Charger 102

[GallenH]

I'm contemplating a system where I'd have:

alternator-->B2B charger-->200AH Lithium.

How do I know how to size the components. IOW: What amp output
alternator is needed? What amp-size B2B is necessary?

Is there some formula used to calculate these requirements?

thx.glenn

[avanti]

As I understand it, if you are specifying all components (as opposed to having an existing alternator), you start by picking a battery and looking up its recommended max charge current. This is expressed as a percentage of the battery capacity, and should be available from the battery manufacturer. If it is, say .5, your max charging target is (in your case) 200ah X .5 = 100amps. You would then pick a B2B whose output is rated at that value. Then you look at the input current of the B2B (which will be larger than the output, due to inefficiencies). Maybe that will be 120 amps. So, you need an alternator that can supply that much current.

[all of the numbers in the above are hypothetical]

[folivier]

Don't forget to add what components you plan to run, microwave?, air conditioner?, or just overnighting?
For microwave you also need an inverter, a bigger one for air conditioning + more batteries.
If you just plan on overnighting a 30 amp B2B charger should replace your isolator and you should be able to use the same cables going to your batteries. But you should put your Lithiums inside so may have to reposition that end of your cables.

[GallenH]

Thanks. Good comments from both of you. I feel less like I'm shooting in the dark.

[GallenH]

ok. Avanti's comment about battery charging value makes sense with all of the batteries that I've looked at: they're all 0.5. Which means I need 100A coming from the alternator. My existing alternator is 140A and I'm guessing that the 40A excess gives me cushion to run basic chassis functions while I'm driving.

What I'm still confused about is the B2B sizing. The Renogy and Kisea specs don't say anything about input amps, only voltage ranges. And the output specs are 20-40-60A or (with Kisea) 30-50A all depending on the model chosen. So how do I figure the size/model needed to charge a bank of 200AH?

thxthxthx.glenn

[hbn7hj]

Thirty amps is more than enough. Choose the 50 or 60 amp depending on brand. I have the Kisae dialed back to 40 amp. More than enough.

I would think you need an A/C charger, too, even if you don’t have a generator.

Enough thinking, time for you to cut and crimp wire!

[GallenH]

So if you have enough alt amps to support a 30A or 50A would the advantage to the 50A be that it would charge faster? Why did you dial back to 40A if your alternator would support it?

[avanti]

To be clear: the ".5" value gives a MAXIMUM recommended charge current, not a required one. It is perfectly OK (even good) to charge at a lower rate. it will just take longer to charge the battery.

As for sizing the alternator against the B2B: the difference between the output and the input current is simply the efficiency. Sometimes this is given in the specs. If it isn't, something like 20% should be conservative.

[booster]

Originally Posted by GallenH

ok. Avanti's comment about battery charging value makes sense with all of the batteries that I've looked at: they're all 0.5. Which means I need 100A coming from the alternator. My existing alternator is 140A and I'm guessing that the 40A excess gives me cushion to run basic chassis functions while I'm driving.

What I'm still confused about is the B2B sizing. The Renogy and Kisea specs don't say anything about input amps, only voltage ranges. And the output specs are 20-40-60A or (with Kisea) 30-50A all depending on the model chosen. So how do I figure the size/model needed to charge a bank of 200AH?

thxthxthx.glenn

The big deal is that you can't run an alternator full out for long or it will overheat. Your 140 amp one will be happiest at about 50 amps to the coach along with the 15-40 for the chassis depending on if you are running things like heater fans, headlights etc. Once you get past about 50% of rating it is best to cut back on something if you can.



40 amp b to b will take about 50 amps so that is where I would go.

[hbn7hj]

Originally Posted by GallenH

So if you have enough alt amps to support a 30A or 50A would the advantage to the 50A be that it would charge faster? Why did you dial back to 40A if your alternator would support it?

Easier on the alternator. Battery is always charged before the driving day ends. Mercedes specs their electrical system for no more than 40 amps to the coach battery. Sounds like a good number to me.

[reilym]

Originally Posted by GallenH

I'm contemplating a system where I'd have:

alternator-->B2B charger-->200AH Lithium.

How do I know how to size the components. IOW: What amp output
alternator is needed? What amp-size B2B is necessary?

Is there some formula used to calculate these requirements?

thx.glenn

I have a similar set up in my 2016 PW Lexor and have installed a Renogy 60 amp DC-DC, using the old cabling from my battery separator (which I replaced with a manual switch). I am monitoring the input and output current, along with the alternator case temperature while charging. Typically, I will use about 60 amp-hours/day of boondocking, so the next day's drive will easily top off the batteries and keep my absorption fridge going (about 15 amps). While bulk charging up to about 98% SOC, the input current is 24% higher than output. Up to about 70% SOC, the DC-DC is putting out close to the full 60 amps with 74 amps coming in from the alternator. I have a 220 amp alternator. So far, on the highway the alternator runs between 110 and 130 F. In heavy traffic and Florida heat, I have never seen it go above 190 F. I usually either drop the charge current to 50% or turn the unit off completely in such conditions and back on once I have air flowing through the engine bay again. After our winter trip, I'll post more of the logged data, but the above is a summary of what I know so far.

