Debunk: Alternator DC to DC charger not required for big Lithium Battery

[trad]

This video seems to go against the current state of advice regarding using the alternator to charge lithium directly. I haven't seen this discussed here (maybe I can't find it) @Will Prowse is recommending a Renogy DC-DC convertor for RV charging LFP from the alternator.

Does this guy know what he is talking about?

 

[Mia]

Anybody can do what they choose, obviously… but… I’d personally rather have a DCDC charger in between to limit the power a bit.

 

[Mia]

Anybody can do what they choose, obviously… but… I’d personally rather have a DCDC charger in between to limit the power a bit.

Plus, also would not want to forget about turning the alternator off…

 

[BentleyJ]

It depends on the use case. In an RV where the batteries could potentially be discharged completely thereby requiring a large amount of current for several hours, a DC DC charge limiter would be highly recommended. The other issue is if the automotive alternator/regulator has the proper voltage. If not then a DC DC charger would solve this issue.

On the other hand I've installed 2 x 50Ah LFP starting batteries in a Dodge Ram, Diesel without changing anything and have had good results. Been about 3 years now. In this situation the batteries are only used for starting and are never deep cycled so the charging requirements are minimal. The output voltage of the stock alternator is 14.0 to 14.1V which is perfect for the LFP chemistry.

To be honest, the alternator did recently fail on a trip through TX but it already had 330,000 miles on it and was fine for the past couple of years so I don't think the batteries had anything to do with it.

 

[sunshine_eggo]

Not going to bother with the whole video but given that it's in an RV, it's almost a certainty that the wiring to the LFP battery is as worthless as it is in most RVs including the five I've owned.

LFP take on maximum current for the vast majority of their charge and do so under 13.8V. This can exceed the duty cycle of a typical alternator as they must achieve absorption voltage before current begins to taper.

There is always substantial voltage drop due to wire gauge/length between alternator and coach battery ESPECIALLY at 12V.

The voltage drop causes the alternator to "see" a higher voltage than is present at the battery terminals, thus it will begin to taper current. It's not uncommon to see voltage that the battery to be 0.6V below the alternator output voltage.

Some alternators also regulate their own output based on temperature and will protect themselves.

If one can verify that the current is within acceptable limits, one can charge directly from alternator.

For smaller installations where there is not excessive voltage drop, a DC-DC may become necessary.

 

[trad]

Not going to bother with the whole video but given that it's in an RV, it's almost a certainty that the wiring to the LFP battery is as worthless as it is in most RVs including the five I've owned.

LFP take on maximum current for the vast majority of their charge and do so under 13.8V. This can exceed the duty cycle of a typical alternator as they must achieve absorption voltage before current begins to taper.

There is always substantial voltage drop due to wire gauge/length between alternator and coach battery ESPECIALLY at 12V.

The voltage drop causes the alternator to "see" a higher voltage than is present at the battery terminals, thus it will begin to taper current. It's not uncommon to see voltage that the battery to be 0.6V below the alternator output voltage.

So you are saying that small wiring and poor connections back to the LFP house battery from the Lead Acid starter battery is one way of protecting the alternator. Adding a resistor to the connection would have the same result?

He basically says that the 250a alternator in his rig is able to handle it. Your alternator may be smaller/weaker. YMMV. And finally many alternators will not fully charge LFP. His puts out constant 14.4 so he gets almost full charge. Most alternators do not.

 

[trad]

It depends on the use case. In an RV where the batteries could potentially be discharged completely thereby requiring a large amount of current for several hours, a DC DC charge limiter would be highly recommended. The other issue is if the automotive alternator/regulator has the proper voltage. If not then a DC DC charger would solve this issue.

On the other hand I've installed 2 x 50Ah LFP starting batteries in a Dodge Ram, Diesel without changing anything and have had good results. Been about 3 years now. In this situation the batteries are only used for starting and are never deep cycled so the charging requirements are minimal. The output voltage of the stock alternator is 14.0 to 14.1V which is perfect for the LFP chemistry.

To be honest, the alternator did recently fail on a trip through TX but it already had 330,000 miles on it and was fine for the past couple of years so I don't think the batteries had anything to do with it.

Wow, those must be special LFP batteries? What amps can they put out for cranking and heating the glow plugs?

