DC to DC Charging

[Airmapper]

I'm building a portable power pack, one of those plastic Ammo boxes with a 12v battery in it for miscellaneous power needs when camping and around the house.

I'm using a 12v 18ah/20hours battery. Common size, inexpensive ($30) and easily replaceable, but also not a pain to haul around.

I intend to use it while camping / traveling, so if I'm in a situation where I don't have down time or handy access to 110AC for a while, I might run it down and want to re-charge it from the vehicle.

My vehicle battery is a 100ah/20hours Odyssey 2150.

I've been researching DC to DC charging, and to the best I can figure, I should be fine connecting it directly into my 12v DC port (I have a handy always hot one in my rear cargo hatch area) and I can charge my small battery from that while driving around.

One concern I have is that power port is not intended for heavy loads, can a low battery draw enough to be an issue? I think it is fused for 15amps but it runs the entire length of the vehicle on small wires, so if the battery can pull a large load attempting to charge that might not work. Without regulation I assume the battery will attempt to draw as much power as it's able to absorb, at higher charge that won't be much, but if it's low I'm not sure how much it can draw.

Since my port stays hot when I turn off the key I should probably unhook it if stopped for a while, as the different size batteries will probably be constantly trying to drain each other due to the mismatch. My Odyssey likes to settle to about 12.7v-ish, and this little battery seems to settle around 13.1-ish, so I'm figuring without the alternator raising the voltage the little battery will feebly and unsuccessfully attempt to charge the Odyssey until the voltage matches. That is assuming I vaguely understand how that works, which I probably don't.

From what I read using a DC-DC charger is good as it regulates the charging process, particularly topping off, which might normally take a very long time off the alternator alone. In my case I may not get a 100% topped off battery, but that will be okay so long as I hook it up to my normal charger next time I am able.

One more question, is an 18ah battery enough for a jump start in a pinch? I didn't particularly intend to use it as a jump pack, but if it would that might be nice.

 

[zelatore]

If you have it in the budget an actual DC-DC charger would be much preferred and would do a lot to maximize your little battery. It will also give you a spec on how much current it will draw at maximum output so you can do the math and figure out how large your wiring needs to be for the distance it's running. You can certainly just plug it into your outlet when the car is running and recharge somewhat, but you won't get the best out of it. Will an 18 A/h battery jump start you? hmmmm....don't know - maybe? If your main battery is just a little down it might do it. If you're really flat, I highly doubt it.

Here's a handy chart for sizing wire:
http://www.westmarine.com/WestAdvisor/Marine-Wire-Size-And-Ampacity

 

[Airmapper]

For a project the budget is somewhat limited as I could easily put more money into it than it's worth, but the expensive chargers are also too large to put in the box, and I'm going for a clean compact setup. I'm also doing it just to learn how to work with these kind of systems.

This is the only thing I've found I think might work, not too expensive, compact, says it's for charging a battery at constant voltage...

http://www.powerstream.com/Product3.htm

I need to check what my voltage is with the engine running, I think it's 13.8v but I'm not sure.

 

[zelatore]

I don't think I'd go with the product in the link. If you're going to do a DC-DC charger might as well do a proper one that will give you the best charge on the aux battery. I don't think you'd get much better performance with that than just connecting it directly to your existing 12v system and letting the alternator's regulator do the job.

 

[Airmapper]

Well I'm back after some real world use.

Charging from the vehicle did not seem to work, at all. If anything I think it discharged the portable battery. When I got back to some electric, my NOCO charger indicated it was quite low.

It may have been that I didn't kill the connection every time I shut off the truck. It was a busy day, I didn't have the time to babysit it, so I plugged it in when I took off and unplugged it when I stopped for the day, and there were some periods in there where I shut off the engine for a short time (30 min at most, more likely less than 10.) I watched the built in voltage meter, and it would climb up into the 13.4V or so while the engine was running, so it had opportunity to charge.

After the attempt to charge off the vehicle, it held about 12.7V, which was annoying as I have a battery protector unit attached that allows the portable battery to be used if it's at 12.8V or above on switching on, and will cut everything off at 12.1V. Since it didn't have a solid 12.8V the unit wouldn't activate it's relay. But this makes some sense as my truck battery likes to hold about 12.7V at rest, although I've noticed it holds a surface charge for quite some time, so within the time I removed it from the truck I don't see why it would pull it down.

