dc-dc CHARGING

smoothJoey said:

Personally I would use one of the profiles from page 19 that has float.
The lithium profiles on page 20 explicitly claim that they don't float...

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Why would you use a lead acid battery profile to charge LFP4?

smoothJoey said:

Because it works.
Kind of a silly question.
Its seems like you don't understand this stuff very well.

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So, what's the purpose of a float charge on a LiFePo4?

No need for personal attacks. Why would you resort to comments like that?

This forum is to help each other out with diy solar energy solutions. If you have valid reasons for using a different charge profile over a proven LiFePo4 profile then I'd like to know what it is. Saying, "because it works" without explanation is unhelpful and rather curt.

Calling it a silly question is deflecting the response back at the questioner in order to avoid the question.

BTW, page 22 does not say anything about charge termination logic.

Mango boat said:

So, what's the purpose of a float charge on a LiFePo4?

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Power assist.
Please read back through the thread.

smoothJoey said:

Power assist.
Please read back through the thread.

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You will definitely need a "power assist" from somewhere. It's not good and here's why...

After setting the Renogy DC-DC 20A charger to a lead acid battery profile, as you recommend, it will only put < 60A of charge to your LiFePO4 battery bank over the 3 hour bulk/absorption time. That's fine if that's all you need to bring the battery up to near full capacity but if you are down 100A at the start you still require >2 hours to charge to 100%. But now your battery charger has entered "float" or "power assist" [sic] since you aren't using the correct charge profile. Because of this, your charger is trying to charge at a lesser rate with a parasitic load adding more stress to the charge cycle. Not good but probably not going to cause any fires. There are some other detrimental things shortening the life of your batteries if you use the wrong profile for your battery chemistry. I can go into those things if you would like.

In the LiFePO4 charge mode you are still charging batteries with a load but at a much more optimal voltage for the battery chemistry. You don't need a "float" for LiFePO4. They take what you give them. That's why you are protecting your alternator with a charger/isolator in the first place.

If you are charging LiFePO4 batteries then you should use a LiFePO4 charge profile for optimum charging. The charger will still "load share" (a more precise term) much like charging your cell phone or laptop while using it. It will cause less stress on the batteries this way.

So, although a lead acid battery profile will work it is not the best way to charge LiFePO4 batteries. If you are charging with a parasitic load, like I am most of the time, you want to get to ~85% and then let the batteries carry the load. That's what they are there for, not the charger. With LiFePO4 you can always charge again. They work well like that. They love to cycle through between ~30% and 85%.

Nol, does your BMV700 shunt connect to the battery or the BMS? From your diagram it looks like you are going through the BMS. Also, how much PV do you have? I'm just curious.

Nol said:

Oeps lot of answers today I try to read them all
I know that the Renogy need a D+ wire and I can make a switch in the line but I also have a BMV700 and maybe that one can switch the Renogy if the battery is 90% ful.
Otherwise the LifePo4 have a BMS which is between the battery and the BMV700 shunt. The otherside of the shunt is connected to the frame of the RV what mean that the BMS regulate the charging of the Renogy or am I wrong ?
Maybe my drawing clear what I mean.

I still have not bought a Renogy because they are not so common here in the Netherlands but the Victron Orion TR is very populair but much more expensive, twice the price of a Renogy.

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I connect the shunt to the BMS so all electricity goes through the shunt and BMS
I have a 265Wp PV on the roof in combination with a Victron MPPT 75/15

By the way, the Renogy DC-DC 20A don´t stop charging the LifePo4 battery if it is full
This is what Renogy wrote me :
It will not stop automatically when the LifePo4 is full. It will continue to charge with a small current if it keeps on charging.
Does that not harm the LifePo4 battery ?

I have a 20A and a 40A Renogy dc/dc in a couple of vehicles. They work as advertised. I use the BMS (Chargery at the moment) as the primary disconnect. Chargery provides a signal for low voltage and high voltage disconnect. A very small relay is connected in-line with the D+ signal to disable charging. Chargery disconnects alt charging at 3.48V (13.92V) and enables charging again at 3.30V (13.2V). Renogy is programed to charge to 14.4V (3.60Vpc). Should BMS disconnect fail, the second line of defense is Renogy dropping down to a trickle charge at 14.4V. Should that occur the BMS will sound an alarm.

How did you do that disconnect and enable charging ?
I think something swich on/off the small relay but what ?
I like that and let the BMV700 do that.

Nol said:

I connect the shunt to the BMS so all electricity goes through the shunt and BMS
I have a 265Wp PV on the roof in combination with a Victron MPPT 75/15

By the way, the Renogy DC-DC 20A don´t stop charging the LifePo4 battery if it is full
This is what Renogy wrote me :
It will not stop automatically when the LifePo4 is full. It will continue to charge with a small current if it keeps on charging.
Does that not harm the LifePo4 battery ?

