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

[SV THIRD DAY]

Originally Posted by goboatingnow

The issue of cell balancing is often presented as "a problem" in practice cell balancing is merely an issue about extracting max efficiencies from a bank. It really doesn't matter a lot if you have small cell imbalances, all that results in a small loss of overall capacity , so what

Dave nailed it folks.
With all the talk in this thread (and other LiFePO4 threads) about cell top balancing, bottom balancing, and high wire balancing, the "New to LiFePO4"
guy gets the impression that LiFePO4 Balancing is something like having to change your engine oil, or keep your Lead Acid batteries topped off with water.

Watch out...if your BMS doesn't have automatic Cell Balancing or you don't manually balance every few months, you will trash your gazillion dollar battery bank or blow it up and burn down you boat. And your wife will leave you and your soul will face eternal damnation

That's the take away in the sound bite nature of chat room postings, then the next thing you know all the experts can't talk enough about cell balancing. Of course they have never actually RAN a LiFePO4 battery bank, but hot damn do they know that only a fool would spend $1700 for a 400AH LiFePO4 bank and not have automatic cell balancing. Heck...the only real way to roll is to have it all controlled from your Ipad or Smart Phone...everyone knows that! Why is it that those that have actually RAN a LiFePO4 battery bank keep reporting that cells getting out of balance is a non-issue yet I can't make it a few posts through a LiFePO4 thread without reading about the necessity of cell balancing all the time?

[deckofficer]

Rich,

In fairness to the "need to balance" folks, maybe their "new" cells were like mine, 3+ year old stock that came to me with each cell's voltage different.

After I balanced they have stayed tight for 400+ cycles, not needing any further balancing.

[OceanSeaSpray]

Originally Posted by Maine Sail

I agree they will eventually need balancing but I am already at 25% of the total 2000 rated cycles and in need of zero balancing. 12mV to 16mV at voltage and 10A net accepted current on a 400Ah bank. This is where I deem it "full". I glance at the cell balance regularly and even at 120A of charge current the spread is next to nothing even after nearly 500 cycles.

...

Originally Posted by Maine Sail

I still don't need balancing even at 497 cycles. So yes I would agree with "rarely"...

If people choose to charge at higher voltages the game changes a lot and cell balancing will be more frequent. For frac "C" use at reasonable charge voltages my cells have stayed in balance well beyond what I had ever anticipated. Won't be the same for ever cell brand but these Winstons have done remarkably well.

Your experience is always interesting. At lower top voltages, imbalance doesn't show as much, so this could partly explain success, but looking at a typical curve, they certainly wouldn't be out by a great many Ah even at 3.475V/cell. If you tried to charge once to 3.5V/cell, you could discover greater differences that weren't there initially.

When it comes to balancers, high voltages, reliability and the like, well, there is a reason why I am building new hardware. The BMS will track any imbalance developing as the bank climbs into the upper knee and if it exceeds a threshold, generate a signal. You could put a LED or small buzzer on it if you wanted. It is good information. It will do nothing else.
But, if it exists, a balancing module will be able to pick up this signal and shunt some current over the higher voltage cell(s). It could spend years without doing anything, or do a little now and then.

Imbalance is a problem primarily when it becomes bad enough to cause a cell to go HV upon charging, so the lower the charging voltage, the more tolerant the whole arrangement is. But there is a point where unreasonably low charging voltage results in unusable capacity with no benefits. Charging high enough to be able to see differences between cells is sensible because a low cell not showing up at the top will eventually show at the bottom causing a LV condition and this determines the effective capacity of the pack.

I think it is high charge rates that promotes imbalance, long before end voltage (within reason). The less efficient cells don't absorb as quickly, the others charge faster and every charge skews things a little further. What the higher end voltage does is highlight the imbalance much more readily.

[evm1024]

Pulled out my 2 gc-15 golf cart batteries and installed the LiFePO4 700 ah bank. Using MainSail's MC-614 parameters I programmed my MC-612. The MC-612 drives a 100 Amp 90 series balmar alternator. Using an Amp Manager setting of 200 gave me 66 amps at 13.64 v (target was 13.9 in bulk).

