12V DC-DC Chargers

[Buliwyf]

I don't see how you can overcharge a battery with an alternator. The alternator only controls voltage with it's voltage regulator. It's up to the battery to actually draw amps.
A 12v to 12v charger seems excessive if you already have a solid 12v source from the alternator. Is this a solution to an odd style alternator that fails with multiple battery setups? The only plus to running these, that I see, is that when I connect my weak aux battery to my running truck, the alternator will have a softer load because of the slower charger.
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If the batteries are in parallel, even though the alternator has no idea one is charged and the other isn't, the alternator outputs 14.5 regardless. Each battery sees 14.5v. Nothing to it. You can feed a 12v battery 14.5v forever and it'll be fine. As long as alternator output remains at the proper voltage you're good to go.
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You can only overcharge a battery if it sees more voltage than that. If the alternator starts going wonky and outputs 18v you're in trouble. I've never seen that happen because of one dead battery in a string of fresh batteries.
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The average prime mover I work on has 4-20 batteries. Just one alternator. Maybe two chargers if the power's on. No isolators needed anymore.

 

[4x4junkie]

It has to do with the battery's temperature dynamics. A battery at say, 120°F begins to overcharge (gas out) at almost a whole volt lower than a battery at 60°F. If the charging source isn't tracking the battery's temperature, it's likely to overcharge the 120° battery, but won't charge the 60° one to full charge (at least not in any reasonable amount of time).

The better a charging source can track the battery's temperature (and can adjust the charging voltage accordingly), the faster and more complete the charge will be without it causing the electrolyte to gas out.

Monitoring my Ford 3G alt's voltage output, it actually is able to track temperature surprisingly well given the environment it operates in next to the engine (outputting about 13.9V on a 110° desert summer day, and 14.6 on a 40° winter morning after the engine's fully warmed up), however it would be a lot more precise if there was a sensor that could be attached directly to the battery's post or case or something (an output of 14.8-14.9V would better force a charge into a 40° battery... Maybe they figure with the battery under the hood next to the engine, it'll eventually be warmed up and will more readily take it's charge at a lower voltage). This is where there can be some potential to have issues with batteries mounted away from the engine compartment in cold environments (especially if your alt is a bit stingy on the voltage, which I've seen on some remanufactured units).

 

[DiploStrat]

UM...what am I missing here? Why get another piece of gear for $$$$ when you can only charge your batteries at the maximum rate that your alternator can put out? There is no way that I know of that you can get more energy out of you alternator than it is rated at without burning it up.?????

Go look at the Sterling (or competitor) site. There are two issues here:

-- In most solenoid isolation set ups the first problem is that the alternator brings the starter battery back up to proper voltage too fast. At this point, the alternator drops its output and does not continue to charge the second battery.

-- The second problem is voltage drop between the starter battery and the camper battery, especially if the camper battery is a long way away and the cables are too small. A battery will not charge (will not accept current) unless the charging voltage is higher than the voltage of the discharged battery. (Think about trying to charge a high pressure tire with only your own breath.) In most cases, a diode isolator is even worse, introducing a .75v or more drop. On a 12v system, is comes out to around 10%.)

The net result is that often the camper battery receives neither adequate voltage or current. You have to get the alternator to continue to produce adequate current and voltage, even after the starter battery has been recharged. Obviously, the greater the difference in size, distance, and type, between the starter and camper batteries, the greater the problem.

A B2B corrects this by dropping the voltage across the starter battery, which causes the alternator to ramp up to maximum current. Then that current is amplified to a higher voltage to charge the camper battery. In some cases, there is even a separate sense wire to measure the voltage at the camper battery to assure that the regulator compensates for any voltage drop.

This gentleman is quite opinionated, and his comments are directed at fifth wheel users, but his site is well worth reading for anyone who is serious about this stuff: http://handybobsolar.wordpress.com

 

[verdesardog]

ok, yes that makes a lot of sense!

 

[DiploStrat]

Chief,

Love the hound!

I have been a bit obsessive about this issue for a year or so, trying to get the real world truth for my camper, Ndeke Luka. http://www.pbase.com/diplostrat/ndeke_luka

Many folks will tell you that there isn't a problem and that all you need is to parallel the batteries and that, at that point, the alternator will treat them both as one big battery. This is what should happen, but, for the reasons mentioned, doesn't always happen.

Similarly, others will tell you that they have never had a problem. The never-had-a-problem may contain elements of the following:

-- Two similar batteries, close together - probably no problem.
-- Plug in every night, especially with a modern, multi-stage charger - certainly no problem.
-- Good solar kit or good genset - probably no problem.

But if you want a camper with:

-- No genset,
-- No access to shore power, and,
-- Cloudy weather,

then you want to be sure that you are getting all you can get from your alternator(s). The best answer that I know is a dedicated alternator/regulator for the camper battery, as described by G35vortec, above. There are, of course, some issues here, as in mounting brackets and/or the need to separate a pair of factory alternators. This latter task is harder and harder in the world of computer controlled vehicles.

In my case, I have gone with a Sterling A2B (http://sterling-power-usa.com/12volt-210ampalternator-to-batterycharger.aspx) and the first tests are very positive:

Should this not work in the real world, I will find a way to split the alternators and install a dedicated regulator for the camper batteries.

Shoot me an e-mail and I will send you a white paper that I have drawn up on the subject.

All the best!

 

[verdesardog]

No problem for me, my truck has dual batteries in the engine compartment with temp sensors, I hook up my camper trailer to the positive line while traveling, then disconnect and connect the trailer battery to a solar panel with a good battery charger/minder/conditioner.

 

[DiploStrat]

A test you might perform.

-- Discharge you camper batteries to 12v or a little less.

-- Start up your truck and measure the voltage across your starter battery. With the alternator running, should be over 14v.

-- Now connect the two batteries. What is the voltage across the starter battery? What is the voltage across the camper battery?

-- If you have access to an ammeter, determine the amperage flow to the camper battery.

This will give you a rough idea of how much you are getting from your truck's alternator.

In the end, it may not matter much if you top off the camper battery with the solar unit or a good shore power unit when at home.

Best wishes.

 

[verdesardog]

While in the Navy I maintained several large battery arrays, one good thing about them is thay were all in the same room. Most of the were 6v golf cart type wet cells. Some were large fork lift type.

I just never thought about the temperature differences in remote battery set up, that why I was questioning 12v to 12v chargers.

My set up works just fine for my needs. I haven't built my Vardo trailer but it will only need electricty for LED lights and maybe a water pump. I have one blue top optima which I feel will be enough. I will probably never be stationary for more than a week at a time.