fully charged.

I am curious as to how you determined my bank has a Peukert of 1.12?

Just last spring I loaded a 400Ah bank of Odyssey PC2150 batteries at at 100A +/- to draw the bank into bulk for setting up and load testing the alternator to dial in the temp sensing. I started with a full bank and applied 100A +/- to get her down into the 60-65% range. In a little over an hour the inverter went into low voltage. These batteries have a Peukert of 1.112. My 400Ah bank ran for over 5 hours, at the same load, and was still at 11.8v at the end even with less than zero her rated capacity left in it........

Just curious as to how two banks, both 400Ah, and both with supposedly similar Peukert's do so differently under nearly the same exact loads....

I understand that LA batts are rated at 20 hours and the LifePO4 are just rated at the Ah capacity. But two battery banks, with nearly identical Peukerts, behaving so differently seems odd to me...

How can we accurately, and preferably easily, determine the Peukert of a LiFePO4 bank. I don't believe my analyzers are very accurate on this chemistry so have pretty much thrown that out the window..

[deckofficer]

Originally Posted by mbianka

Not sure of the time but, I expect it to be in the millisecond range. Never put a scope on it. It's probably done to make sure the battery (bank) is not over charged and is also used in some algorithm in the charger to detect state of charge too. This is fine for an individual battery but, in a series string like my EP bank it can not detect an overcharged battery in the string from an undercharged battery. So that's why I switch to the individual charging before it reaches full charge. Has worked well for the past five years.

Maybe a simpler reason for the pause and that is 48 volt along with 36 volt are the common voltages of golf carts and as such, the charger's side of the plug is male. For safety, golf cart chargers will not turn on unless it sees a voltage from the battery. My vacation residence has a lot of golf carts and when someone asks me to check their charger because it isn't working, the first thing I do is check the voltage of their cart's batteries. If too low to trigger the charger I'll use jumper cables on my cart to theirs, then their charger sees my voltage and turns on, job is done.

[tsmwebb]

Originally Posted by Maine Sail

I plan to use this point, probably somewhere around 1-2A, and the target voltage to do an "auto" reset of my amp hour counter...

If a load comes onto the system near the current cut off point (eg. the fridge or a pump switches on) will its current draw cause a misread?

Tom.

[Maine Sail]

Originally Posted by tsmwebb

If a load comes onto the system near the current cut off point (eg. the fridge or a pump switches on) will its current draw cause a misread?

Tom.

Yes it can but you can also adjust the duration at voltage & duration at current and manually re-set as I do anyway.

[Lagoon4us]

Does a BMS throttle back all sources of charge presented at the battery to what's acceptable to the battery in it's present condition?

[mbianka]

Originally Posted by deckofficer

Maybe a simpler reason for the pause and that is 48 volt along with 36 volt are the common voltages of golf carts and as such, the charger's side of the plug is male. For safety, golf cart chargers will not turn on unless it sees a voltage from the battery. My vacation residence has a lot of golf carts and when someone asks me to check their charger because it isn't working, the first thing I do is check the voltage of their cart's batteries. If too low to trigger the charger I'll use jumper cables on my cart to theirs, then their charger sees my voltage and turns on, job is done.

Deckofficer

I'm sure there is a check on power up too on the Zivan for low battery. But, I think the voltage check while charging is to check the SOC of the battery (in my case the battery bank) It happens so fast it is not noticed on the current meter. But, as I monitor the voltage when it reaches a certain point I see the current drop down and the charge LED goes from red to flashing yellow. Which represents the bank at approx 94% soc which is when I switch to the Dual Pro charger to top each individual battery to full charge. They usually finish charging within minutes often seconds of each other. Which lets me know they are in pretty good balance with each other.

I did have one anomaly last year with one of the EP AGM batteries that my Paktrakr Battery monitor was drawing power (25ma) from. It was timing out on two different battery chargers when charging:
THE BIANKA LOG BLOG: BATTERY INVESTIGATION PART THREE: Overnight Charge & Battery Analyzer Test
Some might have pitched it out at this point. But, when I disconnected the Paktraker and with a little TLC it came back nicely and is still working fine. Lot's of quirky things happening in batteries sometimes. Perhaps LifePO4 may not be so.

