Vehicle charging via second inverter (DC-AC-DC)

[avanti]

I have been preparing to start a thread on the topic of B2B chargers vs using a dedicated inverter feeding 120VAC into the shore-power input of your house charger (the so-called DC-AC-DC approach).

So, here it is [moved from another thread].

[markopolo]

I'd vote new topic for that.
[done]
I'm curious to know if/how that could be seamlessly automated.

[booster]

Originally Posted by markopolo

I'd vote new topic for that. I'm curious to know if/how that could be seamlessly automated.

I would agree, with the inefficiencies in both, and differences in stand alone and vehicle controlled alternators, that probably should be in the mix also and that would bring the remote regulator controls into any automation.

[hbn7hj]

That is how B to B chargers work, isn’t it? How else are they gonna get 14.4 volts out of 13.7 volts from the alternator?

[booster]

Originally Posted by hbn7hj

That is how B to B chargers work, isn’t it? How else are they gonna get 14.4 volts out of 13.7 volts from the alternator?

Put a different regulator on the alternator.

[GeorgeRa]

Originally Posted by hbn7hj

That is how B to B chargers work, isn’t it? How else are they gonna get 14.4 volts out of 13.7 volts from the alternator?

Yes, the difference is operating frequency.

Alternator > Inverter 120V 60 Hz > Charger vs

Alternator > B>B (Voltage/Frequency tuned of efficiency) > Battery

[avanti]

OK, so our new Transit is going to have dual alternators and dual chassis batteries. This option is capable of delivering very high continuous currents (at least 150-200 amps) without any modifications. At the high end, there is a load-shedding signal that you are supposed to use to remove the external load if the chassis power system is overloaded.

The "traditional" way to utilize this system to charge a large lithium bank is to use a battery-to-battery (B2B) charger, which is basically a charger that is powered by 12VDC rather than 120VAC. The bad news is that these devices tend to have relatively small current capacities (30A is popular). Larger ones are starting to show up, but are currently pretty pricy). The good news is that these devices can be used in parallel, so you can use multiple 30A devices to increase the capacity, though only in coarse increments. Some people have used as many as four of these in parallel.

The alternative, which seems to be gaining in popularity, is to forgo the B2B units, and instead use the alternator DC output to power a largish 12VDC to 120VAC inverter. These are readily available in 1500 - 2000 watt capacities and are much less expensive than the equivalent capacity in B2B. The idea is to use the 120VAC output of the "engine" inverter to power the "shore power" input of your coach's normal inverter/charger.

At first glance, this seems a little silly--going from DC to AC just to immediately turn it back to DC to charge the battery. But, the more I have thought about it, the more sense it makes. To wit:

--This is certainly less efficient, since an extra conversion is involved, with its associated inefficiencies. However, the penalty for this is only paid when power is coming from the engine, and (at least in the case of the Transit) is not at all in short supply. So, except for a tiny fuel consumption penalty (which seems insignificant), this really isn't much of a negative.

--If you position the engine inverter near the engine and the inverter/charger near the battery, you avoid having to run a huge DC cable long distances, since that part of the run is 120VAC, which needs only a much smaller cable.

--As I mentioned, the total cost of a high-current system is quite a bit lower, as is the required space.

--Most importantly, you have much more control of current consumption. A B2B will happily provide current to the battery right up to its rated capacity. With a large, empty battery, this could easily be unacceptably large. This is prevented by choosing a B2B setup with a total capacity within the safe zone of your alternator. This is fine, except that you only get very coarse adjustments (say 30A increments), and there is no easy way to throttle it back if, say, your alternator is getting overtaxed. With a second inverter, however, you can have very fine control. The reason is that most good chargers have parameters to allow you to adjust the maximum current it will attempt to get from shore power. So, for example, if you are plugged into a 15A outlet, it will not attempt to draw 30A to charge the battery. This very same feature works perfectly as a throttle to the alternator draw, since the engine inverter will not draw more current than is being demanded by the charger. This is a very appealing feature, and can easily be automated if you have a "smart" charger.

--What really tipped the balance for me is the fact that this approach centralizes the control of the charging profile at the main charger where it belongs. With a B2B, you have to coordinate the behavior of two (three if you have separate solar) chargers, each with different profiles. This can certainly be done, but the ability to centralize these smarts is very appealing to me.

So, I am close to being sold on this approach. Anybody see any major downsides that I have missed?

[booster]

Originally Posted by avanti

OK, so our new Transit is going to have dual alternators and dual chassis batteries. This option is capable of delivering very high continuous currents (at least 150-200 amps) without any modifications. At the high end, there is a load-shedding signal that you are supposed to use to remove the external load if the chassis power system is overloaded.

