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

[Maine Sail]

Originally Posted by deckofficer

Using the specs for a 80% DOD, doesn't seem like much of a test. Specs say my batteries are 100 a-hr but we all know these cells give about 15% more than their spec sheet. I took 80 a-hr out on this cycle and these are the cell voltages....
3.26, 3.25, 3.25, 3.25

Quite a ways from the discharge knee, but to be fair to Winston, this will be the cycles for the future tests.

Those match my 80% DOD numbers pretty darn closely......

[deckofficer]

Originally Posted by Maine Sail

Those match my 80% DOD numbers pretty darn closely......

That's good to know. Who would think a 12 volt battery when pulled down to 80% DOD would have a resting voltage of 13.01 volts?

Whats not to like about these cells?

[T1 Terry]

I think there is a bit of mixed learning going on here. Don't try to carry over what has been learnt from lead acid batteries to lithium batteries, the whole technology is different so the rules are different.
The cell min voltage is 2.8v.... that is the number, nothing else taken into consideration or after you blah blah blah. The rules, are don't drag the cell below 2.8v. It the load is 1 amp then 2.8v will be a fully drained battery, if the load is 5C then the same 2.8v limit applies, reduce the load to maintain 2.8v min, don't assume anything, that is the min voltage before you start doing damage.

Every time you drag a cell below 2.8v you are heating the electrolyte, simple math of volts below 2.8v x current will give you the value of the resistance in watts that is changing electrical energy into heat energy. The same deal the other way around, voltage above 3.4v x current.
The rest of the lecture I'll save for my book :lol:

T1 Terry

[goboatingnow]

Originally Posted by T1 Terry

I think there is a bit of mixed learning going on here. Don't try to carry over what has been learnt from lead acid batteries to lithium batteries, the whole technology is different so the rules are different.
The cell min voltage is 2.8v.... that is the number, nothing else taken into consideration or after you blah blah blah. The rules, are don't drag the cell below 2.8v. It the load is 1 amp then 2.8v will be a fully drained battery, if the load is 5C then the same 2.8v limit applies, reduce the load to maintain 2.8v min, don't assume anything, that is the min voltage before you start doing damage.

Every time you drag a cell below 2.8v you are heating the electrolyte, simple math of volts below 2.8v x current will give you the value of the resistance in watts that is changing electrical energy into heat energy. The same deal the other way around, voltage above 3.4v x current.
The rest of the lecture I'll save for my book :lol:

T1 Terry

I think the process is a bit more involved then " heating the electrolyte"

In an overcharge what's basically happens is

", the battery becomes unstable if inadvertently charged to a higher than specified voltage. Prolonged charging above 4.30V forms plating of metallic lithium on the anode, while the cathode material becomes an oxidizing agent, loses stability and produces carbon dioxide (CO2). The cell pressure rises, and if charging is allowed to continue the current interrupt device (CID) responsible for cell safety disconnects the current at 1,380kPa (200psi).

Should the pressure rise further, a safety membrane bursts open at 3,450kPa (500psi) and the cell might eventually vent with flame. The thermal runaway moves lower when the battery is fully charged; for Li-cobalt this threshold is between 130–150C°C (266–302°F), nickel-manganese-cobalt (NMC) is 170–180°C (338–356°F), and manganese is 250°C (482°F). Li-phosphate enjoys similar and better temperature stabilities than manganese.

"

Deep discharge issues involve issues around plating of the electrodes causing internal shorts , which then cause overheating on recharge. Hence the primary danger on very deep discharges is the recharge cycle not the discharge one.

Dave

[goboatingnow]

It's should also be mentioned that , charging of Li batteries should never take place at 0 degree C , or below


Dave

[goboatingnow]

Egg heads will enjoy this recent article on LiFerrous , which has shown a " memory effect" exists in these cells


http://phys.org/news/2013-04-memory-...batteries.html

Dave

[FlyingCloud1937]

So in essence, we are talking about 12.4 volt nominal banks. That is really easy to understand.

Once you extricate your own self from 12 volt nominal.

So we have a new kid on the block.

It falls down at at 11.2 volts and skins it knees, anything below 11.2 extended, can cause a catastrophic health issue.

It gets to high at at 13.6 volts for extended, can cause a catastrophic health issue.

So really it's simple, I'm not your dads OLDSMOBILE, I have rules that you must follow.

It happened in the 80's when kids said FCUK to the rest of the world.

Now we see advertising dollars being spent, and consumed, chasing the same kids we determined derelicts.

