Alternator output

[sharmy]

I have recently bought my wife a new 1.4 litre Skoda Octavia station wagon and have fitted a 60 litre Kings drawer fridge and a 100 ah lithium battery. I am in the process of trying to fit a 40 amp dc to dc charger in it. Reading the fitting instructions it says to ensure the alternator power does not exceed 720W with the output current within the range of 75A TO 100A. I am pretty sure it has a 140A alternator. I would have thought you would multiply the amps by about 14 to get watts which does not work out. Any help appreciated, or is there another way to charge the lithium battery without the dc to dc. The car has a smart charger and shopping is about 25 ks each way.

[JDNSW]

You are right - those numbers do not make sense!. I think you need to talk to the supplier of the charger about it. As far as the car is concerned, in my view the alternator and its system should handle the load OK, but it is another matter whether the smart charging system can handle it wiithout doing something silly - remember that the smart charging system is designed to give best possible fuel economy by only switching the alternator on when necessary to keep the battery charged. Talking to the dealer, and specifically someone who understands the system would seem to be a good idea.

[TonyC]

As John said, those numbers make no sense.

The DC-DC won't care what the alternator output is, as long as it's enough.

Being a smart alternator, it will be important to connect it correctly, on a D4 it's important to never connect anything to the negative post of the battery, as this interferes with the car controlling the alternator, you must use one of the negative posts that are supplied.

I've never had anything to to do with smart alternators, DC-DC chargers or lithium batteries, but this is my understanding of them.

Hopefully Tim/Drivesafe will be along shortly.

Tony

[drivesafe]

Hi sharmy and as both the above posters have stated, that does not make sense.

You may have misunderstood what is in the instructions.

Next, regardless of that strange info, make sure your DC/DC device has an IGNITION sense and is not just a voltage sensing setup.

John was close to how SMART alternators work.

They do not actually turn off.

If the cranking battery is better than 80% charged, while you have your foot on the accelerator peddle ( or your cruise control is running your vehicle ) a SMART alternator will lower its output VOLTAGE to reduce the load on the motor while the motor is working.

Any time you take your foot off the accelerator, the voltage will rise.

The voltage can lower to as little as 12.2v, but usually around the 12.5v mark while the motor is under load, and it can rise to 14.7v while the motor is coasting.

If your DC/DC device is set to voltage sense, with a SMART alternator, your DC/DC device may spend more time off than on.

When set to IGNITION, the DC/DC device may reduce to OUTPUT current when the alternator voltage is low, BU it will still keep charging the lithium, all be it at a reduced charging current.

One more point, make sure the DC/DC device is mounted as close as possible to the lithium battery, as this will guarantee the correct charging voltage at the battery.

[sharmy]

I was also baffled by the request, it doesn't seem to make sense. I will go ahead and fit it and hope for the best, the vehicle hand book gives no technical information at all. Here is the offending page.

[drivesafe]

I was also baffled by the request, it doesn't seem to make sense. I will go ahead and fit it and hope for the best, the vehicle hand book gives no technical information at all. Here is the offending page.
Hi again sharmy, before you fit that device, please post up more info about it.

There are DC/DC devices that are specifically designed for Alternator to Battery charging and have an INPUT CURRENT LIMIT.

This type of DC/DC unit is most likely NOT SUITABLE for charging lithium batteries.

If you can, post up a link to the manual for your DC/DC device.

Your vehicle has a 140 amp alternator which will easily exceed the limit of such a charger if it has a maximum input current of 720w.

[drivesafe]

NOTE, that DC/DC device is most likely intended for use with a marine outboard motor, which has a much smaller alternator.

[sharmy]

Thanks for your info Drivesafe, I probably bought the wrong unit. I hope you can scan this, I can't, I am hopeless at such things.

[Tins]

I hope you can scan this, I can't, I am hopeless at such things.

I can (https://store-fhnch.mybigcommerce.com/content/RBC40D1U-US/RBC40D1U%20Manual.pdf).

[drivesafe]

I can (https://store-fhnch.mybigcommerce.com/content/RBC40D1U-US/RBC40D1U%20Manual.pdf).
Hi Tins, I can not, if you can, please post up a copy.

