Lifepo4 Charging

[Ip485]

For a 600 or 800 AH bank what is the highest sensible charge rate (I understand they can pretty much absorb everything you chuck at them) and if I am correct are there any really "larger" chargers available or is it a matter of combining multiple chargers?

[smac999]

How big is your shore cord or gen? That will likly be your limit.

I would say 300a but A 30a 120v shire cord will only run about 200a (with nothing else on)

200a for an 800ah agm bank is normal.

[Ip485]

240V 32 Amps and 8Kw Genset

[PaulCrawhorn]

The weasel word "sensible" there determines the answer.

If the skill set of the electrician allows for a safe infrastructure, then 1000 amps or 2000 will be safe, in other words a 20 minute refill. But most would say that is not sensible, and instead of X cycles life span you might get only 70% of X as a result - but still many many years.

For maximum cell lifespan, keep to 200-300A, 3-4 hours recharge is nice and gentle for lithium.

All that is at low voltage DC of course.

The current capacity of your upstream source will usually be the limiting factor. If this is a 24V system, the latter range above might be 7kW, which is rarely available off a single shore circuit.

Ganging multiple heavy duty chargers in parallel requires a specialist electrician and units designed for that, especially if fed from separate circuits or "legs" - really you could be getting into 3-phase power there.

If this is not just a dream, maybe look at the world of DIY electric car conversions for expertise.

[Frode]

Originally Posted by PaulCrawhorn

If the skill set of the electrician allows for a safe infrastructure, then 1000 amps or 2000 will be safe, in other words a 20 minute refill. But most would say that is not sensible, and instead of X cycles life span you might get only 70% of X as a result - but still many many years.

For maximum cell lifespan, keep to 200-300A, 3-4 hours recharge is nice and gentle for lithium.

Paul's advice is generally agreed to with respect to LiFePO4. Used to be that manufacturers would indicate 1C as the max sustained, repeated charge rate, but that now appears to be have been reduced to 0.5C. Very little real-life experimentation exists to be able to predict what the lifespan will be when charging at 0.5C vs 1C.

On the practical side of things, coming up with 400A for an 800AH bank, let alone 800A, will prove challenging, as commercial solutions are not readily available. For my own 900AH bank, I'm building a five-stage charger, max 200A per stage, using retired power supplies from high-end servers (240VAC, single-phase). The idea is to be able to supply up to 12kW, to match the output from the generator, on the very rare occasion when the solar output has been inadequate for an extended period of time.

[Ip485]

Thank you.


Its 12V.


The largest chargers readily available appear to be 100A / 12V.


I was thinking of two of these;


https://www.es-store.co.uk/product_d...0&show_menu=21


and running them at the same time.


Any idea with say a 600AH bank run down to 100AH, what the realistic recharge rate would be please?

[markcouz]

With a 100amp charger you will replace the 500Ah requied in 5 hours. (100amp x 5h = 500Ah)

[markcouz]

Also with a 600Ah bank I personally would not charge at above 120amps (0.2C), 100amp charger is just about perfect in my opinion.

You will get lots of opinions that will differ.

[Frode]

Originally Posted by Ip485

Any idea with say a 600AH bank run down to 100AH, what the realistic recharge rate would be please?

Recharge rate (current) is a function of rate accepted by the battery bank and rate available from charge sources. Since LiFePO4 batteries are both documented and tested to readily accept 3C, it means the rate dictated by the bank would be as high as 1,800 amps. This is not dependent on depth of discharge, nor does it vary noticeably as the batteries approach full charge, in contrast with lead acid.

In contrast with marcouz, I would have no qualms about charging at 0.5C. This is not just an opinion, but is based on real-life trials, several of which have been documented here on this discussion board.

Assuming you can get each of two connected chargers to actually put out the full 100 amps each, until batteries are full, you'd be able to supply 200 amps.

If you're really looking for recharge time, it would take 2.5 hours to add 500 AH, at a rate of 200 amps (there is virtually no loss in the charge process for these batteries).

Now, the Victron chargers you listed are not for lithium-ion batteries, so you need to find something else. Most of the chargers I have seen listed as LiFePO4 capable (not enough to just state lithium-ion capable), will not charge correctly, by themselves, and will require either manual intervention or additional control provided by an advanced BMS.

With no intent to be disparaging towards your current knowledge or capabilities, if you're not entirely sure why that last paragraph is crucial, you're nowhere near ready for a switch to this battery chemistry.

