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Do I really need a DC-DC or can I go with a high power alternator?

Luk3jay

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So I'm still looking at the best way of charging my 200ah LFP aux battery in my camper conversion.
The more research I do, it seems like all a DC-DC charger really does is limit the current from the alternator to protect it. From what I can tell, it seems like if I had an alternator that was capable of supplying enough amps, I could just connect the LFP directly to it and it would charge without issue.
This would be via the starter batt of course so there wouldn't be a worry about dump spikes.

I'd much rather spend a few hundred quid on a high power alternator and have the ability to recharge a lot quicker vs a few hundred on a DC-DC charger which would recharge a lot more slowly.

I'm an experienced electrical engineer, but this is my very first time with LFP so I appreciate all the info I can get. Thank you!

P.S. bonus question: If i went down the DC-DC route - Is there a reason why I couldn't use a 30A 12v (or 14v) regulator to charge the LFP battery? They cost a fraction of the price of a DC-DC charger - I'm not necessarily considering this as an option but it was a thought I had.

Thanks!
 
In addition to limiting amps, the DC-DC charger also supplies the correct voltage for the battery. Some are programmable. Although depending on the wire length and size you may not have an issue. I mostly recommend measuring the issue and then determining the solution.

Larger alternator could be the solution. Although the battery can absorb 200 amps and the vehicle needs 20 to 60 amps to run.

Yes you could use a regulator. Many of these are designed to supply a certain amount of current and will overload if the battery asks for more. They expect the device to have a limit. Some can be used as a battery charger and will allow voltage sag to avoid overload. Verify what you are buying.
 
Is there a reason why I couldn't use a 30A 12v (or 14v) regulator to charge the LFP battery?
These are good questions.
My only addition would be to wonder is such a regulator could/would be turned off when engine not running?
Most DC-DC chargers are turned on with an IGN signal voltage or sense increased voltage by alternator.
 
In addition to limiting amps, the DC-DC charger also supplies the correct voltage for the battery. Some are programmable. Although depending on the wire length and size you may not have an issue. I mostly recommend measuring the issue and then determining the solution.

Larger alternator could be the solution. Although the battery can absorb 200 amps and the vehicle needs 20 to 60 amps to run.

Yes you could use a regulator. Many of these are designed to supply a certain amount of current and will overload if the battery asks for more. They expect the device to have a limit. Some can be used as a battery charger and will allow voltage sag to avoid overload. Verify what you are buying.
Thanks. What is the correct charging voltage for a 12v 4s LFP pack? I Don't have the spec sheet of my cells to hand but I feel like it was around 13.8v? is there much leeway for voltage deviation or could it ruin the pack? I guess the BMS would kick in worst case.

If I'm going to have to spend several hundred pounds on some form of charger, it makes sense to me to spend it on a better alternator which would charge the battery at a better rate. My current one is probably only around 80A so I'd have to go with a 30A dc-dc charger to stay safe. with a 300A alternator, I could charge my 200ah pack in no time...

Good point about the regulator. I hadn't considered the output protection on it.
 
These are good questions.
My only addition would be to wonder is such a regulator could/would be turned off when engine not running?
Most DC-DC chargers are turned on with an IGN signal voltage or sense increased voltage by alternator.
I was planning to wire it after my existing VSR. the VSR(Voltage Sensing relay) Switches on when the engine is running (Voltage above 3.7v) and off when it's not (Voltage below 12.7v)
 
Also keep in mind that one of the risks that is often talked about with LFP and alternators is that if the BMS shuts off charge while the engine is running, an unprotected alternator will burn up quickly due to no load. A good regulator should have temperature sensors on the alternator and prevent this. I think the Balmar and Wakespeed regulators take care of that, but those are expensive add-ons.
 
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Also keep in mind that one of the risks that is often talked about with LFP and alternators is that if the BMS shuts off charge while the engine is running, an unprotected alternator will burn up quickly due to no load. A good regulator should have temperature sensors on the alternator and prevent this. I think the Balmar and Wakespeed regulators take care of that, but those are expensive add-ons.
Thanks. I have considered this and I believe that having the lead-acid starter battery connected will take care of this issue. There are also protection devices such as this which I could install if needed.

