Home Made Voltage Booster charges TT battery off 7-pin

[eb145]

My TT batteries charge really slowly from my truck alternator through the standard 7-pin trailer wiring harness. This is due to the large voltage drop while current is flowing on the long thin battery charging wire from the truck alternator to the truck 7-pin socket. This is normal behavior for probably all STANDARD TT / TV combinations.

Some people have successfully run thicker wires on their truck from the alternator to the truck 7-pin socket (or even added a separate connector from TV to TT) to reduce voltage drop and increase TT battery charging rate.

I took a different approach that I can use when camping (not while driving). I built this DC-DC voltage booster out of two ebay DC-DC adjustable voltage boosters (about $6 each - wired in parallel to increase current and power throughput), a computer fan and a 7-pin RV plug.



The output voltage is adjustable with a screwdriver. I have it set to 14.8V right now. This picture was taken with the truck turned off so the voltage on the input side of the contraption is 12.6 while the output is 14.8.



If my solar panel can't keep up with my battery use after a few days of dry camping, I can use this to do some bulk charging Instead of a generator that would be used maybe 3 times a year (based on my current camping style).

Here are some closer pictures of the voltage booster:




Ed

[BFL13]

Good report. I admire your approach to getting by while RVing. Some "purists" just don't understand. 🙂

[eb145]

OP here to finally report back on some real world results from last July when I was dry camping in the Adirondacks in very dense shade and I had to use my voltage boosting contraption.

After 2 days of getting no charging from my solar panels (hopeless in my 100% shaded site) I hooked up my contraption and it worked out fairly well - once I figured out the process.

So with my 2 Group 24 batteries down about 60 AHs, I was able to transfer about 40-50 AHs into the TT batteries in a few hours. Yes it was labor intensive (fun for me) to set it up, monitor it and tweak the voltage setting over time. I have a Turnigy meter and a sears DC clamp on ammeter. The goal was to pump about 30 AHs from the truck battery to the TT batteries with the truck turned off, then let the truck idle for 20 minutes to quickly charge the truck battery and repeat till done.

I would first let the voltage booster run for an hour or so without the truck running. I first set the boost voltage to something like 13.6 volts and it would pump 15 or 20 Amps into the TT batteries - but it would soon blow the fuse on my truck 7-pin charging wire - so I hooked up jumper cables between the truck battery and the contraption.

With the jumper cables connected to the voltage booster, I could let the voltage booster run for an hour or so at 30 amps charging from the TV battery to the TT battery. Amps would taper to 10 or less and I would have to increase the voltage set point on the booster to raise it back to 30 or so Amps. All without the truck running.

After transferring about 30 AHs from the TV to TT batteries, I would start the truck and let it idle for 20 minutes while keeping the contraption running. The truck battery was accepting something like 70 Amps from the alternator when I first started the truck.

So basically, I was slowly moving Amps from the TV to the TT (at about 20 Amps), and periodically quickly recharging the TV battery with the truck alternator (at about 70 Amps). And later that day, I would drive the truck on some errands for an hour or so to get the truck battery more fully charged.

Lesson Learned: a current-limited voltage booster would be better. If I had used a current limited voltage booster I could have set the output to 14.4V with a 20 Amp max output. Then I would only have to remember to start the truck every 60 or 90 minutes for 20 minutes and not have to monitor the amps and adjust the voltage when they tapered.

I did this same procedure every 2 or 3 days to keep the batteries charged enough to keep the the propane fridge, water heater and LED lights working for our 8 day stay.

Ed

[Redbelly]

eb145 wrote:
...........As far as my special use case, I intend to use this 2 or 3 times a year while camping and my solar can't keep up (due to deep shade). It works better than my cheap jumper cables alone. And I don't want to get a generator. And I'm really enjoying this whole battery charging game.
................Ed

Ed, very creative. I actually do the same thing with my hybrid Escape. I can charge my trailer battery off the traction battery in the Escape without running the engine.

Your setup using eBay parts is the first that I've seen that has actually worked. My initial interest was to make something that other people could mount permanently and save rewiring the tow vehicle(and not cost $200), as this is a frequently reported problem.