[Tins]

It should have come up in the link, but I'll try again. Might be a Mac thing I suppose.


https://store-fhnch.mybigcommerce.com/content/RBC40D1U-US/RBC40D1U%20Manual.pdf

[Tins]

It should have come up in the link, but I'll try again. Might be a Mac thing I suppose.


https://store-fhnch.mybigcommerce.com/content/RBC40D1U-US/RBC40D1U%20Manual.pdf

Tested this link, it opens an online, downloadable, PDF.

[drivesafe]

Thank you for that Tins.

Sharmy, that is NOT and Alternator to Battery DC/DC device.

It is a standard Battery to Battery DC/DC device and as such, I have no idea about the reason for that wattage statement.

There is a contact e-mail address at the bottom of the instructions. Try sending them a message and see what they have to say.

BTW, they have a pretty good name, so should do what you want.

[sharmy]

Thank you for that Tins.

Sharmy, that is NOT and Alternator to Battery DC/DC device.

It is a standard Battery to Battery DC/DC device and as such, I have no idea about the reason for that wattage statement.

There is a contact e-mail address at the bottom of the instructions. Try sending them a message and see what they have to say.

BTW, they have a pretty good name, so should do what you want.
Thanks, I will contact them to see if they know what it's about. They are a good mob, I have dealt with them before. I will let you know the outcome.

[sharmy]

Well the answers aren't much chop, I am starting to wonder if I am talking to real people or not, or maybe some Chinese person with a western name. I will go ahead and fit it and hope for the best.192450192451

[drivesafe]

That's a bit of a worry.

With older type DC/DC devices, as the INPUT VOLTAGE from a SMART alternator drops, the DC/DC device compensates for the lower voltage by increasing the INPUT CURRENT draw. As the INPUT voltage goes down the INPUT current goes up to maintain a constant OUTPUT current and voltage.

If the INPUT voltage from the alternator gets very low, the DC/DC device may draw VERY HIGH INPUT CURRENTS.

If your DC/DC device is this type, it could draw in 70+ amps to be able to charge your battery at 40 amps.

You do not have to be a rocket scientist to realise the danger this poses.

New DC/DC devices actually limit the MAXIMUM INPUT current to avoid this danger.

But this means that in a vehicle with a SMART alternator operating at around 12.5v, your 40 amp DC/DC device might only be supplying 15 amps the the lithium battery.

I was given the job of testing a new DC/DC device for a company a few years back, and while this specific DC/DC device had some spectacular features not found in any other DC/DC device, the Australian company decided not to sell the unit based on my test results, where the input currents were just not safe.

That device has been upgraded but it should never have been marketed in its original state in the first place.

[sharmy]

I don't understand the ins and outs of how these things work, but could they be talking about the 750W maximum draw after the car has taken what it needs to run its self. I believe it is a new model charger. and requires an 80 amp ANL fuse between the starter battery and charger. I don't know what ANL stands for, but I have ordered one.

[sharmy]

The link to the item

Renogy 40A 12V DC to DC Battery Charger Lithium AGM Gel Dual Battery Maintainer | eBay (https://www.ebay.com.au/itm/296374408690)

[drivesafe]

Hi again Sharmy and that, in my opinion, is way too high an amperage for the INPUT current.

That device obviously does not have an automatic input current limiting function and, again in my opinion, is not suitable for use with any vehicle with a SMART alternator function.

Your vehicle has a SMART alternator operation and has a 140 amp alternator.

This means, that once your vehicles own electrical needs are covered, and that would be less than 40 amps, there is around 100 amps available to power that DC/DC device.

That is nearly double the maximum limit set by the DC/DC manufacturer.

As for the fuse, you could have got an 80 amp MIDI fuse and MIDI fuse holder from your local Jaycar store.

[sharmy]

Thanks Drivesafe, I will see if I can send it back. John

[drivesafe]

Hi Sharmy and I will leave it up to you on whether you use that device or not, but I would NOT use it in any vehicle with a SMART alternator.

If you had a Toyota, that device would be fine, but not in a VW ( Skoda ) or any new Land Rover.

If you are sending it back, first check and see if Renogy make a DC/DC device with automatic input current limiting and see if you can swap what you have.

Unfortunately there is a lot of misinformation about DC/DC devices and the differences in the types available.

PLUS, few sellers have any real idea how they actually work, and this thread is a perfect example.

While there are safe DC/DC devices available for vehicles with SMART alternators, they are a trade-off.

To make them operate safely, they need to reduce the actual charging current, so while you might buy a 40 amp DC/DC device, it may actually only be charging at around 15 amps in a vehicle with a SMART alternator.