 

[trad]

Plus, also would not want to forget about turning the alternator off…

Yes, he says that solar is his main way to charge the LFP. The switch to charge the LFP from alternator is mostly off.

 

[justinm001]

Skimmed through the video. How is he limiting the alternator's current? What gauge wire is he using and whats the alternator's output rating? Also what is the alternator regulator set to? My guess is the guy's wiring is hot and limiting the current. Also an IR temp sensor is extremely unreliable in engine bays or areas with dramatic temp changes, It'll show the cooler dirt on the alt and not the actual internal temps.

I'm working on my alternator setup right now on my current coach but on my old all 12V RV I reconfigured my aux start button (connects house to chassis) to only work when pressed for the BIRD type device. One day on the freeway I drove 5 miles with it on and had the wife drive while I lifted up the bed (DP with engine under bed) and took this with the flir while going 65 down the freeway. Was a bit too dangerous to adjust it to just get the alt temps but that ALT was too hot for my liking.

On my current coach I have a wakespeed and my industrial grade 50DN oil cooled alternator which is designed for very high duty would still get hot and I could watch it control the field to cool the alternator down and it would push close to the 250a limit I set and that was on 2/0 gauge wire 40ft.

I should get the rest of my 6 DC to DC converters tomorrow and planning on finalizing that build but my testing showed that my 24/48v 8.5a DC to DC converter got to 135 degrees after 1-2 hour drive and it pumped the full amperage the entire time until my batteries got to 100%

 

[sunshine_eggo]

So you are saying that small wiring and poor connections back to the LFP house battery from the Lead Acid starter battery is one way of protecting the alternator. Adding a resistor to the connection would have the same result?

Yep.

He basically says that the 250a alternator in his rig is able to handle it. Your alternator may be smaller/weaker. YMMV.

Of course. A 250A alternator @ 14.4V is going to have a wicked voltage drop. 20' run of 2awg 14.4V/250A will have a ... 1.56V drop, so the alternator will almost never output 250A to the battery thus ensuring the alternator is never stressed. I would be surprised if output current is ever much over 100A.

Please note:

If one can verify that the current is within acceptable limits, one can charge directly from alternator.

If one's installation is running an alternator at near capacity, it's likely not going to work, unless the alternator can regulate its output based on temperature. Not all do.

And finally many alternators will not fully charge LFP. His puts out constant 14.4 so he gets almost full charge. Most alternators do not.

As an absolute statement, I can't agree. I assume you are not aware that LFP can be nearly (95%+) fully charged at 13.6V and fully charged at 13.8V? Most will charge to these levels; however, the voltage drop issue is the actual cause of failing to achieve full charge.

 

[Vigo]

Charging from a 'stock regulated' vehicle alternator will generally 'work' other than the issues Sunshine mentioned, which in reality means people almost never get the current they think they're going to without doing a lot of wiring upgrades. If you WERE to wire the circuit so that the alternator COULD get anywhere near its max current rating across that circuit, then the alternator would have a fairly short duty cycle before overheating and would be very sensitive to being operated at too low of an rpm (since its internal fan functions poorly at low engine speed). If you were to alter the regulation of the alternator to raise voltage in compensation for voltage drop or as a way to charge the lithium beyond ~14.1 or so, then you run into the additional possible issue of the lithium BMS disconnecting at ~14.6 and the alternator suffering a massive voltage spike that fries something in it, unless you are also somehow in parallel with a battery that does NOT disconnect, or implement something in the control scheme to prevent that from ever happening.

So yes, it can be done and at times it's dead simple, as long as you're willing to settle for much lower charge current than what the alternator is rated for.

 

[justinm001]

The biggest part that seems ignored here is that alternators don't typically smoke and blow up after a few hours. Its typically after months of them being overworked and overheated. Whenever you stress an alt over 70% is when they start overheating, like driving a car at redline for hours at a time. Worst part is you'll likely lose the alt after driving for hours on a long road trip rather than right away.

