Experiences charging LFP from alternator WITHOUT a DC-DC charger

lolailando said:

Thank you bzzt. Do you mind to share the size of your battery LFP bank? 280ah? do you know if your alternator is temp regulated? I've read that almost all of them are; so it could be the alternator limiting the current on high temp. I just found one alternator from Leece-Neville that is: "High Temperature rated to 125oC with No throttle-back regulator, maintains output at high temperatures" and continous duty at 12,000rpm (alt rpm). Trying to figure out what the limit on current is cause if it is not going to charge more than 80A most of the time no need to though more money at it lol. Thank you!

Click to expand...

The battery is a 240Ah unit made up of 120Ah cell pairs (2p4s)? I wanted some redundancy! I suspect the alternator has protection BUT the voltage at the van battery has been more or less constant during charging, so the alternator has not entered any protection mode.

The one time I saw 160 amps was after drawing the lifepo4 down to 12 volts. This is lower than I normally go! The voltage at the cells rose quite rapidly and the current dropped to the 80-100 amp range for the duration of the charge.

I recently added the 10mR resistor to the system just because I don't need to charge so quickly and with summer coming up I thought it would be nicer to the alternator. That said, I can easily bypass the resistor for higher charge rates (or if it's -30 and I am not worried about the alternator overheating!).

It is hard to find hard numbers out there to support or debunk direct charging! I went with the direct connection based on a single bit of 'internet evidence' suggesting that below 300Ah you probably don't need to throttle. Unfortunately I failed to collect data with the base setup (bad coding). I will post the charge rate data that I collect with the resistor (~20mR TOTAL resistance) since this question keeps coming up.

lolailando said:

I just read in some other forum that the alternator regulator also wont put the alternator voltage up unless there is enough headroom between the amps pull out and the max amp of alternator.
Then I also read a Ford owner who has the Leece_Neville no temp throttle alternator and has proved/tested that it does throttle on temp.
There seems to be so much confusing info out there.....then there's people building dc generators with a junk 7hp motor and junk alternator pulling 200a or more. It would be nice to put in 400A in an hour of driving instead of 4 on a truck that already has 250HP lol.

Click to expand...

Yep, the power required to charge the batteries is trivial compared to a cars engine! In my case, putting away almost 200Ah in a couple hours of driving is great and one of the main reasons I upgraded to lithium in the first place! A 40 amp DC-DC charger would almost be a downgrade to my old lead-acid charge rate.

Alternator regulators (with a few rare exceptions) have no way to measure output current directly.

The regulator supplies up to battery voltage to the field coil. The number of windings, gauge, and rectifier design determine output current. For example many alternators use the same regulator for both 100 and 200A units.

The regulator simply tries to attain its voltage target. This target can change with alternator temp, or with time (some start high and taper down a bit. It does this by changing how much voltage is applied to the field coil. More voltage on the field coil (up to the battery voltage) will be applied. Once that max field voltage is attained, the alternators current cannot rise any further. This self regulator behavior is the reason that a starter/primary battery must always be connected to the system. Otherwise a sudden load shed would cause a positive feedback loop, where the voltage could spike to 2-5x normal. The starter battery suppresses these spikes.

If a demand greater than the alternators output is applied, such as a big battery or similar, the system will find an equilibrium at less than the alternators target voltage. For example a big discharged LFP pack may pull an alternator down to 12.5V for the initial part of the charging. As the battery SOC rises so will the alternator voltage. Once the system equilibrium drops below the alternators current limit, the voltage will then rise to the alternators setpoint.

Wiring resistance and alternator temperature will affect charge rates, as will battery SOC. These all work to produce an equilibrium in both voltage and current. The high end of voltage being limited by the alternators regulator setpoint, and the high current limit being a function of the alternators design.

My 500AH 12V bank will take all my alternator has to spare right until about 95% SOC. This is about 100A at idle, and 150A at 2k rpm. This is a 200A unit, and it will make about 180-190A hot and at high idle. Engine and vehicle systems consume some of that, so I have 100-150A available for charging.