[Blknight.aus]

Ive fitted a few of these as replacements for the kings 40A DC/DC units that have died just out of warranty.

fit it as per the instructions, use 2x fuses one near the start battery, one near the aux battery.

Set it up to the battery you have (lithium) and cross reference the charge profile page of the DC/DC manual and your battery data sheet. IF the numbers line up its now time to forget about the whole thing and enjoy the amps in the aux battery.

If the charge profile doesnt line up to the data sheet, in the monitoring app on your phone adjust the required paramaters and refer to the last half of the previous instructions.

Maximum input draw Ive seen on a 40A dc/dc was 70 amps and it was kicking 45amps into the aux battery the battery was big and flat after about half an hour or so the unit was running hot but the rates dropped off to more reasonable number . This was neither a kings or renogy unit

Normaly input draw for a kings 40a is in the 45A range for about 35 out (Ive never tested it to limits) in rush is upto 65 although that might have a slight meter lag on it.

The only renogy IVe tested pulled very similar numbers.

So long as your alternator can produce more demand than all the loads combined it doesnt matter what its maximum output amps are so long as it can maintain voltage, your start battery will take care of any short term drops caused by the smart alternator going to a reduced voltage state. So long as you hook up correctly and the smarts that drive the alternator can see the load of the DC/DC trying to charge the battery and run your accessories it will sort itself out.

That said...

be careful how you earth the accessories and the aux battery. I've heard tell of AUX appliances upsetting the apple cart for the smart alternator smarts, Never seen it and dont put much faith in it but most stories start somewhere. My reccomendation is currently anything that runs on the AUX battery gets connected to the earth of the aux battery and not chassis earth. (this doesnt apply for the current kings unit as its a common earth unit.)

[sharmy]

Ive fitted a few of these as replacements for the kings 40A DC/DC units that have died just out of warranty.

fit it as per the instructions, use 2x fuses one near the start battery, one near the aux battery.

Set it up to the battery you have (lithium) and cross reference the charge profile page of the DC/DC manual and your battery data sheet. IF the numbers line up its now time to forget about the whole thing and enjoy the amps in the aux battery.

If the charge profile doesnt line up to the data sheet, in the monitoring app on your phone adjust the required paramaters and refer to the last half of the previous instructions.

Maximum input draw Ive seen on a 40A dc/dc was 70 amps and it was kicking 45amps into the aux battery the battery was big and flat after about half an hour or so the unit was running hot but the rates dropped off to more reasonable number . This was neither a kings or renogy unit

Normaly input draw for a kings 40a is in the 45A range for about 35 out (Ive never tested it to limits) in rush is upto 65 although that might have a slight meter lag on it.

The only renogy IVe tested pulled very similar numbers.

So long as your alternator can produce more demand than all the loads combined it doesnt matter what its maximum output amps are so long as it can maintain voltage, your start battery will take care of any short term drops caused by the smart alternator going to a reduced voltage state. So long as you hook up correctly and the smarts that drive the alternator can see the load of the DC/DC trying to charge the battery and run your accessories it will sort itself out.

That said...

be careful how you earth the accessories and the aux battery. I've heard tell of AUX appliances upsetting the apple cart for the smart alternator smarts, Never seen it and dont put much faith in it but most stories start somewhere. My reccomendation is currently anything that runs on the AUX battery gets connected to the earth of the aux battery and not chassis earth. (this doesnt apply for the current kings unit as its a common earth unit.)
Its already on the way back, I wasn't prepared to take the risk, even the input of that 70 amps you talk about is way more than their limit of 720 watts. I am going to leave it go for now, we went to town yesterday with a fully charged battery, did some shopping and a bit of running around and it used 10% so I might just recharge when it gets down to about 30-40%. On longer trips we can just run it off the power socket in the back and change over to the battery if we stop for any length of time. Town and back is about 50 ks.

[drivesafe]

With your SMART alternator, when you start your motor, your vehicles BMS will sense that the CRANKING battery is down a bit and it will run at a high voltage for a longer time, particularly if the battery is under 80%, to replace as much used capacity in the shortest drive timed and if you are doing 50km trips, for what you want, you should be fine.

As you put it, things might be different for long trips.

Rapid recharging is one of the benefits of SMART alternator operation. It will run at the maximum SAFE voltage for short periods of time. This is usually around 14.7v.