[PaulCrawhorn]

So long as the CV voltage setpoint is customisable to say between 13.8 and 14.2V

and the "hold time" can be shortened or even eliminated

There is no need for a special LiFePO4 charger, let alone one controlled by the BMS.

[PaulCrawhorn]

Normal cycles use the lower settings to be gentle and get longer life

When you see cells getting out of balance, by say 70 mV then increase the voltage to get above BMS start-balance setpoint

and increase the Absorb hold time until you see the cells are back in balance, within 20-30mV

With a well matched set balancing may only be needed very rarely.

A BMS or balancer with a high balance rate will take less time to get there, over 1A can be useful with well worn or poorly matched cells.

[Frode]

Originally Posted by PaulCrawhorn

So long as the CV voltage setpoint is customisable to say between 13.8 and 14.2V

Repeated testing, both in controlled laboratory settings and real-life scenarios, has shown this approach to damage the cells.

The fully charged resting voltage of a LiFePO4 cell is around 3.35V, or 13.4V for a 12V (nominal) bank. Keeping these batteries at, or near, full charge for extended periods of time, even without an active charge source connected, is damaging. These results are well documented in other threads on this discussion board.

The current prevailing thinking is that longer-term storage should be done with cells at or below 50% SOC, and that keeping the cells, for any length of time, especially with elevated temperatures (above 30 degrees Celcius), at or near 100% SOC will reduce both capacity and expected lifetime.

Overcharging of cells can result in heat generation and electrolyte evaporation, causing cell bulging and capacity loss.

User Maine Sail has quite possibly done more testing and documentation of these cells than anyone else on this board. I highly suggest reading his posts and learning from his research.

In summary, the conventional chargers, and even many claiming to be ready for LiFePO4 batteries, will not cut off charging at an appropriate time, and will not wait until the batteries are sufficiently discharged before starting another charge cycle.

[CarlF]

With an 800amp 12v bank you don't have to worry about exceeding .5c charge rate. 400 amps of charging is hard to do for a reasonable cost. But you'll be much happier if you can charge above 200amps.

And if you have the budget, you'll be much happier with 800 amps than 600 amps if your boat has a lot of power hungry equipment. You'll have to start the genset much less often. Although 800 is still not enough to be practical for air conditioning.

And the only time you care about fast charging is when running the genset. At a dock, you have all night.

On my current boat I have an 800amp Firefly bank (12v). I plan to convert it to drop-in Lifepo4 one of these days and would use the same chargers. I have two Victron Multi 3000 watt inverter chargers each with 120amp of charge. While these Victron's can be interconnected, mine are wired separately . One is wired just as a charger. The other is the inverter/charger. The Victron Inverter/Charger is only about 15% more costly than just the a charger.

Advantages:

Should the inverter/charger fail - I can throw a few switches and have the good unit take over as inverter/charger. A backup inverter is really important to me.

My genset and shore power is wired as two 120v 50amp legs. But the genset can really only provide 35 amps per leg. Each Victron charger draws about 15amps on its leg. This leaves plenty of capacity for other AC loads.

I would set both chargers with the same voltage settings. When the batteries are low they both will charge at full capacity. As the batteries get close to full, one charger will go to float before the other - but this won't materially effect total charge time.

I have debated adding another Victron 70 amp charger on the AC leg that has the lesser load from other things. This would help when there's no solar adding to the charge rate. It would still be well below .5C.

[Frode]

Originally Posted by PaulCrawhorn

A BMS or balancer with a high balance rate will take less time to get there, over 1A can be useful with well worn or poorly matched cells.

Maine Sail has referred to a BMS with active balancing as a Battery Murdering System. In general, active balancing is frowned upon and seen as highly unnecessary (for batteries not used for propulsion). For batteries not pulled well into either their charge or discharge "knees", the imbalance is not observed, even after years of usage.

[Ip485]

Some useful thoughts, I am very grateful.


I will ocnsider 800AH, I have room.


Realistically I only have room for two chargers. I have two at the moment and can use them at the same time to charge my existing bank, but the output is only 150AH.



Cost isnt a consideration, and I shant do the install myself either.



What would your recommend then as the bast double charger setup and the best batteries to buy. I aim for simplicity of use and reliability.


I do use a lot of power at anchor, and I dont wish to run the Genset more than necessary.


I have 800W of solar array with a Outback MPPT and a big Balmar on the engine.