Is it stupid or will it work? Charging aux battery through 400W inverter?

[Martinjmpr]

An old saying I heard in the Army was "If it's stupid but it works, it isn't stupid."

So I'm tapping into the collective wisdom here to see if this idea is stupid or if it will likely work.

First off, I originally posted this a couple months back as a response to this thread, because what I'm thinking of is similar:

Can I plug a trickle charger into a Toyota factory inverter.

My wife’s car a Toyota 4 runner has an inverter built into the cargo area. It says it is good for 400 watts. I have a 2 amp trickle charger I want to use to keep a spare battery topped off while driving. When I am parked the spare battery will be used for a fridge overnight. I Will plug the...

www.expeditionportal.com

Here's the situation: I have an Indel-B TB-41 refrigerator (Truckfridge TF-41) that we use when camping. The fridge sits in the cab of the truck (2018 F-150.)

My previous vehicle was a Chevy Suburban with a dual battery, and I ran the fridge off the "house battery" which was charged when the engine was on and isolated when it was not. Putting a 2nd battery in the engine compartment of my F-150 isn't possible so as an alternative, I made an inexpensive "power box" with a 12v, 90ah wet cell battery. The fridge stays hooked up to this.

My issue, then, is how to get power to the auxiliary battery to keep it charged up while on long trips? I actually purchased as Renogy 20a DC-DC battery charger last year with the intention of running a pair of wires from the factory battery back into the cab. My only dilemma currently is that I haven't found a good way to get the wire from under the hood into the cab. I do have a nice space under the back seat tha the DC-DC charger will fit neatly into.

HOWEVER, Something else occurred to me: Like many vehicles today, my F-150 came from the factory with a 400W inverter. The inverter goes to a 120vAC plug in the dashboard. This plug is ONLY powered when the ignition is on (that's an important part of my plan.)

So I got to thinking: Seeing as how I have 400W of usable AC power, why couldn't I just get a 120v AC battery charger and plug it into the inverter plug? The charger I'm considering is this one:

https://www.amazon.com/dp/B004LWTHP2/?coliid=I1ECG7FR5RNHPQ&colid=29PHGCD6GGEP&psc=1&ref_=lv_ov_lig_dp_it

Per the specs, this is a 7.2a charger that draws 132W.

Now I know the salty electrical gurus out there are probably clutching at their pearls at the thought of such a sloppy setup. After all, it seems silly to take 12v DC power from the charging system -----> Convert that to 120v AC ---------> Convert that BACK to 12v DC to charge the battery. Seems like there is probably a lot of wasted power.

I totally get that there are much more "elegant" ways to achieve the same goal (keeping my "power box" charged up.)

I totally get that this seems sort of "jury rigged."

But my question for the group, going back to the topic sentence here, is: As 'stupid' as this might be, WOULD IT WORK? And if not, why not? What am I not considering here?

Keep in mind that the inverter only works when the ignition is on, which in practical terms means the engine is running and the alternator is charging the battery. So I don't see much of a concern that the charger will draw down the truck's battery since it won't be drawing any power when the truck is turned off. As for the power draw, the 132W draw is well under the 400W capacity of the inverter.

I'll reiterate that ultimately I want to use the 20A DC-DC charger to keep the power box charged up. And while 7.2A is not 20A, it seems to me that a 7.2a charge ought to keep the power box charged up for several days, at least, assuming that I'm driving for 3 - 4 hours a day minimum. The fridge draws an average of 2 - 4a depending on how hot it is. On an extended trip, it seems likely that th 7.2A charger might not be able to keep up with the discharge of the battery that is constantly running the fridge.

But for a short trip - 3 to 5 days, I can't think of why this WOULDN'T work.

 

[Justin Cook]

But my question for the group, going back to the topic sentence here, is: As 'stupid' as this might be, WOULD IT WORK? And if not, why not? What am I not considering here?

My take: bottom line, sure it'll work. All the things you said, the inefficiency, the jury-rigging, etc etc, 100% agree... but it'll work.

Now, that being said, I will point out that a Victron IP65 Smart 12/7 charger only costs about $8 more, has bluetooth functionality that allows monitoring, settings adjustment, and will -in the next month or so with a FW update that's currently in beta testing- offer user-settable charge voltages, plus will work to charge a completely flat battery (the NOCO seems iffy about that) and has all the optional accessories you could possibly want... plus a 5-year warranty that's backed by Victron's authorized dealers/distributors, so if you have issues within five years you just contact the dealer you bought it from and get a replacement rather than having to deal with the company itself.

