Charging the battery with a generator: AC or DC?

**BobWilson** · 12-09-2003, 02:16 PM

On our last trip to Joshua Tree National Park we dry camped and used more electrical power because it was colder outside 30F to 40F at night during our 6 night stay) and preferred to be inside more. So I brought my Honda EM650 charger to charge up the battery. I plugged the TM's AC cord into the generator to charge up the battery the first couple of nights. Using the ammeter I installed, as recommended by Bill Jeffery, it measured 5 amps of charging with nothing else running on the TM. I wondered how accurate the automotive 20 amp ammeter is since the scale is so small.

I noticed that the generator had terminals rated at 8.3 amps at 12 Vdc expressly for charging a battery. So I made a cable from parts at an automotive parts store and plugged it in. I didn't have a way of measuring the charging current without modifying my ammeter set up. But I was wondering which is the best way to charge the battery. Faster is better, of course, from a time standpoint because you can only charge in that campground at certain hours of the day. And of course you have to be at your trailer to do it at those hours. So time is important. Charging slower is probably better for the battery in the long run. But how much current is too much? I have two Interstate 6v Deep Cycle batteries in series. What if I connected the 12 v cable to the batteries and plugged the TM cord also to use for lights and other appliances at the same time?

Bob W.

**Bill** · 12-11-2003, 02:36 PM

Bob -

Assuming your TM has the 6332 converter/charger, its total DC current capacity is 32 amps. Some of this amperage (either 7 or 12 amps, apparently a factory option) is available for battery charging, and some runs whatever 12-volt loads you have on (lights, furnace blower, etc). The total cannot exceed 32 amps, so if you were running, say, 27 amps of "appliances", then only five amps would be left over for battery charging.

But you said that you weren't running any loads at the moment. In that case, either your battery was reasonably well charged up at the moment, so it wouldn't accept much charge current. Or there is a slightly loose/corroded connection somewhere in the line between the TM converter and the battery. I would suspect the former - your battery wasn't really flat - in which case the direct connection from the generator wouldn't have charged any faster.

FWIW, my battery never charges any faster than about 12 amps as indicated by my ammeter, even when it is quite low. I am reasonably confident in the ammeter accuracy.

Quote:

What if I connected the 12 v cable to the batteries and plugged the TM cord also to use for lights and other appliances at the same time?

In this case, you have two chargers trying to charge the battery at the same time. I can't see any specific reason why this would be bad, but I'll admit that it makes me nervous. My guess, though, would be that one charger would essentially shut off the other, so you wouldn't gain anything.

Neither of these chargers (or even both of them together) put out anywhere near enough current to damage your batteries.

Bill

**BobWilson** · 12-11-2003, 02:56 PM

Thanks Bill.

I have a lot to learn. Bob Strong talked me into buying a digital voltmeter that plugs into the 12 vdc socket in the Tm for battery monitoring. It is interesting to watch how the battery responds to demands and how long it takes to recover after you turn off the appliance using the current. Do you have a recommended source for a voltage table of volts vs. per cent charge? And how useful is this method?

Bob W.

**Bill** · 12-12-2003, 04:16 PM

The plug-in digital voltmeter is a good idea. With experience, you will learn what it means with your particular setup, and that is the best lesson.

To the best of my knowledge, there is not a good hard-and-fast correlation between battery voltage and state of charge. This situation arises from several factors.

Battery voltage depends on battery temperature. A cold battery will register a lower voltage than a warm battery, even at the same state of charge.
Battery voltage depends on the amount of current that is being drawn at the moment.
Battery voltage depends on the amount of current that has recently been drawn - this is the recovery time that you referred to.
Battery voltage depends on what kind of battery you have - flooded vs gel-cell, etc.
Battery voltage is pretty meaningless if the battery is being charged.
The total range of battery voltage, from fully discharged to fully charged, is very small. Thus even a small change in voltage, from measurement error or from any of the above factors, may represent a large change in state of charge.
State of charge (empty, full, half, etc) is not a well defined term. Think of it this way. It is pretty clear when a water bucket is full, half full, or empty. But stuff that bucket with a big sponge, and it is not nearly as clear. If you turn the bucket over and pour out all the water, is the bucket empty? Probably not - you can always squeeze a bit more water out of the sponge. Now slosh in a bunch of water until the bucket overflows. Is the bucket full? Probably not - you can squeeze in a bit more if you do it slowly and let the sponge absorb it.

