Step up + mppt better than battery to battery charger system?

[Takigama]

Howdy All

I have two disparate battery/solar systems (ones about 200ah of agm with its own solar, mppt, etc in a car completely diy) and the other is a roughly 240ah lifep04 setup (own solar, mppt, etc in a caravan) and the two are connected via a battery to battery charger (car -> caravan). Both systems have a 12v -> 48v step up converter attached to them to power some custom 48v stuff.

The short version:
On the road we broke the battery to battery charger. I had a spare mppt charger and as a stop gap I hooked the output of the cars 48v into the MPPT charger, connected it to the caravan's Lifep04's and we had to run like that for about 2 months. But it actually improved the capacity noticeably and it has me wondering why was it so much better?

Reason for all that is with the car we can just kick the engine over if we have to to regain some charge and the caravan's only option on cloudy days is to charge from the car. (Technically, it can charge the caravan's batteries from mains power and the car has an inverter which can provide it, but the van's charge controller will only operate from solar+car OR mains and cant do solar or car + mains)

The 12v->48v step ups are CHEAP, a bit electrically noisy, but surprisingly efficient and the other bonus is the mppt charger has alot more control than does the battery to battery charger so i'd like to leave it all in place and more or less wire it permanently but Im wondering what the downsides are if any?

 

[pollenface]

I used to kick the engine over to regain charge, but got complaints about my cars idling exhaust fumes on one occasion. After that I opted for more solar and more battery

What amperage is your 48v converter?

 

[Vigo]

There is no downside to it. Generally the 12>48 step ups are only cheap if you are talking about a small one, and once you get into the bigger 40-60a stuff you're not saving much vs buying a 'whole' dc-dc charger (which is just a step up converter with some brains to do multistage charging).

As for why it's working BETTER now, i don't think there is any magic in the current setup, so it must reflect that the previous DC-DC had a non-optimal charging strategy for your lithiums.

 

[fratermus]

the mppt charger has alot more control than does the battery to battery charger

During my build I wanted a configurable shore power converter/charger to keep the FLA optimally charged while I was shopping for panels. I gave up trying find something that met my needs and within my budget. I ended up running a 24v meanwell-type power supply into the MPPT PV input. Worked great.

so i'd like to leave it all in place and more or less wire it permanently but Im wondering what the downsides are if any?

I think it's mainly upside if one already owns the MPPT.

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Interested onlookers might also be interested in rickst29's thread on a similar setup, and this thread (or this one) on feeding MPPTs with things other than solar panels.

 

[justinm001]

I'm assuming you're talking about the cheap step up converters like the one below. These aren't great devices and I believe just 4x the input the voltage so 12V=48V but 14V=56V. I had one of these to power my starlink and my 12V voltage ramped up to 16v and it melted my starlink plug. https://a.co/d/dtv6Hki

On my coach I use Victron Orion converters, 4x 24/48v 30a and 2x 24/12v 70/85a. My alt is 24V and have both a 12V lithium and a 48v lithium setup. These devices have adjustable voltage and seem to be the same voltage the entire time regardless if 24v with engine off or 28v with engine running. They also have safety controls to derate when they get warm and others so I feel a lot more comfortable running a bunch of voltage through them to very expensive equipment.

 

[Takigama]

I'm assuming you're talking about the cheap step up converters like the one below. These aren't great devices and I believe just 4x the input the voltage so 12V=48V but 14V=56V. I had one of these to power my starlink and my 12V voltage ramped up to 16v and it melted my starlink plug. https://a.co/d/dtv6Hki

On my coach I use Victron Orion converters, 4x 24/48v 30a and 2x 24/12v 70/85a. My alt is 24V and have both a 12V lithium and a 48v lithium setup. These devices have adjustable voltage and seem to be the same voltage the entire time regardless if 24v with engine off or 28v with engine running. They also have safety controls to derate when they get warm and others so I feel a lot more comfortable running a bunch of voltage through them to very expensive equipment.

I actually do have one of those for powering starlink, but the original ones are more like switched power supplies (originallly i was trying to power starlink from them but they're too noisy for starlink, hence the one like the link you have, but its only about 200w or so)

 

[Takigama]

There is no downside to it. Generally the 12>48 step ups are only cheap if you are talking about a small one, and once you get into the bigger 40-60a stuff you're not saving much vs buying a 'whole' dc-dc charger (which is just a step up converter with some brains to do multistage charging).

