LiFePO4 - The future for off-grid battery banks?

[steveg]

[QUOTE=T1 Terry;38292]Lithium iron cells have a 100% full point, 3.45v, any voltage higher than that is 100% full, just wasted energy that eventually heats the electrolyte and damages the cell. By balancing the load to being slightly higher than the charge rate the excess voltage is burnt off the high cell using up the wasted energy used to get the voltage up there but none of the other cells have any issues with wasted energy, it is all used for charging. Just a hypothetical example, if 0.5Ah is needed to get a cell from 3.45v to 3.75v as soon as the load is applied that 0.5Ah would appear as though it was never there, the voltage on the high cell will immediately drop to below 3.45v. Each time this cycle happens the lower cells get 0.5Ah closer to fully charged.
If the boost voltage was set high enough eventually all the cells would reach above 3.45v together but it's not really necessary to pull all the cells up that high every recharge, they are not damaged at all if they never reach fully charged. Maintained high cell voltage (above 3.5v) and below 2.0v are the thresholds that damage lithium iron cells, they stay healthy between these two limits.
The only reason to bring them all as close to 3.45v as possible at the same time is to know where the 100% charged point is, that

[russ]

I dont want to obsess about my batteries I want to forget about them for months at a time.

Regards,

Steve.

That is the case with maybe 95% of users - they want to use and not have to tinker.

Russ

[T1 Terry]

Lithium iron cells have a 100% full point, 3.45v, any voltage higher than that is 100% full, just wasted energy that eventually heats the electrolyte and damages the cell. By balancing the load to being slightly higher than the charge rate the excess voltage is burnt off the high cell using up the wasted energy used to get the voltage up there but none of the other cells have any issues with wasted energy, it is all used for charging. Just a hypothetical example, if 0.5Ah is needed to get a cell from 3.45v to 3.75v as soon as the load is applied that 0.5Ah would appear as though it was never there, the voltage on the high cell will immediately drop to below 3.45v. Each time this cycle happens the lower cells get 0.5Ah closer to fully charged.
If the boost voltage was set high enough eventually all the cells would reach above 3.45v together but it's not really necessary to pull all the cells up that high every recharge, they are not damaged at all if they never reach fully charged. Maintained high cell voltage (above 3.5v) and below 2.0v are the thresholds that damage lithium iron cells, they stay healthy between these two limits.
The only reason to bring them all as close to 3.45v as possible at the same time is to know where the 100% charged point is, that

[steveg]

How does a dump load take excess amps out of only a few cells ??

It doesnt, it takes current out of all cells including the ones that are too high/low ... reducing the head/foot room until balanced.

[steveg]

Sorry Steve, you can't read accurate cell balance with the cells under 3.45v and the charge rate less than 10 amps, anything below full voltage except for fully discharged voltage is relatively meaningless accept to show a failing cell because the difference will be big. Fully discharged (2.8v or under) is used by a section of the EV people because it gives a more accurate out of capacity reading, saves a cell being driven into reverse charge and killing it.
As far as needing a full time active BMS on an off grid system, why? Monitor the cell voltages with a Jusi Cell Logger 8 does 8 cells and cost about $28, it can record data files you later view on your computer so you can see just how the batteries going and make minor balance adjustments if needed. I have set mine to 30 sec readings and I can record a weeks battery activity at a time. It has an alarm port that can be configured for dump loads to bring high cells down or turn off the charge till the high cell drops back. The dump load system seems to work better because it self balances the cells very quickly and they don't seem to wander much after that.
Here is a sample graph showing 4 cells of a 12v nom. 720Ah battery bank being recharged daily by a 2 kw solar bank.[ATTACH=CONFIG]1613[/ATTACH]

T1 Terry

Huh?

Voltage is not meaningless as this is the real voltage of the cell (at the cell terminals with my very expensive DVM) at that charge state - I am an electronics engineer by trade and have more than a laymans idea of how to read a battery. You are referring to internal impedance under charging/load I am guessing - if my full charge rate is 60A (and no higher in any condition) then this is the true variation I will get as of now 70mV maybe later as it ages I will get a larger result between cells - with a large load I get similar results from what I have seen so there is no 'hidden' condition with my cells that I am aware of.

Why you so against me having a BMS? (that came with my cells in any case) - why would I now go off and purchase a logger etc to replace something that is working now (yes I have tested it in overcharge and discharge modes and cell balancing mode)

Dump loads from battery?? You either have a shunt across the cells that are higher (usually under micro control) or you only push charge to the ones that are lower (very expensive flyback converters for each cell) ... a shunt across *all* cells pulls current from the entire stack, and ages the battery un-necessarily I would think? and if it is up to charge then no current flows into the cell because it is the same potential as the regulator output, in my case 56.4V

Why you so against me having an BMS? It *does work* you know - and you not having one works well for you - great news, but I dont want to obsess about my batteries I want to forget about them for months at a time.

