question

onrbikes avatar image
onrbikes asked

Is this victron trend graph OK for lifepo4

Having just swapped from lead acid (20 years) to lifepo4 really and really don't know what I'm looking at anymore.

Batteries are 200ah self build with a balancer. Batteries were balanced and have a difference .02v each


I've attached the latest trend.

We only use about 1kwh per day as the house was designed well before offgrid became trendy. That's why the usage is so low.


It seems odd it doesnt stay at a high voltage very long and seems to be going into float at 11am every day.

Voltage never goes below 26.5v

Voltages are set as follows

Float at 27.7v

Absorption 28.4v

Charged 28.2v

Everything works perfectly but can't believe it's this easy.

but it's consistent.


Thanks

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Lithium BatterybatteryBattery Balancerstored trends
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screenshot-20240407-082424.jpg (66.0 KiB)
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3 Answers
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pwfarnell answered ·

So you have 200Ah at say 25V = 5kWh of battery and you use 20% of that so you will only get a short bulk/absorption cycle because the batteries will take full current for most of the recharge unlike lead acid and once full the current tapers off very quickly and goes to float. You did not mention charge current but if you plot it you will see the difference vs lead acid. Welcome to the lithium world.

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onrbikes avatar image
onrbikes answered ·

Thanks heaps for that.

Yes lead acid is a much different beast

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onrbikes avatar image
onrbikes answered ·

I've set the charge current to max 40amps.

I've only got 2000w of solar panels

This is all in very cloudy days as well.

Should I use a lower charge?

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klim8skeptic avatar image klim8skeptic ♦ commented ·

@onrbikes most lithium cells are rated to charge / discharge at half their Ah rating, continuous. Ie for you 100a.


You could possibly lower your float voltage to 26.8-27v. At that voltage your battery wont accept further charge current.

At 27.7v your batts will still accept charge current. Dont forget to disable temp compensation, and charge below 5c.

Lithium is a different beast compared to LA.

Li only need a little bit of time in absorp to balance the cells to be fully charged. Time required in absorption is proportional to the charge C rate, and frequency of full charge cycles.

LA needs hours and hours and hours time spent in absorp charging to fully charge the battery.

The only downside of Li is the requirement for decent balancers, and the lack of feedback that voltage provides for judging SOC during discharge. A properly dialed in shunt will provide excellent SOC data should you put the effort in.

Below is a chart that gives a basic SOC vs voltage for a resting (no charge / discharge for an hour or so) Li battery.

crop-b.jpg


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