Camper battery charging system

[Dr.Kayak]

I have a slide-in cab-over camper with it’s own deep cycle battery. For charging I ran 8 gauge back to the truck battery with a “battery switch” that disconnects the camper battery when the key is turned off. I’m using a local ground to the truck frame for the camper battery.



I don’t seem to be getting a very fast recharge on the camper battery. Is it the 8 gauge wire, local ground, or the battery switch that is limiting my charging?



I have a friend with a dealer installed charging system that only used 12 gauge wire.



Thanks,



Chuck

 

[Dieselnerd]

My camper or fifth wheel, depending on which I'm using, always charge quite well plugged into the socket on the factory hitch that comes with the camper/tow package. I don't really know what gauge the wire is but it's always worked fine for me.

 

[HEYBOSS]

How's the battery?

Is the battery old, battery may be weak and no longer hold charge! How's the wiring, good connections? Can the truck's alternator put out for all the batteries? my 2cts.

 

[Dr.Kayak]

good battery

The camper battery charges fine in 24 hours with a 6 amp charger. You would think with a 117 amp alternator the truck could charge the camper battery pretty quick. After a 4 hour drive the camper battery was still less than 20% charged. I woud expect better.

 

[klenger]

You could have a 1000 amp charger and it would not help. Once the battery voltage floats up to about 13. 75 volts, the the charge current drops way down because of the chemical properties of the battery. That just the way an automotive alternator / regulator is designed to work. To properly charge a deep cycle battery, you need some kind of a charging system designed for that purpose. Solar panels and a solar controller is one way. The camper converter will do this too, especially if you have one with a smart controller circuit.



One of the things you can do to see if you have a wiring problem is to measure the voltage at the battery terminals of the camper battery. Compare that voltage with the that at the truck battery. If they are within a few tenths of a volt, the wiring is not the problem. You could also put a ampmeter in series with the battery to see what the charge current actually is. To do this you would need a shunt type meter capable of the possible high currnet.



Clear as mud?

 

[Dr.Kayak]

Ken



Thanks for the info. Thought of an ampmeter but don’t have one that could handle the load. Sounds like I should stick with charging the battery at home with my 6 amp charger. I can camp for 5 days before the camper battery dies.



I did read that if you connect a dead battery into the truck’s system all 3 batteries will try to equalize. Like water between 3 buckets. This implies there will be a very high current going to the dead battery, enough current to melt wires. In the real world, my system does not behave this way.



Chuck

 

[dresslered]

Battery Isolator

I just went with some heavy gauge wire and a solenoid switch myself, but the real answer would be to use a battery isolator. This will divert the required current directly from the alternator to the camper batteries for very quick recharge times. See http://www.hellroaring.com/rv.htm for products and a good description.



I also rigged up an ammeter to monitor how much current the camper battery is drawing. After building my own circuit, I found a nice little kit already assembled for such purposes: http://store.yahoo.com/wind-sun/digdcampmet.html

 

[Dr.Kayak]

David:



I was advised against a battery Isolator by the manufacture of my camper. He said a modern computer controlled engine, like ours, would not like having an isolator installed. He claimed to know several Dodge Cummins owners who had problems using an isolator. I read a lot about isolators but chickened out on cutting into the factory wiring going to the alternator.



I will look into the ammeter. Sounds like what I need.



Thanks!

 

[BK]

Chuck...



Since you don't have an amp meter to measure the current, just check losses... since thats what your really concearned with anyway... .



(following all assumes you have a good battery and connections)

(you should double check your connections for losses too, I've seen a single bad crimp connection drop over a 1 volt)



While the truck is running and battery is charging...

Measure the voltage at the point you wired the 8Awg wire to with ground being at the truck battery or alternator.

Now move to the camper battery and measure voltage across the camper battery.

Now measure the voltage drop across your 8Awg wire..... might need to lengthen your test leads for this. . but since the meter has a very high resistance when measure voltage it wont effect your results enough to worry about it.



Now for a bit of easy math...

If you take the voltage at source point,

subtract the voltage at your camper battery from the source point voltage... you get your total losses. .



Is the majority of the losses from the 8Awg wire?

Or from the ground? or is it 50/50?



Compare the voltage you measure across the 8awg wire to your subtraction result.

If the voltage drop you measured across the 8awg wire is about equal to or 50% of your losses... you need to make both the ground and + conductors bigger.

If most of your losses are in the + 8Awg wire... than you need to just make your + wire larger. .

If the voltage drop you measure across the 8awg wire is only about 20% of your losses, than the other 80% must be in the grounding and you need to run a large ground wire. .



