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

[SV THIRD DAY]

The Honda 2000 can put out 13.3A A.C. continuous, so they will run a 60A 12v DC battery charger without a problem.

[Hoopla]

Originally Posted by CGirvan

I have a quick question as we seem to be on the subject of chargers. I haven't switched to LiFePo yet but am planning on doing so when the current set of batteries die. I was thinking of buying a benchtop power supply instead of a dedicated charger specifically the Volteq HY1550EX.

My charging system will be primarily solar, and alternator when motoring and running the Honda 2000 when the first two aren't enough. My thinking is that the power supply will be handy when initially balancing the cells and any necessary subsequent balancing, also when using the charger I will be monitoring what's going on and I'm pretty much just bulk charging.

Anybody see any problems with this scenario? Does the honda put out clean enough 120v for the power supply? Is there an issue running the full 50amps for an hour at a time or would I be better to cut back to 45?

Thanks for your help

Colin

Colin a piece of lab/bench top equipment won't last long in a sailboat environment.

[transmitterdan]

Colin,

Cheap lab supplies may not be designed for continuous service. They for sure are not explosion proof which is more important for petrol fueled engines. The Honda should make decent enough power for a 600W power supply.

If you go this route try to find a power supply that is conformal coated on all circuitry. Most are not. Marine chargers usually are to handle moisture and salt fog.

[CGirvan]

Thanks for all your comments.
I was thinking of conformal coating the circuit boards to help with longevity.

My hesitation with using it with the Honda 2000 was not so much a having enough power issue as more of a waveform/cycles issue. Are the inverters used in these modified square wave and are the cycles consistent or do they vary a bit with engine revs. Does any of this matter with the benchtop power supply, maybe they're more tolerant of these issues than I'm giving them credit for.

[transmitterdan]

The hondas use an inverter. They don't vary frequency with engine revs. It will power just about anything.

[john61ct]

Yes the inverter Hondas are very commonly used to run PCs etc as well as quality mains chargers.

Another way is a little 5-7HP motor and HO Alt+MC-614 VR. Biggest challenges there is belt/pulley setup and "marinizing".

[greg_kulosa]

Originally Posted by pacific_voyager

Perhaps I am confusing nomenclature: BMS should be "charging source control". The number of posts is approaching 6000 and it would be nice if there were a synthesis somewhere. The best seems to be this. I am just trying to figure out what will work for a 600-700 AH bank without having to go crazy with complexity and expense.

So far it appears to me that the following is a subset of what is good for DIY LiFePO4 vessel battery banks:

...

4. The "BMS" may or may not have top balancing. It is arguable whether, in a properly initially balanced bank, automatic top balancing is needed. There are good arguments for bottom balancing every year or three and not having cell balancers. It may be simpler than we think.

From what I have read here, it seems to me like top-balancing (NOT bottom) is preferred. It's also easier to achieve without endangering the cells.

If you top balance and have your protection system set for 2.9 volts/cell (when any cell reaches 2.9, cut out the bank), then you are limiting yourself to the capacity of the lowest cell in the bank.

Since chargers use voltage set-points to switch from CC to CV charging (bulk to absorb), you would really prefer that all cells reach their top voltage together - hence top balancing is preferred.

I think T1 Terry was recommending doing a manual cycle with manual top-balance every 1-3 years. I know he doesn't like the per-cell balance boards. BTW, this is also working for me (but I have less than a year of use so far).

Hopefully that makes sense (it makes sense in my head, but I am not sure that I am communicating it well).

Originally Posted by pacific_voyager

5. What is more important for a "BMS" (or "safety net management") is the ability to disconnect the charging source at a high voltage threshold and then to disconnect the loads at a low voltage threshold. The general thinking appears, to me, that this is your safety net. Not what I would call a “management system” but more like an emergency disconnect.

I call mine a BPS - Battery Protection System. It disconnects the bank at low voltage, high voltage, (or per cell low/high voltage) or if the difference between cells is too great.

There is no "shunting", and no per-cell circuit boards. Just a wire between each cell to measure voltage.

Those are my thoughts.

- Greg

[pacific_voyager]

Originally Posted by greg_kulosa

From what I have read here, it seems to me like top-balancing (NOT bottom) is preferred. It's also easier to achieve without endangering the cells. ...



I call mine a BPS - Battery Protection System. It disconnects the bank at low voltage, high voltage, (or per cell low/high voltage) or if the difference between cells is too great.



There is no "shunting", and no per-cell circuit boards. Just a wire between each cell to measure voltage.



Those are my thoughts.



- Greg

Thank you, Greg. BPS is a good name. I feel like a full-blown commercial BMS is over-kill, over-expensive, and over-complex. If charging sources "manage" their own output and BPS is the safety net then what remains is to manage the charging cycle to allow the bank a reasonable amount of discharge before another charge is applied.

[T1 Terry]

BMS seems to be a blanket terminology for anything related to batteries. In reality the voltage regulator on the alternator or charger is a form of BMS, without it the battery would be destroyed, stopping discharging or starting recharging when the battery is flat is another form of BMS and often human controlled with all the problems that leads to..... humans are crap at keeping their mind on what they are supposed to be doing.
That means the BMS really needs to be automatic, stop charging when a cell gets high and stop discharging when a cell gets low, not all that complex, if the cell voltages are within the safe range then the battery will be within the safe range. The only value in an alarm function for cell voltage differential is to tell the user the battery might require a rebalance at some stage, just when requires a bit of recording regarding the cell imbalance at the time. Which cell was high and which cell was low, only happens at the end of charging so not something you need to watch all the time. If the same thing happens at the end of the next recharging cycle, again record the high and low cell, was it the same 2 cells? If it happens a third time and it was the same 2 cells, charge the lowest cell till its voltage is the same as the high cell but leave the normal charging running at the same time.... the problem will simply go away. If the same cell goes both high and low and jumps around a lot, check the connections across that cell and the series link to the next cell, a bad joint is the cause of such a problem.
Without cell voltage monitoring you wouldn't know there was a problem, the units that do this job are cheap and they have an alarm output.

