Solving a problem with off the shelf modules

[theoldwizard1]

I'll try to make this short ...

Most travel trailers (caravans for those outside of NA) and motor homes have a "house" battery.  Because modern automotive systems have charging logarithms built into the powertrain control module that assume the stock battery capacity taht keep the charging voltage low (13.5V)  and there is measurable voltage drop back to the house battery, they never get charged properly.

Most of these RVs now come with a AC-DC power converter and a "smart" charger.  Many RVs also now come with built in AC sine wave inverters.  Anyone how has done anything with an inverter knows that they drop out somewhere between 10-11V so trying to run it off of the vehicles charging system is likely going to be a problem.

So hear is my idea. Inside the RV, take the 12V line from the vehicle and run it through a DC-DC boost converter (Cheap on eBay.  They nee a housing and cooling !)  Set the output to about 14V.  Now, insert a DPDT relay between the house battery (input) and the inverter (output) and the output of the DC-DC converter as the second input.  The relay control is voltage from the vehicle.

Finally, through another switch/relay (plug and receptacle ?) wire the output of the inverter and the shore power as either-or inputs to AC-DC converter charger.  Lots of details on sizing (typical RV AC-DC converter/charger puts out about 35A max so an 800-1000W inverter should power it), losses, etc. but ... AM I CRAZY    


(In a previous life over 15 years ago, we could never solve the problem.  The house battery would never get fully charged by the end of a day of driving.)

[ovnr]

Uh. So to be clear, you want to switch an inverter between being supplied from the car and the caravan battery. You then want to switch a charger between the inverter output and short power. Right?

If so, no major issues. You should add another relay so that it doesn't try to charge the battery from the battery (caravan disconnected from car, inverter runs from battery in caravan, inverter supplies battery charger which charges the battery).

It'd be lossy as all hell. Also, you cannot really run an inverter off of a DC/DC brick.


I'd recommend trying to find a 12V-powered battery charger. Something like this, just one that auto-starts, and preferably has a lower minimum input voltage. Run that off of the vehicle power to charge the battery. Buy a beefy 15V PSU to run it off of shore power; yes, you can use a relay to switch between the 15V shore power and the 12V vehicle power.

I suggest using a relay to disconnect the battery from the charger if the charger is not powered.

[theoldwizard1]

Also, you cannot really run an inverter off of a DC/DC brick.

Why ?

I'd recommend trying to find a 12V-powered battery charger.

They do exist, but I have not been able find one that does proper deep cycle, lead acid, multi-stage charging AND will accept input voltage down to 10V !!

[ovnr]

Also, you cannot really run an inverter off of a DC/DC brick.

Why ?

Well, a 800W inverter will take what, 75 amps with losses? A 75A boost converter going from 10 to 14 volts is going to be really hard to find. And since most inverters are boost converters internally (though maybe not with the typical boost topology), that just creates a lot of extra losses and costs a fair bit.

They do exist, but I have not been able find one that does proper deep cycle, lead acid, multi-stage charging AND will accept input voltage down to 10V !!

Build one?

[ovnr]

And on "build one", see page 27 in the LT8390 datasheet for a 25-amp 9-36V to 12V buck/boost converter. Stick on a micro, a DAC for the setpoint and some current/voltage sensing, and voila, you've got yourself a pretty fancy charger.

[theoldwizard1]

And on "build one", see page 27 in the LT8390 datasheet for a 25-amp 9-36V to 12V buck/boost converter. Stick on a micro, a DAC for the setpoint and some current/voltage sensing, and voila, you've got yourself a pretty fancy charger.

A really GOOD deep cycle lead acid battery chargers uses at least 3 stages.  I don't want to spend half of my remaining lifetime developing those algorithms and programming something that already exists.

[djacobow]

I'm a bit confused by the premise of the question. How does the car's voltage regulator control the rate of current to charge / not charge the car's battery, except by varying the voltage? And if it sets the voltage to 13.5V because that sets the mostly topped-up car's battery to just trickle/maintain, and then you add another battery to the mix that is below 13.5V, you will see a lot of current flowing again, and that will charge the second battery. If it is really that the 13.5V at the car's battery is reduced to < 12V at the trailer's battery because of I2R losses in the cable, then it seems the solution is a thicker cable. Why not try running a #6 cable directly from the front battery (or even the alternator) to the trailer battery? That's a lot of copper, but perhaps less expensive and fussy than a pile of power electronics.

