Using a Non-LiFePO4 DC-DC Charger on LiFePO4

[FLICK]

Mainly as a learning experience and as a foot in the door to LiFePO4, (with the view of building an off-grid residence on my property in the future), I'm upgrading my campervan from a 130Ah AGM to a 12v 100Ah LiFePO4 battery, (new Ganfeng cells, Daly 4S 12V 100Ah BMS)... I've purchased a Victron 75/15 MMPT solar charger, Victron IP65 15A mains charger, and a 500A SmartShunt, being a cheapskate I'm reluctant to spend the money on a Victron DC-DC charger also...

I have an (Aussie brand?) Projecta 25A DC-DC & solar charger that I've been using on AGM batteries in my campervan for a few years:
https://www.projecta.com.au/ts16492...Group/1/IDC25+Instruction+Manual+12.03.20.pdf

I'm thinking it would probably be ok to use this intelligent? non-LiFePO4 DC-DC charger set to the Gel profile to charge the LiFePO4 from my alternator:


The GEL Profile is 14.1V Bulk/ABS, 13.7V Float, I "think" this would probably be ok but I'm not chasing a maximum capacity or charge rate, I'm more interested in maximizing the life of the battery... With the research I've done I'll probably set my Victron solar and mains chargers at around 13.8V ABS, 13.4V Storage/float...

An idea that crossed my mind, with the view of being kind to the battery, was to use the AGM profile on the DC-DC charger and add a large power diode in the charge cable to introduce a 0.7V voltage-drop, maybe something like a 70 Amp 70HF120, so in theory the ABS voltage would be dropped to 13.7V, and float to 13V...

I'm wondering if any of the resident experts here have an opinion on this diode idea?, any comments are appreciated ...

Thanks

 

[Goboatingnow]

I would suggest 3.5V per cell as a maximum even less ( 3.45)

No float stage is necessary or desirable

 

[FLICK]

I would suggest 3.5V per cell as a maximum even less ( 3.45)

No float stage is necessary or desirable

Thanks, yep, that's my understanding, and I'm keen to use 3.45V (13.8v) or less, which is why I've been trying to work out a way to lower my non-LiFePO4 DC-DC charger voltage to a more LiFePO4 friendly voltage.... The only settings these chargers have is for the battery type, Gel, AGM, Wet, or Calcium... I've had a good search around and I haven't been able to find any info about people a using power diode to lower charge voltage, so I'm a bit unsure if it's a silly idea or not, I figured it was worth asking before moving ahead just in case I was missing something...

I actually have a couple of these DC chargers so I'm keen to make use of them rather than spending money on a new lithium charger...

So far I've only cycled my new battery a few times tinkering with charge settings on the Victron mains charger, my SmartShunt will arrive later in the week and then I'll start playing with the solar and DC-DC chargers, I'm keen to have accurate measurements and graphs for any further learning/experimenting...

 

[Goboatingnow]

Thanks, yep, that's my understanding, and I'm keen to use 3.45V (13.8v) or less, which is why I've been trying to work out a way to lower my non-LiFePO4 DC-DC charger voltage to a more LiFePO4 friendly voltage.... The only settings these chargers have is for the battery type, Gel, AGM, Wet, or Calcium... I've had a good search around and I haven't been able to find any info about people a using power diode to lower charge voltage, so I'm a bit unsure if it's a silly idea or not, I figured it was worth asking before moving ahead just in case I was missing something...

I actually have a couple of these DC chargers so I'm keen to make use of them rather than spending money on a new lithium charger...

So far I've only cycled my new battery a few times tinkering with charge settings on the Victron mains charger, my SmartShunt will arrive later in the week and then I'll start playing with the solar and DC-DC chargers, I'm keen to have accurate measurements and graphs for any further learning/experimenting...

The diodes are one way albeit somewhat crude to drop the voltage as the forward voltage drop curve needs to studied for a given diode and your expected current range.

You need to consider charge termination strategies. Either dead disconnect at a given set point but for low C ( <0.5V) you really need an absorption time and tail current shutdown

Having said that if your happy with 80 % SOC or approx , there’s no need for anything other than simple disconnect at a setpoiht

 

[FLICK]

The diode voltage drop idea "is" crude, which is why I like it, lol, it's simple and fool proof... I believe these dc-dc chargers switch from ABS to float once the charge current drops below 3.8A which I figure is probably good...

This is the diode I'm looking at using, just because I have a couple on hand:
https://au.mouser.com/datasheet/2/427/vs_70hfrseries-1769215.pdf
Looking closer, it appears the voltage drop might be a bit higher than the 0.7V I'm looking for at the expected max 25A, so I may have to rethink this...

I'm more interested in the DC-DC being a bulk charge without stressing the battery, and I'll setup the solar charger deal with the final charging...

Having said that if your happy with 80 % SOC or approx , there’s no need for anything other than simple disconnect at a setpoint

I have a "XY-L30A Lithium Battery Charge Controller Protection Board", (which I originally purchased to use with my 48V e-bike Li-Ion battery pack so it doesn't charge to 100%),
https://www.ebay.com.au/itm/224059810097
I've considered setting that up with the campervan DC-DC charger to cut the charge off at 3.45V (13.8V), but I'd prefer not to add extra (maybe unnecessary) gear to my setup...

Thanks for your input, good food for thought...

 

[pollenface]

I would be weary of the 13.7v float value. 13.4-13.5v might be better.

 

[Goboatingnow]

I would be weary of the 13.7v float value. 13.4-13.5v might be better.

Well the best is no float at all. I would not float LI at any voltage

 

[Substrate]

Unless you use it to serve as an accidental low-drain catch, rather than letting the vhf radio drain the battery over the course of a month or two down to the bms LVD disconnect. Or perhaps the external LVD one might use. Stuff happens. Sometimes people don't throw all the system breakers they should.

By floating below 3.4v / cell, (say at 13.5v for a 12v nominal bank), assures that there is no way for a long slow float at higher (say 13.6v or above), charges your bank to full capacity behind your back. Because this takes so much time, sometimes this fact is hidden.

The float - no float war rages on only if one is trying to achieve perfection vs an application concern that might override it.

 

[Goboatingnow]

Unless you use it to serve as an accidental low-drain catch, rather than letting the vhf radio drain the battery over the course of a month or two down to the bms LVD disconnect. Or perhaps the external LVD one might use. Stuff happens. Sometimes people don't throw all the system breakers they should.

By floating below 3.4v / cell, (say at 13.5v for a 12v nominal bank), assures that there is no way for a long slow float at higher (say 13.6v or above), charges your bank to full capacity behind your back. Because this takes so much time, sometimes this fact is hidden.

The float - no float war rages on only if one is trying to achieve perfection vs an application concern that might override it.

Stoping the charger is easy the bms can easy open a charge relay or remote stop command.

Hence why float , it degrades Li and is easily worked around

Chargers deal with unexpected loads ( like your radio ) by having a “ re-bulk” setting ie you want for discharge to occur and then restart charging not floating