 
Also keep in mind that one of the risks that is often talked about with LFP and alternators is that if the BMS shuts off charge while the engine is running, an unprotected alternator will burn up quickly due to no load. A good regulator should have temperature sensors on the alternator and prevent this. I think the Balmar and Wakespeed regulators take care of that, but those are expensive add-ons.
The existing chassis battery and general load to run the vehicle is inadequate to allow for proper regulation if the BMS cuts off?
 
There are temperature monitors that limit alternator output via voltage regulator. Cheaper?
 
There are temperature monitors that limit alternator output via voltage regulator. Cheaper?
This could be a good idea. I've not found any yet. are they basically a DC-DC charger which throttles it's load based on alternator temp?
 
This could be a good idea. I've not found any yet. are they basically a DC-DC charger which throttles it's load based on alternator temp?
No, it alters the field of the alternator. More factors considered when alternator voltage regulating.
A charging lead battery quickly builds resistance, raising the voltage making a normal regulator's job easy. Lifepo builds resistance slowly so something other than just voltage is needed to regulate output.
 
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There are absolutely dedicated 2nd alternators that are purpose built to charge battery systems, including Lifepo4. They are way more than a couple hundred quid, but... will all but eliminate your need for solar power if you drive frequently. This is super common for conversion vans with big batteries. Heck, the Winnebago EKKO small C class RV comes with a Balmar. Very common in Marine also. See the link.
 
One of the limiting factors in trying to get a high current charge to a LiFePO4 battery bank is that you may need some very large cables to support those amps. There have been multiple forum members asking about this. If you put too large of a DC-DC charger in, you may not have the cabling to support it. A 50 amp (output) charger could pull well over 60 amps. In a 12v system, you'll need a rather large cable to support that, depending upon the length of the cable. For an RV trailer, it starts to make sense to put in a smaller DC-DC charger, like 15 amps, so that you don't have to spend twice as much as the charger on cabling. For a truck camper, the distance should be less and maybe you can get away with higher amps.

Let's say you have a 100 amp alternator and the round trip distance is 30' in a 12v system. Assuming you pull no more than 80 amps from the alternator, you'll need 1 gauge cable to stay under a 3% voltage drop. 1/0 gets you to a 2.01% voltage drop. Now tell me how you're going to limit the current to only 80 amps without anything else in the circuit.

As @time2roll led with in his post, a direct alternator to LiFePO4 connection has no charge profile. The alternator may push 14+ volts all the time and that's not what you want with a LiFePO4 battery.

For the LiFePO4 battery bank in my RV trailer, I completely disconnected it from the charge coming in from the 7-pin connection. My LiFePO4 battery bank gets zero charge from the tow vehicle, which is rated to have 397 amps of alternator output (only once in a blue moon). I didn't want to chance my LiFePO4 getting jacked up with a lousy charge, plus, my 640 watts of PV on the roof takes care of battery bank just fine.
 
The existing chassis battery and general load to run the vehicle is inadequate to allow for proper regulation if the BMS cuts off?
when you leave the lead battery in the system no problem with cutting out BMS or Fuses.
 
Assuming you pull no more than 80 amps from the alternator, you'll need 1 gauge cable to stay under a 3% voltage drop. 1/0 gets you to a 2.01% voltage drop. Now tell me how you're going to limit the current to only 80 amps without anything else in the circuit.

so the primary issue is - that the LFP can pull very high amps for a long time.
I'm an experienced electrical engineer, but this is my very first time with LFP so I appreciate all the info I can get.

Hello fellow electrical engineer,

so an LFP battery has a very low internal resistance. When the source can deliver unlimited current the LFP will take it.
It's for practical purposes almost like a short in the wiring. So the general idea is - to limit the source.

Myself and others discovered when you add enough resistance in the circuit (like using the chassis of vehicle as return path, or cooper clad aluminum wire, instead of pure cooper) we can lower the amount of amps flowing. (resistors in series)

Actually the voltage drop is a good thing - the closer you get to it's optimal charging volte (depending on the manufacturer 14.2-14.8v) the better and faster it charges.

