I plan on dry camping with my FEH and a small trailer which has a deep cycle battery. The deep cycle battery will need to be charged every couple of days.

Can I just attach jumper cables between the FEH 12 volt battery and the deep cycle battery and turn the car to the "ON" position without running the engine? I have a Scan gauge, so I can read SOC on the traction battery.

My objective is to not run the FEH needlessly, yet of course, I don't want to flatten the traction battery, either.

Anyone tried this?

 

I haven't tried that but I don't think that one 12vdc battery could sufficiently charge another 12vdc battery, the charging voltage just isn't high enough. I'd probably take my 110vac battery charger plugged into the accessory outlet and use that to charge the camping battery. Maybe not very efficient but you would get a full charge that way.

Maybe someone will have a better idea.

 

When the ignition is on, the DC/DC converter is operational and you are supplying 14.5 volts to both batteries from the traction battery, even if the engine is not running. You can verify this yourself by measuring voltage at the cigarette lighter socket with the ignition in various positions.

 

As you said, the HV battery should feed the 12v battery as long as the key is in "on" position. There are a couple of threads where someone calculated the watt-hours of the HV battery that are accessible. You should be able to take those numbers and the size of the deep cycle battery and figure out what it will take to "refill it". It would probably require you to run the engine a little bit to keep the HV battery in its happy range.

https://www.greenhybrid.com/discuss/...15/index2.html

Basically, he figured
"5.5Ah pack x 330v x 13% useable ( Engine on at 40% SOC, off at 53% SOC )"

That works out to about 150 watt-hours (at 12v)

Here's some other useful figures:
https://www.greenhybrid.com/discuss/...c-plugs-12702/

 

Just as a followup.....I had an opportunity to test this theory out. My tractor had a dead battery, so I attached jumper cables to my FEH, then turned the key into the ON position without starting it. I observed that the voltage on the Scan gauge jumped to 14.2 v, SOC was over 50. Later I came back to check on it and the found that the voltage was now 12.1 and that the SOC had fallen to around 40, so I assume the FEH kicks off the DC/DC inverter when SOC falls to some limit.

The tractor battery was sufficiently charged to start it. Given the small amount of gas that it uses while idling, I would probably just idle it another time, but it was interesting to see what the limits are.

 

Thanks, l've got a ac invert I plan on using if our power goes out.

 

The proper way of doing this is to use a DC/AC inverter (400W, ~$40.00) and then a GOOD, not cheap, battery charger connected to the inverter's AC output. Otherwise the jumper leads to the deep cycle battery will drop enough voltage that the deep cycle battery will take an excessive long time to FULLY charge.

RV's with a TOAD often experience this very same problem. The 12 volt line from the RV battery to the TOAD battery will suffice to maintain a charge but will NEVER "top-off" the TOAD battery charge as would be needed in your case with a deep cycle battery.

 

In which case the "jumpered" battery would also never reach a "FULL" charge, especially so for a deep cycle battery.

 

Well, the electrical efficiencies of an inverter to a transformer to a rectifier, or a high speed SS switch won't ever catch up to a bigger piece of wire. Cheap #8 or #6 jumper cables 25' long have never been a good idea, and maybe some electrical basics like those need to be emphasized.

Absolutely, undoubtedly, unequivocally, own and carry a good set of jumper cables, not less than #4 copper with sturdy strong bronze or copper clad teeth and no more than 18' length. Any conductor or device is no better than it's connections: minimize them. If available power is limited: DON'T WASTE IT. If you wish to buy a good battery charger do so, and select, store, and treat it like the valuable tool it is, and consider the expense of time and money it will save you. DC/AC Inverters are power handling devices and though their prices have dropped and availability has risen over the last few years, Moore's law has nothing to do with it. If you can't recite Ohm's law in your sleep, and explain it to a child, keep working on it or stay the heck away from electrical stuff. At least know the meaning and correct use of the terms "watt" , "watt/hr" and "VA" in DC electrical calculations.

I hope none of this comes across poorly, I'm trying to state facts clearly and give good, solid advise to those who may need it! Even those that had good high school science teachers may have been distracted or absent when this sort of stuff was covered. All the data you could want is available online. I won't defend 18' #4 jumpercables, do the math. I don't care to handle anything substantially bigger than that if I'm not on the clock, but to each his own.

 

Yet another problem is that most modern day automotive battery charging system will NEVER top off the battery charge. In order to extend the battery life they are now never topped off, only charged to the level required to facilitate 1 or 2 starting cycles. And with modern day reduction geared starters, EFI, and direct ignition it doesn't take much battery power for an engine cold start.

Probably especially true of an HSD battery since there is never a starter equivalent load.

So don't be surprised if the HSD system doesn't charge your deep cycle battery beyond 50%