[goboatingnow]

Originally Posted by deckofficer

Maybe a simpler reason for the pause and that is 48 volt along with 36 volt are the common voltages of golf carts and as such, the charger's side of the plug is male. For safety, golf cart chargers will not turn on unless it sees a voltage from the battery. My vacation residence has a lot of golf carts and when someone asks me to check their charger because it isn't working, the first thing I do is check the voltage of their cart's batteries. If too low to trigger the charger I'll use jumper cables on my cart to theirs, then their charger sees my voltage and turns on, job is done.

Yes I suspect its a simple safety device o ensure the battery is present. Reading the voltage in a few milliseconds of a pause would be useless for the charger algorithm.


Dave

[goboatingnow]

Originally Posted by Lagoon4us

Does a BMS throttle back all sources of charge presented at the battery to what's acceptable to the battery in it's present condition?

Not simple ones no. , some attempt that by using shunt resistors etc but these are not much use in high C situations

Dave

[deckofficer]

Originally Posted by goboatingnow

Yes I suspect its a simple safety device o ensure the battery is present. Reading the voltage in a few milliseconds of a pause would be useless for the charger algorithm.


Dave

36 and 48 volt golf cart chargers won't turn on unless there is voltage present at the male plug of the charger.

[T1 Terry]

The 48v system is an interesting one. It actually measures out nicely to 15 lithium ferrous cells. As 3 groups of 5 cells, each group has a voltage operating range of 15v to 17v, this is very close to the perfect operating voltage for 12v solar panels.
I plan to run a 48v system in my big bus project, both house power for the 5kW inverter and power for a hybrid drive assist electic motor, most likely AC so I can gain the benefit of regen braking. I plan to use 3 individual Junsi cell loggers and timer circuits and simply toggle the solar on and off to stop any cell going over 3.6v, the charger side of the inverter will be set to 50v cut off, 3.3v per cell, the solar can do the rest.
There is no reason why the same system could be set up for the two chargers, the Zivan is a good programable charger and the Dual Pro I believe can be also programmed for each chanel to stop charging just that little bit before all the cells were full, then the solar finishing off the job.

The C20 rating for a lead acid battery is Peukert's effect being taken into account. Good manufacturers give tables showing the reletive capacity at different C rates. Peukerts isn't really the method for calculating the losses in lithium ferrous batteries although the effects are similar at very high discharge rates. Proffessor Jay Whitacre refers to it as the Warbird (sp) coeffecient, it is very reliant on the cell build, the thickness of the material coating, each design build is different. The thinner the material, the higher the C rating before the Warbird kicks in but the lower the cell capacity, the thicker the coating, the earlier in a the C rate the Warbird kicks in but the greater the storage capacity. This is one thing they have in common with lead acid batteries, it's the difference between starting batteries and deep cycle batteries, just on a much higher scale. Some of the high current cells are up around the 20C mark before the Warbird kicks in, but their capacity is only around 10Ah.

T1 Terry

[Lagoon4us]

A Lithium Trove of info here:-
Li-Ion BMS - Lithium-Ion Battery Management Systems and large battery packs

[ebaugh]

Somehow it's not this complicated to charge LiFePO4.

It's clear that SOC may be somewhat difficult to determine precisely. But only more testing will discover the answers, not speculation. And +/- 10% is close enough.....

[Lagoon4us]

I guess the resistors generate heat but i would think part of the process of the BMS that switches the 'overvoltage' to a resistors has cut charge off to the pack simultaneously?

Cheers

[Lagoon4us]

Originally Posted by ebaugh

Somehow it's not this complicated to charge LiFePO4.

It's clear that SOC may be somewhat difficult to determine precisely. But only more testing will discover the answers, not speculation. And +/- 10% is close enough.....

Exactly Bob, how long has your system been running and with nil incidents? Cheers