(8x12) 96V 180 Ah BYD lithium pack... Any useful applications?

[cracovian]

The charger I'm using is the clunky but respected in its day 48V QuiQ 1500 charger that has at least 10 algorithms built-in, mainly for lead brands but probably one or two for lithiums too. They have a bunch of custom updates but I'm not doing it - it's like $300 for the USB dongle... probably not worth it. I still need to call their support and see which one they'd recommend for my application; hopefully there's one but I can't set voltages myself aside from picking one of the ten algorithms at this point.

But here's what I don't understand and I'm trying to learn from all of your stickies but still... I let it charge again, it was running at 1 kW and voltage reached around 60. Once I disconnected the charger with my beautiful wifi outlet switch, battery pack voltage dropped to below 55 Volts within minutes. This makes me think that 60+, while it's charging, is not really the voltage in the batteries themselves and it's not harming them yet, or is it?

[Sunking]

I will try to help you understand. Most all chargers, at least ones worth having, are Current Limited. Example a 10 amp charger can only deliver up to 10 amps maximum. The voltage is just a Set Point and is leading you and many others to the wrong conclusion.Example assuming your batteries need to be charged, I can connect a 48 volt 50 amp charger set to 56.8 volts. If I measure the charger voltage before connecting to the battery I will read 56.8 volts. When I connect it to the battery, the voltage is going to Fold Back, and the charger pumps 50 amps into the battery.

In order to charge a battery, the charger voltage has to be higher than the battery Open Circuit Voltage aka OCV. So lets say your battery was resting with an OCV of 50 volts. Bear in mind we set the charger to 56.8 volts and it can only supply to 50 amps. So why does the voltage Fall Back? It has to do with Ohm's Law and the relationship of current voltage, and resistance. All batteries have resistance or internal resistance we will call Ri. If we pass current through resistance we develop Voltage. Ohm's Law say Current x Resistance = Voltage.

Using a realistic Ri number for your batteries we will say Ri = .04 Ohms. So if we apply 50 amps x .04 Ohms = 2 volts. So if your battery OCV is 50 volts and we pump 50 amps into the battery, the voltage will go up from 50 to 52 volts despite we set the charger voltage to 56.8 volts. In order for the charger not to destroy itself it current limiter had to Fold Back the voltage to 52 volts to limit the charge current to 50 amps. If we had a magic charger with unlimited current, the charger would have to pump 170 amps. So if you look at the math for a charging battery, the Battery Voltage = Battery OCV +[Charge Current x Battery Ri] From above example 50 volts + [50 amps x .04 Ohms] = 52 volts

As the battery charges over time the voltage is going to rise. The charger will continue to pump 50 amps until the battery voltage reaches 54 volts. At that point, current is going to Taper down as the voltage continues to rise. When the battery reaches say 56 volts the current will have Tapered Off to 56.8 - 56 volts / .04 Ohms = 20 Amps. When the battery voltage is 56.8 volts, current = 0 Amps. The battery is Saturated to 56.8 volts and charging stops. You can prove that with the math. Current = Charge Voltage - Battery Voltage / .04 Ohm's = 0 Amps. There has to be a difference in voltage for current to flow. If the charger and battery voltage are equal, current cannot flow.

So to answer your question it is normal for voltage to drop when removed from a charger. Your voltage drop was a lot higher than normal for a LFP battery. I do not know if it was a problem for you or not because I do not know how much current was flowing and your batter Ri values. That is a moot point because what I do know assuming you have LFP battery the charger voltage should never every be set higher than 56.8 volts.

Lithium batteries have some unique characteristics. One of the interesting characteristics if the batteries are allowed to saturate until the point charge current stops, the voltage will not relax like a Pb battery. So if you charge them to 56.8 volts saturation, they will hold 56.8 volts after taken off a charger. On the other hand if you were to charge a 48 volt Pb battery to 56.8 volts until it saturates, when you disconnect the charger and allow the battery to rest will drop to 50.4 volts.

So if you charged your battery to 60 volts until it saturates you have severely over charged the batteries. You hope and pray when you disconnect the battery it falls back to around 56.8 volts. So assuming the batteries did not saturate and charge current was still flowing falling back to 55 volts is a good sign indicating the batteries did not fully charge.

If you take anything away from this let it be this. You never want to fully charge LFP batteries. They are not Pb batteries which require to be fully charged. LFP batteries perform best when in a Partial State Of Charge (PSOC). Real damn easy to make that happen. Turn down the charger voltage. So instead of 56.8 volts try 55 to 55.2 volts. When charged up no battery should be less than 3.4 volts and no higher than 3.5 volts.

[cracovian]

Thank you so much for your time and willingness to explain it all to me. I'll be watching and learning and try to avoid burning my beautiful, almost zero emissions, house down

[Sunking]

You are welcome. Just remember there is no reason to fully charge LFP batteries. Doing so is just asking for trouble. So resist going over 55.2 volts, never go below 48, and keep your eye on individual cell voltages.

[cracovian]

I have an automatic schedule to run the microinverter for only 3 hours in the evening for now. This means two hours of charging at 1 kW during the day which is peanuts for my 10 kW peak system. Baby steps and nothing that the pack (only 8 kWh for now) can't handle.

It's a perfect match for the nighttime idle speed. Once I add the other 4 packs, it will be plenty to run all night and save ~150 kWh or more of my solar each month instead of giving it away for almost nothing just to buy it back at 5x the cost...