Charging devices can be either controlled at the input of the output or internally by using either the temp sensor or battery voltage sensing, so lots of methods and the one most suitable to the device and system can be selected.

Forget about bottom balancing, this is for electric vehicles where they need all the cells to reach 2.8v at the same time as they use every scrap of storage held in the pack, if you are doing that to house batteries you need a bigger capacity pack or recharge more frequently.

T1 Terry

[senormechanico]

Originally Posted by T1 Terry

If the same cell goes both high and low and jumps around a lot, check the connections across that cell and the series link to the next cell, a bad joint is the cause of such a problem.

T1 Terry

Imho, excessive internal cell resistance would cause the same symptom.

[T1 Terry]

Originally Posted by senormechanico

Imho, excessive internal cell resistance would cause the same symptom.

Fair point, but it won't cause rapid changes between high and low readings, only a greater difference between load voltage and charging voltage compared to other cells in the same battery pack. Flickering voltage change is the result of a poor connection creating an intermittent high resistance joint.
I'd be checking the tightness of all the bolts across the battery pack first, then the faces of the cell terminal and link plates for a bright clean surface with no oxidisation (and that can only be done by cleaning them with an abrasive pad to see if they really do look better afterwards), then coating with Alminox paste . If the first remedy didn't solve the problem nor did the second, only then would I test the cells individually to determine if the cell really did have a high internal resistance.
Of course the give away would be a pregnant looking cell indicating either excessive voltage charging or very low cell voltage discharging, then the cell would be suspect and should be tested individually as the odds are fairly high that the internal resistance would be affected.
The Winston cells in particular are really tough, they don't just die, they are murdered and it takes a lot to kill them in one go.

T1 Terry

[downunder]

Would a 400 AH Li bank be OK size for 1500W of solar panels. If I was to stick with AGM batteries what is the minimum size bank recommended. Would 800ah be OK?

thanks

[botanybay]

Originally Posted by hellosailor

dan-
" The actual fuse opening rating should be based on the expected max load and wire size."
I would suggest rephrasing that, i.e. " The actual fuse opening rating should be based on the expected max load and wire size, whichever calls for the lower fuse value in order to be protected." Depends on what you're trying to protect, and whether there's just one load involved.


But last time I checked, if a fuse wouldn't blow under a 5kA load, there was no way in hell that putting in a 10kA fuse would make it blow any better of faster. The small fuse should blow faster, and potentially even explosively, well before the larger fuse opens. And once the fuse has blown--unlike a breaker, there's nothing left to weld itself shut. Or is there?

They are talking about the "interrupt rating" of the fuse, not the point where the fuse blows.

The concern is that with a wrench dropped across the lugs or someone is moving around the main feeds hot by accident that the short provides enough current to weld the wrench or lugs down. In this short condition it is possible for the let's say 300A rated fuse to blow but since the current is very large (say 5000A) that the fuse draws an arc as it blows. The interrupt rating of the fuse is it's ability to stop the current from flowing when current is well above the point where the fuse blows.

As an example, a 400A "T-Series" Fuse has an interrupt rating of 20,000A while a small "ATM" fuse has an interrupt rating of 1,000A.

I prefer to have the primary fuse near the battery which has a amperage blow rating which is less than the lowest "interrupt rating" of the rest of the fuses in the system.

Thus, if the "ATM" fuse blows at high amperage and is arcing (so the current is over 1,000A) at which point the 400A "T-Series" fuse will blow and extinguish the arc.

Style Interrupt Rating
----- -------------------
T 20,000A
ANL 6,000A
AGM 2,000A
AMI 2,000A
MAXI 1,000A
ATM 1,000A

So, I prefer to have either a T or ANL fuse near the battery bank with a fusing level of less than 1,000A (nominally around half of that for margin) such that all of the remaining fuses in the system are protected from a short which is large enough to draw an arc across the less expensive fuses.

[T1 Terry]

Originally Posted by downunder

Would a 400 AH Li bank be OK size for 1500W of solar panels. If I was to stick with AGM batteries what is the minimum size bank recommended. Would 800ah be OK?

thanks

As far as the solar charging, lithium Depending on the load not exceeding 40amps at any one time (5 amps per 100Ah = C20 discharge rate) 800Ah of AGM is the equivalent to 400Ah of lithium if the end of discharge is 12v under load. If you want more than 40 amps at any one time divide the highest required load by 5 (amps) and multiply that by 100 to give you an idea of how much battery AGM battery capacity you would require to equal the 400Ah of lithium.
Sound like B/S, well as long as you don't want more than 400 amps at any one time from the 400Ah of lithium these are the real deal figures. You can draw more than 5 amps per 100Ah from an AGM battery, but you can't get the full 50% of the advertised capacity, the longer the load remains above the C20 rate the less you can get from that 50% stored capacity before the voltage drops below 12v.... even if the load drops to the C20 rate for the last bit. Argued this bit more than once on more than one forum, but do the test and you will find I'm right, that capacity is lost and can not be regained with full recharging the battery to 100% SOC.

T1 Terry

[andreas.mehlin]

Does anyone know of any LiFePOY4 suppliers in mexico or panama? Or any suppliers who ship there?