How much remaining alternator capacity?

[neatvan]

My question is:
Is there a way to (somewhat) accurately determine the amount of available remaining amperage capacity of my alternator?

I ask this question to determine what current limit I should use for charging my lithium batteries and not burn up the alternator.

Here is an example of my current thinking. This is a factory alternator sized to keep the starting battery charged and factory electronics running. If the headlights take 10Amps on high beam, I would presumably have 10 Amps of available capacity with the lights off. I could continue down this line of thinking and turn everything on, measure current, and back calculate, but is there a better or easier way?

I can look at the alternator and find specs regarding max current, idle current, etc. but I'd like to to know what the van currently requires/uses in order to determine capacity available for my house batteries.

Although I hope this question can remain general, my specifics are:
2005 E350 V10
Renogy DCC50S MPPT charger (can set charge limits from 10-50 in 10A steps)
2 100AH Renogy lithium house batteries in parallel

Thanks in advance.

[BroncoHauler]

Two items to remember about your alternator, any alternator, is that the stated output is the maximum output at a particular RPM, and that the alternator output goes down due to ambient heat. Engines generate a lot of ambient heat.

I would start by finding out what alternator you have and find the associated output specification sheet.


Herb

[Scalf77]

My general quick checkout procedure is run van at idle with lights on, heater of air-conditioning on high and any other 12 volt items that you run while driving, this could be stereo or entertainment system. You want a reasonable worst case power usage off of the van 12 volt system.

Then I would turn on the DC to DC charger, and monitor the output of the alternator. If the voltage level is holding, you will be supplying enough power. If the voltage level drops then you might want to back off the current setting on DC to DC to where it does not drop the voltage.

-greg

[neatvan]

Thanks Greg, that's exactly the sort of thing i was looking for!

Any recommendation on allowable voltage drop before considering it too taxing for alternator?

[Pntyrmvr]

I would prefer to run down the road all day with no more than a 50% load on the alternator. Peaking near maximum for a few minutes is acceptable, but they were designed to top up batteries not be generators running flat out constantly. And then, as noted above, there’s the self heating effect under a jammed packed van hood.

[Scalf77]

You don't want to see much of any drop in voltage. There will be a quick drop as you turn on the DC to DC, it should recover.

To put this in some perspective most ACR's or automatic isolators will disconnect at 12.8 and connect at 13.2. Also a 50% SOC 4D battery will pull around 50 amps. If you noticed your ACR dropping out at idle, then you will probably have to throttle it back some.

The reason we try to set this up at idle, is that it will be your worse case condition as far as alternator output. If set up correctly you should be running down the highway in good shape.

Not sure about your unit but many DC to DC have a built in pause, to reduce load for a bit, and adjust output on the flay also.

greg