Home Made Voltage Booster charges TT battery off 7-pin

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

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

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.

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-/351004581852?pt=LH_DefaultDomain_0&var=&hash=item51b980dfdc

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

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

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.

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.

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-/351004581852?pt=LH_DefaultDomain_0&var=&hash=item51b980dfdc

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.

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