Is a dc-dc charger really needed for a van setup?

[oldspice]

Just have some confusion around the topic, after doing as much reading as I could find.

I gather that it limits the amperage so that it doesent over heat the alternator? Is this the main reason for using it?
it also gives a “charge profile” but I’m curious how this is different from an alternator as it also adjusts .
Is this something you only need for lithium batteries or is it for agm too because in all the YouTube videos I’ve seen they only show you how to install smart isolators/ relays and never this dc-dc thing.



thanks for helping me understand as I get asked a lot about what to do for vans and I’m not quite sure what to recommend people (or myself)

 

[MisterSandals]

Is this something you only need for lithium batteries

yes
LiFePO4 accepts current so readily that it pretty much makes an alternator run full blast which causes them to overheat and fail.

 

[Hedges]

That seems to be the safest packaged solution. And it could provide for proper charge voltages.
Alternator has a temperature/voltage curve for lead-acid, so in cold weather it charges to higher voltage. Potentially that would exceed LiFePO4 specs.


You could probably come up with a clever cheap way around the excessive current issue. Some people say they've had no problems with direct connection, but that involved a long wire due to location of house battery. If battery was deeply discharged, it would draw higher current. One could engineer suitable resistance. If only trickle charging is needed, higher resistance, possibly a light bulb, would do.

There are some charge controllers with both PV and alternator input, might be a good package for a van.

 

[Posplayr]

That seems to be the safest packaged solution. And it could provide for proper charge voltages.
Alternator has a temperature/voltage curve for lead-acid, so in cold weather it charges to higher voltage. Potentially that would exceed LiFePO4 specs.


You could probably come up with a clever cheap way around the excessive current issue. Some people say they've had no problems with direct connection, but that involved a long wire due to location of house battery. If battery was deeply discharged, it would draw higher current. One could engineer suitable resistance. If only trickle charging is needed, higher resistance, possibly a light bulb, would do.

There are some charge controllers with both PV and alternator input, might be a good package for a van.

As MrSandals describes the alternator will run all out to provide the amount of current a LiFePO4 will accept. The technical term here is internal impedance which for LifePO4 is about 1/10 of what it is for LA. So expect higher currents all around for LiFePO4, which when coupled with long-distance for connections t house batteries and different charging profiles there is no good way to passively connect the LiFePO4 to the alternator to not have problems. Even going back to MrSandals point about how much current the LiFePO4 will accept, even with a DC-DC charger you can overload the alternator.

I'm an EE had have gone through a major revision in my van build after switching from LA to a 200 Amp-Hr LiFePO4 for my 1997 Ford E-350. The solution involves a new smarter 300Amp alternator (with over-temp current limiting) and a 40 amp Renogy DC-DC charger.

The only other alternative is to keep your solar separate from alternator charging and only charge (besides solar) using shore power which is you guessed it yet another charger.

300 Amp Heavy Duty High Output NEW Alternator For Ford E350 Super Duty E150 | eBay

100% New Premium Quality 300 Amp Alternator. Will Replace Factory 3G Series Alternator Only. This Is Truly A High Amp Premium Quality Well Built Alternator. ( Compare the pictures with the original alternator).

www.ebay.com

12V 20A DC to DC On-Board Battery Charger

The best 12v battery charger. Free shipping. The DC-DC Battery Chargers are the most effective method to charge your home batteries from the starter battery

www.renogy.com

 

[schmism]

Its my understanding that this device was designed for this issue.

Amazon.com

No need to add a Dc-DC converter and can use the stock alternator.

 

[Alphacarina]

This is the one most folks use for charging lithiums with a vehicle alternator

Amazon.com

Biggest problem I think is when the engine is running at lower RPM's while the Lithiums are drawing full current from it. The alternator needs plenty of cooling air when it's working hard and at low engine RPM's it's most likely to overheat and burn up

There are other advantage too - The DC to DC can supply an exact voltage, chosen by you when you set it up, so even if the input to the DC to DC is 13.5 volts and you have it set up for a higher voltage, it will supply that higher voltage to the batteries. Also, it regulates current to a manageable amount. You buy a 20 or 30 or 40 amp unit and it regulates it's output to that amount, which ensures the alternator won't be overworked

A very worthwhile investment for both your alternator and your batteries

Don

 

[Posplayr]

Its my understanding that this device was designed for this issue.

Amazon.com

No need to add a Dc-DC converter and can use the stock alternator.

The DC to DC is built into this unit; it has dual inputs for Solar and/or Alternator. The 50amp unit is rated for 25A solar and 25A Alternator.

