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

[ebaugh]

Originally Posted by bill good

OK I need to supply more info!! Boat has 2 X 7.5HP Yanmars Prefer to keep standard alternators if I can. There is a point of view that any voltage external to the cell "may" contribute to problems within the cell & since the self discharge is not an issue why take any chances with float charging. Except for one batch of faulty cells the only other failure mode that I can trace is where charging is involved. To my way of thinking a simple addition of an external electro is not only cost effective but universal to any alternator with internal regulation & no ext field connection. That is why the idea is on the table.

Regards

ps daddle tks to a B+ of an alternator a battery is an equiv. large capacitor which does the filtering & adding a suitable value of capacitor maybe provides the same filtering. Well thats the thought!!

A "sacrificial" Group 24 start battery kept in parallel with the Li will solve the problem for minimum investment and engineering? The float current on such a battery would be little more than a capacitor. Or configure the electronics somehow so the Group 24 is isolated until either key switch on the Yanmars is turned on. That +12V signal should be easy to identify.

[daddle]

Forget capacitors. The output of a proper alternator is ripple free DC. The tiny bit of diode switching noise is not an issue.

[hellosailor]

Bill, 2 x 7.5hp sounds like outboards on a cat. With toy alternators, compared to most inboards. I would suggest finding out from Yanmar exactly what alternators you have, and to be sure they are alternators. The company that makes them should be able to tell you if they can be disconnected from the battery while running: Some alternators can be, quite safely, while most cannot. If they cannot, then again the maker is the first one to ask, they may know how to rig this based on past experience.

The Delco companies and Leece-Neville are very good with customer support. The Japanese (Hitachi and Denso) tend to get really upset by customers who think the product might not be perfect as is.

In the worst case, you will wind up treating this like a wind generator, and perhaps even using the controller from a wind generator, cutting over the alternator output to a hot water heater as a "dump load" once the batteries are charged.

"But honey, we have to have copious hot water on the boat, it prevents the batteries from exploding."

[daddle]

LFP should not be floated. For maximum cycle life they should not be charged any higher than necessary for the next expected demand. If the demands are low consider charging only to 60 or 80% with discharges to 40 or 20%. When near either charge limit the electrolyte is very reactive. The life of the plates is reduced.

Or alternatively do not charge as often. Allow the cells to spend a greater amount of their life around 50% charge.

This is quite different than what we are used to with most other cell types. Takes some planning.

[CharlieJ]

daddle
What technical data backs your assertion that LFP cells should not be floated? Every major cell manufacturer provides a float voltage.

[bill good]

[QUOTE=hellosailor;1171194]Bill, 2 x 7.5hp sounds like outboards on a cat. With toy alternators, compared to most inboards.

In the best of conditions 7.5 Knots approx 3ltrs per hr & yes they are called "Aux power"

Regards

[FlyingCloud1937]

Originally Posted by CharlieJ

daddle
What technical data backs your assertion that LFP cells should not be floated? Every major cell manufacturer provides a float voltage.

They also tell you to do charge cycle management. That doesn't mean float charge.

There is only one US production company that is not not china build, they build for USA mil-spec.

They also recommend charge cycle management.

They project 3k cycles, or 10 years, they count each discharge/recharge as a cycle, and don't speculate on % of discharge, all though they caution against a discharge of more then 70%.

What they say is 80%, 50%, 60% what ever the discharge value is. IS counted as a cycle, and it assumes that at the end of each dis-charge will come a re-charge, ='s one cycle.

Their recommendation is to charge daily.

Lloyd

[NahanniV]

How long before a float voltage becomes a problem ?

Engine:
I plan to set my alternator to 13.4V. And I won't be motoring more than 10hours maximum this season.

Shoreside:
I have a Xantrex Truecharge 20+ shorepower charger that I can set to charge to 13.8V and the documentation says it will switch to 13.4Vfloat or 12.5Vmonitor-mode after the current drops below 4ADC for 3min. Problem is it takes longer than three min. to switch to float or monitor-mode, sometimes several hours.
I have another charger that can work in a power supply mode @13.5V I could hook that up to run my loads while plugged in.

Alternate:
I also have 170W of solar. I have an SES Flexcharge NC25A that can be adjusted to stop charging @13.8V, looks like it will turn back on @13.15V. I have not tested it yet.

