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

[NahanniV]

Originally Posted by OceanPlanet

"Drop voltage"...I like it. Soon we will have to come up with a whole new terminology for dealing with Li (or at least LiFePO4)...;-)

"drop voltage" = Hysteresis

In electronics you would define two thresholds: Upper and Lower with the difference being the Hysteresis.

[NahanniV]

Originally Posted by Maine Sail

I was not suggesting charging in full cycles only just that this has worked very well for us. If I know we are going to be on the hook for a while I will charge when I can.

It was GOBOATINGNOW who said that any cycle ("mini cycle") counts the same as a full cycle in the life of the cell.

[SteveSails]

Rereading my previous post I see that it is a bit of a ramble, I apologize for that. Perhaps that was due to thinking in terms of a LA system and not how a Li system operates....Still learning

Doing a Google search for papers on Li batteries I came across this;

"DEGRADATION ANALYSIS AND HEALTH MONITERING OF LITHIUM ION
BATTERIES By Nicholas Dane Williard "

Various charge/discharge methods were tested and their affects on life capacity were determined. Continuous charging (float voltage) and shallow cycle degredation would be of the most interest to us. While these tests were performed on LiCoO2 the paper does mention LiFePO4 and implies results should be similar.

Pg. 77,78
"Continuous Charging
The continuous charging test was performed on an “A” battery to examine the effects of float charging on battery reliability. The cell subjected to this test was given a float charge at 4V for an extended period of time and was periodically discharged to measure capacity. A float charge refers to a low current charge which counteracts the battery’s self discharge effects.
The float charge allows the voltage to remain constant over the duration of the rest time. Figure 34 shows voltage vs. time plots collected during various discharge times throughout the continuous charging test. There was no significant change in capacity during the continuous charging times. In a previous study [14], extended periods of float charging at voltages above 4.2V resulted in accelerated capacity fade. This is likely cause by the
instability of the cathode material at higher voltages.
Because the float charge in this test allowed the battery to remain within the battery’s stable operating region below 4.2V, no significant degradation was observed. This would be an important consideration for BMSs that wish to keep a battery maintained during down time. By applying a float charge in the battery’s stable operating voltage, the battery can remain fully charged and ready for immediate use without undergoing significant degradation during storage."

The paper cited [14] is the one by Choi & Lim that Dave mentioned where the float voltage was 4.2V and caused damage to the cell. So it would seem that a float voltage in the stable voltage region which for LiFePO4 a bank voltage of 13.1V or 3.275V/cell would be safe.

Shallow cycle capacity degradation (pg.79) is also of interest in a house bank. Shallow cycles did cause some capacity degradation but that can be recovered after a rest period or by a full 80%DOD and 100% charge. I believe that was exactly what was seen by those of you that performed bench tests.

There is also a good explanation of Peukert's exponent on pg. 7,8. While Peukert did develop his equation in 1897 using LA batteries the equation is independent of battery chemistry and does apply to Li batteries. It just may be that for LiFePO4 used as a house bank the exponent is close enough to 1.0 so that it is of little practical use.

I agree that terminology has been misused or inconsistent and I have been guilty of that myself. A carryover of LA terminology does cause confusion and there should be something different used for LiFePO4. I do like Maine Sails term "drop voltage" for when the pack voltage has dropped to a point where charging can be reconnected.

Here are the voltages that I have come up with so far with using LiFePO4 as a 12V nominal house bank.

>14.4V >3.6V/cell HV fault, alarm on, disconnect all charge sources
13.9V 3.45V/cell Charge complete, disconnect charge sources
13.1V 3.275V/cell Drop Voltage, reconnect charge source (solar/wind)
11.6V 2.9V/cell Low voltage warning, recharge immediately
<10.4V <2.6V/cell LV fault, alarm on, trip contactor to disconnect

The HV fault and LV fault are conditions that should never occur in a properly functioning system and are used to protect the cells from damage.

The exact voltages above are open to discussion and may change with different cell manufacturers or even as the cells age. A BMS with adjustable settings would accomplish this but at the cost of increased complexity and reduced reliability. Maybe best to keep it simple and use conservative fixed settings.

Hope this helps,

Steve

[goboatingnow]

Originally Posted by dlentz

+1 13.8 Bulk/absorb 13.2v float here. and No worries!

Again how do you know ? It's a serious question , how do you know you are or are not slowly loosing capacity or cycle life

Dave

[s/v Thea]

Re 13.2 float voltage causing damage

13.2v is less than pack resting voltage, if that caused damage than cells connected in parallel would cause damage to each other even when no charging taking place.

I don't think so.

[goboatingnow]

I think steves summary was excellent and I agree with his charging patterns.

In my own case I re-engage charging after a 20 % drop in capacity

Dave

[SteveSails]

Dave,
Determining charge capacity, SOC and SOH (state of health) was a problem for the authors of the mentioned papers even with expensive lab grade equipment. For us that is compounded by using low cost or 'hobby' instruments that may not have the accuracy or repeatability to determine small SOC differences. Maybe set it to what works and forget it, or at least back of mind, is best.
In a way I'm sorry that I'm retired (not!) and no longer have access to lab grade test equipment. And a shop to change surface mount components in case I ever wanted to 'hack' the BMS boards. Did I also mention the paycheck that could finance the experiments? Maybe just as well.

Steve

[goboatingnow]

I agree Steve, with the very flat voltage slope of a high C Li bank , its notoriously difficult to use voltage in the linear part to determine SOC. In my case I am using 18650 cells to to power GSM equipment so I see high C discharge. Here the slope is a little more pronounced and the voltage a better ( though not exact ) SoC indicator.

