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

[DotDun]

Originally Posted by dannc

The definition of Net Metering varies by state and/or power company.

I have not checked the recent laws so things may have changed, but in NC I could sell power to my power company but they would pay whole sale rates while I was buying at retail rates.

A few years ago I attended a PV class and the instructor said that it was simply not worth it for a homeowner in the state to try to sell surplus power back to the power company. If one designed and built their PV system to match their power needs, the cost and hassle of selling back to the power company was just not worth it. The instructor had built subdivisions full of PV installations and as the time limits expired on the power buying agreements, the home owners were no longer selling back to the power company.

Later,
Dan

My point is if the "sync up" period is annually, the grid is your battery. If I send the grid 2000kwh and I suck 2000kwh out of the grid during any particular "sync up" period, the net charge is $0, you are actually selling power to the grid at retail price, i.e., they are storing it for you and giving it back when you need it, that is the definition of a battery. If I send more power to the grid than what I suck out of it, the power company pays me at wholesale rate. If this is the case, what's the issue?

Am I missing something?

[Hoopla]

Originally Posted by JAMcDonald

Revised for clarity.

Are you sure LFP batteries being sealed don't comply with the rules and therefore an AGM or sealed LA is superfluous??? That aside In light of your revision the above solutions I put forward no longer apply.

You don't mention how you deal with charging the LA bank? If utilising a Balmar Duo or slave style charging arrangement a VSL on the charging link between the two banks would work. That arrangement works even better with GEL batteries having a lower charge voltage than other LA's and lower than LFP's. The Balmar Duo does have a current limit, 30A I recall.

However if the BMS cuts the alternator regulator field during a HVE then you lose charging to the second LA bank with this Duo arrangement. If that is a problem you can direct alternator charge to LA bank first and link to the LFP bank via a LFP style HVE VSR. Such an arrangement does not require regulator field wire being cut by the BMS in the event of a HVE.

[hellosailor]

JAM-
"I didn't hear the alarm warning "
You need a wheel shaker. Aircraft sometimes have them, to heavily vibrate the wheel in order to make Real Damn Sure the pilot gets the message. Cadillac and some other car makers have adopted the same idea, either shaking the steering wheel or vibrating the whole seat.
Dunno if Edson sells one yet (G) but basically they are just cheap robust electric motors with an unbalanced weight attached to them, so as it rotates unbalanced, the whole thing shakes.


Lots of ways to make an alarm noticed.(G)

[JAMcDonald]

Originally Posted by Hoopla

Are you sure LFP batteries being sealed don't comply with the rules and therefore an AGM or sealed LA is superfluous??? That aside In light of your revision the above solutions I put forward no longer apply.

You don't mention how you deal with charging the LA bank? If utilising a Balmar Duo or slave style charging arrangement a VSL on the charging link between the two banks would work. That arrangement works even better with GEL batteries having a lower charge voltage than other LA's and lower than LFP's. The Balmar Duo does have a current limit, 30A I recall.

However if the BMS cuts the alternator regulator field during a HVE then you lose charging to the second LA bank with this Duo arrangement. If that is a problem you can direct alternator charge to LA bank first and link to the LFP bank via a LFP style HVE VSR. Such an arrangement does not require regulator field wire being cut by the BMS in the event of a HVE.

Good points, thanks. I'm not planning to use the AGM battery at all save for emergencies but have solar and alternator charging covered. I have a Balmar MC614 regulator, not the Duo unfortunately. All batteries have to be sealed. There is (was) still a requirement for a separate engine start battery for the Pac Cup at least in 2014. Have to review this year. I may just ditch the AGM reserve idea for now and do it manually. I did get a loud alarm. I thought of one way to do it using a second EV200 contactor with or without an auxillary output, but these guys come with significant parasitic load (as will every solution).

[JAMcDonald]

Thanks for the thought, but I have a light displacement boat that planes downwind and a tiller not a wheel. I'll add an alarm in the cockpit.

[Cottontop]

Originally Posted by hellosailor

Putting it on a boat would be a waste unless the boat was a dock queen.


Whatcha gonna do with all that expensive and bulky AC interconnect and transfer equipment, if there's no mains power company tied into you?(G)


Not to mention, a convenient refrigerator-sized space, conveniently located so it doesn't tip over the boat.