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Ding-Ding Ding, we have a winner! The reason for "small current" is NOT the charger it's the LFP resisting the amps due to being in the "end of charge voltage". This elevated voltage will cause lithium plating, With time. Bad JuJu.

grizzzman said:

Ding-Ding Ding, we have a winner! The reason for "small current" is NOT the charger it's the LFP resisting the amps due to being in the "end of charge voltage". This elevated voltage will cause lithium plating, With time. Bad JuJu.

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My Mean Well power supply does something strange when it gets to the end of CV and it will over charge it I'm not careful. I was avoiding it but yesterday I decided to see what it would do. The power supply puts out amps for a second then turns off and the display goes into minus numbers for a second then it puts out a few amps again. I don't think it ever turns off and it will over charge if left alone.

So the best way is to stop de charge by disconnecting the charger with a relay in the D+ cable
I think that is what Cal means ?!

Nol said:

So the best way is to stop de charge by disconnecting the charger with a relay in the D+ cable
I think that is what Cal means ?!

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Yes, that's it. The D+ signal uses very little current. Just a small relay is used. It consumes minimal power.

Do you activate the relay by hand or automatic for example by a BMV700 ?

The BMS controls the relay. However, there's also a manual switch to disable D+ signal.

The BMS also controls solar panel power. Same scenario, the mppt charge controller is set to 14.4V. But relay disables solar charging at 13.92V.

There is no float voltage. Float is not ideal for LiFePO4.

But do you want to charge the battery to 100%? I also have the Renogy (40A version, currently on sale for 135 eur at the German Renogy store, and they ship outside DE (I'm in NL)).

I haven't finished my LiFePO4 build yet, so not sure which setting i'm going to use. I have to do some testing to see what's the best. I might just go for the lead-acid profile, since a absorbtion of 14.1V and a float of 13.5V (or maybe 13.2) sounds good to me: Charging not to 100%, and with a lower float voltage, it will stop charging at all basicly (If the cells reach 13.5 (3.375V/cel) no current will flow).
If the LiFePO4 profile indeed is 'it will not stop automatically when the LifePo4 is full. It will continue to charge with a small current if it keeps on charging.', I won't be really happy to use that.
Off course I can add relay from my BMV to just disconnect the D+, but if I can use the lead-acid profile with a 14.1V absorbtion and lower float, it might be easier.

Just finished the first step: Finding an actual D+ on my 2007 Ducato.

@Cal, that is a good idea, the D+ to the relay and from the relay to the BMS so when the alternator runs the relay switch the DC-DC charger !
A manual switch in the D+ cable is alway good just incase.

@DJSmiley, I don´t have a D+on my 2011 Ducato and I don´t know if you have one otherwise you can buy a simulator like this
Or you can find the wire of the cigarette lighter but the the DC-DC start running before the alternator give power

DJSmiley said:

But do you want to charge the battery to 100%? I also have the Renogy (40A version, currently on sale for 135 eur at the German Renogy store, and they ship outside DE (I'm in NL)).

I haven't finished my LiFePO4 build yet, so not sure which setting i'm going to use. I have to do some testing to see what's the best. I might just go for the lead-acid profile, since a absorbtion of 14.1V and a float of 13.5V (or maybe 13.2) sounds good to me: Charging not to 100%, and with a lower float voltage, it will stop charging at all basicly (If the cells reach 13.5 (3.375V/cel) no current will flow).
If the LiFePO4 profile indeed is 'it will not stop automatically when the LifePo4 is full. It will continue to charge with a small current if it keeps on charging.', I won't be really happy to use that.
Off course I can add relay from my BMV to just disconnect the D+, but if I can use the lead-acid profile with a 14.1V absorbtion and lower float, it might be easier.

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I would like to suggest a connection in line with the D+ yo the BMS charge disconnect as a fail-safe.

Not sure if the 2011 ducato’s are much different from the 2007 versions, but on my Ducato (2.3JTD) i found in E-learn that pin 25 on connector A on the module in the cab (fuse panel left to the driver side), B002/M001 is the actual D+.
Verified with a multimeter, and connected the wire to my renogy to it.

Nol said:

@Cal, that is a good idea, the D+ to the relay and from the relay to the BMS so when the alternator runs the relay switch the DC-DC charger !
A manual switch in the D+ cable is alway good just incase.

@DJSmiley, I don´t have a D+on my 2011 Ducato and I don´t know if you have one otherwise you can buy a simulator like this
Or you can find the wire of the cigarette lighter but the the DC-DC start running before the alternator give power

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LOL yah beat me to it.

Thanks DJSmiley for the hint, I also have E-learn so I check it out soon and my Ducato is also a 2.3JTD so I think similair to yours.