If anyone's considering not using a DC to DC charger at least get a BIM like battle born Li-BIM-225. This will at a minimum give the alt time to cool

 

[12vDC]

yeah, didnt bother with the video. some people just need attention and clicks.

as stated above, perhaps the most compelling reason for a specific B2B mechanism is to regulate the alternator charge voltage (and current) in programmed steps to protect the battery and with respect to temperature.

sure a direct connection would be relatively faster and especially at a constant speed all things being equal. but thats the problem without the charger in between, nothing is consistent.

if youre not concerned with maintenance or longevity of your aux batt system then straight alternator charging is a swift alternative.

 

[BentleyJ]

Wow, those must be special LFP batteries? What amps can they put out for cranking and heating the glow plugs?

They are specifically designed for automotive starting and each battery has a CCA rating equivalent to a LA battery. With 2 of them, each only has to supply about 300A to 350A.

Normally you wait for the glow plug indicator light to go out before cranking but if I'm in a hurry sometimes I don't.

 

[trad]

Thanks all for the ideas on how this might be able to work. It would seem that leaving a lead acid starter battery in the loop helps, and being certain that the alternator will never output higher than the LFP battery BMS cutoff voltage also helps.

Finally putting wiring a little too small and a little too long will help lower the voltage at the LFP battery terminals and help keep the Amps from going up into dangerous terrritory. Worst thing would be for your alternator to be able to supply more amps than your battery can handle, and your wiring being top notch, finally the LFP battery BMS tripping from too many amps. This could result in a dreaded magic smoke event, and bye bye alternator.

 

[trad]

They are specifically designed for automotive starting and each battery has a CCA rating equivalent to a LA battery. With 2 of them, each only has to supply about 300A to 350A.

Normally you wait for the glow plug indicator light to go out before cranking but if I'm in a hurry sometimes I don't.

3 yrs and 330,000 miles says that it worked for long enough for most everybody. What year of RAM diesel was that? Do you know what the alternator is rated for?

 

[BentleyJ]

2005.5 model year. The factory alternator is 130A I believe.

 

[Aussieguy]

M 2 cents...

for an RV with a chassis and lifepo4 house
only upgrade to a dc-dc if you drive each day 3 hours or more..
if you are unsure of your alternator disconnect the wiring in the BIM which allows the batteries to parallel

I would spend the $$$ first on upgrading the solar
which will be more value for money... works while at the camp site or while driving.

DC-DC will protect an alternator if the house battery is real low and the engine is at or just above idle speeds
when there is minimal cooling air flowing over the alternator ... Highway speed plenty of cooling

 

[justinm001]

M 2 cents...

for an RV with a chassis and lifepo4 house
only upgrade to a dc-dc if you drive each day 3 hours or more..
if you are unsure of your alternator disconnect the wiring in the BIM which allows the batteries to parallel

I would spend the $$$ first on upgrading the solar
which will be more value for money... works while at the camp site or while driving.

DC-DC will protect an alternator if the house battery is real low and the engine is at or just above idle speeds
when there is minimal cooling air flowing over the alternator ... Highway speed plenty of cooling

In a DP there's basically no cooling and on a front engine the radiator is heating all that air to 180+ so basically no cooling either. Typically alts have a max temp rating of around 200f and it easily gets there.

3 hours is a looong time to run an alternator at full power. 30 minutes is a long time.

It's not worth burning out your alternator

 

[scubadoo]

Just repeating a previous post of mine on this forum.
Take it or leave it. 9 years full-time on the road and no smoke yet.
The alternator is regulated to 14.5V. I added a Victron Argodiode Battery Isolator in series
a few years ago to limit the maximum output to nearer 14.1V.

Our setup:
The "non smart" original 100A rated alternator in our Mitsubishi Canter 3.9l turbo diesel truck based motorhome has survived 9 years of full-time travel direct charging the 4 cell 300Ah LiFePO4 Sinopoly battery at 70-80A without releasing any smoke yet.
No standard "BMS" circuitry involved.
I recently paralleled a 4 cell 280Ah LiFePO4 EVE battery.
As well as powering our house it has also started the truck perhaps a few thousand times over the years. Perfect performance so far.
The "BMS" involves no charging source exceeding 14.1V at which point the battery is always at 100% SOC regardless of charge current. 80A alternator, 50A solar and 30 battery charger.
20% SOC alarm and Victron BatteryProtect disconnect at 12.5V. Never triggered.