Bulk & Float Solar settings for LiFePO4 batteries

[asimba2]

I am completing the design of my electrical system and have a single 300W LG solar panel charging a 200aH GBS LiFePO4 battery bank. I have not yet selected a solar charge controller but am considering the Victron 100/30 and the Midnite Solar "the Kid." Clearly I need adjustable charge parameters for the LiFePO4 battery.

The Victron 100/30's ($195) absorb time is only adjustable in hour increments, which is not acceptable, so if I use that charge controller I'm thinking I should set the BULK charge to terminate at 13.7-13.8V (to under-charge for long life) and perhaps set the FLOAT to 13.4V. Is that a reasonable work-around to prevent the long absorb time that would no doubt harm the battery?

Or, alternatively, the Midnite Kid ($330) has more adjustable parameters including the ability to switch to float when current reaches a % of C (usually 5%). As I understand it, the measured output includes loads, which means loads would need to be disconnected when charging, which is not workable. Adding the optional Wiz Bang Jr sense module might fix that (?) but I already have a shunt measuring amps/in out.

What's my best bet here for a semi-compact safe solar charge control of my LiFePO4 batteries?

[john61ct]

Since other charge sources are also an expensive challenge to get right, consider routing all of them through a lead "reserve" bank, maybe used as Starter, and then use a Sterling/ProMariner as the ONLY LFP charge source.

Very reliable, fully programmable, only one bit of kit to worry about.

This way your standard alternator VR and shore charger (maybe wind?) can also contribute to the LFP bank, later expand the solar by just adding more rather than replacing, lots of cost-effective flexibility.

And/or your coulomb-counting battery monitor could have a relay to disable all inputs based on SoC, tuned lower rather than mfg-rated (too) full.

[asimba2]

^^ I didn't want to complicate the conversation too much but I already have the shore power/alternator charging handled. I essentially have the Magnum inverter/charger set to very conservative charge levels since I almost never hook to shore or switch on the alternator charging.

Solar will be my main source of charging.

[SV THIRD DAY]

For the last 3yrs I've been playing with this issue and have settled in at 13.8v for Bulk and 13.4v. So you sound on track to me.

[john61ct]

Yes on the bulk/absorb, 3.45V for longevity, can go a bit higher if current <.3C, bit lower if > .6C.

Maybe lower 13.2V on the float, much better to just STOP when Absorb drops to .02C

Resting full - no charge or load for five hours - should be around 13.5-13.6.

Go a LITTLE higher voltage, maybe 14-14.1 periodically to eliminate LFP memory bump and check cell balance.

Don't be afraid to routinely go right down to 20% SoC, but **never** even close to empty.

Never charge while temps anywhere near freezing.

That's pretty much it?

[Bleemus]

Can someone provide a link to the Sterling Promariner solar charge controlller? I cant find it.

[john61ct]

Collaboration on design, same main boards, separate companies and sales channels.

​http://sterling-power.com/products/b...ers-waterproof

http://www.promariner.com/en/product...arging-systems

[Bleemus]

Originally Posted by john61ct

Collaboration on design, same main boards, separate companies and sales channels.

​http://sterling-power.com/products/b...ers-waterproof

http://www.promariner.com/en/product...arging-systems

Thanks. I have found those but not MPPT so I will stick with a different brand.

[john61ct]

MPPT is a solar controller thing.

DCDC battery to battery charging is an infrastructure piece not to do with solar per se. These two are the most flexible and robust I've found, only ones I'd put on my LFP bank.

The MPPT controllers tacked on to some lower end DCDC chargers are cr^p, avoid.

But if you can replace all your usage from solar, don't need shore, genny or Alt input to the LFP, keep looking at just the many excellent solar controllers.

The ability to stop Absorption when current gets down to .025C is great.

If you find one that just Stops rather than continuing to float, let me know.

Meantime the SoC-based relay like Victron BMV70x can do it.

But only one relay on that, need a bank-level LVD too.

[T1 Terry]

Originally Posted by john61ct

Since other charge sources are also an expensive challenge to get right, consider routing all of them through a lead "reserve" bank, maybe used as Starter, and then use a Sterling/ProMariner as the ONLY LFP charge source.

Very reliable, fully programmable, only one bit of kit to worry about.

This way your standard alternator VR and shore charger (maybe wind?) can also contribute to the LFP bank, later expand the solar by just adding more rather than replacing, lots of cost-effective flexibility.

And/or your coulomb-counting battery monitor could have a relay to disable all inputs based on SoC, tuned lower rather than mfg-rated (too) full.

