dc-dc vs inverter charging

[Aphers]

Hello again!
If I want to charge LFP from a lead-acid bank, or vice versa, it seems the two options are a) DC-DC charging; b) an inverter and AC charger.
As far as I can tell, all of these pieces or kit are about 85% efficient, so going via an inverter works out at 72%.
But the huge advantage of going through the inverter is that I would be reusing existing equipment.

Other than efficiency and perhaps a degree of automation, is there anything else to consider? I might only need to set this up as a contingency anyway, although I'm still very early in the research and design phase.

 

[snoobler]

All of this is a game of cost, convenience and efficiency. If 72% doesn't bother you, since you're just throwing away free energy when you don't need it, who cares?

The inverter has an "idle" consumption from just being on, and it can be a substantial portion of the total. I run off-grid "shore power" for my 5th wheel, and it's in vs. out efficiency is in the 65% range primarily because of the large idle consumption compared to my typical "idle" load (about 45-75W). The converter itself is hardly using anything because it's just floating the battery, so that efficiency doesn't matter.

When I'm actually using meaningful loads, the efficiency jumps into the 90%+ range.

An inefficient solution already in hand may be superior to a more efficient solution that costs more especially if the waste doesn't cost you anything.

 

[Aphers]

Thanks, I hadn't thought about idle consumption on the inverter, which might lower efficiency somewhat.
I'm juggling with different ideas about where to connect up loads and chargers on what will almost certainly be a mixed FLA/LFP system. It seems that simply running them in parallel is a massive no-no unless they are permanently connected that way, since you could get a big surge flowing one way or the other when one bank is discharged compared to the other. Hence thinking about ways of transferring power in a more controlled way.

 

[oh_thatballoffire]

Thanks, I hadn't thought about idle consumption on the inverter, which might lower efficiency somewhat.
I'm juggling with different ideas about where to connect up loads and chargers on what will almost certainly be a mixed FLA/LFP system. It seems that simply running them in parallel is a massive no-no unless they are permanently connected that way, since you could get a big surge flowing one way or the other when one bank is discharged compared to the other. Hence thinking about ways of transferring power in a more controlled way.

Hi,

Did you ever solve your dilemma?

I am also trying to increase my bank size and looking at the DC/DC solution.

I was hoping to add a 24v lifepo4 bank in front of the FLA bank, with a DC/DC charging and down converting 24 -> 12v. on a 10 or 20 amp converter, with the existing inverter and DC Loads.

Thanks

 

[Aphers]

Hi,

Did you ever solve your dilemma?

I am also trying to increase my bank size and looking at the DC/DC solution.

I was hoping to add a 24v lifepo4 bank in front of the FLA bank, with a DC/DC charging and down converting 24 -> 12v. on a 10 or 20 amp converter, with the existing inverter and DC Loads.

Thanks

I was offered a Sterling 60A B2B charger for a price that I couldn't turn down, so I am going down that route.
I have actually still to buy my cells, I'm based in the UK and with Brexit the shipping and tax situation has been a bit up in the air.

 

[Tomthumb62]

Thanks for this question, as I have the exact same one.

I'm planning on going the inverter route, it will be cheaper to do so and I'm okay with 72% efficiency as I will only be using it while driving.

Plus we already have a decent Victron AC charger and space is TIGHT in there for any more equipment. Plus, being a 12v system, 80+ feet roundtrip for 2- to 4-AWG pure copper cabling is $$$ as well as a decent (Victron) DC-DC charger. Going from 12v to 120v to 12v will lose some efficiency, but the upfront cost (for how rarely we will need it), will be much less. About $60 for a decent-but-cheap Giandel 300W pure-sine inverter, plus 12AWG wiring to the trailer. Just need to figure out how big our alternator is so I don't need to upgrade it or over-tax it.