Charging curve

We have been traveling full-time for almost 8 years with our 4 cell 300Ah LiFePO4 battery. It is charged via 800W worth of solar panels and directly from our Canter 100A rated alternator while driving.

Solar can be anything from zero to 50A and the alternator typically 75-80A. No smoke yet despite all the naysayers and irrelevant YouTube videos.
All charging sources are set at 14.1V bulk and the battery will take all the current pushed at it (c130A while driving) until it is 100% SOC. When the battery terminals reach that 14.1V at whatever current it is ALWAYS full and the current quickly drops to all but zero within a minute or three. Increasing the voltage to 14.5V takes about 10 seconds and pointless with no added capacity.
During the day once full the solar controller is set to drop the voltage (float) to 13.45V where the battery maintains 99-100% SOC until the sun goes down and it is typically 80-85% SOC by dawn waiting to start the cycle again.

My advice FWIW leave the voltmeter in place but invest in a quality battery monitor that will set you back somewhere between $40 - 200.

I have to play with here. Juntek 1300 ($40), Junctek KG140 ($80) and a Victron SmartShunt.
They all work perfectly and agree with each other to within 1-2% SOC.

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Hi qldcamper,

For any battery to accept charge, the charging voltage must be above the battery's nominal 'at-rest' voltage otherwise current will not flow. Because of this, the terminal voltage during charging does not offer any meaningful indication of battery State of Charge. This is especially so for lithium batteries because of their low internal resistance and almost flat voltage/SoC relationship.
Control of lithium charging is therefore achieved by monitoring the charge current then terminating charging when this falls to a setting of 3-6 Amps.
The battery terminal voltage does rise during the charge cycle but it is a small change and the actual value may vary due to reasons such as temperature and battery condition so it is not reliable for determining the SoC during charging.
On the other hand, the low resistance of both the charger and battery will reflect significant current change as the rising battery voltage approaches the charger voltage which then is used to terminate the charge.

If the charger then goes to a 'float' or 'supply' mode, the terminal voltage will be maintained at 13.0-13.8v which may be taken to indicate that the battery is fully charged.

In summary, it is difficult to determine the battery SoC during the charge cycle by voltage observation. Only battery monitors which incorporate 'Joule counting" where the charge and discharge current is integrated and used to indicate SoC will provide meaningful indication of the SoC.

"Does it achieve near full voltage pretty much straight away then continue to take substantial amounts of current with very little voltage increase or is it a little more linear?"

I have not done any measurements on lithium batteries but I have done a bit of research from more reliable sources than web forums. Go to battery manufacturers literature rather than any travellers/campers forums if you want good info and not duff-gen. From what I have seen, whilst there are differences there are also similarities between SLA and lithium batteries, just like there are with nicad batteries. If there was no similarity then you would not be able to use SLA chargers (with care) to charge lithium batteries.

There is a lot of conflicting information above so I am providing some manufacturers advice for you so you can sort out the wheat from the chaff.

Lithium battery State of Charge

How to charge lithium-iron-phosphate lifepo4 batteries

Charging Lithium Batteries: The Basics

Depends on the charge source, all chargers will act as constant current source if you exceed their max output capability, ie 20A. Two 100Ah Lithiums in parallel and in a discharged state will suck a lot of current when the charger is turned on, the charger will then limit its output current and hence voltage to protect itself so the battery voltage will reflect this, ie if the terminal voltage of the battery was 13V before you turned on the charger then the charger pumping in 20A may raise it to 13.2V. On the other hand if the battery is connected directly to the alternator and it is pumping in 80A the terminal voltage may be 13.8V for example. On the other hand if the battery is near fully charged then the terminal voltage will jump to say 14.4V if the battery is taking less than the chargers max output. It all depends on how much charge voltage and current is available and the state of charge of the battery.

qldcamper,
Voltmeter is far from useless. A voltmeter plus ammeter is ideal.
I have been running Lithium batteries in both Troopy and caravan and monitor voltage and amps continuously on both.
The voltmeter sits on 13.3-13.2V all the time I'm using them. Just running lights and fridges most of the time. But if the voltage were to drop to say 12.8V, it tells me that I'm down to the last 20% and I have to do something about it. Does not indicate what's going on between say 90% and 40% SOC but that doesn't worry me.
While driving, the Troopy batteries easily take 40A charging and the voltage only rises about 0.2-0.3V above the resting voltage. But when approaching fully charged, the voltage rises quickly and once the charging voltage gets above 13.8V I find that not long after the absorption voltage of 14.4V will be reached, the amps drop right off and the batteries swing into float. So basically the voltmeter tells you when you are nearing full charge.
I find the caravan is a bit different because it has 240W rooftop solar via a separate controller. The rooftop solar keeps up easily with my 12V demands, so little is drawn from the alternator (via a 25A DCDC charger) - the caravan voltage typically is getting up towards absortion voltage when it is approaching fully charged then gradually drops to it's 13.5V float.
I also run a Victron smartshunt in both and the % remaining is something I just look at occasionally out of curiosity.

Briste,
What type of information is available to you via the bluetooth connection?

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Hey Briste,
On the subject of storing LiFePO4 batteries, Enerdrive say that their batteries should be either.....
1) Left connected to a lithium charger on float. In this situation a load such as the fridge may be connected.
Or
2) Fully charged and disconnected, then monitored and maintained at or above 90% SOC.