No battery temp probe, it does have an alt temp probe. Using a single 1/2" gates green stripe belt.

The start battery is an agm. It is powered by an echo charge and was at 13.4 volts. I'll get a rest voltage later.

The Xantrex SW3000 inverter/charger appears to be an older one (model 815-3000) that does not have a programmable battery type. (just has OEM Preset instead). I've a note off to Xantrex asking whats up.

The key with the inverter/charger is that the bulk/absorbtion voltage is 14.4 for flooded and Gel and 14.3 for AGM. A bit too high for me. Also, you need to turn "Force Charge" to off. With Force Charge "on" the charger will come on anytime power is restored (like after a power failure).

I have charged with the inverter/charger and a close eye on the pack voltage. The charger was putting 130 amps into the pack with the pack at 13.67 v. You can de-rate the charger if need be.

I leave the inverter/charger battery connection turned off when not on the boat (so far).

[simat]

Originally Posted by OceanSeaSpray

The aim is fairly simple: a LiFePO4 bank that can be charged and discharged without having to think about it and without the risk of losing it due to some complicated installation- or chemistry-related glitch. Just like a SLA bank.

I think this hits the nail on the head. I have been hesitant to recommend LiFePO4 batteries to non technical people because I don't think there is a BMS around that is flexible enough to let me program it for what I think is the best charge regime and protects the battery from the glitches and stuff-ups that do occur without intervention from the user.

So the idea is a black box that is straightforward to install, robust and takes the complexity of the protection side away.
In this instance it will also help with first balancing the battery pack.

I hope you will be able to buy a bigger boat with the proceeds from developing a BMS along the lines you have described in previous posts.

Originally Posted by goboatingnow

With Lithiums, YOU MUST monitor cell voltages, monitoring systems that simply monitor the total charge voltage across a series string are very poor , liable to mislead and can result in individual cell damage. If you cell monitor then small cell imbalances can be ignored

In practice in fractional C situation, typical of boats, you get no imbalances over time.

I agree about individual cell monitoring. I can see a possible fault where one cell could loose capacity due to an internal fault which reduces the storage capacity of that cell so that the battery will go out of balance and the individual cell could be overcharged with all the consequences of this. Individual cell monitoring should also catch any bad terminal connections and maybe even internal cell resistance problems.

[simat]

Originally Posted by evm1024

The Xantrex SW3000 inverter/charger appears to be an older one (model 815-3000) that does not have a programmable battery type. (just has OEM Preset instead). I've a note off to Xantrex asking whats up.

The key with the inverter/charger is that the bulk/absorbtion voltage is 14.4 for flooded and Gel and 14.3 for AGM. A bit too high for me. Also, you need to turn "Force Charge" to off. With Force Charge "on" the charger will come on anytime power is restored (like after a power failure).

You might be able to get around the lack of program-ability and high voltages by fooling the charge controller into thinking that the battery is at a high temperature so it will drop the charge voltage. These two posts from the Wind & Sun forum, Lithium iron phosphate battery (LiFePO 4) battery for Solar applications ? - Page 5 and Lithium iron phosphate battery (LiFePO 4) battery for Solar applications ? - Page 5 go into more detail

[rfadler]

The latest discussion/mention of cell level voltage monitoring made me think of the next obvious question;

Suppose you have a cell that goes bad after 500 or 1000 cycles. Do you replace it with a new one? You now have a bank of cells that no longer "match". What if you can only find a similar but not identical cell. Say you bought cells from Balqon and they no longer have the same size available but you can get a similarly rated cell from Caleb? This goes against all the information I've read so far yet what's the alternative, buy a whole new bank?

When I buy my cells, should I buy a bunch of small cells? Let's say I want to build a bank with around 1000Ahr. If I use 100AHr cells then I could buy a spare cell at minimal cost. If I go the other extreme of buying 4 1000Ahr cells then having a spare cell is going to cost me $1000 or more.

I was all set to buy four very large cells and all the appropriate updates to my charging systems but suddenly I'm realizing that may not be the best idea on many levels.

For those who have installed LiFePO4 systems made up of relatively small cells, did you buy a spare cell?

