Let's talk float voltage.......

[boywonder]

I’ve somewhat recently upgraded to a Victron smart solar controller which allows very easy changing of the battery charging parameters and now I’m addicted to changing the float voltage while camping.


With my old setup (and with the new one as well if I leave the float voltage alone) when arriving at camp fully charged and then running some heavy loads like the electric water heater, microwave, etc it’s a little annoying that I’m parked in the sun but since the house battery is nearly topped up the controller isn’t sending much energy/power to the battery until the battery voltage drops below the float voltage to get into bulk charge mode.

I can be camping for a good part of the first day (well ….any day really if the battery is charged) without putting much energy back into the battery if the battery stays above the float voltage.

If I crank the float voltage up to say 14V or so while camping every time the water heater cycles the watts/amps from the solar panel go way up providing all of the current needed for the water heater and preserving the battery state of charge, since the voltage will drop below 14V and get the controller back into bulk mode. If there is plenty of sun I can end the day with the battery still around 95-96% state-of charge. This is vastly different than my old setup where by day three my old AGMs were getting down around 65% SOC or lower.

I understand that my Lithium battery far outperforms the AGMs but in hindsight if I could have easily increased the float voltage on my old controller while camping I could have played this same game.

When I get back home and the van sits in the driveway I set the float voltage back to 13.4V since there is no load on the battery and Lithiums are happy being around 50% SOC when not being used.

I’m curious if other people have tried this and what people’s thoughts are. I think Andy/”Off grid garage” played with this on his youtube channel; that’s where I got the idea.

[Scalf77]

So I am going to need a better power point for bye in?

My first challenge would be that the re-bulk voltage is not the float voltage. It can be indirectly tied float via a re-bulk offset. This is what I believe Victron uses, although I have seen some controller to just have direct set voltage. Some controllers may not even allow you to set it. If I wanted to go back into bulk earlier this is where I would change the setting.

From a second viewpoint, once your battery is fully charged if disconnected it would drop to ≈ 13.6 volts which would be the resting voltage of a lithium battery. So the energy consumed between the ≈ 14.4 charge voltage should be relatively negligible . So I am not seeing a big savings there. But I would think you are at risk of overcharging the battery

So I admit I generally don't run in float voltage very much with Lithium, so I would have to look at some real life situations. Presumably Float as I remember it from AGM charging would provide a small amount of current, and would also increase that current when devices turned on. So you could the current increase with refrigerator, some lights etc. Obviously large current draws would be out the capabilities of float. In the past there was always a concern that a short spike may prematurely put you into re-bulk to soon and cause overcharging.

The third point is at some point , it wouldn't matter as the loss of SOC during the trip would have you in bulk or MPPT mode and any event. So while you may be able to capture a slight amount of more power on the first day, it would not carry over to the next day, and comes at the expense of possibly overcharging the battery or at minimal reducing life cycles of the battery.

[boywonder]

Originally Posted by Scalf77

So I am going to need a better power point for bye in?

My first challenge would be that the re-bulk voltage is not the float voltage. It can be indirectly tied float via a re-bulk offset. This is what I believe Victron uses, although I have seen some controller to just have direct set voltage. Some controllers may not even allow you to set it. If I wanted to go back into bulk earlier this is where I would change the setting.

From a second viewpoint, once your battery is fully charged if disconnected it would drop to ≈ 13.6 volts which would be the resting voltage of a lithium battery. So the energy consumed between the ≈ 14.4 charge voltage should be relatively negligible . So I am not seeing a big savings there. But I would think you are at risk of overcharging the battery

So I admit I generally don't run in float voltage very much with Lithium, so I would have to look at some real life situations. Presumably Float as I remember it from AGM charging would provide a small amount of current, and would also increase that current when devices turned on. So you could the current increase with refrigerator, some lights etc. Obviously large current draws would be out the capabilities of float. In the past there was always a concern that a short spike may prematurely put you into re-bulk to soon and cause overcharging.

The third point is at some point , it wouldn't matter as the loss of SOC during the trip would have you in bulk or MPPT mode and any event. So while you may be able to capture a slight amount of more power on the first day, it would not carry over to the next day, and comes at the expense of possibly overcharging the battery or at minimal reducing life cycles of the battery.

Thanks Greg....Did I miss a power point? I'll go look...or point me to it...

The thought is that if you leave the controller in it's usual settings, you can be drawing power all day long and then get below your float voltage (or re-bulk I suppose) late in the day when there's not much sun left...so you've depleted the battery (at least down to a voltage to start bulk again) all day and now it's evening with no meaningful sun.

The simple idea is that if you crank up the float voltage (or re-bulk as you say....I'll have to play with the Victron app) then the solar panel(s) can provide the juice when the sun is shining instead of depleting the battery during peak sun hours, leaving the battery SOC higher at the end of the day.

My cells aren't getting above 14.4V/4 or 3.6V/cell ......can I still be overcharging them? Obviously the BMS will disable charge above 3.6V.

