package pickup, solid charging

the way there

My amazon package came in so I bopped into town to get it. It was completely overcast this morning so the mounted solar was barely treading water.

I cranked up the engine and the alternator provided ~30A of charging, quite welcome under the weather circumstances.

in town

the way back

It was still overcast but the sun was higher so I was now getting net +10A into the bank from solar alone. Started the engine to head back and got ~40A. Solid.

I hit a patch of open skies for about 5 minutes and the controller maxxed out at 45A. Net into the bank was 68.2A; after 10A of normal background loads this suggests the alt was still contributing ~13A.

When the patch of sun was over the alternator returned to ~28A of contribution.

direct-charging and the voltage delta

When solar and DC-DC charging are combined in Bulk both will output their rated current (2+2=4). With a plain relay the alternator contribution will generally decrease, so we end up in a 2+2=3 situation. .

Current is a function of total resistance and the voltage delta between the battery’s resting voltage and the alternator’s open circuit (“not charging the battery yet” voltage.) DC-DC can tweak the charger’s output voltage to increase current, but the alternator voltage will always be the alternator voltage. Thus direct-charging from the the alternator results in a current taper as bank voltage increases.¹

mppt early float

After a few minutes of strong charging I noticed the battery’s charge acceptance had decreased from ~60A to ~28A. I pulled up the Victron app and confirmed the MPPT was in Float.

The combination of all that current pushed apparent bank voltage to Vabs. MPPT contribution tapered quickly while the alternator stepped up; the end current target for Absorption was immediately met.

heat gun

One of the things I got from Amazon was a low-wattage heat gun. I’m thinking about re-terminating the cables running to/from the voltage-sensing relay. I have a hydraulic crimper now and I think I can do better.

1. and therefore delta decreases ↩