Thanks,

[evm1024]

Originally Posted by simat

You might be able to get around the lack of program-ability and high voltages by fooling the charge controller into thinking that the battery is at a high temperature so it will drop the charge voltage. These two posts from the Wind & Sun forum, Lithium iron phosphate battery (LiFePO 4) battery for Solar applications ? - Page 5 and Lithium iron phosphate battery (LiFePO 4) battery for Solar applications ? - Page 5 go into more detail

Thanks, I'll read those. It would be easy enough to fake out the temp sensor and to come up with a table for de-rating the voltage. There should be a way to program in the voltages. The manual shows this. Perhaps a firmware update....

Watch there be some pots inside.

Regards

[hellosailor]

"With Lithiums, YOU MUST monitor "
And yet, Amazon is full of ~10AH LiFePO4 batteries being sold, with no BMS, sometimes by "brand names" like BatteryTender, for drop-in replacement use mainly on motorcycles and racing and other niche markets.
Are they expecting the users to simply not notice premature failures, since they won't be as explosive with this chemistry? Or...?

That inconvenient question of what purpose a BMS really serves refuses to go away.

[georgec]

Originally Posted by evm1024

Pulled out my 2 gc-15 golf cart batteries and installed the LiFePO4 700 ah bank. Using MainSail's MC-614 parameters I programmed my MC-612. The MC-612 drives a 100 Amp 90 series balmar alternator. Using an Amp Manager setting of 200 gave me 66 amps at 13.64 v (target was 13.9 in bulk).

No battery temp probe, it does have an alt temp probe. Using a single 1/2" gates green stripe belt.

The start battery is an agm. It is powered by an echo charge and was at 13.4 volts. I'll get a rest voltage later.

The Xantrex SW3000 inverter/charger appears to be an older one (model 815-3000) that does not have a programmable battery type. (just has OEM Preset instead). I've a note off to Xantrex asking whats up.

The key with the inverter/charger is that the bulk/absorbtion voltage is 14.4 for flooded and Gel and 14.3 for AGM. A bit too high for me. Also, you need to turn "Force Charge" to off. With Force Charge "on" the charger will come on anytime power is restored (like after a power failure).

I have charged with the inverter/charger and a close eye on the pack voltage. The charger was putting 130 amps into the pack with the pack at 13.67 v. You can de-rate the charger if need be.

I leave the inverter/charger battery connection turned off when not on the boat (so far).

The my manual for SW3012 has a custom battery setting where you individually set the voltages for float, absorption, and bulk. It also allows the float stage to be deactivated.


Sent from my iPhone using Cruisers Sailing Forum

[evm1024]

Originally Posted by georgec

The my manual for SW3012 has a custom battery setting where you individually set the voltages for float, absorption, and bulk. It also allows the float stage to be deactivated.


Sent from my iPhone using Cruisers Sailing Forum

The same in my manual. However the actual charger does not have a Custom battery type (like the manual says) it has "OEM Preset" in its place.

Can you check your SW3012 menu?

I'm wondering is there was a jumper that needed to be removed or firmware or?

My actual model is an 815-3000 made on 09/Aug/2011

Regards!

[simat]

Originally Posted by rfadler

Suppose you have a cell that goes bad after 500 or 1000 cycles. Do you replace it with a new one?

If we have four cells and we loose one cell, we have lost 1/4 of our pack, or one quarter of the value of the battery. If the remaining value of the total battery is less than the price of the new replacement cell it is not worth replacing the single cell. If we think our cells are going to last 10 years then it is not worthwhile when the battery is over 7.5 years old.

Another strategy is to just replace the the cells when they fail or become too weak. This gets the maximum value from the cells but means much more monitoring and work. Might work if we had quick connect batteries

You are right in saying that the characteristics of the new cell will not match the rest of the cells in the battery. I can't see this being a problem if the replacement has the same or a larger capacity, that you top balance the old battery cells and the new one in parallel before rebuilding the battery with the new cell, and you have an individual cell BMS of some description.

When I buy my cells, should I buy a bunch of small cells? Let's say I want to build a bank with around 1000Ahr. If I use 100AHr cells then I could buy a spare cell at minimal cost.