Now "smart" alternators often have a digital bus connection to the vehicles ECU/modules. These systems can vary the voltage to save energy. Some actually have a crude current measuring shunt on the battery. This lets the ECU drop the alternator voltage (with a command to the regulator) once the starter battery is full.

For these systems you can often connect directly to the starter battery for the aux battery. The ECU will see the charging load and keep the alternator voltage elevated. Otherwise voltages below the charging range for LFP may happen.

For these dynamically variable alternator systems where the low voltage situation cannot be remedied, a DC-DC is the only option for charging.

To expand a bit further, most alternator designs are something like this (some models have internal excitation, and most don't need a ignition voltage supply). Some may have more phases, or different configuration stator windings. Older models use a totally analog regulator. Newer models still use analog for high speed voltage response, but its monitored by a digital chip. This supplies the voltage reference signal, reacts to temperature and voltage spikes (some alternators will lock out after a excessive spike), and can do voltage/time curves in some cases.



External regulators can be added, or in some cases retrofitted. These allow limiting charge current, using external signals, or voltage profile adjustment. Not common in the automotive world, and not easily retrofitted to most modern alternators not specifically designed for them.

Bzzzt said:

Time will tell if a simple resistor is an ideal current limiter.

Click to expand...

Ideal is only limiting the current as needed to keep from killing the alternator - which is done automatically for many of us it sounds like (we still need to disconnect when battery full is needed to protect the battery). The issue with just adding a resistor like that is your reducing current at all times to the needs of your worst case. On the other hand if that reduced rate meets your requirements there's nothing wrong with the extra piece of mind that gives.

After hearing from Luthj (who obviously has a bit of experience and knowledge here) I'm of mind to give it a go. If the alternator fails I have a rather large battery and solar to lean on. I wouldn't mind an alternator upgrade anyways.

The disconnect issue really depends on duration and voltage. If your alternator is at say, 3.5Vpc, and you will only see the pack at the voltage for 100Hr a year, then it really isn't a big deal. Current tapers to nearly zero at that point, and the whole overcharge thing doesn't occur. There is some capacity reduction from sitting at high SOC and voltages, but its cumulative, and take a fair amount of time.

Luthj said:

The disconnect issue really depends on duration and voltage. If your alternator is at say, 3.5Vpc, and you will only see the pack at the voltage for 100Hr a year, then it really isn't a big deal. Current tapers to nearly zero at that point, and the whole overcharge thing doesn't occur. There is some capacity reduction from sitting at high SOC and voltages, but its cumulative, and take a fair amount of time.

Click to expand...

Especially with a direct connection I foresee many hours driving with completely full batteries. It could be a manual disconnect I suppose, but I think it nessessary.

geoffire said:

Especially with a direct connection I foresee many hours driving with completely full batteries. It could be a manual disconnect I suppose, but I think it nessessary.

Click to expand...

I have followed Rod Collins' (Marine How To) advice on lifepo4 charging. He strongly asserts that with these cells the charger must STOP after achieving full charge. Unlike lead acid you aren't just boiling off water if you continue. You are permanently damaging the battery if you continue to charge past 100%, even at voltages well below the typical high cutoff we use. In my case, I disconnect at either 3.65 volts or once the current drops below 24 amps. This isn't based on any of my own experience, just Rod's assertions, which are definitely worth a read. https://marinehowto.com/lifepo4-batteries-on-boats/

geoffire said:

Ideal is only limiting the current as needed to keep from killing the alternator - which is done automatically for many of us it sounds like (we still need to disconnect when battery full is needed to protect the battery). The issue with just adding a resistor like that is your reducing current at all times to the needs of your worst case. On the other hand if that reduced rate meets your requirements there's nothing wrong with the extra piece of mind that gives.

After hearing from Luthj (who obviously has a bit of experience and knowledge here) I'm of mind to give it a go. If the alternator fails I have a rather large battery and solar to lean on. I wouldn't mind an alternator upgrade anyways.

Click to expand...

I totally agree. I think of the resistor as ideal in the sense that if you want to limit current it is simple, cheap, and (with the flat voltage profile of lifepo4) should provide a fairly constant current to the batteries.

So far I haven't had any issues with my straight 'non-throttled' connection though! I suspect my alternator is smarter than me.

