48v to 12v with decent sized loads

[Enigma86]

Hello. I've tried to look but its kind of hard to find this kind of information. Most of the time people talk about converting 48-12 its for low draw loads. I put an SP6548 and 3 100 ah batteries in my 5th wheel but want to ditch the 120 lbs of the lead acid in there. I don't really feel like wasting money on more lithium 12v to go in there and would like to use the battery area for other things since I've taken up so much front storage with the server rack batteries. I'd much rather simplify and run everything off of the 48volt stuff and was looking at buck converters to run the whole 12v system off of but the biggest I can find from what looks like a reliable source is 60 amp. That could probably handle the water heater but I'm not so sure about the lippert leveling system. I've heard that can go up over 100amps at times.

I've also read that paralleling 2 converters is not that straight forward. Right now there is still a 120 - 13.6 (I think 80 amp but might be 60) that is on the trailer. From what I'm understand the problem with paralleling 2 converters is that one will always take most of the load? But I'm not sure why that would be an issue. Wouldn't that just mean that 1 runs the majority of the time until it cannot handle the load and the other would inevitably have to start supplying power?

 

[justinm001]

Why not just use a lead acid/agm battery and charge it with a 48 to 12 adapter or even 120v to 12v? Doesn't have to be a massive battery just something to boost the high loads. Also kinda works as a ups for 12v stuff as if you lose 48v/120v power you still have electronics working.

Most 12v loads are small (like lights) or if large then are rarely used (slide motors, awnings, jacks).

Didn't know they had 12v water heaters but I'd look into swapping the element to 48v if possible. It's gotta be like 80amps.

 

[Symbioquine]

If you just wanted to reduce the amount of lead-acid batteries, you might be able to go pretty small if your loads are mostly instantaneous/periodic. Even a fairly small lead-acid battery can supply a lot of amps for a short time and will last pretty long if it is not drained significantly and is immediately charged back up (from your other battery system) - that's why car batteries last as long as they do even with big starter motor draws. The lead-acid battery mostly just acts as a buffer for loads that exceed what the alternator can provide.

I've also read that paralleling 2 converters is not that straight forward. Right now there is still a 120 - 13.6 (I think 80 amp but might be 60) that is on the trailer. From what I'm understand the problem with paralleling 2 converters is that one will always take most of the load? But I'm not sure why that would be an issue. Wouldn't that just mean that 1 runs the majority of the time until it cannot handle the load and the other would inevitably have to start supplying power?

I think this comes down to how the converters behave when they are overloaded. Most converters will either a) shutdown/restart or b) go into constant current mode and reduce the voltage.

Those that shutdown will cause a cascading failure where the other converter(s) get the load from the one that shutdown and then shutdown themselves. If the converter(s) restart then they could toggle back and forth or on and off very rapidly.

Converters that go into a constant current mode probably can be paralleled because each unit will protect itself by reducing the voltage as needed to not overload and the remaining one(s) will take up the rest of the load's demand. The problem, even then, is that one unit may still be serving more of the load than the other(s) and - depending whether it is intended to run at full load all the time - may have a shortened life-expectancy.

Ideally, converters that are designed to be paralleled will either coordinate with each other to share the load equally or will be designed to operate in constant current mode at a level that they can run with a 100% duty cycle (on at that level all the time).

 

[Symbioquine]

See https://diysolarforum.com/threads/24v-setup-with-onan5500.64990/post-828135 where it seems someone has the same leveling system and dilema.

 

[Enigma86]

Why not just use a lead acid/agm battery and charge it with a 48 to 12 adapter or even 120v to 12v? Doesn't have to be a massive battery just something to boost the high loads. Also kinda works as a ups for 12v stuff as if you lose 48v/120v power you still have electronics working.

Most 12v loads are small (like lights) or if large then are rarely used (slide motors, awnings, jacks).

Didn't know they had 12v water heaters but I'd look into swapping the element to 48v if possible. It's gotta be like 80amps.