I know this stuff is not optimal, but I already have it, so the price is right.

[CharlieJ]

Lloyd-

They also tell you to do charge cycle management. That doesn't mean float charge.

I am sorry, but I do not understand your point. Several of the LFP cell manufacturers lists a float voltage on their spec sheet. They also provide nominal voltage, nominal charge current, some of them provide pulse charge current, standard charging method; e.g., "C/3 constant current to 3.65VPC then Constant 3.65VPC with current taper to C/20"; cutoff voltage, etc., etc. Taken as a whole, I am assuming that this is "charge cycle management". But it includes a value for float voltage so, my take is that the manufacturers of the cells intend on the users of the cells to maintain them at the prescribed float voltage.

[diugo]

Originally Posted by CharlieJ

Several of the LFP cell manufacturers lists a float voltage on their spec sheet. They also provide nominal voltage, nominal charge current, some of them provide pulse charge current, standard charging method; e.g., "C/3 constant current to 3.65VPC then Constant 3.65VPC with current taper to C/20"; cutoff voltage, etc., etc. Taken as a whole, I am assuming that this is "charge cycle management". But it includes a value for float voltage so, my take is that the manufacturers of the cells intend on the users of the cells to maintain them at the prescribed float voltage.

Could you please provide one or more manufacturers that list a nonzero float voltage on their spec sheet?

Because how can a lithium ion cell have both a taper/cutoff current AND a float voltage???

[FlyingCloud1937]

Originally Posted by CharlieJ

Lloyd-


I am sorry, but I do not understand your point. Several of the LFP cell manufacturers lists a float voltage on their spec sheet. They also provide nominal voltage, nominal charge current, some of them provide pulse charge current, standard charging method; e.g., "C/3 constant current to 3.65VPC then Constant 3.65VPC with current taper to C/20"; cutoff voltage, etc., etc. Taken as a whole, I am assuming that this is "charge cycle management". But it includes a value for float voltage so, my take is that the manufacturers of the cells intend on the users of the cells to maintain them at the prescribed float voltage.

Some call it Ching-lish.

I remember when Victron came into the North American market they specified an equalize voltage.

They were calling equalize the same as we knew absorption, not recognizing that equalization is not the same.

When GM introduced the Chevy, Nova into the Hispanic market the Nova, was not something they valued.

Lloyd

[CharlieJ]

Diugo-
I thought that there were more manufacturers with a float voltage spec but it appears that CALB, Winston and Sinoply do not. Forzatec (cells that I am using on 2 x 1000Ahr packs on a sailing cat) does provide a 3.4VPC float voltage spec: FORZATEC | LifePo4 batteries manufacturer | Lithium ion batteries manufacturer

That said, my experience with a the 4P4S battery pack I have in the shop is that the CALBs float very well @ 3.35VPC with < 1 A flow into the battery pack. As power boats routinely float there LA batteries for extended periods under analogous conditions (alternator voltage very close to battery bank voltage for low current flow), I am not sure I understand what the proponents of not floating LFPs are concerned about.

[daddle]

Okay. I will find the tech paper re floating the cells. Yes, the current will be close to zero, but that is not the point. What I recall is the paper said the electrolyte at full charge and at discharge is "very reactive". That is a problem for the chemistry. The point was that they will last much longer at middle charge levels. Note that posters here have said the cells are shipped near 50% SOC. The tech paper said it would be advantageous to only charge the cells enough to satisfy the expected demand. Admittedly difficult and unusual for yachties. It was what made the difference between a life of 3000 cycles or 10,000.

The manufacturer datasheets are complete BS in this industry. Fortunately there are many peer reviewed papers describing cell behavior.

[CharlieJ]

daddle-I, and I am sure others, appreciate your effort to find the tech paper.

As an aside, I also agree that the LFP datasheets leave much to be desired. As an example, most (if not all) cell resistance values listed on cell datasheets are actually AC impedance. Some manufatures note that the "resistance" was taken at 1kHz, therefore implying that the value recorded is an impedance value, many don't. What we are most interested in is pure DC resistance, which is more difficult to measure and not available.

[s/v Jedi]

You can always set the float voltage of the charger at 13.2V and problem is solved, i.e. as you steer away from the highest allowable, you avoid the "danger area", if it exists.