There is a high chance that a failing alternator in a modern vehicle will take some/all of the vehicle computer modules with it.

Bzzzt said:

So far I haven't had any issues with my straight 'non-throttled' connection

Click to expand...

Do the sums and calculate the resistance of the path between the alternator and the battery, factor in the battery internal resistance and the alternator output resistance. This is limiting the current together with the alternator volts drop as it approaches full power output.

Mike

mikefitz said:

There is a high chance that a failing alternator in a modern vehicle will take some/all of the vehicle computer modules with it.

Do the sums and calculate the resistance of the path between the alternator and the battery, factor in the battery internal resistance and the alternator output resistance. This is limiting the current together with the alternator volts drop as it approaches full power output.

Mike

Click to expand...

Yup, this is exactly what I have seen. The 9mR resistance between the van battery and house battery keeps the charge rate at 80-100 amps and the increased 20mR setup is keeping the charge rate at 40-50 amps (so far). In both cases the 220 amp alternator has not dropped it's voltage. I do see a few tenths of a volt drop at the van battery but this is likely due to resistance in the current path from the alternator.

These are the charging rates I have seen with an alternator-battery direct hookup to a 240Ah LiFePO4 battery. In the first case the cables/fuses/connections/shunt add up to around 10mR and in the second case the addition of a resistor bumps it up to around 20mR. The voltage from the alternator sits at around 14.37 at the van battery (prior to the wiring that leads to the 'house' battery).

In my case I am pretty happy with the direct connection. However, I use so little power in summer that the lower rates with the resistor are still more than enough. For folks with lower output alternators, or doubts about the alternator thermal regulation, it seems like the use of a resistor is pretty effective. Although you have to accept lower charge rates at a high state-of-charge it stays surprisingly high up until 90%!

Bzzzt said:

These are the charging rates I have seen with an alternator-battery direct hookup to a 240Ah LiFePO4 battery. In the first case the cables/fuses/connections/shunt add up to around 10mR and in the second case the addition of a resistor bumps it up to around 20mR. The voltage from the alternator sits at around 14.37 at the van battery (prior to the wiring that leads to the 'house' battery).

In my case I am pretty happy with the direct connection. However, I use so little power in summer that the lower rates with the resistor are still more than enough. For folks with lower output alternators, or doubts about the alternator thermal regulation, it seems like the use of a resistor is pretty effective. Although you have to accept lower charge rates at a high state-of-charge it stays surprisingly high up until 90%!


View attachment 41913

Click to expand...

Bzzzt, good info, I appreciate it,

I have giant alternators in both my Diesel and Ferd 3.5 Eco,
100 Amps or 50 amps, wow, 2 awg cable huh?

My Ferd 3.5 Eco is wired for 60A behind my driver's seat.

So adding a shunt on the negative wire will be enough to raise the resistance of the system to 20mR?

sshibly said:

Bzzzt, good info, I appreciate it,

I have giant alternators in both my Diesel and Ferd 3.5 Eco,
100 Amps or 50 amps, wow, 2 awg cable huh?

My Ferd 3.5 Eco is wired for 60A behind my driver's seat.

So adding a shunt on the negative wire will be enough to raise the resistance of the system to 20mR?I

Click to expand...

If you have 'giant alternators' maybe you can comfortably skip adding extra resistance? However, If your battery pack is massive then you might end up hoping the alternators can protect themselves.

The current path to my batteries includes around 10 feet of 1 gauge, a dab of dual 6 gauge, a switch, two mega fuses, two 5/16" steel bolts, and a 0.00075 ohm shunt. Altogether that is just a hair under 0.009 ohms without the added resistor. Clearly everyone will have a different value for their wiring. What I can say is that adding a 10 milli-ohm resistor will make a sizable dent in the current. That resistor has to dissipate a fair bit of heat though - be sure to mount it up on an appropriate heat sink with ample airflow or it won't last long.

I used the following (https://www.digikey.ca/en/products/detail/ohmite/TGHGCR0010FE/1817153). The terminals on this are M4 (!!!) which are NOT reassuring! I had to use some short single 6 gauge leads to tie it into the system and even with that small wire the resistor is dwarfed. I'd love to see if anyone has found a 10 mR resistor with a larger case and beefier leads!