Honestly I forget what it was before I tried to charge it off the truck, I think it was 12.8V or above as I didn't run it so low my protector was locking me out yet before attempting to charge it.

If I were charging more than a portable power pack a $200+ DC to DC charger would be a valuable purchase for me, and if I ever go to a second battery setup that will be on my list, but for this purpose, any creative ideas?

Maybe another voltage sensitive relay unit to put inline between the truck power and portable battery? If the voltage is not higher than say 13.2V on the truck side it cuts off? Does such a thing exist....

Perhaps even more simple, switch to using a key on hot 12v port, which I'm in short supply of but have access to.

 

[Walrus]

I am experimenting with a similar set up. The best solution would be to get yourself an inexpensive battery isolator like this: (https://www.amazon.com/PAC-PAC-80-80-Amp-Battery-Isolator/dp/B001EAWS6Or) and wire yourself in another power point with larger gauge wire. This will solve all your issues, and is much less expensive than a DC-DC charger.

 

[ajmaudio]

The real issue here is that you cannot charge that battery effectively with the voltage available from a power socket. To get power into a lead acid type you need to bring the Voltage up to a higher point...generally just under 15v, let the batteryear rise to that voltage, hold it there until the current accepted is about 2 percent of battery capacity, and then do down to a safe float voltage to maintain the battery. Batteries have resistance and that resistance rises as the battery approaches fully charge. This is why you charge at a higher voltage.... to increase the current into the battery..ohms law. It takes a very long time to charge at 13v....it will get there...but it will take forever...and remember that any time the battery isn't fully charged it's sulfating.... they don't like to sit not charged.

 

[Airmapper]

The real issue here is that you cannot charge that battery effectively with the voltage available from a power socket.

Yes I think you nailed it. I got to looking, and I like the old alternators where it cranked up the running voltage to like 14.6V or something, like the Ford 3G I put in my Bronco, that thing is no slouch on the voltage, you have something to work with. I think the wimpy setting for the alternator on my Xterra is like 13.4V.

I've actually already had thoughts of replacing it with an aftermarket waterproof alternator, but that doesn't necessarily mean I can tune the voltage as desired. This has actually cropped up as an issue, I already have to charge my vehicle battery every weekend or two already, my charging system can't properly top off the Odyssey 2150 under the hood. (More a battery issue than a charging system issue though...) During weekend downtime, my rig spends some quality time plugged into the grid as frustratingly hilarious as that is. I'm waiting for someone to see it and ask if I have a plug in hybrid 4x4. It won't rum my battery down, it just never tops off, hence leading to the sulfation issue mentioned.

Okay, in a pinch how about this. Use my 400w Inverter running off the vehicle 12V to run a charger to charge the battery while I'm driving. Assuming the modified sine wave won't cause a problem, it's not efficient but might work, or open a space/time rift.

Even more fun, power the inverter off the battery I'm charging creating either a perpetual motion machine, automatic battery discharger, or explosive device. Guess I won't try that.

 

[Airmapper]

I've yet to test it, but I think I found the cheaper alternative DC to DC charger.

I got a multi chemistry charger for my Quadcopter, run about $100. It has a Pb mode which if I read the manual correctly is intended to charge a normal lead acid battery. It can do 7.0A or 80 Watts, I'm sure the programming varies each value as necessary.

However, it lists the input voltage as within the range of a vehicles DC system, and it accepts DC input, not just AC. It's designed to be run off a DC battery or AC. I charged my Quads battery off the portable pack mentioned earlier weekend before last. I believe the feature is intended just for allowing the charging of a portable battery so you can use one charger for maintaining all your RC related batteries, even the Pb field batteries.

Now I'm not sure about how you get higher voltage from lower voltage, but if what I learned in my ham radio studies stays with me, perhaps it uses transformers and therefore could deliver the necessary voltage at the expense of lower amperage. However, I charge my portable battery at .9A anyway, and since the vehicle battery can deliver loads more than that, it might just make sense.

I would only need 13.23 watts in order to deliver .9 Amps at 14.7V. Assuming the charger steps up the voltage received from the vehicle, and can use all the wattage it desires, this leaves a pretty wide margin.

I may experiment later, but if it works, at $100 it's about half the cost of a proper DC to DC charger, and does the same thing. Plus if you fly an RC aircraft like I do, you need it anyway.