As is evident by my signature, I'm clearly biased, but it's something to consider. Otherwise, yeah man, go for it... not the first time I've seen someone doing exactly this!

 

[Martinjmpr]

My take: bottom line, sure it'll work. All the things you said, the inefficiency, the jury-rigging, etc etc, 100% agree... but it'll work.

Now, that being said, I will point out that a Victron IP65 Smart 12/7 charger only costs about $8 more, has bluetooth functionality that allows monitoring, settings adjustment, and will -in the next month or so with a FW update that's currently in beta testing- offer user-settable charge voltages, plus will work to charge a completely flat battery (the NOCO seems iffy about that) and has all the optional accessories you could possibly want... plus a 5-year warranty that's backed by Victron's authorized dealers/distributors, so if you have issues within five years you just contact the dealer you bought it from and get a replacement rather than having to deal with the company itself.

As is evident by my signature, I'm clearly biased, but it's something to consider. Otherwise, yeah man, go for it... not the first time I've seen someone doing exactly this!

Just sent you a PM.

 

[Martinjmpr]

UPDATE: So, doing a little research on the F-150 forum, it appears that the factory inverter is NOT a pure sine wave inverter. So am I right in thinking that trying to run a battery charger through a modified sine-wave inverter would not be a good idea? Would it be potentially dangerous (i.e. risk of fire) or just not work very well?

There are a couple of write-ups on the F-150 forum about replacing the factory inverter with a PSW inverter but the difficulty/complexity of that would seem to tip the scales back towards using the 20a DC-DC charger instead.

As I said, my MAIN objective is to get sufficient power to the aux battery to keep it charged while it is running the fridge. I'm looking for the simplest way to do that. If the only difficulty with the Renogy is running the power wire into the cab, that's something I can deal with.

 

[luthj]

There are varying levels of modified sine. Any idea what waveform or harmonic distortion is?

 

[Martinjmpr]

There are varying levels of modified sine. Any idea what waveform or harmonic distortion is?

Not certain if that is a "knowable" thing but over on the F-150 forum there's a lot of chatter that the factory "400w inverter" is a cheap POS and not suitable for much more than maybe charging a computer or camera battery.

As I said the Renogy DC-DC charger (which I already have) is starting to look like the better option to keep the fridge battery charged up. The biggest obstacle to that is getting a power wire into the cab but really a few minutes with a drill and a decent rubber grommet and that problem is solved.

 

[gtitonta]

For modified sine inverter, it might work (it depends on the charger), but definitely not efficient to supply power to the charger. I think that hard wiring B2B is best option since you already have. However, you still do not want to drill hole and hard wire them, you can use small pure sine. Hook them to cigar lighter and use charger. like this,

https://www.amazon.com/BESTEK-300Wa...+inverter&qid=1586465703&s=electronics&sr=1-3
This inverter is pure sine $45 producing 150w from hooking cigar lighter. If you hook this by hard wire, it will produce 300w. I believe that 150w is enough for NOCO, right? Then, you can hook NOCO charger you mentioned to charge your battery. This is the easiest way to charge battery forr you. However, it is not 20amp like B2B so it is slow to charge like you mentioned. For this option, you can use this inverter for other stuffs like charging sensitive devices since it is pure sine inverter. Also I believe that your cigar lighter is key hot, right?

 

[calicamper]

Not certain if that is a "knowable" thing but over on the F-150 forum there's a lot of chatter that the factory "400w inverter" is a cheap POS and not suitable for much more than maybe charging a computer or camera battery.

As I said the Renogy DC-DC charger (which I already have) is starting to look like the better option to keep the fridge battery charged up. The biggest obstacle to that is getting a power wire into the cab but really a few minutes with a drill and a decent rubber grommet and that problem is solved.

Is your “inverter house plug like my Expedition and in the 2nd row?” Do you use it? Is the fridge in the 2nd row? Or out back?
The solar controllers don’t really care what power is fed to them as long as its not more than their ratings. Why not pull the cheap 400W out they are small about the same size as a Solar controller and drop the solar controller in the same spot using the same power drop the 400w was using? Then replace the house socket with a nice plug you can plug the porta battery box into? The charge controller runs when it has power and tops off your porta box?

 

[luthj]

400W is probably the surge limit? Lots of folks don't realize how much startup power certain devices need. About the only way to figure out for yourself is to put a scope on the output of the inverter.

