Lithium battery & component suggestions?

[Delfin]

For the battery obsessed, like me, the following might be interesting as it illustrates why LiFePO4 batteries are so different from Pb chemistry in terms of their charging behavior. The charge cycle below occurred after a typical usage cycle for us of around 300 amps. This would discharge the 600 amp bank around 50%. When re-charging our prior AGM bank, I always strove to get them back to 100% capacity to increase longevity by reducing sulfation and if we still had those, because of the extended taper on Pb batteries, the time to get back to 100% would have been 6 to 7 hours or so, with the last 3 hours just having the current dribble into the batteries at less than 30 amps or so on a 1280 Ah bank.

On our current Li bank, that taper period is reduced to 3 1/2 minutes. Note that the acceptance rate is totally flat - the bank absorbs what is put to it at around .3C with no fall off until the very end. The total re-charge time is 1 hour and 54 minutes, during which time house loads were drawing off some of the available charging current (we made bread, freezer and refer, etc.).
This also illustrates how difficult it is to adapt even sophisticated Pb charging equipment to Li chemistry, if what you want to do is ignore the charging cycle like you can when a Pb bank is connected to charging sources and those sources are managed by a smart regulator. If I had left the Li batteries charging, the voltage would have risen to the 28.8 bulk setting on both the Trace 4000 charger/inverter and Balmar 624 regulator. But to what end? The acceptance rate doesn't change materially after it drops below around 2.5% of capacity (in my case 15 amps), so having any absorption period where the bulk voltage is held over time is pretty pointless in terms of practical usage, and harmful in terms of just generating heat. After all, that charging current has to go somewhere, and once the Li battery is full it goes to heat, which will kill the battery. Partial recharging is just fine since Li batteries don't sulphate, and while some argue that if you don't fully re-charge memory effects will reduce total capacity, but I am not sure that is proven. If I wanted to get the batteries to 100% for drill, instead of the 95% or so achieved in the cycle described below, I would hold them at constant voltage for another 15 minutes or so, but again, there is no point in that, or at least one I am aware of.

In my mind, the perfect charging system for a Li battery would be able to automatically disconnect the Li bank from the charge sources at a user defined voltage, and automatically switch over to a running battery bank if underway or just start using the Li bank for loads if at anchor. I manage this manually now by using the BMS to disconnect the bank from charging or loads and use the Blue Seas ACR (automatic charging relay) to direct current to the Pb starter bank if we are underway once the Li batteries are full, or simply turn off the genset if we are recharging at anchor. In other words, all the existing charging equipment suited for Pb batteries can be used, but it has to be used differently to reflect the unique charging profile of this chemistry.

 

[Delfin]

It's interesting to see Delfin's charge voltages and compare to what other manufacturers recommend. Delfin, do you happen to know who's cells are in the Lithionics product? I wouldn't expect much difference in recommended charging between vendors, assuming LFP is both cases, but you never know.


You are charging at/to 28.8V, which is interesting. One system I have looked at a bunch, mostly because a friend is building one, and their Allow To Charge signal shuts off at 28.1V. When that signal goes away, all charge sources are supposed to shut off. And they recommend charge settings of less than that so charge sources switch off on their own, and ideally the Allow To Charge signal never shuts off.


Float is another area where I scratch my head. I've heard people say to charge to a certain voltage/return current, then STOP. And I have heard people also say not to Float. All this only makes partial sense to me. Maybe it's just unclear language - I'm not sure.


What makes sense to me is that you want to charger at an elevated voltage and positive current up to some point, then stop charging. But to me that doesn't mean no Float, it means no Absorb or otherwise holding the batteries at an elevated voltage for a prolonged period of time as you would with LA. That's the "Stop Charging" part that makes sense.


But float I think is still a very desirable thing, assuming to have the voltage set right. With Float correctly set to the battery's resting voltage when near full charge, the zero current will flow, and there will be no charging. However when loads get switched on and the battery voltage is otherwise drawn down a bit, the charger kicks in to carry the load. To me this is very desirable to prevent gratuitous cycling of the batteries. Otherwise while on dock power, the chargers would stop, you would run on batteries until they are drawn down, then you would go through another charger cycle, stop, and repeat again. Float prevents this, and lets the chargers carry the load when a power source is available.