40 feet (20 feet x 2 pos and neg) of #6 copper is 0.016 ohm. At 20A, that's 0.32V, which seems fine for charging a battery discharged enough to pull 20A. As the battery charges the current will go down and the IR drop will reduce. I don't see why the battery won't charge fully.

I'm also a bit surprised by the 13.5. both of my cheap, but late model vehicles land steady state around 13.8V. sometimes more or less, but only temporarily.

What am I not getting?

[theoldwizard1]

Well, a 800W inverter will take what, 75 amps with losses?

Okay, I'll give you that.

A 75A boost converter going from 10 to 14 volts is going to be really hard to find.

1200W DC-DC boost converters are cheap on eBay.  Yes, they need additional cooling and maybe even some better power transistors.

[theoldwizard1]

I'm a bit confused by the premise of the question. How does the car's voltage regulator control the rate of current to charge / not charge the car's battery, except by varying the voltage? And if it sets the voltage to 13.5V because that sets the mostly topped-up car's battery to just trickle/maintain, and then you add another battery to the mix that is below 13.5V, you will see a lot of current flowing again, and that will charge the second battery.

It is not that simple.  The battery charging algorithms in the PCM ASSUME a certain capacity battery (what is installed at the factory) and will NOT "overcharge" it.

If it is really that the 13.5V at the car's battery is reduced to < 12V at the trailer's battery because of I2R losses in the cable, then it seems the solution is a thicker cable. Why not try running a #6 cable directly from the front battery (or even the alternator) to the trailer battery? That's a lot of copper, but perhaps less expensive and fussy than a pile of power electronics.

Been there, done that (note my comment at the end of my original post).

40 feet (20 feet x 2 pos and neg) of #6 copper is 0.016 ohm. At 20A, that's 0.32V, which seems fine for charging a battery discharged enough to pull 20A. As the battery charges the current will go down and the IR drop will reduce. I don't see why the battery won't charge fully.

Your WAY TOO SHORT on your length !  Double it !!  #6 "marine" grade wire is about $1/foot.  Don't forget the disconnect at the trailer coupler and the lugs at each end !

Plus, if the vehicle battery is seeing 13.5V then the 0.32V drop will result in 13.18V, not nearly high enough.

What am I not getting?

What you are not getting is that proper charging of a deep cycle, lead acid battery to its full capacity takes a lot more than just more than 2.2V per cell.

[djacobow]

I'm a bit confused by the premise of the question. How does the car's voltage regulator control the rate of current to charge / not charge the car's battery, except by varying the voltage? And if it sets the voltage to 13.5V because that sets the mostly topped-up car's battery to just trickle/maintain, and then you add another battery to the mix that is below 13.5V, you will see a lot of current flowing again, and that will charge the second battery.

It is not that simple.  The battery charging algorithms in the PCM ASSUME a certain capacity battery (what is installed at the factory) and will NOT "overcharge" it.

It sounds like the PCM in your car is a lot cleverer than the ones in mine. I can't even think of what this cleverness is designed to achieve. If the battery is charging more than XX amps at a reasonable charging voltage, then it is either in need of charge, or is already broken somehow. Is this to avoid a fire or something? Not to mention that people have been replacing car batteries with new ones of higher capacity forever.

What you are not getting is that proper charging of a deep cycle, lead acid battery to its full capacity takes a lot more than just more than 2.2V per cell.

From the diagram you showed, it looks like 2.3V per cell will do it, which is 13.8V, which is in fact what you see in most charging systems.

I understand that every battery has its own optimal charging cycle for maximum charge and lifetime, but also that most batteries don't receive such good treament. Certainly the ones in boats don't, and I suspect RV's too. Let's say your battery only gets to 90% charge and needs to be replaced 20% sooner because of the abuse of being connected directly to the car's electrical system. How does that compare to a multi-stage string of converters that have their own price tags and failure rates (including the potential for "exciting" failures)?

[theoldwizard1]

It sounds like the PCM in your car is a lot cleverer than the ones in mine. I can't even think of what this cleverness is designed to achieve.

2 primary "goals".  Replaced the used power (like from cranking) quickly and minimize any possible overcharging.  Overcharging shortens the life of the battery AND wastes fuel (yes, every 1/10 of a MPG makes a difference in the EPA test)

BMW actually uses a "battery sensor" (?) that most be "re-initialized" (with a special "tool") when the battery is replaced.

What you are not getting is that proper charging of a deep cycle, lead acid battery to its full capacity takes a lot more than just more than 2.2V per cell.

From the diagram you showed, it looks like 2.3V per cell will do it, which is 13.8V, which is in fact what you see in most charging systems.