You have issues of balancing the current going into each cell in a battery if you do parallel any of the cells, the extra cost of the interconnection hardware and more connections that could be points of failure. I personally like the idea of having two cells in parallel (my off-grid system actually has four, but that is another story) and then each paralleled pair put in series to get the battery voltage you want. The main reasons for this is that if one cell does fail you don't loose the whole battery and it does give you the option when you are going to retire the batteries of keeping the best cells with maybe a few more years of use in them and only replacing the duds. There might also be a benefit in averaging out the performance differences between the cells you put in parallel.

If I go the other extreme of buying 4 1000Ahr cells then having a spare cell is going to cost me $1000 or more.

The problem with this, besides the cost is that the spare on the shelf will age differently to the ones that are in use, so they will be mismatched if and when you do have to use the spare.

[OceanSeaSpray]

Originally Posted by simat

I hope you will be able to buy a bigger boat with the proceeds from developing a BMS along the lines you have described in previous posts.

I agree about individual cell monitoring. I can see a possible fault where one cell could loose capacity due to an internal fault which reduces the storage capacity of that cell so that the battery will go out of balance and the individual cell could be overcharged with all the consequences of this. Individual cell monitoring should also catch any bad terminal connections and maybe even internal cell resistance problems.

A bigger boat?? Hell no! I had already tried to make the present one smaller, but I couldn't really. Length/displacement.

A BMS is all about watching the cell voltages, otherwise a simple HV/LV cut off would be enough. It is also grabbing two temperatures to have that side covered. On a 4-cell pack, this allows monitoring all the cells with sensors between 1-2 and 3-4.
I don't see too many situations where cells would go over-temperature while reading normal voltages, but maybe on heavy sustained discharge or individual cell failure.

[witzgall]

How are you going to know when the cell first goes bad?

I think the first clue you will get that one of the paralleled cells is bad is when it has been bad for a bit, and has damaged its sister cells as well.

Chris

Originally Posted by rfadler

The latest discussion/mention of cell level voltage monitoring made me think of the next obvious question;

Suppose you have a cell that goes bad after 500 or 1000 cycles. Do you replace it with a new one? You now have a bank of cells that no longer "match". What if you can only find a similar but not identical cell. Say you bought cells from Balqon and they no longer have the same size available but you can get a similarly rated cell from Caleb? This goes against all the information I've read so far yet what's the alternative, buy a whole new bank?

When I buy my cells, should I buy a bunch of small cells? Let's say I want to build a bank with around 1000Ahr. If I use 100AHr cells then I could buy a spare cell at minimal cost. If I go the other extreme of buying 4 1000Ahr cells then having a spare cell is going to cost me $1000 or more.

I was all set to buy four very large cells and all the appropriate updates to my charging systems but suddenly I'm realizing that may not be the best idea on many levels.

For those who have installed LiFePO4 systems made up of relatively small cells, did you buy a spare cell?

Thanks,

[OceanSeaSpray]

Originally Posted by witzgall

How are you going to know when the cell first goes bad?

I think the first clue you will get that one of the paralleled cells is bad is when it has been bad for a bit, and has damaged its sister cells as well.

Chris

Chris,

The only way of damaging a good cell is by taking it outside its voltage limits or temperature limits. Even if a bad cell develops in a series pack, the BMS still enforces the limits, so a bad cell won't damage the other ones in a series configuration.
A failing cell would show in terms of voltage, being imbalanced or becoming imbalanced during charge, getting warm etc. The internal resistance and capacity of a bad cell would be quite different.

If you connect multiple cells in parallel, then a bad cell could arguably discharge the other parallel cells outside the control of the BMS, but you would still get an alarm at some point, LV or temperature. 4-cell packs would be the more reliable configuration, because otherwise you would need to make multiple packs with a BMS each.
I haven't heard of single failing cells in parallel configurations so far. In fact, I don't know of any instance of cells that failed for no reason at all.
Trying to protect against something that has never happened is a matter of personal choice or commercial risk. If I built a 2000Ah system with 1000Ah single cells, I might consider splitting it into two parallel banks of 1000A fully protected, because of the value of each cell in relation with the cost of individual protection.

4 cells is simple and robust and I wouldn't look any further for any standard small yacht installation.

Eric