The "traditional" way to utilize this system to charge a large lithium bank is to use a battery-to-battery (B2B) charger, which is basically a charger that is powered by 12VDC rather than 120VAC. The bad news is that these devices tend to have relatively small current capacities (30A is popular). Larger ones are starting to show up, but are currently pretty pricy). The good news is that these devices can be used in parallel, so you can use multiple 30A devices to increase the capacity, though only in coarse increments. Some people have used as many as four of these in parallel.

The alternative, which seems to be gaining in popularity, is to forgo the B2B units, and instead use the alternator DC output to power a largish 12VDC to 120VAC inverter. These are readily available in 1500 - 2000 watt capacities and are much less expensive than the equivalent capacity in B2B. The idea is to use the 120VAC output of the "engine" inverter to power the "shore power" input of your coach's normal inverter/charger.

At first glance, this seems a little silly--going from DC to AC just to immediately turn it back to DC to charge the battery. But, the more I have thought about it, the more sense it makes. To wit:

--This is certainly less efficient, since an extra conversion is involved, with its associated inefficiencies. However, the penalty for this is only paid when power is coming from the engine, and (at least in the case of the Transit) is not at all in short supply. So, except for a tiny fuel consumption penalty (which seems insignificant), this really isn't much of a negative.

--If you position the engine inverter near the engine and the inverter/charger near the battery, you avoid having to run a huge DC cable long distances, since that part of the run is 120VAC, which needs only a much smaller cable.

--As I mentioned, the total cost of a high-current system is quite a bit lower, as is the required space.

--Most importantly, you have much more control of current consumption. A B2B will happily provide current to the battery right up to its rated capacity. With a large, empty battery, this could easily be unacceptably large. This is prevented by choosing a B2B setup with a total capacity within the safe zone of your alternator. This is fine, except that you only get very coarse adjustments (say 30A increments), and there is no easy way to throttle it back if, say, your alternator is getting overtaxed. With a second inverter, however, you can have very fine control. The reason is that most good chargers have parameters to allow you to adjust the maximum current it will attempt to get from shore power. So, for example, if you are plugged into a 15A outlet, it will not attempt to draw 30A to charge the battery. This very same feature works perfectly as a throttle to the alternator draw, since the engine inverter will not draw more current than is being demanded by the charger. This is a very appealing feature, and can easily be automated if you have a "smart" charger.

--What really tipped the balance for me is the fact that this approach centralizes the control of the charging profile at the main charger where it belongs. With a B2B, you have to coordinate the behavior of two (three if you have separate solar) chargers, each with different profiles. This can certainly be done, but the ability to centralize these smarts is very appealing to me.

So, I am close to being sold on this approach. Anybody see any major downsides that I have missed?

I think all of this is very true and well thought out, and especially if you have already taken the effort of making the shore charging right for lithium. Probably with careful attention to shutting of various charging sources to limit charge maximum to leave headspace, way to recover cold shutdown, etc. With the van electronics controlling the alternators the proposed system would need to do essentially nothing to limit what the inverter or charger see from the inverter. IMO, that would be a big plus as it would eliminate nearly 1/3 or the automation. If you chose to, you could even run the solar, if you have it through the same way, although the inefficiencies would hurt worse.

[GeorgeRa]

Since I joined the Sprinter Forum David Orton was promoting DC > AC > DC approach. There was a lot of critique but mainly efficiency, about 75%. Since, his technique gained momentum.

There are 3 sources for chargers for a motorhome:

1. Alternator
2. PV
3. AC / Generator

With your plan to have alternator and AC this AC>DC>AC makes a lot of sense, single charger.
B>B would be a good option for users using alternator and PV, still need second AC charge controller.
Ideally, there would be a charger capable to take PV and alternator DC and AC.

[markopolo]

For performance comparison, consider this:

You previously mentioned considering 700Ah lithium. A 1500W to 2000W inverter might only be suitable for up to 70A DC charging. 80A DC chargers often require 120V 20A plugs.

Mastervolt Charge Mate Pro 90 units can be used in parallel. 180A DC charging potential. They switch on at 13.5V and off at 12.75V so there's rudimentary alternator protection there. I wonder if the Transit load shedding signal could be used to turn one or both Charge Mate Pro 90 units off? Switch input to ground turns unit off. 0.2V max drop on output might be good for the lithiums. 700Ah lithium should easily take all 180A at 13.8V+.

Keeping them cool is a consideration https://www.classbforum.com/forums/f...tml#post133654 (140 °F max I think but warning led on at 167 °F) It would be great if they could be installed under the hood.

Posted just for a performance comparison.

[avanti]

Originally Posted by markopolo

For performance comparison, consider this:

You previously mentioned considering 700Ah lithium. A 1500W to 2000W inverter might only be suitable for up to 70A DC charging. 80A DC chargers often require 120V 20A plugs.]

That doesn't sound right. 70A @12VDC is only 840 watts. Even with the various losses, that shouldn't even be close to a problem with a 1500W inverter.