[eb145]

Redbelly wrote:

eb145 wrote:

This device definitely increases the amps going into the battery. something like 2 Amps without it, and up to 20 Amps with it (till truck fuse blows). It does this by raising the voltage closer to the battery terminal. Your idea of boosting to 72 volts for the long distance run on the existing truck wire and then dropping to 14+ volts is very interesting. Increasing the voltage by a factor of 5 or 6 will reduce the current by a factor of 5 or 6.

300W/72V = 4.2 Amps on the truck wire which is way under the 15 Amp limit for 14 gauge wire.

Ed

Ed, I'm impressed with what you've done as I've been messing around with a similar concept, mounted as close to the trailer battery as possible.

A couple of what-if questions.

Ebay sells a $25 version of that DC-DC booster that is rated at 400 watts and has potentiometers that allow you to adjust both maximum voltage and current. What if you used one of these instead?
http://www.ebay.com/itm/111177068259

Second, why couldn't one use a solar panel controller in series with the battery to regulate the battery charge rate? You can pick up a charge controller for $15 that will measure the battery state of charge and regulate the input voltage to keep from overcharging it.
http://www.ebay.com/itm/12V-24V-5-10-15-20-30A-PWM-Solar-Panel-Battery-Regulator-Charge-Controller-/...

The DC-DC booster you link to above is still a max of 10Amps output, but it can be set to limit current which is better than what I put together. It looks like it has a microprocessor that lets you set the voltage and current. Pretty cool.

Putting a $15 PWM solar controller would work but it is not necessary for the specific use case I have in mind for it.

If I was going to wire this in to be a permanent part of the trailer charging circuit I would buy one of the commercial products where they (hopefully) have built them with safety in mind. Ctek builds one that is interesting: CTEK D250S for $180 on Amazon that includes a MPPT solar charge controller feature.

CTEK D250S


As far as my special use case, I intend to use this 2 or 3 times a year while camping and my solar can't keep up (due to deep shade). It works better than my cheap jumper cables alone. And I don't want to get a generator. And I'm really enjoying this whole battery charging game.

I actually intend to let the trailer batteries charge off the truck battery (with the truck turned off for the first hour or two). My gizmo can transfer 10 or 20 Amps from the truck battery whether the truck is running or not. I'll only let maybe 10 or 20 Amp Hours move from the truck battery, then start the truck and let it run for another hour. My 180 Amp truck alternator can easily charge the trailer batteries and the truck battery simultaneously.

Or I might transfer 20 or 30 Amp hours *silently* from truck battery to trailer batteries right before I plan on going for a one or 2 hour drive in the truck. That will let the truck batteries recharge from the alternator without having the truck running at the campsite for an hour or two.

If I screw up and drain the truck battery too much to start it, then I'll get a jump from a friend and accept a lecture from my DW.

I don't plan on leaving this gizmo operating when I am not around to closely monitor it. Westend is right to point out safety issues using wood and cheap plastic for this type of thing. Obviously I have no metal working experience - my father is a carpenter so I know how to do basic wood stuff.

And mex points out the lack of heatsinks. The dc-dc converters I bought specify to do something about additional cooling if you want to operate at the max rated 10 Amps. That is why I added the muffin fan.

Now I'm thinking of some changes in construction... using metal to provide heatsink capability and fire resistance.

Ed

[Redbelly]

westend wrote:
Really a specific use for your booster because unhooking and using jumper cables to the trailer's battery from the truck is going to trump it every time........

I think the desire is to have a device that one could install permanently in the trailer near the battery so that it charges faster when you tow it with a tow vehicle (like the Dodge) that comes with a small 14 gauge charge wire. These are commercially available, but $277.

http://www.allbatterysalesandservice.com/browse.cfm/4,7750.html

This version is $185
http://www.powerstream.com/DCC.htm

[MEXICOWANDERER]

I just gave a quick glance at a couple of these eBay gizmos and have to ask, how are they managing the heat sink issue? Even with MOSFET PWM control heat sinks are mandatory and I do not see any. On one, I see 16 amperes going in and 10 leaving. Call me not a skeptic but something doesn't fit here.