Yeah if I kept any 12v battery I think that the converter in there would be sufficient enough. The other consideration is weight since I'm adding about 1000 lbs for the solar system for this thing and lead acid is heavy. I will at least look into what sized batteries I can get though. I know that the lithiums I looked at did not have enough amp capacity to be enough help at the smaller sizes. I think I was wrong about the water heater, I think you're right and that the actual heating is done by 120. Wasn't aware.

If you just wanted to reduce the amount of lead-acid batteries, you might be able to go pretty small if your loads are mostly instantaneous/periodic. Even a fairly small lead-acid battery can supply a lot of amps for a short time and will last pretty long if it is not drained significantly and is immediately charged back up (from your other battery system) - that's why car batteries last as long as they do even with big starter motor draws. The lead-acid battery mostly just acts as a buffer for loads that exceed what the alternator can provide.



I think this comes down to how the converters behave when they are overloaded. Most converters will either a) shutdown/restart or b) go into constant current mode and reduce the voltage.

Those that shutdown will cause a cascading failure where the other converter(s) get the load from the one that shutdown and then shutdown themselves. If the converter(s) restart then they could toggle back and forth or on and off very rapidly.

Converters that go into a constant current mode probably can be paralleled because each unit will protect itself by reducing the voltage as needed to not overload and the remaining one(s) will take up the rest of the load's demand. The problem, even then, is that one unit may still be serving more of the load than the other(s) and - depending whether it is intended to run at full load all the time - may have a shortened life-expectancy.

Ideally, converters that are designed to be paralleled will either coordinate with each other to share the load equally or will be designed to operate in constant current mode at a level that they can run with a 100% duty cycle (on at that level all the time).

Thank you for this run down. I did not know this about converters.

 

[justinm001]

There's all kinds of weights and sizes of batteries with different types. Even baby lithium batteries. I'd just go with some AGM battery. I'm actually doing this now with a optima bluetop (the largest) as my 12v lipo batteries are getting replaced. Just put some cheap 120v to 12v charger that has a switch for 5/10/20a and it works fine. Optima says 10 amp charge only though so that's a bit confusing. It'll be my dedicated genny charger when I get my batteries back

 

[Enigma86]

So far I have not seen anything that is small be able to supply enough amps though

 

[justinm001]

How many amps and ah do you need? 50ah with a 50a bms should be plenty. 30amp charger would recharge in 2 hours and give you 80amps 15-20lbs id guess. I have some 24ah and they're small and light

LiTime 12V 50Ah LiFePO4 Lithium Battery, Build-in 50A BMS, 640Wh Energ

Similar Usable Energy but 5 Times Faster Charging: Our 12V 50Ah LiFePO4 battery has 640Wh energy (12.8V×50Ah×100%DOD=640Wh ), which is close to the real energy of 12V 100Ah lead-acid battery (12V×100Ah×60%DOD=720Wh), as the depth of discharge (DOD) of lead-acid is about 60%. It means 12V 100Ah...

www.litime.com

 

[Enigma86]

the lippert hydraulic system is known to spike over 120 at times from everything I've read on this forum and people checking it

 

[justinm001]

the lippert hydraulic system is known to spike over 120 at times from everything I've read on this forum and people checking it

get 2 of them then 100amps plus the charger and you're all set. I'm sure there's others with larger BMS as lithium can handle 2C so 50ah can handle 100a

 

[toms]

In this situation (12V native system with occasional high current 12V load), i have had success with using 4 individual 12V batteries, each with their own charge controller. Link them in series for your 48V inverter, and take 12V loads as evenly as you can from individual batteries.

It doesn’t matter if one battery gets used more than the others as long as they all reach full charge often enough.

In mobile systems there is rarely the need to go greater than 24V for the main inverter, and this is generally the simplest and most cost effective method of setting up a system.

 

[justinm001]

In this situation (12V native system with occasional high current 12V load), i have had success with using 4 individual 12V batteries, each with their own charge controller. Link them in series for your 48V inverter, and take 12V loads as evenly as you can from individual batteries.

