Boondocker's 7 Pin Battery Charger (& two questions)

[profdant139]

There have been quite a few questions about the easiest way to charge batteries while boondocking, and it got me to thinking about a really cool homemade device that was sent to me a few years ago by one of the forum members (smkettner). This thing (I call it a “pigtail,” for lack of a better name) enables me to charge my spare battery in the bed of my truck while driving around. It consists of a plug that sticks into my seven pin adapter, and then there are two cables coming out of the plug that go to the battery terminals. (Obviously, since the pigtail occupies the seven pin adapter, I can only use it when I’m not towing my trailer.)

When I had an SUV, I didn’t use the pigtail much because I was a little worried about “outgassing” from the battery, since the back of the SUV was (of course) the back of the passenger compartment. Now, with the truck, the cargo bed is separate from the passenger area, and fumes aren’t a problem. I’m intending to carry the spare battery inside a plastic tub, just in case some battery acid sloshes out when I’m driving.

Anyway, here are my questions: I hooked this thing up, and I noted that my multimeter shows that the voltage on my spare battery is 13.9 V. Just for the sake of comparison, I hooked up the power cord for my trailer to my seven pin plug, and the battery on the trailer was also registering 13.9 V. And then (just for the sake of more comparison) I plugged my trailer into shore power, and the battery registered only 13.6 V.

Are those normal differences? I don’t understand why the trailer battery reads 13.9 when plugged into the truck but only 13.6 when plugged into shore power – doesn’t the power get filtered through the converter, in either case, before reaching the battery?

My second question has to do with charging the spare battery in the bed of the truck – how long can I leave the spare battery hooked up to the pigtail while I drive around? Is there a way to calculate ahead of time how much “juice” the spare battery needs, and how much the spare battery is getting from the pigtail? I don’t want to “cook” the battery. Of course, I will check the battery with my multimeter to see when it reaches 12.7. But that is kind of a trial and error method, not very scientific.

Thanks in advance for your suggestions, and thanks to smkettner for sending me the pigtail! (He swears he did not invent it, but I have not seen it anywhere else.)

(Note to moderator: I posted this on the Boondocking forum, rather than the Tech forum, since there has been a lot of recent discussion here specifically about how to charge batteries while boondocking or dry camping. If you have to move this to Tech, I understand, and I apologize for any inconvenience.)

[profdant139]

Roy, here is a wild idea -- solar panels on the roof of your truck!! They would go to a controller in the bed to charge your two extra batteries. I don't know if there is enough real estate on the roof to do the job -- probably not. I guess it depends on how much usage those two batteries would get. But this would avoid putting a big drain on the alternator. Getting stuck in the middle of nowhere with a dead truck battery can ruin your whole day.

[RoyB]

This is my upcoming problem - I want to have two 150AH batteries on the tongue of my trailer and an additional two 150AH batteries in the bed of my truck. This layout gives me 300AHs capacity to run my two way radios when I am just in my truck alone or 600Ahs capacity when connected to my OFF-ROAD POPUP trailer when camping off the power grid.

According to the experts I ran into discussing all of this is what we come up with. The answer was to get a SMART RELAY setup...


BLUE SEA SI-ACR SMart Relay

My concerns now is will I blow up my truck alternator system for adding additional 40AMPs current drain over what it currently is doing with its single truck start battery installation.

The smart relay monitors the DC VOLTAGE from the truck start battery and when it gets to an adjustable trip point showing the alternator has started a 14VDC charge mode the smart relay will switch the remote battery circuit into the truck alternator charging circuit paralleling the truck start battery. This is suppose to charge both battery systems while the alternator is in its charge output mode. This is how the big boys get around the sensing problem. When the truck alternator eventually reverts back to its 13.2 float mode then the smart relay disconnects the remote battery connection. This is how the big boys keep you from running down your truck start battery. The downside of this system is it cannot detect the remote batteries being discharged below the trip point to tell the alternator to start going into charge mode . They added a manual battery switch to overcome this situation. Sounds like a good plan to me...

All of the experts I talked to indicated the remote battery being so far away from the truck alternator would not provide good sensing information to overcome the already charged truck start battery sitting within inches of the alternator. You also do NOT want to drain the truck start battery down and get you stranded somewhere.

In the old days the DIODE BASED battery isolators did this just fine but the 0.7VDC voltage drops associated with these heavy duty 200AMP DIODE units really messes up the proper amount of DC charge voltage you want to go to the remote deep cycle battery banks if you want to re-charge them to their 90% charge state in a short three hour time frame.