As for why it's working BETTER now, i don't think there is any magic in the current setup, so it must reflect that the previous DC-DC had a non-optimal charging strategy for your lithiums.

I was having a bit of a think about this and theres a couple of things that jumped into my head and now i want to lab it once i get back to a place where thats possible. My car setup is entirely victron (orion/smartsolar/multiplus) and my caravan setup is a bmpro (came with the van, had no choice and very common in Australia) except for the inverter which is also victron (was installed after the fact).

The bmpro has almost no configurability (you get to choose if you have lifepo4 or agm batteries, and that is all). But it also occurs to me that going from the car to the bmpro over 12v is about a 8 meter run, the wiring in the car is well over spec'd and im going to assume the caravan's wiring should at least be fit to purpose, but pulling form the from the cars 48v into an mppt charger (victron smartsolar) thats almost sitting on top of the batteries would probably mean somewhat less line loss if nothing else.

Anyways, what i plan to lab is a cheap 12 to 48v converter feeding into a smartsolar charger vs an orion tr 12/12 when i get the chance, curious what else i should add to that if anyone has any ideas?

 

[Vigo]

12v and 8 meters would require some pretty large conductors to not have considerable losses. That's probably the main factor in the differing performance of the two approaches.

I dont know if the DC-DC charger in question will 'throttle' its output to avoid pushing the input voltage below the cutoff point, but i assume your DC-DC was either throttling or cutting in and out repeatedly whereas the MPPT approach simply operated at capacity full time.

 

[Takigama]

Ok, while im on road i did a quick experiment. From a 12v power supply i went into two chargers:
1) Victron Orion tr isolated 12/12-18
2) Cheap 12/48 step-up to Victron smartsolar mppt 100/20

All the cable runs are under 20cm's and the thinest ones are the ones on the cheap 12/48 step-up

Set both to charge at 18a (actually can't change this on the orion so it just charges at 18a anyways) and measured the current draw on the power supply when the battery was actually receiving 18a and the orion definitely appears to lose out here, to provide 18a, the orion setup drew 20.4a and the mppt drew 18.7a.

Not exactly the most amazing and comprehensive test and when i get back to home i'll do something a bit more sophisticated, but it equates to about 96% for the mppt/step-up combo vs 88% for the orion in terms of efficiency which ins't what i would call an insignificant difference really.

If I can get my hands on a non-isolated orion, i'd be keen to check it against that as well cause im assuming the non-isolated ones would be a bit more efficient

 

[Vigo]

I think you're right that the non-isolated ones would be more efficient, but i don't really see the efficiency difference as a big deal in the context of alternator charging because using a 'huge' engine to power a relatively inefficient alternator into a 12v system is a really inefficient thing to do in general, so the differences here are a 'drop in the bucket' in my eyes. If i were you i'd probably just run both systems simultaneously for the added charge rate!

 

[justinm001]

Ok, while im on road i did a quick experiment. From a 12v power supply i went into two chargers:
1) Victron Orion tr isolated 12/12-18
2) Cheap 12/48 step-up to Victron smartsolar mppt 100/20

All the cable runs are under 20cm's and the thinest ones are the ones on the cheap 12/48 step-up

Set both to charge at 18a (actually can't change this on the orion so it just charges at 18a anyways) and measured the current draw on the power supply when the battery was actually receiving 18a and the orion definitely appears to lose out here, to provide 18a, the orion setup drew 20.4a and the mppt drew 18.7a.

Not exactly the most amazing and comprehensive test and when i get back to home i'll do something a bit more sophisticated, but it equates to about 96% for the mppt/step-up combo vs 88% for the orion in terms of efficiency which ins't what i would call an insignificant difference really.

If I can get my hands on a non-isolated orion, i'd be keen to check it against that as well cause im assuming the non-isolated ones would be a bit more efficient

Are you sure the Orion was outputting 18a? I routinely get more power from them then rated for a while then less when they get hot. I think the 20.4 draw was more like 20a output than 18a.