I had drafted a lengthy post but I figured most people are visual learners
This started with a friend and fellow tabber asking me to help with his persistent solar charging issue. He has a 160W Zamp portable panel that he had used for years with an AGM battery. After replacing the battery with a LiFePo type and changing the Zamp controller to a newer one with LiFePo setting, charging would stop seemingly at random long before the battery was fully charged (the battery has bluetooth monitoring). It would restart after unplugging/replugging the panel from the trailer but stop charging again. He then changed the charge controller to a Victron MPPT 75/15 but that did not fix the problem. The absorption LED would come on within a few seconds after plugging in the panel even with a fairly discharged battery. I tracked it down to the voltage drop in the cable from the panel to the trailer.
Below are two pictures showing my verification setup using a Victron Blue Smart charger instead of the MPPT controller. The first picture is the charger directly connected to the battery, the second picture is with the extension cable in between the charger and the battery (yes it looks like I hooked it up wrong, don't get me started on those stupid SAE connectors...). Note the charger went into bulk mode in the first picture but absorption mode in the second. Absorption mode times out after 2 hours and the float voltage is too low to charge the battery. Basically his battery was only charged for 2 hours a day.
The reason is that voltage drop affects both ends of the connection. The battery tries to pull as much current as it can and draws the voltage down to 13.5V on its end. Because of the wiring resistance the charge controller detects a higher voltage on its end and decides to go into absorption (14.21V is above the default absorption threshold of 14.2V).
There are a number ways to fix this, better wiring, move the charge controller close to the battery or changing the configuration of the charge controller (higher absorption voltage, longer absorption time).
Comments
2024 T@B 400 Boondock Black Canyon
2024 Kia EV9
2020 Tacoma TRD Off-Road
2020 nuCamp T@B 320S * Jeep Wrangler
That’s a too-dang-high voltage drop for the situation.
Guessing that the cooler draws 5 amps gives the wire a connector resistance as 2V/5A or 0.4 ohms. Estimating 25’ of wire (12-ish there and back again) gives the per-foot resistance at 0.4/25 -> 16 milli-ohms per foot. That would match 22 gauge wire, which is pretty thin. 16 gauge copper zipcord is only about 4 milli-ohms per foot and ought to be good. Ideally it would have a ½ volt drop here.
Perhaps a connector is too loose or tarnished?
2018 T@B 320 CS-S; Alde 3020; 4 cyl 2020 Subaru Outback Onyx XT
The original wiring is 25ft of 12AWG wire. The Victron MPPT accepts 10AWG so a new quality copper wire would reduce the voltage drop. We'll probably relocate the charge controller to the tub near the battery which is the proper solution. The solar panel generates higher voltage at lower current so with the original wire it would only minimally reduce the wattage from the panel. Another option is to do a custom configuration on the charge controller with higher absorption voltage and longer absorption time.
Ken / 2023 Tab 400 “La Bolita” (23,000+ miles) / 2024 Toyota Sequoia
2024 - 3 Trips - 35 nights - 9 National Parks, 3 National Forests
This brings up an issue I found somewhat puzzling which is that many charge controllers use the concept of absorption charge and float charge with LiFePo batteries. Most battery manufacturers just state a single charging voltage of 14.4V or 14.6V and that is all that is needed. For a lead acid battery you need some intelligence in the charger to avoid overcharging but for a LiFePo battery the intelligence is in the BMS.
Ken / 2023 Tab 400 “La Bolita” (23,000+ miles) / 2024 Toyota Sequoia
2024 - 3 Trips - 35 nights - 9 National Parks, 3 National Forests
2020 Tacoma TRD Off-Road
2020 nuCamp T@B 320S * Jeep Wrangler