This is why I'm a little skeptical of the SOC measurements that some people are doing. Determining SOC by using voltage as terminator points is very error prone.

Dave

[sparrowhawk1]

Originally Posted by NahanniV

I think the question boils down to:
How should you use low amp alternate charging sources (wind and solar) with Lithium.

The problem is:
Some are suggesting to charge then discharge the batteries in full cycles only.

But:
The low amp alternate charging sources (wind and sun) are not always available when the battery is discharged.

Also:
While these sources are available, we would like to keep the battery at %100 SOC because we don't know when they will be available again.

For example:
On a sunny day solar might top off my battery by 10:00am. If I stop charging (or Floating) at that point the battery will be partially discharged by the end of the day even though the solar was available. Next day might be cloudy and now I may need to run the engine to charge. This might have been unnecessary if the battery had been fully charged at the end of the previous day.

Disclaimer. Not a technical guru. On balqon's website (LiFePo4 battery distributor) they have a chart showing life expectancy verses depth of discharge. My interpretation is that you can do a bunch of small discharges or one big one( for example you can take 20 percent out 3 times or 60 percent out once and it's the same thing or close to it) they also show if you do shallow discharges you can get over 5000 cycles.

[SteveSails]

It would be nice to reuse or re-purpose our existing chargers, solar controllers, alternators, regulators and monitors for use with LiFePO4 batteries. If the charger, controller and regulator have adjustable voltage levels in the range required for Li then they should be usable. The SOC function based on voltage used by most battery monitors is meaningless with Li batteries, however they can keep track of Amp hours and that is very useful. Maybe the best way to have a 'fuel gauge' is to keep track of the Amp hrs. When the monitor shows, say, -300Ah on a 400Ah bank it's down 75% and time to start the generator and recharge.

Sterling Power Products has what they call a "Battery Chemistry Module" that takes any charge source and converts it into the voltage levels required by the chemistry of the battery used. There are settings for FLA, AGM, gel and several others including LiFePO4, however the voltages listed for Li are 14.6V bulk, 14.4V adsorption and 14.2V float, much higher than what has been discussed here. So what seems like a solution just raises more questions and adds to the confusion.

The ultimate solution is to just buy one of the commercial Li systems. West Marine is selling the Mastervolt 180Ah Li battery system for $10,300...

I'll keep looking.

Steve

[OceanPlanet]

That $10k for the MV 180Ah must be for a 180Ah x 24V. Yes...that is imposing.

For all the talk about Genasun GLi being in the same stratosphere, the MSRP for a GLi 180Ah x 24V is just under $7k. Still a chunk, I know. Just sayin...

[SteveSails]

Yes you're right. The 180Ah is 24V, and the 12V is 360Ah, same price, makes sense since the number of cells would be the same.

I'm trying to see what can be done for about $2K and a bit of DIY time.

[Maine Sail]

Originally Posted by SteveSails

Sterling Power Products has what they call a "Battery Chemistry Module" that takes any charge source and converts it into the voltage levels required by the chemistry of the battery used. There are settings for FLA, AGM, gel and several others including LiFePO4, however the voltages listed for Li are 14.6V bulk, 14.4V adsorption and 14.2V float, much higher than what has been discussed here. So what seems like a solution just raises more questions and adds to the confusion.


Steve

Steve,

The BCM will only boosts voltage thus you set your charger for the lowest voltage bank you need to charge. In my case the Li bank charges with a custom setting at 13.8V. The BCM boosts the voltage to my AGM reserve bank to 14.6V so I don't chronically undercharge it at 13.8V.... It will not drop the voltage of your charger.. If you want 14.1 out of the BCM but your main charger is set for 14.4V you will get 14.4V out of the BCM.

We should be very leery of any company slapping a Li charge profile on their equipment. Much of this is done without a clue of which Li chemistry or brand you are charging. Based on some of the profiles I've seen I assume these are decided on by the marketing departments not the actual engineers.

I have yet to see one I would use on my own boat, not one. This means a charger, controller or regulator, that is fully programmable is almost always necessary. Usually they are never close to a good voltage for safe Li charging for house bank use. For example Charlie has the BCM set for 14.6V then a 14.4V float.. There is no "float" with Li and especially not at 14.4V /3.6VPC...

You could not pay me to charge my Li pack at anywhere near those voltages because it is absolutely not necessary and only puts you closer to the danger zone. For a house bank there is no need to push into the knee ranges voltage wise. The BCM on Charlies Li setting won't even work with a House Power BMS as the HVC triggers at 14.2V.

These days it seems to be the "cool thing" to have a Li setting but it is usually pure garbage and nothing more than a label to make people think it works with Li... Buyer beware.....

That said the BCM is a great little tool to boost charge voltage to a second LA bank when charging a Li bank at 13.8V - 14.0V...... It also works well when you have a GEL house bank and an AGM or flooded starting bank that need more than 14.1V.

The BCM is only designed to work on an unused output of a battery charger and the charger signals it on.. This output needs to be diode isolated for the BCM to work correctly.

[goboatingnow]

Though there is no technical why someone couldn't design a high power SEPIC. DC DC convertor to handle voltage above and below the required Li range. This would allow legacy devices to connect to the Li bank.

Dave

[Singleprop]

I think this one from Mastervolt might work, it has adjustable output voltage between 10V and 15V:

Magic 12/12-20 | Mastervolt Marine