TAG 320 lithium battery conversion

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• Grumpy_G Member Posts: 687

  March 3

  Way to open a can of worms ... There is only one voltage regulator and it is designed/computer controlled for the type of battery in the tow vehicle. LiFePo batteries need a higher charging voltage so they won't be fully charged. They also have a higher resting voltage when fully charged (e.g. via solar or a dedicated charger at home) so they can backfeed into the tow vehicle. That is not an issue as modern vehicles can ramp up voltage to over 14.5V so the electronics are designed with that in mind. 
  DC-DC converters do solve the charging issue but also drastically increase the current draw from the tow vehicle which typically blows the trailer power fuse. That's why people end up installing dedicated trailer charging wiring in the tow vehicle.  
  
  

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• eddycurrents Member Posts: 3

  March 4

  Danke schön very much. This corroborates my existing bias. Re/ the fuse blowing issue, I presume you're talking about the excess load presented by the trailer, rather than the DC-DC converter's own internal circuitry, which I imagine would only add negligible current draw like any other switch-mode power supply..?

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• pthomas745 Administrator, Moderator Posts: 4,490

  March 5

  Millions of owners travel with basically the same "stock" alternator setup with no issues.  The "exploding car alternator" videos on You-Tube, etc, have been around for years now, and I've tried hard to find any examples of it actually happening.  

  If this was such a big issue: car manufacturers and battery companies would have warned us about this sort of thing from the very start.  (I could be wrong....since I really haven't wanted to study new car manuals much these days!)

  2017 Outback

  Towed by 2014 Touareg TDi
  

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• HikerTravelers Member Posts: 6

  March 5

  I can't imagine the 7 pin's 12-14 gage wire will/could pass enough current to cause an alternator issue.  I'd agree with Grumpy that in most cases current would pass from the lithium battery to the TV, which was my experience.  A lithium iron phosphate battery at 10% SOC is still around 12.5 volts.
  
  I suspect horror stories about burning up alternators are from folks running heavy gage wires directly from an older "non-smart" alternator attached directly to a larger lithium battery setup.
  
  My 2016 Honda Pilot would barely offset the draw from my 12v fridge when towing with the original lead acid deep cycle battery via the 7 pin plug.  I installed a lithium iron phosphate battery along with a 30 amp DC-DC charger. It is connected to the TV battery via a 4 AWG cable, then connected to the trailer via an Anderson plug.  I discovered quickly that the DC-DC charger back fed current to the TV and I ended up pulling the 7 pin power fuse.  The Honda alternator's voltage throttles back and was so irregular that turning on the headlights was the only way to keep the Victron software on and charging the battery.  I picked up a digital voltage reading USB phone charger and it frequently goes below 12.6 volts without the headlights on.
  
  The 30 amp Victron smart charger draws up to 45 amps and I have a 60 amp circuit breaker near the battery.  It all works well and typically get 26-28 amps going down the road when the battery is depleted.

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• Grumpy_G Member Posts: 687

  March 6 edited March 6

  eddycurrents said:

  Danke schön very much. This corroborates my existing bias. Re/ the fuse blowing issue, I presume you're talking about the excess load presented by the trailer, rather than the DC-DC converter's own internal circuitry, which I imagine would only add negligible current draw like any other switch-mode power supply..?

  The issue arises from how the DC-DC converter tries to reach the target output voltage. Say it is set to put out 10 A at 14.4 V, that is 144 W. If it receives 12 V from the tow vehicle it would try pull 12 A (ignoring losses for simplicity). So far so good but in most vehicles the wire run from the fuse box to the connector at the rear is fairly long so voltage drop raises its ugly head. The DC-DC converter tries to pull 12 A but if doing that drops the voltage to e.g. 11 V it now increases the current draw even further. Which in turn drops voltage even more (voltage drop is proportional to current) until either it reaches its lowest operating voltage or the fuse blows. 

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• eddycurrents Member Posts: 3

  March 13

  My bad, and with considerably less excuse than most.
  
  I wasn't thinking in those terms on account of having dealt mostly with the kind of current you encounter in electronics, which is generally negligible except in audio output circuits.  So I never consider that the resistance in 15' of light-gauge power supply wire has has no meaningful effect in matters of mA, but gets critical when you remove the "milli" thanks to Mr. Ohm and his damned Law.
  
  Shame on me, and thanks.

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