[westend]

Really a specific use for your booster because unhooking and using jumper cables to the trailer's battery from the truck is going to trump it every time.

That said, some suggestions: use bigger heatsinks or a larger device that will handle more current, using a thermistor in line with the power source will set a current limit to the device.

This is maybe a little hypercritical but a heads-up since I was there once, too: There are all kind of chassis' available and making standoffs are very easy. When i see plastic tubs and wood used for electronics, it always makes me think, "Did the builder have so little respect for his work that he couldn't make a good case for it?". This isn't just about small experimental devices but for devices built to a purpose for fair amounts of money that I see. A good aluminum chassis and PTFE for standoffs is where I usually get to with small boards and devices. An aluminum case will act as a passive heatsink, too.

[Redbelly]

eb145 wrote:

This device definitely increases the amps going into the battery. something like 2 Amps without it, and up to 20 Amps with it (till truck fuse blows). It does this by raising the voltage closer to the battery terminal. Your idea of boosting to 72 volts for the long distance run on the existing truck wire and then dropping to 14+ volts is very interesting. Increasing the voltage by a factor of 5 or 6 will reduce the current by a factor of 5 or 6.

300W/72V = 4.2 Amps on the truck wire which is way under the 15 Amp limit for 14 gauge wire.

Ed

Ed, I'm impressed with what you've done as I've been messing around with a similar concept, mounted as close to the trailer battery as possible.

A couple of what-if questions.

Ebay sells a $25 version of that DC-DC booster that is rated at 400 watts and has potentiometers that allow you to adjust both maximum voltage and current. What if you used one of these instead?
http://www.ebay.com/itm/111177068259

Second, why couldn't one use a solar panel controller in series with the battery to regulate the battery charge rate? You can pick up a charge controller for $15 that will measure the battery state of charge and regulate the input voltage to keep from overcharging it.
http://www.ebay.com/itm/12V-24V-5-10-15-20-30A-PWM-Solar-Panel-Battery-Regulator-Charge-Controller-/...

[NinerBikes]

Park the hood of your truck by your batterys, and apply some jumper cables while the motor is running. I think my Touareg TDI diesel alternator is 170 AH rated? Something about it has heat that's electrical while the diesel engine is warming up for the internal vents.

[eb145]

Specularius wrote:
I personally think you are just creating a parasitic load that adds nothing to your batteries. If you measure the actual current going into the batteries, I bet you find it higher without your device. Having said that, if you were to put a dc-dc switching power supply at the front of the truck and upped it to 72 volts and then another one at the rv to drop it back to 14+ volts, then you have done something.

This device definitely increases the amps going into the battery. something like 2 Amps without it, and up to 20 Amps with it (till truck fuse blows). It does this by raising the voltage closer to the battery terminal. Your idea of boosting to 72 volts for the long distance run on the existing truck wire and then dropping to 14+ volts is very interesting. Increasing the voltage by a factor of 5 or 6 will reduce the current by a factor of 5 or 6.

300W/72V = 4.2 Amps on the truck wire which is way under the 15 Amp limit for 14 gauge wire.

Ed

[eb145]

LittleBill wrote:

eb145 wrote:


The little voltage boosters are designed for laptop power supplies that can plug into 12V cigar lighter outlets in cars. They are designed as constant voltage output (variable voltage). With no apparent limit to the current. They will burn out quickly if allowed to pass lots of current - thus the fan to cool them in the summertime. Max power is 150W per device.



Ed

Not really sure what you mean by there is no limit to current . right in the listing it says 6 amp. of course they have no limit but you better push some serious airflow if you expect them to last


" DC-DC 150W Boost Converter 6A 10-32V to 12-35V Step-Up Voltage Power Supply"

fan would do alot better also if you actually had it aimed over the heatsinks

This is closer to what I got - 16Amps input max, 10 Amps output max:
150W DC-DC boost step up converter

"You can use it to power all your electronic devices, as long as the rated input voltage and rated input current is not exceeded."
I see they cost $11.99 now.