It doesn’t matter if one battery gets used more than the others as long as they all reach full charge often enough.

In mobile systems there is rarely the need to go greater than 24V for the main inverter, and this is generally the simplest and most cost effective method of setting up a system.

That sounds like a nightmare to manage and no way to get even loads from each. Then some will need a lot of charge while others are floating so tons of wasted power

If going 12v batteries then I'm surprised a Vanner isn't mentioned often here. Not sure if they support lithium but my chassis batteries from the factory has 4 12v lead acid and starter and many components run 24v while other runs 12v and the vanner is 100amp equalizer that allows the coach to pull from 12v batteries but charge via 24v. Which is pretty cool but that means I have 5 DC power systems on the coach (2 chassis, 12v inverter, 48v inverter and 12v genstart)

 

[toms]

That sounds like a nightmare to manage and no way to get even loads from each. Then some will need a lot of charge while others are floating so tons of wasted power

If going 12v batteries then I'm surprised a Vanner isn't mentioned often here. Not sure if they support lithium but my chassis batteries from the factory has 4 12v lead acid and starter and many components run 24v while other runs 12v and the vanner is 100amp equalizer that allows the coach to pull from 12v batteries but charge via 24v. Which is pretty cool but that means I have 5 DC power systems on the coach (2 chassis, 12v inverter, 48v inverter and 12v genstart)

And yet, surprisingly in practice it is the simplest and most cost effective layout I’ve seen. The key is understanding your load profile. It obviously isn’t suitable for all scenarios, but for the OP who is predominantly using inverter for load with an intermittent high current 12V load it would work well.

 

[Symbioquine]

Maybe worth mentioning, but there isn't really a reasonable way to put batteries (or cells) in parallel and in serial at the same time. If you draw a wiring diagram you'll see how it yields a short-circuit. (In case you're temped to get around the problem with diodes - check out this.)

 

[justinm001]

Maybe worth mentioning, but there isn't really a reasonable way to put batteries (or cells) in parallel and in serial at the same time. If you draw a wiring diagram you'll see how it yields a short-circuit. (In case you're temped to get around the problem with diodes - check out this.)

I believe this is how mines ran. But unsure as there's a ton of thick wires all over and I covered the batteries with a Milwaukee packout mount and keep the tools in there

http://www.vanner.com/wp-content/uploads/2011/10/D013444-F.pdf

 

[Symbioquine]

Yeah, you can draw power from one of the 12v batteries, but not both in parallel.

Even with the equalizer (which would solve the short-term imbalance problem) I'm pretty sure the "A" batteries will tend to wear out before the "B" batteries.

 

[Enigma86]

Yeah, that battery setup scares me to even think about. Plus I really just don't want to go for more batteries anyways. I'm over 3k$ in on battery stuff as it is. If I buy anymore I'd rather it be a 4th 48v. I was digging through another thread and did finally find this...

48v to 13.8v 150A

If I'm worried about it I can put a voltage protection on the other side of it to make sure that 48v never hits the 12v system. This I think should let me completely remove batteries and old converter/charger system and reduce all the weight and open up some free space.

 

[toms]

With LiFePO4 it’s not an issue to have one battery in a series string at a different SOC.

In the situation if the OP, the levelling motors would draw less than 5% capacity. The next time the batteries reach float this is recovered.

 

[cinergi]

I have a victron orion 48/12 30 amp keeping a 100 AH LFP battery that I already owned floating ... the converter covers all my DC loads except jacks and slides which need 120 amps.. so the battery covers that momentary excess load. Plus that 12v battery covers the emergency breakaway situation (powers the brakes).
The orion maxes out at 40 amps briefly and then holds steady at 30 amps (even where there's demand for more). So the battery (100 amps) and converter (30) can give me 130 amps. The orions can be paralleled but I'd need 4 of them and a lot of wiring and they're not cheap.

 

[Zapper77]

What about a Victron 150/35 MPPT w/Bluetooth?