I see a lot of this stuff as an answer in search of a problem.

Well, I don't know. House has solar panels, boat has solar panels. House has grid, boat has alternator. Size and weight, similar to a gen set and a lead-acid house bank, I suppose. Advantages: no gen set. Cost compared to gen set and conventional batteries: ?

[Hoopla]

Originally Posted by JAMcDonald

Good points, thanks. I'm not planning to use the AGM battery at all save for emergencies but have solar and alternator charging covered. I have a Balmar MC614 regulator, not the Duo unfortunately. All batteries have to be sealed. There is (was) still a requirement for a separate engine start battery for the Pac Cup at least in 2014. Have to review this year. I may just ditch the AGM reserve idea for now and do it manually. I did get a loud alarm. I thought of one way to do it using a second EV200 contactor with or without an auxillary output, but these guys come with significant parasitic load (as will every solution).

In closing and my opinion only but I’m not sure subjecting your backup battery bank automatically to potentially the same over voltage or under voltage event without manual intervention and first checking the problem is such a good idea. With the exception of a voltage spike or drop in a wayward cell those events could instantly render your backup instantly useless. While I appreciate the demands of short handing there are a lot of other important activities that you undertake without automation.

By the way I think having redundancy in the form of a simple LA bank that doesn’t have the complex monitoring and switching of a LFP bank like your contemplating is a good idea. A manual battery bank switch providing it doesn’t inadvertently cut the alternator field wire is also a good idea. The alternative of working over a very dense energy source with metal tools attempting to rig a backup power supply in a seaway is probably a recipe for disaster.

Good luck

[JAMcDonald]

Originally Posted by Hoopla

In closing and my opinion only but I’m not sure subjecting your backup battery bank automatically to potentially the same over voltage or under voltage event without manual intervention and first checking the problem is such a good idea. With the exception of a voltage spike or drop in a wayward cell those events could instantly render your backup instantly useless. While I appreciate the demands of short handing there are a lot of other important activities that you undertake without automation.

By the way I think having redundancy in the form of a simple LA bank that doesn’t have the complex monitoring and switching of a LFP bank like your contemplating is a good idea. A manual battery bank switch providing it doesn’t inadvertently cut the alternator field wire is also a good idea. The alternative of working over a very dense energy source with metal tools attempting to rig a backup power supply in a seaway is probably a recipe for disaster.

Good luck

Well put! Besides the cost and complexity factor, the fact that I have good AGMs and an appropriate genasun solar regulator make this attractive. Neglecting the voltage spikes (hopefully minimized by having diode relays but who knows), here is a schematic showing one way to do this with the HousePower BMS.

[katsumi]

Question regarding capacity, I am trying to establish size of LiFePO4 capacity I will need. I currently have 720AH AGM with and 760 watts of solar 60amp MPPT. 50 amp charger, 2x 80 amp alternators.
I figure our current average hourly usage would be 15-20 amps underway and 6-10 amps at anchor.
We run 2 fridges with small danfoss compressors and 1 x 80litre waeco. which are the biggest consumers, we will eventually add a water maker.
The options I am considering are either 4x 400 ah or 8x 300 ah so 400 or 600 ah.
I like the idea of the smaller capacity as their are less cells. Just worried with more cells in the system can it cause the cells to go out of balance.
Appreciate hearing from anyone who has simular set up.
regards
Nick

[OceanSeaSpray]

Nick,

For whatever it is worth, I built a 400Ah system on a 43' cat about 3 months ago, the overall situation was very similar to what you describe here except that you have more solar capacity. 400Ah has been ample, almost excessive.
Solar capacity and consumption are your only real design parameters as ideally you don't want to be relying on shore power or combustion engines for charging. In Queensland, this should hardly be an issue.
A 100% step up in consumption under way would be unusual, you may want to have another look into that figure and also factor in any motoring.

Following very clear advice from a manufacturer, I don't use cells larger than 200Ah for marine banks, they are not strong enough mechanically in relation with their weight and would give reduced life.