What I meant to say is that there seems to be no over-current protection for these little things. There is a limit on the specifications, but there seems to be no current-limiting control (other than letting the smoke out).

For the airflow I am going to cut out some plastic baffles to force all the air to flow over the voltage converters and their heatsinks. I don't know if you can tell by the pictures, but the air is drawn in by the fan, blows under the wood block and exits the other side. Plastic and duct tape baffles will force all air to exit where I want it to. When I did my initial testing on this, temperature was in the low 40s (F) so cooling wasn't so critical. In July, cooling will be a big deal.

Ed

[Specularius]

I personally think you are just creating a parasitic load that adds nothing to your batteries. If you measure the actual current going into the batteries, I bet you find it higher without your device. Having said that, if you were to put a dc-dc switching power supply at the front of the truck and upped it to 72 volts and then another one at the rv to drop it back to 14+ volts, then you have done something.

[LittleBill]

eb145 wrote:


The little voltage boosters are designed for laptop power supplies that can plug into 12V cigar lighter outlets in cars. They are designed as constant voltage output (variable voltage). With no apparent limit to the current. They will burn out quickly if allowed to pass lots of current - thus the fan to cool them in the summertime. Max power is 150W per device.



Ed

Not really sure what you mean by there is no limit to current . right in the listing it says 6 amp. of course they have no limit but you better push some serious airflow if you expect them to last


" DC-DC 150W Boost Converter 6A 10-32V to 12-35V Step-Up Voltage Power Supply"

fan would do alot better also if you actually had it aimed over the heatsinks

[eb145]

drsteve wrote:

RoyB wrote:
What do you do when the battery gets hit with your 14 PLUS VDC and wants to draw around 20AMPS of DC current. There may not be that much DC current available in your 7-way connector wiring.

I wondered that myself.

To the OP, have you used it yet to see how well it charges your batteries?

Short driveway testing only so far. In 40%F weather.

My initial testing showed 20+ Amps to my pair of Group 24 Marine batteries when I dropped them to about 75% SOC. In fact, they were cranking too much current when they were both running in parallel and I set the output voltage high (14.8V) and blew a fuse in my truck. I need to lower the voltage output to reduce the current draw on the truck.

The little voltage boosters are designed for laptop power supplies that can plug into 12V cigar lighter outlets in cars. They are designed as constant voltage output (variable voltage). With no apparent limit to the current. They will burn out quickly if allowed to pass lots of current - thus the fan to cool them in the summertime. Max power is 150W per device.

Other things to do for testing/refining/playing:

1. reduce output voltage to reduce current draw from truck 7-pin. I'll probably try 13.6V when camping to see how that goes.

2. bring lots of fuses for the truck so I can have fun experimenting with this when camping 🙂

3. hook up jumber cables from truck alternator to input side of these converters eliminating the 7-pin from the circuit. Then I have to worry about letting the smoke out of these voltage boosters with too much current / power.

rough power estimates below:

150W x 2 voltage boosters = 300 Watts max power before component failure and smoke let out. When current is flowing, the 7-pin will have reduced voltage due to the high current.

Output (charging side): 300W/14V = 21.4Amps

If Input side (7-pin) is @12V: 300W/12V = 25 Amps
If Input side (7-pin) is @11V: 300W/11V = 27.2 Amps
If Input side (jumper cables) @13V, 300W/13V = 23.1 Amp

EDIT: I did the testing a month or two ago. And actually, now that I think about it, the truck fuse would blow only when I had both voltage boosters operating. With only one wired in I would get half the amps and it wouldn't blow the truck fuse. And what is disturbing is I just found this:
Dodge Trailer Battery Charging Wire

where someone states Dodge uses 14 gauge wiring for the trailer battery charge wire! While Ford and Chevy use 10 gauge. 15 Amp max on 14 gauge wire.

It looks like I'll use jumper cables to power this thing or use just a single booster and drop the voltage to keep the current under 15 Amps from the 7-pin.

Ed