Voltage

Use a regular digital voltmeter to measure the system voltage right at the battery posts. My bet is it will agree with the plug-in tester. Dash gauges are never perfectly accurate. Their purpose is for you to notice when something has changed drastically over what you normally see.

Also be aware there are voltage drops in all electrical wiring. That is due to resistance and the current flowing through the circuit. If you are going to compare two voltmeters to each other, you must take the readings at the same place and at the same time.

Your dash gauge is not actually measuring system voltage directly. The instrument cluster is a computer module that is powered by multiple 12-volt feed circuits. On one of those, it reads the voltage, then translates it into a series of voltage pulses that set the "Volts" pointer to the desired position. Due to the current flowing through all the other circuits tied to the 12-volt feed circuit, there will be voltage drops, and the voltage seen at the instrument cluster will be lower than that right at the battery.

Two different gauges reading slightly differently is to be expected, and is typical. What is important are two readings, depending on what type of problem you are trying to solve. The first is battery voltage with the engine off. A good, fully-charged battery will read 12.6 volts. If it is higher than that, it is due to the "surface charge" from excessive electrons that have not been absorbed into the plates yet. That occurs right after charging the battery. Turn on the head lights or heater fan for about ten seconds, then do any battery testing, for accurate results. You will see the voltage has dropped to 12.6 volts.

If you find the battery's voltage is around 12.2 volts, it is good, but totally discharged. Charge the battery at a slow rate for about two hours with a portable charger, then test it again. If you find the voltage is around 11 volts or less, it has a shorted cell and must be replaced.

The second important voltage is battery voltage again, but with the engine running. That must be between 13.75 and 14.75 volts. More than around 15.0 volts will tend to boil the water out of the battery's acid. With less than 13.75 volts, the battery will not fully recharge after cranking the engine. There are exceptions to this rule, so do not get excited if the battery voltage drops a little. Generators are very inefficient at low speeds, so with a lot of loads turned on, like the heater fan, rear defroster, and head lights, most of the current to run them will have to come from the battery at low engine speeds. That will draw the voltage down for a short period of time. That is totally normal and to be expected. In fact, all charging system testers require engine speed to be raised to 2,000 rpm for accurate results.

For my final parting thought, all voltmeters have a tolerance rating that is usually listed in the paperwork that comes with it. A typical tolerance is something like "+/- 2% plus one digit". That means if you are measuring exactly 100.0 volts, the meter could read 2.0 volts, (2 percent), and.1 (one digit), higher or lower. The meter could read anywhere between 97.9 and 102.1 volts, and be in specs. Many meters have much larger tolerances, although you will still find most them to be pretty accurate and consistent.

"Battery" gauge and "Volts" gauge are the same thing. They're reading system voltage, and as I mentioned, they're only approximate. Most of them only show the lowest and the highest readings, typically 8 and 18 volts. Don't use them to make a diagnosis. They're only useful to notice when something is abnormal.

A 15-amp charger is a slow charger and will work just fine. That only means it is capable of delivering 15 amps continuously over a considerable period of time. In actual practice that isn't going to happen. As the battery charges, it's voltage goes up slowly, and that voltage opposes the voltage from the charger. Think of a water pump filling a municipal water tower. As the tower fills up, the water pressure goes up, and it opposes the pressure from the pump. Once the two pressures are equal, no more water will flow up to the tower. As the battery charges up, no more current will flow from the charger.

That over-simplifies the story. In reality, charging current will never go to 0 amps, but even though current continues to flow through the acid, no more electrons will be stored in the plates. All continued charging will do is heat up the acid and potentially boil the water out of it. As a general rule, you can consider the battery fully-charged when the charging current drops to five amps. If you start with a totally discharged battery, that will take around two hours with most small chargers.

I've never used a meter like the one you're looking at. As an electrical specialist, logic tells me they can't be that accurate when trying to provide so much information, but they do seem to work.

I have no use for automatic battery chargers. My friend has a few in his shop, and he is always fighting with them to get them to just charge a battery. I have some older small units that work fine for slow, overnight charging, and I have some larger ones for getting an engine started quickly. The problem for me with those larger ones is I've damaged more than one battery by forgetting to turn them off. I left one on for over a week before I smelled the rotten egg smell of the boiled acid. I can't be trusted with any large charger that doesn't have a timer on it.

