Help with Travel Trailer

[cotrailhead]

I derived 29.2 (the bulk charging rate) from the distributors website. See attached picture or link for details. I don’t really understand the float voltage but I though having had it set at 27 maybe I wasn’t achieving full charge.

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[tictag]

Float voltage is to a very large extent irrelevant to Lithium-ion batteries - a Lithium-ion battery does not need a Float charging stage and can, in fact, be damaged (prematurely aged) if the Float voltage is too high. A battery (Lithium-ion or lead-acid) will exit the Absorption stage fully charged. The Float stage is only relevant to lead-acid batteries to counter the effects of self-discharge. Lithium-ion batteries do not self-discharge (well, not like lead-acid batteries do).

So for a Lithium-ion battery, the only parameter of any importance (to fully charging it) is the Absorption voltage or, to use BattleBorn's terminology, the "Charging Limit Voltage". For 1S, that's 14.6V, for 2S that's 29.2V. Once the terminal voltage of the battery reaches 29.2V and the current flowing into the battery has reduced to around C/100, your battery is fully charged - no Float required. At this point, a good charge controller, designed for Lithium-ion, will stop charging and monitor the battery voltage ready for the next charge cycle.

You must never allow a charge controller to perform an Equalisation charge stage for a Lithium-ion battery.

 

[cotrailhead]

This charger seems different. If I set the bulk charge to 29.2 and the "float" charge to 27.2, or 27, or 28, the voltage never exceeds that "float" charge. In other words, this charger bulk charges until it reaches whatever the "float" is set at and then it just maintains. This is why I'm tempted to set both at 29.2 as 3.65 per cell is the peak voltage suggested by the distributer.

 

[tictag]

Replace the charger. No charger I've ever experienced does this.

Permanently maintaining the battery at 29.2V will damage it.

 

[cotrailhead]

I appreciate your time. I'll try to keep this quick. I keep coming back to this paragraph from here when researching how to charge lithium batteries. "During the conventional lithium ion charging process, a conventional Li-ion Battery containing lithium iron phosphate (LiFePO4) needs two steps to be fully charged: step 1 uses constant current (CC) to reach about 60% State of Charge (SOC); step 2 takes place when charge voltage reaches 3.65V per cell, which is the upper limit of effective charging voltage. Turning from constant current (CC) to constant voltage (CV) means that the charge current is limited by what the battery will accept at that voltage, so the charging current tapers down asymptotically, just as a capacitor charged through a resistor will reach the final voltage asymptotically."

I think I can achieve this with this charger by setting the bulk charge to 26.2 (70% SOC?) followed by a float charge of 29.2. Hear me out. I've watched it charge, and as it approaches that "float" voltage, it decreases the amps flowing to the battery "asymptotically" as mentioned above. TicTag mentioned that "permanently maintaining the battery at 29.2V will damage it." But this is an off grid solar system the battery will cycle every night. Will floating for a few hours per day be a problem? 29.2 is not a number I am committed to at all. It's just a placeholder for conversation purposes.

 

[John Frum]

I appreciate your time. I'll try to keep this quick. I keep coming back to this paragraph from here when researching how to charge lithium batteries. "During the conventional lithium ion charging process, a conventional Li-ion Battery containing lithium iron phosphate (LiFePO4) needs two steps to be fully charged: step 1 uses constant current (CC) to reach about 60% State of Charge (SOC); step 2 takes place when charge voltage reaches 3.65V per cell, which is the upper limit of effective charging voltage. Turning from constant current (CC) to constant voltage (CV) means that the charge current is limited by what the battery will accept at that voltage, so the charging current tapers down asymptotically, just as a capacitor charged through a resistor will reach the final voltage asymptotically."

I think I can achieve this with this charger by setting the bulk charge to 26.2 (70% SOC?) followed by a float charge of 29.2. Hear me out. I've watched it charge, and as it approaches that "float" voltage, it decreases the amps flowing to the battery "asymptotically" as mentioned above. TicTag mentioned that "permanently maintaining the battery at 29.2V will damage it." But this is an off grid solar system the battery will cycle every night. Will floating for a few hours per day be a problem? 29.2 is not a number I am committed to at all. It's just a placeholder for conversation purposes.

This is kind of my thing.
the way cc=constant current, cv=constant voltage really works is this.
During the cc phase the charger adjusts the voltage to keep the current rate at the configured level.
When the battery voltage is close enough to the configured target voltage the current is limited by the battery, that is the start of the constant voltage phase. The charge is considered complete when the current flow approaches 0, meaning the potential difference between the battery voltage and the charge voltage approaches 0.

tl;dr
cc current is limited by the charger
cv current is limited by the battery

 

[tictag]

Will floating for a few hours per day be a problem?

This is a question for your battery manufacturer.

My advice would be to never leave the battery connected to a charger at 3.65V per cell (i.e. 4S:14.6V, 8S:29.2V) after the CV part of the charge cycle has concluded. Without further advice from your battery manufacturer, I would not set a float voltage above 3.3V per cell (i.e. 4S:13.2V, 8S:26.4V).

 

[cotrailhead]

I'm happy to admit that I was wrong. The charger does achieve the bulk voltage setting before settling back to the float setting. So I will set the float at 26.4. Can you tell me why that is your voltage of choice?

 

[tictag]

Tbh, it should be as recommended by your battery manufacturer, however, 3.3V/13.2V/26.4V is the float voltage recommended by BattleBorn. I trust them to know what they are doing.

 

[cotrailhead]

I haven’t used the generator since March 23. I used all of the advice I received here and I finally found this page. But I still don’t have my 2 bms installed. However I did parallel my 2nd 8s battery into the system and have been monitoring cell voltages. I’m not coming close to the low end and I’m not coming close to 3.65 per cell on the high end,so I think I’m fine for now. Also I understand now that though the battery may register a certain voltage it isn’t fully charged at that voltage until it won’t accept amps (basically). So relying on the voltage was my problem back in March. Anyway, I’ve now added induction cooking into my electrical usage mix. The stock refrigerator is still running on AC. I haven’t tried to run the water heater and probably won’t until I get the bms installed. And might not ever. My goal is to start tapering off in terms of monitoring the system. We shall see. It’s just so darn fun to check in on!

 

[thomascleary66]

Your ties into the load center sound right, and are more or less how mine is set up (though with Victron gear and 12VDC, so no conversion).

I am also going 12v, can I tie in my 12v loads this way:

Can the 12v distribution panel connect directly to the MPP (along with the battery leads?
@Defenestrator

 

[Defenestrator]

This question/discussion probably makes more sense to have as a separate thread though, rather than hijacking this one.

By distribution panel you mean the BlueSea fuse box? Yeah, seems completely reasonable to me.

Wiring sizes and fuses don't quite match up, though.
Your controller tops out at 40A, so there's no reason to have a fuse over 50A there even if the wiring's OK at 100A.
You probably want a 100A fuse rather than 125A leading to the fuse box, both to protect the wiring and the box itself.
You either need bigger wire or a smaller fuse between battery and busbar. Realistically you shouldn't see more than 100A in/out, so a 100-115A fuse and 2/0 is OK, or you can also size for the future and go up to 4/0 with that 225A fuse.

 

[thomascleary66]

This question/discussion probably makes more sense to have as a separate thread though, rather than hijacking this one.
By distribution panel you mean the BlueSea fuse box? Yeah, seems completely reasonable to me.

Thanks for the advice/confirmation and the hijack etiquette feedback; my bad