I can send you an Excel spreadsheet for wire losses at a given currents if you interested.

Based on Ohms law, the higher the current, the more voltage drop over the lenght of given wire you'll see... . so the battery may very well charge slow... but as it charges it's current draw reduces and the voltage losses over the wiring drop and the eventually the battery comes up to it's "float voltage" , which is the point that it draws very little current. . this is usually 13. 6 - 13. 8Vdc... but most car/truck alternators will raise the voltage voltage during cold temps (battery capacity reduces has the temperature drops) ... so you may see 14 - 14. 6 (14. 6 is getting a bit too high for my taste,,, but for deep cycle use , like what our heaters tend to represent... there's some reasoning to be that high)



Now getting to another point that popped into my head when reading your post... what are you using for a "switch"?





Bob

 

[Dr.Kayak]

Bob,



I will try the measurements you recommended. For a switch I'm using a big solenoid sold by the RV dealers. They call it a battery switch. The instructions said you could use as small as 12 gauge wire, but you would not see very fast charging with such small wire. I have a chart from Sure Power (maker of Isolators) that show the length limitations and load for different gauge wire. My + wire is almost 20 feet of 8 gauge with a 50 amp fuse. This is the length limit for 8 gauge. Anything longer and they recommend 6 gauge.

 

[BK]

Chuck,



The relay sounds fine, just wanted to make sure...

If it's silver and kinda look a lot like a Ford starter solenoid, most of those I've come across are made by Prestolite. They are good for about 100 amps all day long. 600 amps in rush and 200amps break current



I didnt have 8awg on my table, I stopped at 6 awg when I compiled the table so I added it.

Now we know your not drawing more than 50 amp, since your not blowing your fuse... .

But again, I only went as low as 50 amps on my table...

I could add lower current examples. . but not worth it for this excersise.



But we'll stick with that (50amps) as a worse case at the moment. .

based on 20' of #8 your max loss @50amps would be about . 67Vdc across that one wire (not counting connection resistance of connections to the wire, crimps, battery lugs, wire lugs and soo on).



Since we cant charicterize your ground path and it's resistance... I can't calculate a loss in the ground path. But with the measurements you'll be making we can quantify the ground losses.



Again, not to beat a dead horse, or a lame horse, but be thinking about losses in connections while out making the measurements.

You can measure from the battery post to the wire (by poking through the insulator) to make sure the battery lug and connection to it from the wire aren't lossy, if you find that the losses of the wire a a lot more than the . 67Vdc. Any high current connections should be soldered and not crimped. Crimps maybe ok now, but in time they will corroid and the resistance of the connection will go up a lot. At an 1amp or so you may not notice it, but at 10's of amps a crimp will become a PITA.



Have fun, if you want you can always email or just post here and wait for me to show up again. .



Bob

 

[RedRamer]

I'm guessing, but maybe your alternator/computer is reading the charge to the truck mains. If and when they tell it they are charged up, then it would cut back on the charge rate to all batteries.

where are you hooked up at the engine end?

 

[bbutler]

I installed a solar charger, $161. 00 from CAMPING WORLD. Works great, it keep the batteries top off all the time.

 

[Bill Stockard]

Bob,



I have been keeping up with your and Dr Kayak's discussion of getting power back to the camper. I am working on getting 20 Amps back to the Dometic fridge running on 12 volts and keeping the volatge above 12. 6 volts at the heating element. I am running 10 and 8 ga wires in paralell from the driver side battery through a 40 Amp circuit breaker, 100 Amp capacity relay, back to the camper receptacle in the truck bed. The camper has 8 gauge wire from the camper plug to the 12 volt distribution system(I have wired around the camper battery isolator) and to the fridge. The ground side of the camper is wired the same with 8 gauge wire all the way. On the ground side, I have 8 gauge wire coming from the camper recptacle to the truck frame and then to the ground post on the driver side battery. Right now, the best voltage I get is 12. 3 at the heating element with the fridge in operation and the engine at 1500 RPM. The fridge will maintain the temp with outside temps below 75 degrees, but if above 80 degrees air temp. the temp inside the box rises. The heating element is very voltage sensitive in relation to heat output and the fridge depends on an exact temp for the boiler to work fast enough to keep up in warm weather.



You are right about voltage drops in crimp connectors and terminal posts. I have checked voltages at each connection--all the wire and connections are less than 6 months old. It's amazing how much just a little drop at each connection adds up to a lower vlatage at the end of the circuit. My next project will be to solder all the connections and see if that will improve the voltage at the fridge.



