Beware the (voltage) drop

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

  • Bill&SuzeBill&Suze Member Posts: 87
    Interesting that the same voltage drop presumably would have existed in his original setup, yet the AGM battery's charging parameters allowed for that. And presumably both his original controller and the new Victron 75/15 were located at the portable panel and not in the camper. When you mention better wiring as a fix for his problem, what do you have in mind, heavier gauge wiring? What's the existing wiring gauge and what is the length of cabling? Given that there will always be a voltage drop over 20-25 feet of cable, it would seem that the overall best solution would be to install the controller at the camper near or adjacent to the battery. I think that a number of forum members have mentioned doing just that with their suitcase or portable panels. Thanks for your explanation of the problem and for running through the thought process!
    Susan & Bill, Yarmouth, Maine
    2024 T@B 400 Boondock Black Canyon
    2024 Kia EV9
  • manyman297manyman297 Member Posts: 1,363
    Yeah, voltage drop (especially on low voltage systems) is a definite concern. We have a 12v cooler we power using the 7-pin connection on the camper. You wouldn’t think there would be much voltage drop from the batteries under the bed to the front of the camper but I was getting a 2v drop in that short span. Part of it is the thinner wire I used to make my 7-pin adapter to power the cooler but the drop is just amplified in low voltage systems.
    2021 400 BD
    2020 Tacoma TRD Off-Road 
  • AnOldURAnOldUR Member Posts: 1,419
    edited December 14
    We've had the same problem @manyman297. The voltage drop from higher draw accessories like a 12V chest fridge is a problem. I replaced our 3-way with a 12V using the dedicated wire that fed the 3-way. I still get two or more volts drop. It hasn't been a problem. The cheap fridge has lasted four seasons. It has settings for low voltage before shutting down. I bought a different brand as a second fridge, but found it shut down too often because it thought the battery was low when it was actually voltage drop and there was no way to adjust for it.
    Stockton, New Jersey
    2020 nuCamp T@B 320S * Jeep Wrangler

  • ckjsckjs Member Posts: 74
    …. We have a 12v cooler we power using the 7-pin connection on the camper. You wouldn’t think there would be much voltage drop from the batteries under the bed to the front of the camper but I was getting a 2v drop in that short span...

    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?

    Charles & Judy, Santa Cruz, CA
    2018 T@B 320 CS-S; Alde 3020; 4 cyl 2020 Subaru Outback Onyx XT
  • pthomas745pthomas745 Moderator Posts: 3,986
    edited December 14
    I have an Alpicool 20 QT 12v portable fridge, and even connected to a lithium battery on a very short cable it can stop working when it senses a "voltage" drop of almost any kind.  The Alpicool has "adjustable" sensitivity settings for this sort of thing, and it helps, but I would guess the fridge itself is simply not designed well to handle this sort of thing.

    2017 Outback
    Towed by 2014 Touareg TDi
  • Grumpy_GGrumpy_G Member Posts: 546
    Bill&Suze said:
    Interesting that the same voltage drop presumably would have existed in his original setup, yet the AGM battery's charging parameters allowed for that....
    I was pondering that as well. I don't have the original Zamp charge controller or an AGM battery to test but here is my theory: According to the Zamp documentation the absorption voltage for AGM is 14.4V (higher than the 14.2V of the Victron) and the absorption time is 4h (twice as long as the Victron). Add to that that lead-acid batteries taper the charge current as they get closer to full (remember voltage drop is proportional to current), and the controller will spend at least some time in bulk and also twice as long in absorption mode. That might just be enough to keep the battery topped off. 

    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. 

  • elbolilloelbolillo Member Posts: 421
    @Grumpy_G I am not sure what brand of LiFePo4 battery is being used, but the recommended absorption charge for BB is 14.6 (Acceptable range of 14.4-14.6)
    _____________________________________________________
    Ken / 2023 Tab 400 “La Bolita” (23,000+ miles) / 2024 Toyota Sequoia
    2024 - 3 Trips - 35 nights - 9 National Parks, 3 National Forests
  • Grumpy_GGrumpy_G Member Posts: 546
    elbolillo said:
    @Grumpy_G I am not sure what brand of LiFePo4 battery is being used, but the recommended absorption charge for BB is 14.6 (Acceptable range of 14.4-14.6)
    The battery is a "Powerurus" brand typical Chinese LiFePo battery. As I wrote above increasing the absorption voltage can at least delay the transition from bulk to absorption but the timeout issue remains unless manually changed as well. 

    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. 
  • elbolilloelbolillo Member Posts: 421
    Grumpy_G said:
    elbolillo said:
    @Grumpy_G I am not sure what brand of LiFePo4 battery is being used, but the recommended absorption charge for BB is 14.6 (Acceptable range of 14.4-14.6)
    The battery is a "Powerurus" brand typical Chinese LiFePo battery. As I wrote above increasing the absorption voltage can at least delay the transition from bulk to absorption but the timeout issue remains unless manually changed as well. 

    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. 
    Yeah, LiFePo4 batteries don't need a float charge since there is very little self discharge. It seems that most chargers still have the Bulk/Absorption/Float routine. BB just recommends to set Float to 13.6v.
    _____________________________________________________
    Ken / 2023 Tab 400 “La Bolita” (23,000+ miles) / 2024 Toyota Sequoia
    2024 - 3 Trips - 35 nights - 9 National Parks, 3 National Forests
  • manyman297manyman297 Member Posts: 1,363
    @ckjs My guess is the wire I used in the adapter is too thin…possibly 22 awg. Not sure. The cooler has a low voltage setting so it still works fine. And with my new lithium batteries will help offset that low voltage situation a little bit. 
    2021 400 BD
    2020 Tacoma TRD Off-Road 
  • AnOldURAnOldUR Member Posts: 1,419
    ... And with my new lithium batteries will help offset that low voltage situation a little bit. 
    Been wondering about this. I still have the old 12V cooler, but haven't tried it since switching to lithium. The only thing I did before replacing it was to shorten the cord in hopes of lessening the voltage drop. It helped, but it wasn't enough to consistently prevent it from shutting down.

    Stockton, New Jersey
    2020 nuCamp T@B 320S * Jeep Wrangler

Sign In or Register to comment.