A lot of folks use the GIANDEL 2200W Pure Sine Wave Inverter, which has a useful remote control capability, and pair it with the Victron Energy MultiPlus 3000VA inverter/charger. They then dial things back empirically until everything is happy in their particular configuration. I think with my setup I will see charging amps well into the 3 digits.

[markopolo]

I'm just looking for downsides as requested.

I'm not sure why some need 20A outlets once you get to 80A.

Example: the PD 80A - Input: 105-130 VAC, 1,300 Watts
Note: Requires 20 amp rated outlet

I had an 80A Samlex charger that also required a 20 amp rated outlet.

You'd think that derating a 15A 1800W circuit - 1800*80% = 1,440 would be enough. It might be worth finding out the reason why. Maybe it's a code or the presumption of a shared circuit .........

Other potential downsides - Two devices at high output vs one might be expected to create more waste heat and more noise from the greater number of fans.

[avanti]

Originally Posted by markopolo

Other potential downsides - Two devices at high output vs one might be expected to create more waste heat and more noise from the greater number of fans.

Yes. As you said earlier, it would be really nice if the first inverter could be in the engine compartment.

The good news, though, is that it will only operate while driving, so both heat and noise are less of an issue than they otherwise would be.

[booster]

You can't rely on fusing requirements to guess at power use in most cases as they are always higher and have varying degrees of oversize based on the type of load. Sensitive electronics that don't have start surge may be quite close to running amps and with fast blow fuses, but big, slow starting, motors may have 30/50 or even higher percent of over sizing of fusing. Industrial heavy motors tend to use fusing and motor starters that let quite a bit of high amps because of starting surge, but they also are thermal in the sense that if they run a long time at just over rating they will blow from heat. This way they can allow a short burst of way over amp without tripping but will trip on continuous at 10% over in maybe 30 minutes. The only good way to evaluate load it to test it, and second best is use the running amp and starting surge amps to determine what it will really take in the real world I think.

[HarryN]

The concept is in fairly wide use and yes there are some inefficiencies, but at least they happen when there is a fair amount of power around (van is driving down the road generating power)

As you mention, it solves a number of challenges that can come from trying to charge the house battery pack from too many different charger types and settings.

I was most definitely not on board with this concept for a long time and considered it to be sort of "brute force" vs more refined / sophisticated approaches. Over time and having spent more money than I care to admit on battery charging testing on my 2 test stands, this method consistently works, as long as you don't go too cheap on that 12 volt input inverter.

Most of the reported issues have been with the Victron inverter / chargers and having the sensitivity set too high.

Depending on the project, I use this method on some customer builds. It also helps to deal with the setups that I build, which are mostly 48 volt battery packs for larger systems and 24 volts for smaller setups.

Space wise, it is kind of a wash. An extra 2000 watt inverter is not small, but a DC - DC charger with that kind of power isn't all that small either.

[diesel_77]

Question: will the dedicated inverter provide the necessary voltage to excite the alternator so that it functions?

This configuration seems very sensible, but I don't see how the alternator gets the required excite voltage to engage.

[avanti]

Originally Posted by diesel_77

Question: will the dedicated inverter provide the necessary voltage to excite the alternator so that it functions?

This configuration seems very sensible, but I don't see how the alternator gets the required excite voltage to engage.

In this case, I am going with the Ford factory dual alternator (which is also dual-chassis-battery). The Ford engineers dealt with the excitation issue so I don't have to worry about it.

Your question is a good one in the context of an aftermarket second alternator, though.

[booster]

Originally Posted by avanti

In this case, I am going with the Ford factory dual alternator (which is also dual-chassis-battery). The Ford engineers dealt with the excitation issue so I don't have to worry about it.

Your question is a good one in the context of an aftermarket second alternator, though.

As far as I know, the alternator will not start as it does not have a battery in the system if direct to an inverter. Avanti will have a system from Ford to handle that. Our van is two alternators in parallel, so it also would be OK, but nearly all other 2nd alternators would not be, AFAIK.

[diesel_77]

Thanks for the clarification.

I have a 2016 MB 3500 diesel, so if I choose this configuration I would need to figure out another way to excite the dual alternator. Any suggestions?

[avanti]

Originally Posted by diesel_77

Thanks for the clarification.

I have a 2016 MB 3500 diesel, so if I choose this configuration I would need to figure out another way to excite the dual alternator. Any suggestions?

I think on a clean installation on a Sprinter, I would probably go with a Balmar regulator, which deals with the issue by itself.

This doesn't get you the "single charge controller" advantage, though. I guess the easy alternative would be to add a small second chassis battery and put it in parallel with the inverter input. The Sprinter has provisions for such a battery, unless it has been taken by a Rixen's glycol tank or some such.

EDIT: On second thought, I suspect that the Balmar probably requires a battery reference, and may not be able to drive the inverter directly. if so, then you may need a second battery in any event. It could probably be fairly small.