My RVs charging system - after the initial 10 minutes of 14.4V after startup drops to 13.8-14.0V - while the battery only sees 13.6V at that voltage my 200ah of LFP are self limiting to about 60A - which is easy to handle for my 180A alternator. I got 25ft of 0/2 gauge wire and a engine ground strap in the return path.

13.6V is the "90% no load" voltage of many LFPs, so with my system - I never charge the battery full with the alternator.
Disclaimer - using resistance to limit amps - is inefficient. But done frequently with things like blower fans (the one in your car) and electric motors (golf cart)

DC-DC - more efficient, safer, and programable.

High power alternator - cheaper, brute force, if you don't measure and get your resistance right you got the chance of burning up stuff.
 
Many modern vehicles have a "smart alternator" that by design does not fully charge the battery. Instead it tries to keep the battery at a lowered state of charge so that when deceleration occurs, the alternator can ramp up and charge the battery, thereby taking advantage of the deceleration mode to essentially charge the battery at "no cost." Such a system is incompatible with properly charging a house battery.

The B2B charger isolates the house battery from these charging "anomalies" because it will charge correctly even at a lower input voltage.
 
Thanks. What is the correct charging voltage for a 12v 4s LFP pack? I Don't have the spec sheet of my cells to hand but I feel like it was around 13.8v? is there much leeway for voltage deviation or could it ruin the pack? I guess the BMS would kick in worst case.

If I'm going to have to spend several hundred pounds on some form of charger, it makes sense to me to spend it on a better alternator which would charge the battery at a better rate. My current one is probably only around 80A so I'd have to go with a 30A dc-dc charger to stay safe. with a 300A alternator, I could charge my 200ah pack in no time...

Your current alternator probably shouldn't have more than a 10A load to charge another battery.

As for installing a large alternator, you will need large cables to get power over any distance. My current project is installing a 2200w inverter in the back seat area of my dual alternator (355A) equipped GMC truck. From the inverter to the rear, I have a 30 amp service mounted at the rear bumper. A short cord allows me to feed 120V AC to the 24V Growatt AIO AC input mounted on the truck camper. This allows charging thru the GW AC to DC charger. The most I would consider for the charge rate is 60A (1440w) more like 30A (720w) will be primary.

It is taking over 15 feet of cable to run from the junction block for batteries to the inverter. The max input of the inverter determines how many amps required and using a voltage drop calculator, I ended up running 2/0 cable.

The reasons for this setup are simple, I have a backup power source and the ability to charge from a 12V source to a 24V load. I already have the charger built into the GW so no additional charger was needed. This setup allows a fairly large number of watts to be moved over a longer distance.
Good point about the regulator. I hadn't considered the output protection on it.
You will need to have a FLA battery between the alternator and LFP battery, you won't be able to charge directly from the alternator to LFP without the possibility of a battery (load) dump if the battery BMS shuts off the connection. You will fry the alternator and rectifier in a hurry. The FLA battery acts as a capacitor and will absorb the transient spike that occurs when a battery dump occurs.
 
I'm not sure if that is true.
A high power alternator will only draw a tiny bit more power for, say a 50W load, than a low power one would.
DC to DC charger, B2B battery to battery DC to DC charger. Should be more electrical efficient in charging a battery.

You are right - for the just providing low end power - it doesn't make a significant difference if you got a large or small alternator.
If you got a 130A alternator or 250A - when you only use 50A - your fuel efficiency will not change.


Alternator directly to LFP battery charging - like I'm doing is - wasting a bunch of power in resistance heating, to limit the current.

Hope someday we get cheap direct for LFP designed alternators. Or even better that we get electric Vans/RVs/Caravans with a small range extender engine.
 
My understanding is that you can go direct from alternator to LFP if you have a DC-to-DC charger connected to a lead acid battery. That will always pull some current, which should prevent the alternator from being damaged if / when the BMS cuts of charging to the LFP.
 
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