I'm not sure how to interpret your last comment. Car batteries have cold crank amp ratings in the range of 650 CCA to 850 CCA. Those numbers are used to compare one battery to another, and refer to how many amps it can deliver to the starter, for 15 seconds, at 0 degrees, or 32 degrees Fahrenheit, I can't remember which. Chargers do not have a cold crank amp rating. They are simply listed as the number of amps they can develop under maximum load conditions. The really huge industrial charger I used at a battery store was capable of developing 300 amps to slow-charge 24 batteries at once. (We were supposed to slow-charge every battery in stock once a month). Larger wheel chargers for home and shop use develop a maximum of around 200 amps, but that is just for cranking an engine. Even in your van, your alternator is capable of developing over 100 amps, but it is never going to be asked to do that. The battery is still going to charge at less than 20 amps when starting from nearly fully-dead. So, the CCA ratings for batteries have no relation to the size of battery chargers. A 15-amp charger will work just fine. A smaller charger just might take a little longer to do the same job.

You are really over-thinking this. Also, it depends on what you are trying to do. If you allowed the battery to run completely dead, the three-amp charge rate will fully charge the battery if you leave it on overnight, possibly for as long as 10 - 15 hours. The 15-amp range will do the job faster, in as little as two or three hours. If you trust the automatic controls, you should be able to leave the charger on overnight at that rate too, then the unit will taper off the charge rate on its own to prevent over-charging the battery.

If you are going to leave the van sit for the winter, the numerous computers' memory circuits will drain the battery. Unless specified differently by the manufacturer, the industry standard is to allow up to 35 milliamps, (.035 amps), for that "ignition-off-draw" current. Chrysler says at that rate, a good, fully-charged battery will still be able to crank the engine fast enough to start after sitting for three weeks. I have found with my 1993 Dodge Dynasty, (with less than 5,000 miles, so it sits a lot), the battery will be good enough for engine cranking after six weeks, but that is not guaranteed for all models.

If this is going to present a problem, your van will have a fuse to pull out to stop that small drain. I think you will have two fuses in a bright yellow holder that will pull part way out. Older cars like mine have a bullet connector in a fat cable to unplug. There are also battery maintainers made for this. Your three-amp charger should not be used for this purpose. Over a period of multiple days, it is going to over-charge the battery unless automatic controls turn it off. A better solution for that is a small solar-powered battery maintainer. 35 milliamps is not very much current, so the smallest solar charger you can find will do the job.

If you simply ran the battery dead by listening to the radio, use the 15-amp range for 15 - 30 minutes. That will be enough to start the engine, then the van's charging system will finish the job. Same thing if you need a jump-start in a parking lot. Normal driving for five to ten minutes will charge the battery enough to restart the engine next time.

Some people mistakenly think when they "upgrade" to a new battery with a higher CCA rating than the old one, they need a battery charger with a higher current rating. In fact, those two things have nothing to do with each other. That is like saying you need a water pump that can supply more gallons of water per minute because you are going to take longer showers. The charger manufacturer is exploiting the general lack of knowledge, or is trying to overcome the confusion about this by putting that, "Rated for 800 CCA). It is just as appropriate to say, "Rated for black batteries", or "Rated for lead-acid batteries". They are trying to assure you the charger will work on a replacement battery that has a higher CCA rating than the old battery had. When you understand batteries and chargers, you will realize that notation means nothing.

Besides the CCA rating, which indicates the maximum current the battery can supply while still maintaining at least 9.6 volts, you have to consider how long it can do that. No dog has a stronger bark than a Saint Bernard, but it can only do that once a minute, then it has to regroup and build up to its next bark. That does not accomplish very much. A chihuahua can yap, yap, yap all day long, and once it starts to become irritating, you can blow on it and knock it over. That does not accomplish much either. With car batteries, you need one that can develop a good 300 amps to get a starter motor going, and once it is up to speed, it only takes around 100 amps to keep it going. Putting in a battery with a higher CCA rating is not going to change anything. Even though it is capable of supplying more current, it is not going to. The starter will not crank the engine any faster, and the trade-off for that higher current capacity is likely to be that it cannot supply high enough current to the starter for as long as your old battery could. I just put a new battery in my Grand Voyager this morning. It used to run an inverter and my laptop for two hours, then it could still start the engine. That time became shorter and shorter until today, with the colder temperatures, it could only crank the engine for about three seconds; not long enough for it to start. I knew the end was coming because for the last five or six weeks, it could only run the inverter for about two to five minutes. It has a really high CCA rating, but that did not make it crank longer, crank faster, or have a longer life.

The bottom line is the charger you have will be fine for any 12-volt battery, and there is no advantage to buying a more expensive replacement battery with a CCA rating higher than necessary.