I would be tempted to say that 400Ah as 8x200Ah in a 2P4S configuration or 300Ah as 12x100Ah in a 3P4S configuration would give you all the capacity you actually need. Most LFP banks built these days are in fact oversize and then spend too much time at high SOC, which will reduce their life - that is essentially all you get for spending too much money up front. They don't work like SLAs.
My own setup is just coming out of the winter now, starting to recharge faster. I haven't run the engine to charge once and I spent months between 20-50% SOC. Never had a low voltage once, never ran out of power.

Connecting several cells in parallel means that their differences in characteristics have a chance to average out, so it is in fact of benefit. If you run into balance drift problems, you can also swap cells between parallel blocks to try correcting the situation.
Cells only stay acceptably balanced if their self-discharge rate and internal resistance are near-identical for a start: you need some luck when you buy them. Next, running high currents doesn't help, because it gets the cells within the pack to run slightly warmer, increasing their self-discharge rates.

Best regards,

Eric

[Hoopla]

Nick your AGM's have around 300Ah usable capacity assuming your charging between 50% and 80/100% SOC. You are therefore best placed to determine if that capacity is adequate having regard to usage and charging. That equivalent usuable capacity in LFP is a bank size of around 400Ah. About the only charging modification you may want to consider is to increase the size of your alternators if you want to take advantage of the improved ROC with LFP compared to the slow acceptance charging with your AGM's above 80% SOC, noting their practical charging limit with LFP as is is around 80% or say 120A max combined without over stressing them. That said with the amount of solar you have the existing alternators are probably fine.

[katsumi]

Thank you for the reply Hoopla and Eric. Eric can you please tell me what is SLA.
Just to give a brief explanation of my current system I have on board.
Solar 760 watts, MPPT 60 amp charger.
2 standard Hitachi 80 amp engine driven alternators which both feed the house bank and each individual start battery via some sort of battery isolator each.
1 Master volt Charge Master battery charger (shore power or small portable generator)

The following questions may have already been answered here, I apologise for asking them again but I have spent every evening for the last week reading so much information, which I really appreciate, I do find myself with a bit of Analysis paralysis at the moment.

Alternators. I am not keen to change the alternators as they are not my primary charging source more of a back up and replenishing some load whilst motoring
1. Can these alternators and batteries handle long periods of underway with engines running?
2. If a high voltage cut off event occur would the alternator be able to handle sudden disconnect whilst charging? or would the fact the alternators which are also attached to the start batteries via the isolators be able to buffer against this?

Cells
1. I like the idea of the Jusi cell monitor discussed here, can it be left attached or will it cause issues with the cells going out of balance?
2. For those of you who have not got the cell balancing system have you had issues with the batteries going out of balance?
3. Is there an easy balance procedure one could do as a preventative routine maintenance?

I will have more questions, I really appreciate hearing from those of you with simular set up as mine.
Thank you for your time
Regards
Nick

[Hoopla]

Nick

SLA = Sealed lead acid. You will need to either have your alternators charge voltage changed or better still install external regulators such as Balmar etc (providing they are P style alts) and then you can reduce output so they won't cook themselves. Spend time and read this thread and your answers will come particularly Mainesails views on balancing. Google Compass Marine and read Mainesails article on LFP. It is first class.

[katsumi]

Hi Eric
Thanks again for the info. Can you tell me what you did with regards to alternator charging? Also what if any did you do to address Low voltage cut out and High voltage cut out?
Thanks again
Nick

[Hoopla]

Nick

Alternator(s) cannot be subjected to the charge relay opening with a HVE unless the LFP bank is charged via a SLA bank (to act as a buffer as you are already aware of) OR if no such SLA bank exists, the Alternators(s) are disabled before the HVE by the BMS cutting the voltage regulator field wire (not to be confused with alternator field wire). Another good reason for using external regulation.

Nick you would have found this easy answer with some quick reading as I suggested. If you don't wish to put that energy into research you will probably find that in embarking upon DIY LFP, you will hurt yourself, your boat and your wallet. You are probably better off either using an engineered LFP system like Victron, Mastervolt etc or even a "drop in" SLA alternative to AGM's like Firefly's. With the amount of solar you have the latter would be a very attractive and hassle free proposition I would have thought.

I am glad to help and don't wish to sound rude, but unfortunately a plethora of rudimentary, unresearched questions such as yours is why extremely experienced LFP people like Mainesail, Terry and others no longer participate in this thread and it has become a dead end.