The deep cycle battery science rules indicate your need to get around 14-15VDC charging voltage with a capacity of around 20AMPS of DC current for each battery you are trying to charge if you want to re-charge this battery in a short three hour time frame.

I do not think just taping the start battery 12VDC cable 15-feet away from using 12-14 gauge wire is the way to go. I guess I am guilty of over-thinking this problem. Maybe that works OK with a single deep cycle battery... Maybe not...

If I am camping way back in the woods like I usually find myself doing I have to add the word "SUCCESSFUL" to the situation.

Just my thoughts on the subj... Of course this is my problem not anyone elses... Just passing on some of my experiences here...
Roy Ken

[pinesman]

When we are dry camping in our fiver, I usually carry 2-3 extra batteries in the back of the truck. I now use two sets of heavy jumper cables with winch type connectors on one end of each set and ring terminals on the other ends. Before that, I had a lighter set of jumper cables which I cut into about 2 feet from one end. I then put a male and female rv plug on the cut ends. While at the campsite, I plugged the trailer pigtail into the longer section which was connected to my extra batteries in the truck. While riding around, the shorter section was plugged into the truck's in bed plug and hooked to the batteries so they might charge a little while driving. How much it really helped-who knows? It did make me feel like I was doing something.

[Less_Stuff]

Yes charging from your tow vehicle works great!

In fact Lance slide on truck campers use a special connector with larger charge wires for auxiliary (camper) battery charging.
Mine has worked swell for the last 10 years.

A simple continuous duty relay that is on when your truck is running then shuts off when the engine stops does the job. They are called battery isolators and make the system automatic.

Battery Isolator

Wire it in like this:


If the main battery is charged and the auxiliary battery is discharged then the main battery along with the trucks alternator will charge your auxiliary battery when you run the truck engine.
Charging is based on voltage. Dead battery's draw down the system voltage increasing current flow into the discharged battery.

Note: A regular 7 pin trailer connector has all the same sized wires limiting charging current. Using larger gauge wires in the charging circuit will improve results. Notice the red lines in the diagram.
You can leave out the surge protector and the rocker switch.

[profdant139]

Thanks, Jay -- that is encouraging news!

[J_herb]

Dan,
some people make things that are simple hard and it doesn't need to be that way.
I have been doing what you are wanting to do for many years and it works great. I have a 06 Tundra with trailer package, I installed a 7 pin RV socket in the left rear of my truck and have a pig tail with a connection to the extra battery.It will charge the battery up if you are driving a few hours not just short trips. My pig tail is only 2' long and 10 Gage wire. We did a 3 week trip this last spring and only camped with elect one night, we have a 17' Casita and we used lights, water pump, heater and the fanatic fan as needed and used our shower too.

[profdant139]

Wow! Lots to think about -- some great answers. But I am still confused about one thing that Roy said: "What will happen is the alternator will only keep your truck start battery charged up and all your remote battery will ever see will be the 13.2 float voltage coming out of the alternator."

When I started my truck, and the spare battery was plugged into my 7 pin via smkettner's amazing pigtail device, I measured the voltage at the spare battery, and it was reading 13.9. Does that mean that my alternator somehow recognized that it needed to bump up the voltage due to the extra draw of the spare battery?

Or is it possible that this is just a temporary phenomenon -- maybe after I run the truck for a while, the truck battery will be fully charged and the alternator will dial back on the voltage??

If that's true, and the voltage slips from 13.9 down to 13.2 or whatever, then it will take a long time for the spare battery to get from a 50% charge (12.1 V) to 90%.


Also, how would I determine the amperage of the output from the 7 pin adapter? Is there a standard number? I would guess that since the 7 pin usually feeds into the converter in the trailer, there must be a "sweet spot" for the converter -- too high and a fuse would blow, too low and there would not be enough power.

Roy mentioned that the apparent reading on a newly-charged battery can be misleading -- I can tell you that he is exactly right. Just after taking a battery off a short time on a charger, it'll often read 12.7. Come back an hour later, and it is down to 12.3 or whatever.

And the link to the "homework" -- the 12 Volt side of life -- is useful but very daunting. Over the last several years, I have read through that website several times. Only a little bit of it sinks in each time. There is really too much info for the novice reader to absorb.

[RoyB]

There is yet another downside for using the truck alternator system for charging your remote located deep cycle batteries.