What you really want to know is stuff like "When the digital voltmeter reads 12.5 volts, my furnace will run for another hour, but not much longer." You learn this from experience, not from a chart or formula.

I suppose that, as a general rule, a resting battery may present something on the order of 13-13.5 volts when fully charged, and 12 volts or so as it dies. If you search around the web, you will find several rules of thumb like this - and they don't agree.

Bill

12-31-2003, 09:02 AM

Good info from Bill. He's right on. Where most novices (that is anyone who's total battery experience is changing double As in their Walkman) make their biggest mistake is thinking a 12 volt battery is at its best when it reads 12 volts. The converse is true -- it's about ready to fail or be completely discharged when it reads 12 volts. So what you really need to know is what is the battery's reading when its fully charged... and that will be well above 12 volts. Second thing you then need to know (as Bill points out) is how long will that last depending on what you are using. Then the third thing you need to remember is reading the voltmeter while the battery charger is connected and operating is just telling you what the charger is producing, not what is stored in the battery.

All this then leads to one of those many life imponderables -- why do they call it a 12 volt battery when it is near dead at 12 volts?

Ed

RockyMtnRay · 12-31-2003, 05:29 PM

[quote author=efelker link=board=2;threadid=1568;start=msg11224#msg11224 date=1072882971]

All this then leads to one of those many life imponderables -- why do they call it a 12 volt battery when it is near dead at 12 volts?

Ed
[/quote]
The best answer to this possibly rhetorical question is to blame the chemists (a group I sorta belong to since I have a M.S. in the subject). If I'm correctly recalling my inorganic chem studies (from about 35 years ago), the nominal voltage (electromotive force or EMF) of an electrolytic cell using Lead & Lead Oxide plates and Sulphuric Acid as the the electrolyte is 2.2 volts. This was usually rounded to 2 volts as a typical working number when the cell was under a load. Hence, 6 cells = 12 volt battery, 7 cells = 14 volt battery (small aircraft, some boats), 12 cells = 24 volt battery (trucks) etc. Using 2 volts as a typical cell's working voltage is just a handy & easy to remember/easy to multiply number.

**Sinclue** · 03-17-2004, 04:27 PM

OK gang, another charging related question...

I usually rely on my single solar panel connected directly to my batteries (generates about 4 amps per hour in good sunlight). During the winter I've supplemented the charging with a Honda 2000 generator that I plugged the TM into. When I did that I disconnected the solar panel "just in case". My question is did I need to do that? Would anything be harmed by leaving the solar panel charging/connected to the batteries? Alternatively, should I connect the generator directly to the batteries with the cables and either leave the solar panel connected or disconnected?

jp

**Bill** · 03-17-2004, 07:12 PM

The answer is that it is almost certainly OK to connect both at the same time.

To convert "almost certainly" to "certainly", there is a multi-part quiz.

Question 1. Is your solar panel connected directly to the battery, or is it connected through a charge controller (a small blocky box that should say something like "controller" on it)? If there is a controller, then you are "certainly" OK to connect everything at once. [To answer your second question, you may connect the generator directly to the battery, or you may plug in the TM and let the TM's charger top up your battery.]

If you don't have a controller:
Question 2. Do you leave your solar panel connected at night? If so, is your battery dead when you get up in the morning?

To be continued if necessary ...

Bill

**Sinclue** · 03-18-2004, 08:09 PM

Ok Bill, thanks again. I do have a charge controller so I guess my question(s) are answered. I thought as much, but its always good to get confirmation from a trusted source.

jp

**Bill** · 03-18-2004, 09:28 PM

You bet, JP. I have been learning a lot about solar lately, most of it from the Wind-Sun forum at http://www.wind-sun.com/forum/

The concern that you have expressed is legitimate. A charge controller takes care of it. It prevents any source of energy (including a battery charger) from feeding power backward through the panels.

You are good to go, with both connected - no need to disconnect anything.

Bill