Granted all of you running second batteries in a trailer shouldn't get too excited, it's one thing charging a 18Ah battery at .9A, but it opens up some possibilities for folks like me just using small batteries for supplemental duty like running tent fans, charging phones and cameras, and odd stuff you don't want to risk connecting to your starting battery while sleeping overnight.

 

[andytruck]

Also, unless someone said already - The slightly lower voltage on the vehicle battery will always be draining off voltage from the slightly higher portable battery, until the portable is at or below the vehicle battery. Even if vehicle battery is fully charged it will try to take voltage from the higher battery. You just have to have the car running with the alternator giving the 14.7 volts to make it work, then of course the little wire to the outlet is not good enough.
This similar situation happens on RVs when running two coach batteries (in parallel) where one battery is bad (had a bad cell or just is older) and tries to suck juice from the good battery. The good battery is providing the 12V to both the RV and the dead battery until it soon runs out of juice. Always match batteries when yo can, buy both new that have the same manufacturer date, or if using used one, use two that have the same voltage etc…
A battery isolator would be the way to go for your charging needs. They are around $30 from an RV parts place. It will connect the portable battery to the car battery when the alternator is running, but disconnect it when the vehicle is off, so not to run down your vehicle battery.
These are available in solenoid type, or diode type. The diode is much easier to install for the challenged.

 

[Haf-E]

For this application I think a solenoid type is better as there will be a 0.6 volt or higher voltage drop through the diode type isolator which will reduce the charging rate and will never get the house battery fully charged.

 

[andytruck]

But depending where one inserts an isolator into the system it may prevent battery voltage return & not allow the alternator to "turn on".

That is interesting. I have not heard this. Can you explain more about where this isolator can be located to get different effects?
Typically the isolator is wired after the alternator and splits into vehicle and RV batteries. To put it anywhere else seems like it would just be connected to the vehicle battery and always have current going into the "input" terminal and always sending voltage out (as opposed to the alternator going into input and the alternator shuts off when vehicle off and thus send no voltage….

 

[MNtal]

What Is your intended purpose for the 12V power pack? It seems kind of heavy to be moving around a whole bunch. have you ever considered a battery boost product like a NOCO?
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NOCO Genius Boost Plus GB40 1000 Amp 12V Ultra Safe Lithium Jump Starter $100 @ Amazon
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https://www.amazon.com/NOCO-GB40-Ul...=1471383209&sr=8-1&keywords=noco+genius+boost
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I have the 400A unit and it works very well, its light, compact and very effective. My primary use is charging devices via the USB port and jumpstarting vehicles, it does both of those things very well. not knowing your intended use I would recommend the device if you think it might fit your need.
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I might also add in that I love the 7.2A Charger from NOCO and wouldn't hesitate to purchase any of their other products.

 

[Airmapper]

What Is your intended purpose for the 12V power pack? It seems kind of heavy to be moving around a whole bunch.

It's not heavy. Your probably thinking an automotive battery, mine is much smaller. It's more like the battery that goes in a home computer battery backup unit, it's smaller than an ATV or lawn mower battery even. It all fits inside a small ammo box. (It's plastic but similar in size to a 60 cal ammo can.)

Honestly it was a goof off project gone wild. I bought the battery just to have sitting around for ham radio and assorted projects, but it's just a handy way to power 12v and USB stuff without dragging out an extension cord and power supply. It's also handy when camping I've found. It's more about the DIY and learning aspects.

The main advantage to it is the battery is easily sourced, if this battery goes bad, $30 or less and I can swap it out for a new one. I learned that the hard way last time I bought a manufactured power pack, it needed a lot of upkeep and after I forgot to check on it a few times it was dead and useless.

have you ever considered a battery boost product like a NOCO?

Yes, it's highly likely I'll end up with one of those in the future. My power pack is for hobby, non-critical kind of stuff but one of those is on my list for jump starting purposes. I have a NOCO 3.5 amp charger that I use for ATV batteries and the little power pack discussed here and always thought NOCO was a quality product.

 

[pugslyyy]

Honestly it was a goof off project gone wild. I bought the battery just to have sitting around for ham radio and assorted projects, but it's just a handy way to power 12v and USB stuff without dragging out an extension cord and power supply. It's also handy when camping I've found. It's more about the DIY and learning aspects.

I was thinking, if you want to play around with this some more fairly inexpensively why not use a diode or MOSFET at the charging port to prevent discharge? (This is the electrical equivalent of a backflow preventer.)