DCC50S 12V 50A DC-DC On-Board Battery Charger with MPPT

Make sure your service battery has all the juice it needs with Renogy's DCC50S DC-DC MPPT On-Board Battery Charger. 10% off: Renogy10off. Free shipping

www.renogy.com

This 25A limitation will probably avoid burning up your alternator even at idle. Of course, I have no idea what vehicle you are talking about so that is a guess. My E-350 alternate (the high output version) has a rated current of 130A but that rating is typically at 6K RPM and it is probably only 30-50A at idle.

 

[oldspice]

As MrSandals describes the alternator will run all out to provide the amount of current a LiFePO4 will accept. The technical term here is internal impedance which for LifePO4 is about 1/10 of what it is for LA. So expect higher currents all around for LiFePO4, which when coupled with long-distance for connections t house batteries and different charging profiles there is no good way to passively connect the LiFePO4 to the alternator to not have problems. Even going back to MrSandals point about how much current the LiFePO4 will accept, even with a DC-DC charger you can overload the alternator.

I'm an EE had have gone through a major revision in my van build after switching from LA to a 200 Amp-Hr LiFePO4 for my 1997 Ford E-350. The solution involves a new smarter 300Amp alternator (with over-temp current limiting) and a 40 amp Renogy DC-DC charger.

The only other alternative is to keep your solar separate from alternator charging and only charge (besides solar) using shore power which is you guessed it yet another charger.

300 Amp Heavy Duty High Output NEW Alternator For Ford E350 Super Duty E150 | eBay

100% New Premium Quality 300 Amp Alternator. Will Replace Factory 3G Series Alternator Only. This Is Truly A High Amp Premium Quality Well Built Alternator. ( Compare the pictures with the original alternator).

www.ebay.com

12V 20A DC to DC On-Board Battery Charger

The best 12v battery charger. Free shipping. The DC-DC Battery Chargers are the most effective method to charge your home batteries from the starter battery

www.renogy.com

Why did you opt for a 300 amp charger if the dc maxes at 30/40 amps? It’s not overkill?

 

[Posplayr]

Why did you opt for a 300 amp charger if the dc maxes at 30/40 amps? It’s not overkill?

Edited to correct an arithmetic error.

It is a little complicated but I will summarize:

1.) 40 amp DC-DC chargers are known to cause problems unless some type of control over the Charger is effected (e.g. 1/2 power enable on Renogy). I found long threads in a Class B RV forums and on the Sportmobile Forum which is largely E-350 Econolines.

2.) An engineering analysis confirms the evidence: A typical 15ft cable length at 3% load loss would allow for a voltage drop that requires 60 amps (through the alternator) in order to produce 40 amps at 14.5V at the output of the DC to DC. While I do not have any alternator efficiency numbers Victron is claiming a 50% efficiency which would mean that there is yet another 60 amps equivalent power dissipated in the form of heat in the alternator. So this is equivalent to 120 (original 160) amps! This is worst-case but it is a 3 (4 original) :1 ratio with the delivered load current.

3.) The alternator I linked is the only after-market alternator for an E-350 replacement that specified Active thermal control and current limiting along with being substantially cheaper than other high-performance alternators without any more modern electronic control circuity.


4.) My plan had been to implement a charge controller algorithm using an Arduino but after looking at the chip shortage I became somewhat disheartened. The controller would have used direct thermal control by measuring the alternator temperature. The only alternative to get direct thermal control over the alternator is some very expensive marine equipment (>$1000) After finding this relatively cheap smarter alternator with active thermal control I can just get away with a manual switch for control between OFF, 20A, and 40A charging.

I would expect that if I tried to produce 300W that this alternator would go into a self-restricted current output even if it has a higher thermal capacity than the stock 22-year-old alternator that I have. The big selling point to me is that in a worst-case situation it will automatically shut itself down so I don't have to worry about it as much

 

[time2roll]

No, the DC-DC charger is not necessary unless you have an issue to resolve.

Overload the wire? Wire should have a fuse to protect. Then adjust as needed.
Overload the alternator? I doubt it unless you pulled some oversize wire with a 100+ amp fuse.
Wrong voltage? BMS will protect until you get the situation sorted out.

 

[Posplayr]

No, the DC-DC charger is not necessary unless you have an issue to resolve.

Overload the wire? Wire should have a fuse to protect. Then adjust as needed.
Overload the alternator? I doubt it unless you pulled some oversize wire with a 100+ amp fuse.
Wrong voltage? BMS will protect until you get the situation sorted out.

FYI

 

[time2roll]

Yes I have seen the video. You most certainly can engineer a bench test to overload an alternator.
How you going to get 78 amps through a 15' #12 wire fused at 30 amps?

 

[Posplayr]

Yes I have seen the video. You most certainly can engineer a bench test to overload an alternator.
How you going to get 78 amps through a 15' #12 wire fused at 30 amps?

Your question is doesn't even qualify as rhetorical. 78 amps through a 30 amp fuse? Obviously, you need a bigger wire, like Size #22 instead of size #12