A second alternator and a higher amperage B-B or two in parallel would be ideal, but I am happy with this setup the way I use the camper.

[Winston]

Originally Posted by reilym

A second alternator and a higher amperage B-B or two in parallel would be ideal . . .

One of the huge benefits of having a second (independent?) alternator is the ability to select a 2nd alternator with an external/programmable regulator. This permits direct connection of the alternator to the house battery - - with the corresponding ability to set any desired charging current - - all without the cost, complexity, and limitations of a B-to-B inverter.

[GallenH]

Originally Posted by reilym

I have a similar set up in my 2016 PW Lexor and have installed a Renogy 60 amp DC-DC, using the old cabling from my battery separator (which I replaced with a manual switch). I am monitoring the input and output current, along with the alternator case temperature while charging. Typically, I will use about 60 amp-hours/day of boondocking, so the next day's drive will easily top off the batteries and keep my absorption fridge going (about 15 amps). While bulk charging up to about 98% SOC, the input current is 24% higher than output. Up to about 70% SOC, the DC-DC is putting out close to the full 60 amps with 74 amps coming in from the alternator. I have a 220 amp alternator. So far, on the highway the alternator runs between 110 and 130 F. In heavy traffic and Florida heat, I have never seen it go above 190 F. I usually either drop the charge current to 50% or turn the unit off completely in such conditions and back on once I have air flowing through the engine bay again. After our winter trip, I'll post more of the logged data, but the above is a summary of what I know so far.

A second alternator and a higher amperage B-B or two in parallel would be ideal, but I am happy with this setup the way I use the camper.

Thanks. Interesting information. Question though: Isn't the B2B only active when the ignition is on? And if it's inactive it's not drawing from the alternator. Is that correct? If so, I'm wondering why you didn't run the original separator connection directly to the B2B and put a switch on the ignition wire.

[GallenH]

Originally Posted by Winston

One of the huge benefits of having a second (independent?) alternator is the ability to select a 2nd alternator with an external/programmable regulator. This permits direct connection of the alternator to the house battery - - with the corresponding ability to set any desired charging current - - all without the cost, complexity, and limitations of a B-to-B inverter.

I'm curious. Can you ballpark a price for a 2nd alternator with a programmable regulator?

[booster]

Originally Posted by Winston

One of the huge benefits of having a second (independent?) alternator is the ability to select a 2nd alternator with an external/programmable regulator. This permits direct connection of the alternator to the house battery - - with the corresponding ability to set any desired charging current - - all without the cost, complexity, and limitations of a B-to-B inverter.

I completely agree on this, as long as your charging amount maximum is within the alternator output range on the regulator. Baldors can turn down 50% of field, so probably a bit more the 50% of rated for the alternator.

[reilym]

Originally Posted by GallenH

Thanks. Interesting information. Question though: Isn't the B2B only active when the ignition is on? And if it's inactive it's not drawing from the alternator. Is that correct? If so, I'm wondering why you didn't run the original separator connection directly to the B2B and put a switch on the ignition wire.

Sorry, I did not describe my system completely. I am using a voltage sensitive switch that marcopolo turned me on to described in this thread, This device turns the charger on when the input voltage reaches 13.2 volts (i.e. the engine is running) and off after it drops below 12.8. It also allows for a fixed delay after the voltage reaches the 13.2 to give the alternator time to warm up. The switch I mentioned that replaced the battery separator is only off for long-term storage and for servicing the charging system. I got rid of the separator because it has it's own inefficiencies and I have already replaced a failed one. The system is entirely automatic, but I have wired things so that I can turn the DC-DC off or switch to 50% current from the cab if I wish to.

[GallenH]

In my looking around it seems that a 2nd smart alternator would run in the vicinity of $1k-1.5k. While it appears great in terms of advantages, it doesn't seem practical for my 1997 PW which is 24 years old.

I don't have high power demands; have no desire to use the microwave (have actually removed it) and don't camp where I need AC. All I'm trying to do here is provide the power for a ~3.0cf compressor fridge to replace my failing absorption (thermostat appears to have absolutely no functional impact).

I suppose that I could replace my existing alternator (140A) with something more powerful at a much less cost than the 2nd alternator. But right now this is going to be a test: How well can I function on 200AH lithium added to my 100AH of AGM. The 200AH of lithium would be charged by the alternator. The 100AH AGM charged by the lithium (as in the original hybrid systems developed by hb and booster). Using dual input B2B chargers I could use rooftop supplemented by suitcase solar.

All of that plus an Isotherm or NovaKool would require an outlay of ~$4k which is enough for a 24 year old RV.