Most switching mode power supplies can handle MSW pretty well. They are just rectifying the AC to DC and putting it through a DC regulation circuit. You are unlikely to cause any damage that way.

 

[sg1]

I have a 2015 F 150 with the 400 w inverter. I tried to hook my 15 A Victron 110 V battery charger up to it. In theory the inverter should have handled the load easily. 15 A at 14.5 V are about 220 w. Every time I plugged in the charger the inverter signaled overload and stopped working. Even when I limited the charger to 7 A. Apparently the charger draws a high current for a split second to get started and that causes the inverter to shut down. Long story short it didn't work.

 

[Ovrlnd Rd]

I did something similar when I ran the IDBDR. I have an ARB fridge that I ran off the AUX A/C plug in the back seat of my Titan. This was fine when the truck was running but overnight I switched it to my AUX battery in the bed of the truck. Every couple of days I'd hook my NOCO charger up to the inverter plug in the bed of the truck and recharge the AUX battery while driving. Definitely more effort and very inefficient but I don't have room for a 2nd battery in the engine compartment and don't want one in the bed except when I'm out in the wild.

 

[Martinjmpr]

I have a 2015 F 150 with the 400 w inverter. I tried to hook my 15 A Victron 110 V battery charger up to it. In theory the inverter should have handled the load easily. 15 A at 14.5 V are about 220 w. Every time I plugged in the charger the inverter signaled overload and stopped working. Even when I limited the charger to 7 A. Apparently the charger draws a high current for a split second to get started and that causes the inverter to shut down. Long story short it didn't work.

From the stuff I'm seeing online it may not so much the wattage limit but the the "dirty" 120v AC power provided by the modified sine wave inverter as opposed to "clean" 120v AC that a pure sine wave inverter would produce.

Of course I could convert my MSW inverter to a PSW, but that's even more $$ to spend and much more complexity (because it involves cutting wires and soldering new connectors on, etc.) If it was a simple swap-in, I'd consider it, but it's much more than that.

Based on this I'm going to go ahead and scrap this idea and go back to figuring out how to install the DC-DC charger. Since we all seem to have our weekends free now thanks to Coronoavirus I will probably make this my new project. 20a of charge going into my aux battery when the truck is running should be more than enough to keep it topped off. 40a would have been better but would also require me to run heavier wires. For my needs, 20a is likely to be 'good enough to work.'

Thanks for all the input everybody! I've learned a lot on this forum!

 

[Mfitz]

From the stuff I'm seeing online it may not so much the wattage limit but the the "dirty" 120v AC power provided by the modified sine wave inverter as opposed to "clean" 120v AC that a pure sine wave inverter would produce.

Of course I could convert my MSW inverter to a PSW, but that's even more $$ to spend and much more complexity (because it involves cutting wires and soldering new connectors on, etc.) If it was a simple swap-in, I'd consider it, but it's much more than that.

Based on this I'm going to go ahead and scrap this idea and go back to figuring out how to install the DC-DC charger. Since we all seem to have our weekends free now thanks to Coronoavirus I will probably make this my new project. 20a of charge going into my aux battery when the truck is running should be more than enough to keep it topped off. 40a would have been better but would also require me to run heavier wires. For my needs, 20a is likely to be 'good enough to work.'

Thanks for all the input everybody! I've learned a lot on this forum!

I have a 2015 F150 w/ the OEM inverter and it doesn't work with a number of electronics that should be within its range- cordless tool battery chargers, for example. You are right with the idea of "dirty power" and the 400w may also be a stretch by Ford. I have a higher quality plug-in inverter of the same rating that will charge tool batteries through the 12v port if necessary.

 

[Martinjmpr]

I have a 2015 F150 w/ the OEM inverter and it doesn't work with a number of electronics that should be within its range- cordless tool battery chargers, for example. You are right with the idea of "dirty power" and the 400w may also be a stretch by Ford. I have a higher quality plug-in inverter of the same rating that will charge tool batteries through the 12v port if necessary.

Yeah it seems like Ford put the cheapest POS inverter into the truck just so they could say 'It has an inverter!'

I have a 150w inverter that plugs into the 12v power outlet that probably would work as well. We use it to charge camera batteries.

 

[Martinjmpr]

Well, I'm putting this idea to bed. I finally bit the bullet and drilled a hole in the cab of my F-150 (this whole exercise was in an attempt to avoid having to do that.)

Waiting for some electrical parts and cable from Amazon and then I should be able to put this together fairly quickly. I'll do a separate write-up for those who may be interested in following along.

Thanks for all the advice and input from the experts here!