But you are not getting 13.8V at the house battery, even with 6 gauge wire !

You also have to remember that we are talking about a battery that has MUCH more capacity than the vehicle battery.  Even with if we were getting 13.8V, the time to get to 90% state of charge is probably more than 12 hours !

[theoldwizard1]

The other possible solution ...

Install the DC-AC inverter inside the tow vehicle, reasonably close to the alternator/battery and connected with heavy gauge wire.  Run 16/3 "extension cord" back to the trailer and just plug it into the trailer's "shore power" cord.

The down side of this is that once you arrive at your non-powered destination, you will probably want the inverter installed back in the trailer, close to the house batteries so you can power the AC items in the trailer.

[djacobow]

BMW actually uses a "battery sensor" (?) that most be "re-initialized" (with a special "tool") when the battery is replaced.

There we go! The missing piece of the puzzle: BMW. I'm sure has identified that their target market does not pull trailers, and therefore, Mr. Trailer Puller, you may kindly get lost.

The funny thing is that I have a 100AH 12V deep cycle VRLA at home that I use in an portable PV system. I have watched it carefully while charging from a controller that claims to be VRLA appropriate, and the current when it is being topped off at 13.8, or sometimes a bit higher than that, is only about 100 mA. So, though I suspect taking 100 mA at 13.8V in perpetuity would be bad for the battery (and this charger actually does shut off entirely), we're only talking about 1.4W, so it's hard for me to see how that would be detectable in the mileage of the vehicle.

But BMW has not fully optimized their charging system yet. In the complete optimization, the battery monitor is built into the BMW-branded battery, and uses an encrypted communications protocol, and can only be replaced at the dealership with a special tool.

[theoldwizard1]

BMW actually uses a "battery sensor" (?) that most be "re-initialized" (with a special "tool") when the battery is replaced.

There we go! The missing piece of the puzzle: BMW. I'm sure has identified that their target market does not pull trailers, and therefore, Mr. Trailer Puller, you may kindly get lost.

Who said anything about the vehicle being a BMW ?  That was on "extreme" case.  After 30+ years designing powertrain control systems, I can tell you that ALL auto manufacturers selling car in the US have some type of "smart" battery charging algorithm.

The funny thing is that I have a 100AH 12V deep cycle VRLA ...

WOW ARE YOU OUT IN LEFT FIELD !

A 100AH "valve regulated lead acid" battery is very different than a flooded, deep cycle 225 AH battery !

[CJay]

You've invented the batteriser for your trailer.

[ovnr]

A 75A boost converter going from 10 to 14 volts is going to be really hard to find.

1200W DC-DC boost converters are cheap on eBay.  Yes, they need additional cooling and maybe even some better power transistors.

Yeah, they're cheap on eBay. They'll also only do 200W on a 10V input, and I expect they'll catch on fire within 15 minutes if you try that.

The 50A models might do 20 amps continuous. Didn't find anything larger.

[james_s]

You can get a charger that runs on DC and contains a buck/boost converter, generally they will work down to about 10V. An even simpler approach is to install a boost converter right back at the battery that boosts whatever voltage it sees up to a reasonable float voltage for the battery. Generally lead acid batteries are pretty robust as long as you don't hammer them too hard. It's not like charging a lithium ion where precise control is required.

[mikeselectricstuff]

If you want to boost from 12V to 14V you could use an isolated 2V converter with the output wired in series with the input - that way the converter only has to handle 1/6th of the power.
 

[theoldwizard1]

You can get a charger that runs on DC and contains a buck/boost converter, generally they will work down to about 10V.

Do you have a reference/link ?

[CJay]

Plenty out there, at least in the UK but I'm sure there will be others, it might be worth investigating solar management systems, I have a feeling the solar panel 'controllers' will have some sort of buck/boost controller and possibly also have the correct battery management algorithms

https://www.mdsbattery.co.uk/shop/productprofile.asp?ProductGroupID=9257&utm_source=Google&utm_medium=base&utm_campaign=google-base&gclid=CjwKEAjw8OLGBRCklJalqKHzjQ0SJACP4BHrNFWcEJ7Vjue3w1KMGotK2h-bRmsQdLFP1SCihb6YSRoCqyvw_wcB

https://www.roadpro.co.uk/catalogue/02d-dc-dc-battery-chargers

[theoldwizard1]

The first one IS sold in the US on Amazon, although at 2A output it would take much too long to charge a 200+ Ah battery bank !

I have found a couple that are possibilities. This is certainly a much SIMPLER solution !!