I can leave the settings alone next trip and see how the SOC is affected at the end of each day.

[arctictraveller]

Interesting question. In my case, in the day time, loads are powered pretty much from the solar panels, but I don't have any heavy current draws. I do leave the radio, satellite receiver, amp, sub woofer, fantastic fan and the refer / freezer on all day, and my amp meter doesn't show any draw on the batteries, so the solar (300w) is powering it all. When you energize a big load, doesn't the voltage drop enough to bring you back to bulk? I guess with enough amp hours of lithium, that may not be the case though. Anyway, it's an interesting discussion.

[boywonder]

Here is Andy's (off-grid garage youtube channel) video:

https://www.youtube.com/watch?v=6vTaCkTVl7I

This is the video that sent me down this rabbit hole.......and the difference in solar energy that you can harvest during the day is significant if you are running high loads like a water heater and your battery voltage is somewhere between the absorb voltage and float voltage.

My default settings are absorb at 14.4V and float at 13.4. This is where I leave the setting when the van is parked...pretty typical settings. The Lithium battery gets to just loaf along at some medium SOC in the driveway.

When camping I started slowly increasing the float voltage to 13.6, then 13.8 then 14V....this is decreasing the deadband volts between 14.4 and 13.4 and gets the controller back into bulk mode at smaller voltage drops resulting in lots more juice from the solar panel during the day when powering the water heater.

With the float voltage set at 13.4V the water heater will draw about 17A from the battery with nothing coming in from the panel if the voltage is above the float voltage.....which is frustrating when the sun is shining overhead.

I just played with the Victron app...expert mode gets you the re-bulk offset voltage; it's set by default to .1V but as Greg has pointed out it may be another knob to turn.

So the question is.....is there any downside to doing this? It works extremely well so I imagine there has to be a downside

[Scalf77]

Sorry, it was a bad power point joke. I meant that I would need better data to make a decision on that.

So after watching the video, I will point out one important thing. His primary mode of operation is to charge his battery to 80% SOC. Your setting, depending on what your battery wants would be close to or at 100% SOC. By moving your float voltage higher than the resting voltage for a full charge, you will be putting your charger back into bulk then absorption in a constant cycle for the day, we don't want to keep the battery fully charged. This would not be good for battery.

In the video , with his settings he is never charging the battery to 100%. So I don't think it would be a problem for him. I don't fully agree with his thought process, but I think his potential for damage is much lower.

Now with your settings, you a raising your chances of over charging much higher. The other thing I am not convinced on but willing to accept is no current delivered when in float, I agree the battery will not need it , we usually have some always on load that could be addressed. I will have to check that out. In his demonstration had both crappy solar input and didn't really have any heavy loads to convince me, but I will run some test when I can.

[boywonder]

Greg, I can't remember if you have a Victron solar controller, but on a full sun day run your microwave or some other large load, note the power that the panel is producing then increase the float voltage and note the panel output.

I can do the same experiment as Andy in the video, reduce the absorb voltage and essentially make the float the same as absorb (at least for a test) and demonstrate that with zero deadband if the sun is out you'll be maximizing your panel output when drawing high loads.

Thinking about it some more I could do this at 13.6V, but that doesn't buy any more capacity than running absorb at 14.4 and float at 13.6. Well it limits the capacity to whatever SOC 13.6V roughly equates to......as Andy demonstrated in the video.

I'm thinking your point is that setting these voltages above 13.6V may be harmful to the battery even though I'm under 3.65V/cell.

Next trip out I'll set the float to 13.6V (up from 13.4V) and see how that impacts SOC....hopefully without stressing the battery.

[Scalf77]

Yes, It is a combination of time and voltage. Your not overcharging per voltage, but you are keeping it in a high voltage range longer than needed. I agree if you go to his settings it is kind of a wash as far as capacity.

Yes, I have a Victron Solar Controller, I will run some test when I can to see what float does on my Lithium batteries. I normally just turn off the chargers after a full charge cycle.

[Charliesmom]

I have a Blue Sky SB3000i MPPT solar charge controller in our van. The float voltage is set at 13.4 V and the absorption voltage is 14.4 V. Although this is a multistage controller the only time that the van is in absorption mode is for 30 minutes (recommended by Blue Sky) to “balance” the lithium cells. It then charges at the float rate until the batteries reach 100% SOC. When I am not using the van all the chargers are turned off and the batteries sit at ~13.5 V after a full charge. There is minimal decay. (Unlike my Transit AGM batteries that I keep on a trickle charger when the van is not in use to keep the starting batteries from dying!)
When there is sun, our solar panels (230W + 120W portable) easily produce enough power to recharge the batteries to 100% SOC. (0ur “overnight” use is usually ~ 30-40 A). If we’re using more power during the day, more current from the panels is going into the batteries.
The video is well done and certainly proves this point.
It’s like The Rolling Stones song:
“You can't always get what you want
But if you try sometimes, …. You get what you need.”

[boywonder]

Charliesmom: Do you have an electric water heater? Without that, my loads are minimal....