I have put a 30 Amp relay in the 12 circuit which by passes sending the load through the Dometic electronic circuit board. Dometic used 12 gauge wire even though they have a 35 Amp fuse in the line!!



This is a case where 1/2 volt will make the difference!! Wish they would hurry up with the 42 volt systems!!!



Bill

 

[Dr.Kayak]

I checked for voltage drop at my camper battery and could not detect a drop with my old analogue meter. I decided not to solder my connections because of the heat required with 8g wire and those big battery connectors. After crimping the connectors I covered the connection with marine grade waterproof heat shrink. Hate to tear all that loose to solder them now. Guess I will charge my camper battery before I leave home and not expect quick charges off the truck's system.

 

[BK]

Bill,



Sounds as if you did your home work. There's a bunch of lead length there. You could double up on the ground as you did the postive and insure a solid ground.

I'm finding too, that my camper's stock fuse block, internal wiring and trailer plug bite the big one for losses. .



I've also found that my Dodge's stock B+ wiring back to the trailer plug is real lossy too, havent tried to find it yet. When we're out on the beach for an extend period, I use jumper cables for a quick recharge. . I recharge in about 1/3 the time.



For the best and lowest loss B+ and negative connectors... Wrangler products had these great connectors that are commonly used on Fork Lifts and we used to use them on our electric car conversions. It's the same connectors used on the Warn winch multi-mount system. There are 3 or 4 size. So you don't need to get them as large as the ones that come with the Warn winches. If your interested I can post a some pictures of the two most common sizes I use and links to Wrangler's site.



Bill and DR. K. ---Crimps...

The easiest way to solder those huge crimps if you dont have a beastly soldering iron is to use a propane torch and get some good flux and heavy solder. When using the torch, I dont need to say be real carefull when doing it while under your hood or truck. Also you want to get a good feel for getting just enough heat and not too much that you discolor the lug and wire causing oxidation. If you do the solder wont take.

You can buy a roll of heavy solder from Digikey for about 20 bucks I guess. Been ages since I had to buy any.

Then use some brake cleaner to get ride of any left over flux.

Using the heat shrink afterwards is great, but in the long run it doesnt save a crimp. The only time I've used crimp were to hold the lug on to the heavy wire and than dipped the lug with crimped wire into a solder pot for the all around best solder job.



--- other losses----- fyi

The other thing to check, that I dont remember if I mentioned it, is that fuses and ckt breakers are lossy by nature. When looking for losses check to see how lossy or not lossy your safety device is. your common fuse and most ckt breakers work on heat. So... current flows... some resistive element in the fuse or breaker has to drop some voltage to cause the heat, which in turns reduces the voltage you'll see on the load side of the fuse or ckt breaker.

Fusing or using a ckt breaker too close to your load currents can cause false tripping and high losses. But you do need to fuse at the current level your wire will trip the protection device.

 

[merryman]

Not as an answer to the specific question in discussion, but for a great discussion of 12V systems and troubleshooting them (and other discussions of all things RV) see

www.phrannie.org/phredex.html



Vaughn

 

[Dr.Kayak]

No voltage loss in my system

I bought a digital Multi-meter yesterday. Checked for Voltage loss with the truck not running. I have 12. 49 volts testing directly across the truck's battery terminals. Testing across the connections to the camper battery (with the camper battery disconnected) I had the same 12. 49 volts. I had expected some loss because of the local ground to the truck frame back at the camper battery.



BTW. . I bought my 50 amp breaker, 8 gauge wire, and connectors at a marine supply. All top quality. The breaker can be used as a disconnect switch. They recomended against soldering the connectors. Should have know not to listed to a salesman...

 

[BK]

Dr. K... Sorry for the last, late reply. My daughters computer took a dive trying to get the AMD machine to like living with an USB HP CDR... and than after I finished that problem, my machine took a dive... seem a piece of ram started flaking out on me. . destroyed my Windows OS... format C: & do a setup. ugh. .



If no loads were on in the camper and with the truck at 12. 49Vdc... the second battery is drawing very little if any current from the truck's battery, or visa versa.

With the truck running and the alternator producing 14Vdc the camper battery will want to charge and draw current to charge.

Than you'll see a drop.

As it charges and it's voltage riases, the closer it gets to equalizing, the lower the current and less the drops.

(again no loads running in the camper)



As far as not solder... maybe he ment to the ckt breaker? maybe it can be damaged by too much heat? If so, can you solder a lug to the wire and use a bolt, internal star washers and nut to make the connection to the terminals of the ckt breaker?

If not, I would do my best to solder to the ckt breaker's terminals.





Vaughn, that a cool link. lots of nifty other articles too.