You are seeing voltage drop similar to what I got. I think that is why folks that are serious about charging off the alternator run a separate 4 ga to the camper batteries.

Steve

 

You dropped 11.57 volts or dropped TO 11.57 volts. I don't know much about resistors.

It seems like a good thing that you saw 16 amps.

 

Yeah, that may be where I’m headed, something with lug terminals on one end and an Anderson powerpole connector on the other:

https://powerwerx.com/dual-conductor-custom-cable

On our old pop-up camper, I used the truck to run the small 3-way fridge, and PV to separately charge the 2x Group 27 FLA batteries. I could do something similar here, but you can’t always count on nice sunny days here in the Southeast.

I could run the fridge on LP while traveling, but I’ve never been comfortable with the idea, especially when stopping for fuel.

I may bring the camper up a week earlier than planned to give me extra time to measure, order, and install.

Never a dull moment!
Jim / crewzer

P.S. I added some pics to my previous post above.

 

It dropped to 11.57 V across the resistor. The drop in the wiring to/from the resistor was 2.43 V; that’s too high.

16 A is a good thing, but 11.57 V won’t charge a discharged battery.

Jim / crewzer

 

Exactly, the same thing I found. If you want to keep the charge lead voltage coming in up, it is best not to put too heavy a load on it. Even just a 3.5 amp load drops the voltage down to under 13.5 VDC in my truck camper.

Steve

 

This was posted on the DIYSolarForum today:
Jim, your test wasn't checking to see how hard the alternator was actually working, correct? You were focused on how much current/volts is actually getting to the RV battery. Is it safe to imply that the alternator wasn't doing much? It seems to me that the above statement that I quoted is complete BS, specifically the part in bold.

 

This video has the DIYSolarForum in a bit of a tizzy.


My response was:

 

I used #2 wire and a 125a switching controller. Our truck has the dual alternators (397a) and 2 batteries, camper has 3 batteries, and we tow a boat with 2 more batteries that is also connected to the truck (7 batteries total). We can run down the camper batteries with the inverter, and the boat batteries while fishing. And the truck will recharge the system during a half day driving.

The 7 pin does a good maintenance charge for keeping batteries topped off. But you need a lot bigger wire if you plan on actually recharging a bank of batteries on the road.

 

Yes, the alternator was not doing much in that experiment.

As a practical matter, the fuse in the +12 battery charge circuit (typically 30 A) would blow if there was too much charge current.

Another problem with the video experiment is the missing “pulley ratio”. Alternators are typically pulleyed at an ~3:1 ratio where the alternator spins at ~3x engine speed. The higher alternator speed helps produce more current at low engine speed and the higher internal fan speed improves cooling.

Adding this all up, I can see where trying to charge a 300 Ah lithium battery from an ~100 A alternator operating at low speed could be a problem.

The current specs for our truck’s 240A alternator is inserted below; note the pulley ratio.

HTH,
Jim / crewzer

 

I don't understand where people are coming up with the conclusion that a LiFePO4 battery is going to kill your alternator, or that the charge from the tow vehicle is going to kill your LiFePO4 battery.

 

Excellent!

Thanks,
Jim / crewzer

 

Looking through Battle Born’s battery manual, it seems to me that their BMS should do a pretty good job of protecting the battery and loads.

https://1t1pye1e13di20waq11old70-wpe...n-Manual-1.pdf

However, the recommended bulk/absorption voltage spec is 14.6 V, which is probably not feasible from a stock alternator and trailer wiring harness, especially when accounting for voltage loss in the harness and connectors.

Finally, I suspect someone has figured this out, as Lance is now offering a lithium battery option for their campers.

HTH,
Jim / crewzer

 

Duplicate post deleted...

 

Host has offered a lithium option for a while in their options. I think folks get tangled up in the Li+ battery being able to handle more amps while charging. If running the fridge and wanting to charge the batteries while going down the road, there is a case to be made for AC/DC fridges as they are only cycling the compressor for a portion of the time they are operating and I believe the amp is less than the absorption models.

Steve

 

Jim, don't quote me on this because I sort of glance over it in forums seeing that I don't charge my lfp batteries with the alternator and if I'm getting any charge through the 7 pin connector it's very little if any. The solar on the 5th wheel charges even going down the road. But what I recall in the boat forums where folks are changing out lead for lfp they're putting in larger battery banks. As you know a lfp battery will take what ever you throw at it charging all the way to 99%. What was happening is the alternator kept charging at max and cooking itself. Folks in motorhomes I believe had the same issues and change out the bim (I think that's what it is) that was for lfp.

There are more robust alternators out there but come at a hefty price tag.