Bzzzt said:

These are the charging rates I have seen with an alternator-battery direct hookup to a 240Ah LiFePO4 battery. In the first case the cables/fuses/connections/shunt add up to around 10mR and in the second case the addition of a resistor bumps it up to around 20mR. The voltage from the alternator sits at around 14.37 at the van battery (prior to the wiring that leads to the 'house' battery).

In my case I am pretty happy with the direct connection. However, I use so little power in summer that the lower rates with the resistor are still more than enough. For folks with lower output alternators, or doubts about the alternator thermal regulation, it seems like the use of a resistor is pretty effective. Although you have to accept lower charge rates at a high state-of-charge it stays surprisingly high up until 90%!


View attachment 41913

Click to expand...

I=V/R so we can calculate the current. i'll assume the battery is at 13.3v and it has 0.25mR internal resistance:
(14.37-13.3)/(.01+.00025)=104 and (14.37-13.3)/(.02+.00025)=53.
So your numbers are right as expected based on the measured system resistance.

Worst case would be down in the low voltage shoulder (most try to avoid such a deep discharge), like 12v: (14.37-12)/(.01+.00025)=231! At least at idle that would overwhelm the alternator and pull your system down from 14.37V, but the battery won't stay that low very long either.

Your system resistance sounds somewhat high looking at wire charts, but adding in the charge disable relay, two fuses, two cutoff switches, the BMS, and the various connections I Imagine I'll be similar.

And let me just say I'm just throwing numbers out there... I don't know what the internal resistance should be (.25mR I read in a post someone measured, but it seems it should be roughly .1/Ah rating of the battery?) nor what the system resistance should be. But if those numbers are inline with expectations, it seems like this isn't a huge issue unless you discharge those batteries super low.

I love back of the envelope calculations, especially when they work!

geoffire said:

And let me just say I'm just throwing numbers out there... I don't know what the internal resistance should be (.25mR I read in a post someone measured, but it seems it should be roughly .1/Ah rating of the battery?) nor what the system resistance should be. But if those numbers are inline with expectations, it seems like this isn't a huge issue unless you discharge those batteries super low.

Click to expand...

what about the voltage drop, won't it limit V to the battery even more?
not sure how you'll get a full charge i.e. whatever you deem full is 14.4V, 14.2V

mkaye said:

what about the voltage drop, won't it limit V to the battery even more?
not sure how you'll get a full charge i.e. whatever you deem full is 14.4V, 14.2V

Click to expand...

Fortunately, the voltage drop is proportional to the amps flowing. As the batteries increase in voltage the current drops, resulting in a lower voltage drop through the system. I have not had any issue hitting my high voltage cutoff at the end of charge.

Bzzzt said:

Fortunately, the voltage drop is proportional to the amps flowing. As the batteries increase in voltage the current drops, resulting in a lower voltage drop through the system. I have not had any issue hitting my high voltage cutoff at the end of charge.

Click to expand...

Yeah, one of the biggest mistake inpatient people do when charging is raising the voltage to increase the amps, but they forget the drop will not be the same as the current decreases.
Hence the need for sense cables or a better PS

lolailando said:

Thank you for sharing this to get some real understanding of what amp rates to expect in a real direct charging set up. So If we wanted to be able to to charge at let's say 150A and we have an alternator that can handle it the real problem (after lowering the resistance conecting the house battery) would be that it would put too many amps at lower SOC of the battery .
I have seen utubes of people raising the alternator voltage by increasing the reference voltage at the alternator with a resistor or a diode....I wonder how dificult would be to get an arduino microcontroller to raise the voltage instead at the reference voltage line of the alternator reliably when the SOC gets low to lower the alternator voltage and lower the current. That would be a neat set up.

Click to expand...

I don't know where in a modern vehicle the alternator is controlled. You are right though, if you could trick the controller into seeing a different voltage you could take control of the alternator output voltage. The logical step to full alternator control seems to have been taken by Wakespeed (http://wakespeed.com/products.html).