The truck alternator system operates from sensing the truck start battery on when it needs charge etc...

The truck alternator follows the battery science and will produce around 14VDC when it is needed to charge the battery. Since the truck start battery is so close to the alternator then this is mostly the only thing the alternator sensors will see. i.e it probably will never know your remote battery is in the circuit as far as DC voltage sensing is concerned.

What will happen is the alternator will only keep your truck start battery charged up and all your remote battery will ever see will be the 13.2 float voltage coming out of the alternator.

Going back to the brochure you will notice it takes many many hours to return your remote battery back up to its 90% charge state just using 13.2VDC DC VOLTAGE only.. This doesn't take into account the available 20AMPs also required for the remote battery to draw to start the internal charging process...

This brings up another subj on deep cycle battery performance. We have found unless you only re-charge at the 50% charge state and go to at least the 90% charge state each time you re-charge the battery you cannot get the expected battery performance. It may look like it has charged alot but as soon as you start using it will drop rather quickly back to the 50% state of charge. If you continually keep doing a small charge like this your battery will eventually go bad on you.

We always follow the 50% charge state as close as we can and re-charge back up to the 90% charge state before using the battery again. Sometimes due to camp ground rules on when we can run our generator we have to do the 90% charge state in a couple of generator run times maybe only get to do two hours in the morning run and then three hours run in the early evening time frame.

Watching this as close as we do seems to really give us a good battery run through the night until 8AM the next morning running the planned thing we want to run...

We also follow the rules of doing a full deep cycle battery re-charge after about 12-14 charge cycles of the 50% to 90% charge state. The full charge state using the 2KW generator will require at least 12 hours of generator run time and most camp grounds we go to doesn't allow this... This is usually when we pack up and head for the house...

Depending on the battery banks is crucial for us to camp off the power grid so we take it very seriously... Some things you just can't skimp on...

This method can be helped in a big way of just adding 120WATTS of solar power to charge the battery bank during the day time hours when the sun is out. We haven't done this yet but is on the things to do. The more you can re-charge during the day means less time you will need to run the generator which fits into the camp grounds generator run time rules just fine...

just food for thought..

I'm sure others do it differently but this make us be very successful when camping off the power grid with our 255AH battery bank. we don't even think about it much anymore... I do keep a close watch on my meter panel however...

Roy ken

[RoyB]

There is only a couple of ways to charge deep cycle batteries. The big thing for us is how long does it take.

This brochure report from Progressive Dynamics is good to follow for how long does it take to re-charge a deep cycle battery.

"Progressive Dynamics ran this test on the amount of time it took a PD9155 (55-amp) converter/charger set to three different output voltages to recharge a 125 AH (Amp Hour) battery after it was fully discharged to 10.5-volts.

14.4-VOLTS (Boost Mode) – Returned the battery to 90% of full charge in approximately 3-hours. The battery reached full charge in approximately 11 hours.

13.6-VOLTS (Normal Mode) – Required 40-hours to return the battery to 90% of full charge and 78-hours to reach full charge.

13.2-VOLTS (Storage Mode) – Required 60-hours to return the battery to 90% of full charge and 100-hours to reach full charge."

It doesn't matter where you get the charging power from but it looks like you need to have these DC voltages available shown in the brochure report and will also need about 20AMPS of charging power available for each 12VDC battery you are wanting to charge. This is especially required if you want to recharge your batteries in the short three hour time frame.

One downside for taping into your truck alternator system is you have to use a SMART RELAY to isolate the systems and cannot use the old time high current DIODE battery isolator as their internal DC VOLTAGE drop is around 0.7 VDC which will result in not getting the required DC voltages listed in the report to recharge your deep battery in the short time frame they list...

I suspect the 12VDC power you have available at the 7-way connector is not going to be 20AMPs. This will relate drastically on how long it will take to recharge any deep cycle battery connected to it.

We camp alot off the power grid and would never think of using anything associated with the truck alternator system without alot of planning as this is the way you get back home. Once it is dead you may be in alot of trouble. Not a good PLAN B source for me to be successful...

just my take on the subj...
Roy Ken

[Less_Stuff]

Question 1
A converter is designed to replace a battery not charge one. Some newer converters can properly charge a battery however.

Question 2
Your trucks voltage regulator will control the charging of all batteries hooked to it.

Now for your Homework. Lots of reading but very enlightening.

Homework