The down side of this idea is the shortened life of the lead acid battery. It does work and we tried it 6 yrs ago in the very early stages of developing a reliable charging strategy. Current surge was one issue making direct connection and a solid state relay or contactor a no go, so a DC to DC charger was employed. This resulted in a much better current flow but unfortunately each charge cycle appeared to register as a life cycle for the lead acid battery. We didn't even see the advertised 700 cycles before the lead acid battery was no longer capable of holding voltage above 12v once the 60 amp load was applied (40 amps at the house battery @ 14v equalled 60 amps at the start battery due to voltage drop and cable losses).
We quickly moved on to other methods as the losses were greater than we desired and the expense of lead acid battery replacement ruled it out as being financially viable.
What is needed is a long cycle life go between storage device that can absorb the charge current without a sudden voltage increase and release the stored charge at a rate that would not cause a voltage spike in down line equipment. Lots of easy things come to mind until you reach the discharge side, then the complications come into it and destroy the whole KISS principle of any control system design.
As someone on another forum loves to say, "Complicated is easy, simply is hard" not quite true though, just a lot more thought needs to go into the ideas end until the easy path stands out among the crowd of possibles.

T1 Terry

[Kim Klaka]

Originally Posted by john61ct

Yes on the bulk/absorb, 3.45V for longevity, can go a bit higher if current <.3C, bit lower if > .6C.

Maybe lower 13.2V on the float, much better to just STOP when Absorb drops to .02C

Resting full - no charge or load for five hours - should be around 13.5-13.6.

Go a LITTLE higher voltage, maybe 14-14.1 periodically to eliminate LFP memory bump and check cell balance.

Don't be afraid to routinely go right down to 20% SoC, but **never** even close to empty.

Never charge while temps anywhere near freezing.

That's pretty much it?

I am intrigued, not least because I am not very good at electrics. I have had LiFePO4 batteries on my boat for over a year with no problems, I just let them take whatever current is offered to them. Each charging device (just two in my case: alternator and solar panels), has a regulator set at a standard voltage i.e. not specific to lithium, and from memory it might be 14.2V. No bulk/float/whatever settings, just push in whatever current is available at that single regulated voltage level. I add two qualifiers:
1. Cell balancing is taken care of within each "12V" block of batteries.
2. In addition to the regulator at each charger I also have a regulator at each battery which limits both maximum AND minimum voltage.

Am I doing anything wrong/bad? I don't think so, because a friend of mine has had lithiums installed for over 5 years with this type of setup and has had no problems. But I'd like to be sure we haven't missed something.

[SV THIRD DAY]

Oh this will be fun,,,,

[john61ct]

Yes lithium banks sold as canned systems with integrated BMS, active balancing etc are a completely different world from just buying bare cells and DIY the management with "open hardware" infrastructure.

Former is a LOT more expensive, but should less risky, drop into infrastructure designed for lead.

But almost everything I've seen from the LFP vendors, do allow higher charging voltages than I would for longevity, unless maybe they're stepping V down internally?

Some cynics say they don't want their banks to actually get to their maximum possible lifespan.

Which so far looks to maybe be decades if kept within those parameters above.

Since people are happy with 5-10 years, and don't want the complications of replacing all their charging sources, that may be the best course, for them.

[Kim Klaka]

Many thanks for the advice. FYI Drop-in 12V lithium packs with min/max regulator cost roughly 10-20% more than good quality AGMs, here in Australia at least. That's the initial outlay; when you factor in their longer life they become cheaper.

[T1 Terry]

Originally Posted by john61ct

Yes lithium banks sold as canned systems with integrated BMS, active balancing etc are a completely different world from just buying bare cells and DIY the management with "open hardware" infrastructure.

Former is a LOT more expensive, but should less risky, drop into infrastructure designed for lead.

But almost everything I've seen from the LFP vendors, do allow higher charging voltages than I would for longevity, unless maybe they're stepping V down internally?

Some cynics say they don't want their banks to actually get to their maximum possible lifespan.

Which so far looks to maybe be decades if kept within those parameters above.

Since people are happy with 5-10 years, and don't want the complications of replacing all their charging sources, that may be the best course, for them.

Not a bad analogy Jogn, no idea what the drop in replacement people do regarding end of charge or even if they monitor cell voltages. On the surface it looks a lot like the early days charging used by the DIY electric vehicle people, charge to 3.8v per cell and burn off the excess, so 14.8v or less comes in at under a balanced 3.8v per cell. The missing bit of course is the charger dropping back to under 0.5 amps and holding cells above 3.6v causes serious electrolyte heating resulting in increased internal resistance..... death by a thousand cuts. Charging at greater than the 0.5 amps the cell top balancers can handle would result in cell voltage run away of course, but if you don't monitor it you don't know it happened, ignorance is bliss
This fits well with your 5 - 10 yr life expectancy, although cycled like that every day might be a stretch for even the 5yr cycle life even if the cells somehow never go into run away.... but then it would be outside the suppliers warranty period or even their doors still being open

T1 Terry