A BMS monitors every individual cell on charge and discharge, cutting the load/charge if any cells goes beyond 2.7/3.65 - I dont see a down side - I am only drawing a MAX of 150A that's 0.5C per cell and charge is 60A that's 0.2C per cell.

You cant compare home use and EV - EV draws much more current, and hence exaggerates any cell to cell variation - 20kW loads for periods of time and is very hard on cells, and re-gen braking moreso, so the brand (cell construction consistency) and type of cell and BMS is more important in this use case.

Regards,

Steve.

[Mike90250]

It has an alarm port that can be configured for dump loads to bring high cells down or turn off the charge till the high cell drops back. The dump load system seems to work better because it self balances the cells very quickly and they don't seem to wander much after that.

How does a dump load take excess amps out of only a few cells ??

[T1 Terry]

there is 70mV of voltage between all 16 cells (3.360->3.367).

Steve.

Sorry Steve, you can't read accurate cell balance with the cells under 3.45v and the charge rate less than 10 amps, anything below full voltage except for fully discharged voltage is relatively meaningless accept to show a failing cell because the difference will be big. Fully discharged (2.8v or under) is used by a section of the EV people because it gives a more accurate out of capacity reading, saves a cell being driven into reverse charge and killing it.
As far as needing a full time active BMS on an off grid system, why? Monitor the cell voltages with a Jusi Cell Logger 8 does 8 cells and cost about $28, it can record data files you later view on your computer so you can see just how the batteries going and make minor balance adjustments if needed. I have set mine to 30 sec readings and I can record a weeks battery activity at a time. It has an alarm port that can be configured for dump loads to bring high cells down or turn off the charge till the high cell drops back. The dump load system seems to work better because it self balances the cells very quickly and they don't seem to wander much after that.
Here is a sample graph showing 4 cells of a 12v nom. 720Ah battery bank being recharged daily by a 2 kw solar bank.90hr self balance.jpg

T1 Terry

[steveg]

steveg,

Any particular reason you went with Seiden over others like Winston or CALB?

Not a lot of time has passed since you posted the information about your setup, but any new updates would be appreciated.

Hi,

I just found them helpful - and were able to ship CIF to me here in NZ.

My updates are on my blog : http://offgridnz.livejournal.com

System working great still - just measured the cells under full charge (sunny today) at 60A charge there is 70mV of voltage between all 16 cells (3.360->3.367).

Steve.

[SunElectronics]

We use to have a few of these and it worked great, just as you said the prices are unbelievable compare to the most expensive ones.

[john p]

Obviously im wrong , but then I never go anywhere near that far noth and never likely to.
But it was always my belief that while it was "light" up there it is not sunlight as we normally think of sunlight. I thought it was like endless twilight,with just a very short time in middle of day with useful sunlight.??Does that make any sense?? but no sun "power"" .most of the day nothing to put much energy into a solar panel because the angle of the sun is very low provides light but no power/energy..

[Sunking]

didn't say it would produce like a gangbuster just that the days are veeeeery lond at high latitudes during the summer. And I agree atmospheric losses would be substantial

Well if I was not so lazy I would look at solar radiance maps up there for the month of July. I know I would not see 24 Kwh/M2 I would suspect maybe 6 Kwh/M2.

I know for the USA Hawaii and Tuscon are the champs regardless of the time of year. Never really took interest to look at Alaska.

Edit note: OK curiosity killed the cat. I looked at the tables for Alaska. Using 2-axis tracking Bettles AK peaks out to 9.5 Kwh/M2 in July. The you have the rest of the year with almost squat December and January = 0 Burrrrrr

[Naptown]

didn't say it would produce like a gangbuster just that the days are veeeeery lond at high latitudes during the summer. And I agree atmospheric losses would be substantial

[Sunking]

What are you thinking at about 70 degrees north the sun is up for almost 6 months
Ok you need a tracker that tracks at 360 degrees and there is no sun in winter but very lond summer days

Well it would require a tracker to capture, but I can only guess what the very low angles will attenuate or filter out the sun.

[Naptown]

Josko there is no way on earth you have 6 hours solar isolation in summer .. You are a long long long way north of the equator.. I be supprised even if you have 3hours..

The low voltage sensor is VERY important to protect the batteries.. Even one time you go below 2.2v per cell the battery can go DEAD

What are you thinking at about 70 degrees north the sun is up for almost 6 months
Ok you need a tracker that tracks at 360 degrees and there is no sun in winter but very lond summer days

[john p]

yes their web site could be better but if you email them they give usually good information