So to me, "Stop Charging" when full, and "No Float" are two very different things. This first is good, and the second not so much.

I asked for the name of the source of their cells, but all they would tell me is they came from Taiwan.

Winston recommends 28.8, others less. In my prior post the chart sheds some light on the issue. In my regime, the batteries never see 28.8 because they stop accepting material current before then. My impression is that all the agonizing over charge settings is if you want to have a set it and forget it system. I don't find that necessary, but maybe that's just me. When on shore power, the Li batteries are off line and I run off the shore power, with the Pb batteries being trickled at 26.8 volts. I could set that higher to benefit them, since I won't float the Li batteries, but when I check the voltage on the starter Pb batts, they are right where they should be. If underway, I charge the Li batts, then take them off line and run off the alternator, starter bank and inverter. Seems to work fine.

On the question of floating Li batteries, I guess I still don't see the point. I can't find the chart Lithionics sent me on cycle life at different levels of discharge, but from memory at 50% they predict over 5,000 cycles. If so, why should I care if I discharge, then charge as I am? That's about 15 years of daily use, after all. I can't find any data from any reliable source that disconnecting charging when the acceptance rate falls to a few percent regardless of what the bulk voltage setting is has a negative impact on longevity. I see lots of negative information of letting them sit at those voltages and simmer after they are full. I suppose you could take a bit more time and recharge at the maximum voltages John recommends, assuming the bank was on a bench and not a boat. On a boat, you have loads kicking in, kicking out, and if you set the bulk voltage too low, you trip into float almost immediately, or at least that was my experience as related above.

 

[john61ct]

It is not proven that a float voltage lower than batt's resting V does not harm longevity.

It has not been proven that it does either.

It is certainly not "the same" as being isolated, and for me I will follow Maine Sail's reco to isolate, not allow float.

Your rig, your choice.

 

[twistedtree]

It is not proven that a float voltage lower than batt's resting V does not harm longevity.

It has not been proven that it does either.

It is certainly not "the same" as being isolated, and for me I will follow Maine Sail's reco to isolate, not allow float.

Your rig, your choice.

Same voltage = no current


No current = open circuit


FWIW, Victron and CALB call for float. The key is the voltage, which when selected correctly does not further charge the batteries, but does serve to help carry loads.

 

[twistedtree]

I asked for the name of the source of their cells, but all they would tell me is they came from Taiwan.

Winston recommends 28.8, others less. In my prior post the chart sheds some light on the issue. In my regime, the batteries never see 28.8 because they stop accepting material current before then. My impression is that all the agonizing over charge settings is if you want to have a set it and forget it system. I don't find that necessary, but maybe that's just me. When on shore power, the Li batteries are off line and I run off the shore power, with the Pb batteries being trickled at 26.8 volts. I could set that higher to benefit them, since I won't float the Li batteries, but when I check the voltage on the starter Pb batts, they are right where they should be. If underway, I charge the Li batts, then take them off line and run off the alternator, starter bank and inverter. Seems to work fine.

On the question of floating Li batteries, I guess I still don't see the point. I can't find the chart Lithionics sent me on cycle life at different levels of discharge, but from memory at 50% they predict over 5,000 cycles. If so, why should I care if I discharge, then charge as I am? That's about 15 years of daily use, after all. I can't find any data from any reliable source that disconnecting charging when the acceptance rate falls to a few percent regardless of what the bulk voltage setting is has a negative impact on longevity. I see lots of negative information of letting them sit at those voltages and simmer after they are full. I suppose you could take a bit more time and recharge at the maximum voltages John recommends, assuming the bank was on a bench and not a boat. On a boat, you have loads kicking in, kicking out, and if you set the bulk voltage too low, you trip into float almost immediately, or at least that was my experience as related above.

OI think the big difference here is that you are overseeing the charge cycle, and disconnecting the LFP bank when fully charged. Nothing wrong with that, but as you say, it's a manual process.


I definitely need an automatic process since it must work unattended. So the end of charging is controlled by the bulk/Absorb to float transition. For all intents and purposes, it's a two stage charge vs a three stage Pb charge cycle.