I've looked into alternator hacking (likely via arduino - enable a resistor in parallel to the regulator to up the voltage, add resistance in series to lower), but that seems potentially problematic on a modern vehicle and I'm not convinced it's necessary for my application. I'd maybe consider it if I went dual alternator, but I'm pretty sure I'd end up with a wakespeed regulator instead.

I just stumbled onto this thread and want to throw out my dilemma. I am currently building an overlanding rig on a Dodge Cummins with dual 220 amp heavy duty alternators. I bought a truck camper without an electrical system so I can spec it myself. I'd like to build my own battery from Fortune cells and control charging with the BMS. I should be able to get 700ah at 12v into the battery box in the camper. I'll install as much solar as I can fit but I want to get the max charge from the alternators when under way since I'll be running a 12v air conditioner often when parked and I know solar won't keep up. Problem is, I can't add an external regulator to the alternators as they are controlled by the truck's ECU. I don't really want to buy another alternator since I have 2 brand new heavy duty units. The charging voltage of the alternators is (supposedly) set at a solid 14.4v. Do I take a chance and use the BMS to open/close a 500a relay (off of the starting battery) so I can control the charge parameters based on the cell's actual voltage/current? Do I use the BMS to switch on/off something like the Victron Buck Boost 100a where I'd have much less current available? Is there a better option? Been looking at a ton of threads for a solution and getting a lot of conflicting answers. Mercy on this newbie!

upgrayedd said:

I just stumbled onto this thread and want to throw out my dilemma. I am currently building an overlanding rig on a Dodge Cummins with dual 220 amp heavy duty alternators. I bought a truck camper without an electrical system so I can spec it myself. I'd like to build my own battery from Fortune cells and control charging with the BMS. I should be able to get 700ah at 12v into the battery box in the camper. I'll install as much solar as I can fit but I want to get the max charge from the alternators when under way since I'll be running a 12v air conditioner often when parked and I know solar won't keep up. Problem is, I can't add an external regulator to the alternators as they are controlled by the truck's ECU. I don't really want to buy another alternator since I have 2 brand new heavy duty units. The charging voltage of the alternators is (supposedly) set at a solid 14.4v. Do I take a chance and use the BMS to open/close a 500a relay (off of the starting battery) so I can control the charge parameters based on the cell's actual voltage/current? Do I use the BMS to switch on/off something like the Victron Buck Boost 100a where I'd have much less current available? Is there a better option? Been looking at a ton of threads for a solution and getting a lot of conflicting answers. Mercy on this newbie!

Click to expand...

That is a good sized battery! Are these alternators hooked up together for 440 amps?!?

I'd lean towards the bms controlled relay between the starter battery and the lithiums. That said, I don't know what start-stop parameters your bms will allow. If it has a cell based high voltage cutoff then I don't see this being an issue. Would this relay also cut out the connection when the engine stops?

The big unknown is whether your alternators protect themselves from thermal overload. The resistance through your wiring should limit the current to the battery to a large extent but in extreme cases (very low state-of-charge) the batteries might be capable of pulling 100% of your alternators output. You want to know that while idling at high temperatures your alternators aren't cooking.

Likely the biggest issue is your own willingness to accept risk! Plenty of companies will happily take your money in exchange for piece of mind.

440A is quite a lot. If you look up at the math I did you'll need super low resistance between the starter battery/alternator and the lithium batteries to get anywhere near that in the normal range of ~13v. Lets just say you want 300amps, Thats a 1.4v voltage drop so R=V/I 1.4/300 = .00467ohm or half of what Bzzzt measured and it will fall off as the voltage rises.

At your battery size and loads you may want to consider a 24v system anyways and there direct connection isn't an option unless you swap one of those alternators. I have no experience with them, but one of these baddies may be an option for you: https://www.daygreen.com/collection...00w-dc-dc-step-up-converter-voltage-regulator. That should be roughly constant power for you until near the end of the charge and you won't have to worry about the resistance quite so much to get that full power.

As for cooling, I'm thinking of attempting to chill my onboard water with the vehicle AC as I drive and use that to chill the cabin while we sleep (similarly heat the water when we are in the cold and want heat). I purchased 16 280Ah cells thinking I'd run AC on my overlanding rig also, but I'm now going to try running half the batteries and skip that.