Couple of questions about DC-Dc charging

BFL13 wrote:
Not quite how it works, at least AFAIK.

Your Renogy does 40 amps output no matter what the input voltage is within its rating for that. The output watts (if it is like other chargers, which I assume to be the case) uses the battery voltage of the battery being charged, which rises as it is being charged. So it goes from say 13.5 to 14.5 to use as an example.

Battery voltage rises to the rated Vabs and then holds for the Absorption stage while amps fall. So highest watts with highest voltage to go with the 40 amps just before they taper, is just before Bulk ends. Pretend that is 14.5v and you start with 13.5v once the battery sees the charger (up from say 12.3 or whatever it was)

OK, so your range of output watts is 40 x 13.5 = 540w to 40 x 14.5 = 580w

"Efficiency" is watts out vs watts in (higher than out) so if 87% is it (here is another wrinkle-- note that MPPT controllers have more efficiency doing same to same voltage than higher voltage to lower voltages. So the efficiency of the DC-DC could well be like that, and depends on how much different the input voltage is from the output (house) battery voltage.)

Anyway pick 87% for this example and input watts will range from
540 x 100/87 = 621w to 580 x 100/87 = 667w

Now we need the input voltage, which will be from the truck's battery as regulated by the alternator, with some voltage drop. (which depends on the amps--higher amps, more drop)

So pick 14v as a likely engine battery voltage, 667/14 = 47.6 amps with no voltage drop.

If you have 1v drop on the wiring R, then 667/13v = 51.3 amps

That is only 3.7 amps difference with a voltage drop of 1 volt. If you had other watts from calculations with better numbers, it would still show not that much difference in amps from using fatter wire.

It would take a really big voltage drop to get 60 amps instead of 43 amps as was measured in that earlier post comparing installations. IMO there must be more to that comparison than we have data for, but you can see it is not as calculated above using the numbers I picked.

Anyway, IMO you could get the 40 amper and it would draw under the 60 amps and only briefly at that near the end of the Bulk stage. If you found it was over-tasking the alternator, you could choose to drop the output to 20 amps using that Renogy feature. If you got the 20, you can't make it into a 40.

You could use fatter wire from the Renogy to the engine batt, but as seen above, it is worth maybe 4 amps per 1 volt drop, so no need to go crazy on how fat to go.

Somebody who is better at these calculations can "check my work", but I think it is in the ball park.

not sure who your answering as you didn't use a quote, so it makes it hard to tell who you are refuring to,but if it is me then...

it doesn't put out a solid 40amps no matter what the input voltage is. right in the specs it list its max output in watts, not amps. the only reason for the amp rating is for comparason and yes at a spicific input voltage it will put out this many amps, but if that charge voltage it sends to the battery changes the output amprage will vary as it is limited by the wattage output.

Because the output voltage changes with the chemistry and "stage" the charge is curently in, the amprage changes with the voltage change. for example if your hard limit is 500 watts for an output, you can get there at 13.6 V, 14.6V, or 12.5V for the first you will have 37.76amps and for the second you will have 34.2 amps for the last you will have 40 amps. so if there using 12.5 as a referance voltaage for the standard it is a 40 amp (max) charger, but once again advertising tricks us as they left the "Max" out of the discription, but they did list the maximum wattage in the specs.

to get that on the output you would have to add the efficency in to get the input wattage to get that output and then do the same thing with your incoming voltage to get the new wattage and that actualy is how it works. of course there is line loss and such which like I said can be reduced with larger wire and such and yes that does cost money, but it will make your alt last longer so its a trade off that only the individual person can make. for me I have to run new wires anyways as the factory wire is way to small and the difference between a 4ga wire cost and a 0ga would be less than 50 bucks in my case.

Steve