Lloyd




QUOTE=T1 Terry;1413551]I think there is a bit of mixed learning going on here. Don't try to carry over what has been learnt from lead acid batteries to lithium batteries, the whole technology is different so the rules are different.
The cell min voltage is 2.8v.... that is the number, nothing else taken into consideration or after you blah blah blah. The rules, are don't drag the cell below 2.8v. It the load is 1 amp then 2.8v will be a fully drained battery, if the load is 5C then the same 2.8v limit applies, reduce the load to maintain 2.8v min, don't assume anything, that is the min voltage before you start doing damage.

Every time you drag a cell below 2.8v you are heating the electrolyte, simple math of volts below 2.8v x current will give you the value of the resistance in watts that is changing electrical energy into heat energy. The same deal the other way around, voltage above 3.4v x current.
The rest of the lecture I'll save for my book :lol:

T1 Terry[/QUOTE]

[Jd1]

Originally Posted by goboatingnow

It's should also be mentioned that , charging of Li batteries should never take place at 0 degree C , or below
Dave

Huh??? That is the first time I have heard this. Do you have a reference to this ?

[goboatingnow]

Originally Posted by Jd1

Huh??? That is the first time I have heard this. Do you have a reference to this ?

Google is your friend. Info All over the place . Li batteries do have a fairly narrow usable recharge temp range

Ok say here
http://batteryuniversity.com/learn/a...w_temperatures
Dave

[Jd1]

A couple or three questions re Winston batteries:
1) Is there any disadvantage to operating the cells in other than an upright configuration? I know they are supposed to be usable in any orientation but they do have a vent plug so I'd like to make certain.
2) Is there concensus if banding the bank is a requirement for boat applications or am I only concerned about imobilizing them in case the boat goes turtle?
3) Is there a rule of thumb about cell sizes and what size is best? Smaller cells have a lower energy density per volume but a dud cell is cheaper to replace. My choices are 100Ah, 200Ah or 400Ah.

[Jd1]

Originally Posted by goboatingnow

Google is your friend. Info All over the place . Li batteries do have a fairly narrow usable recharge temp range

Ok say here
Charging Batteries at High and Low Temperatures
Dave

Wow, who knew .... (well I guess most people other than me). I am curious how electric cars get around that limitation. I would have thought that in order to be useable in most areas, an electric car needs to be able to accept a charge at a much lower than zero degree temperature. Does this mean that electric cars are not useable in a vast area of North America in the winter? Or maybe you can only buy an electric car if you have a heated garage? I can't argue with the reference cited but I would have thought that this would be a big issue .... I suppose some of the charge energy could be used to heat the battery packs ....

[deckofficer]

EV batteries are contained in a insulated HVAC box to keep them happy from temp swings.

[CharlieJ]

I have a question that is bothering me. Several very knowledgeable contributors on this forum (you all know who you are ;-) ) use the phrase "...80% DOD..." to describe the operating region with a LFP bank.

When educating a client that is contemplating switching to LFPs, I tell him that we are able to use 80% of the installed capacity of the bank by operating in the region from 10% SOC (or 90% DOD) to 90% SOC (or 10% DOD), thus avoiding the discharge and charge knees.

Am I counting the angels on the head of a pin?

[dlentz]

Originally Posted by goboatingnow

Egg heads will enjoy this recent article on LiFerrous , which has shown a " memory effect" exists in these cells


Memory effect now also found in lithium-ion batteries

Dave

Deep down below the headlines-

"The effect is in fact tiny: the relative deviation in voltage is just a few parts per thousand. But the key was the idea of looking for it at all. Normal battery tests usually run deep, and not partial charging/discharging cycles. It thus took a flash of inspiration in order to ask what might happen during partial charging in the first place."

[Maine Sail]

Originally Posted by CharlieJ

I have a question that is bothering me. Several very knowledgeable contributors on this forum (you all know who you are ;-) ) use the phrase "...80% DOD..." to describe the operating region with a LFP bank.

When educating a client that is contemplating switching to LFPs, I tell him that we are able to use 80% of the installed capacity of the bank by operating in the region from 10% SOC (or 90% DOD) to 90% SOC (or 10% DOD), thus avoiding the discharge and charge knees.

Am I counting the angels on the head of a pin?

Charlie I am actually operating to 20% SOC/80% DOD and back to 98-99% SOC. At 13.8 - 14.0V I have not found I have lost much capacity at all despite being well out of both knee's.. The only slight down side is the taper at the very end is a touch longer to less than 5A acceptance.. I guess I am technically utilizing about 78% - 79% of the installed capacity which for conversation I just call 80%...

I prefer to run with a tad longer taper than to push into either knee.. This taper is still 4-6 hours shorter than most LA batts....

Still, like you I am playing around with this technology and will continue experiments over the winter with my bank...