Charging batteries AND running the DC fridge on the road: a DC-input battery charger

[Wavery]

Originally Posted by PopBeavers

fwiw, my Honda 2000 generator will run for 45 minutes on a pint of regular gas and is very quiet. I typically run it for 45 minutes every other day. I have never measure how many amps the converter can put into the batteries.

I have never measured how much gas my GMC 2500HD would use in an hour while idling. I do know that on a good day I can get 13 mpg. 11 when towing.

I did measure the current input to the batteries on my 2720. For the first 15-30 minutes my original converter put out 12A to the batteries then dropped to about 5-6A. That's the reason that I mounted my new converter closer to the batteries.

The new converter put out ~27A for the 1st 15-30 minutes then dropped to ~8A then after the bulk charge it dropped to 2A while mounted where the old converter was.

When I moved the new converter closer to the batteries, it would start out at 45+A then settle to the same 8A & 2A charge.

Now I have solar so it's a mute point but when I was running the generator to charge the batteries, it was a fairly big deal (to me ).

[Bill]

Googling around the Internet seems to produce a rough concensus - an automotive engine consumes about 1 quart (2 pints) of gasoline per hour of idling. Obviously this depends on a lot of factors, and some of the information I found seems much more reputable and scientific than others (one guy computed 17 gallons per hour!) But it all converged at about that value, and I would trust it within a factor of 2:1 or so. As expected, it is more than a Honda generator consumes, but on the other hand, a couple hours of idling doesn't cost much.

In a normal tow vehicle / trailer setup, idling the tow vehicle produces only a little charge on the trailer battery. The reason is that the alternator's voltage regulator, seeing a fully charged tow vehicle battery, cuts the alternator voltage back to a maintenance level. The voltage doesn't get high enough to push a meaningful charge into the TM battery, and the resistance of the wiring back to the TM just makes the situation worse.

But the charger described by Dave / Shrimp Burrito doesn't care about the low voltage from the tow vehicle. Its whole purpose in life is to take a low voltage, whatever it is, and make it into a voltage high enough to charge the TM battery in a hurry. And that is what you want if you are going to rely on the tow vehicle for charging through the Bargman connector.

Bill

Originally Posted by Bill

But the charger described by Dave / Shrimp Burrito doesn't care about the low voltage from the tow vehicle. Its whole purpose in life is to take a low voltage, whatever it is, and make it into a voltage high enough to charge the TM battery in a hurry. And that is what you want if you are going to rely on the tow vehicle for charging through the Bargman connector.

Bill

I am missing something here.

Assume that the voltage at the battery from the TV is only 10 volts, due to voltage drop in a wire that is too small.

This device will boost that voltage from 10 volts to around 13 volts.

Correct me if I am wrong, but that would require drawing yet more amps through the TV wire and we have already determined that the wire is too small. Depending on just how small that wire is and how many amps have to go through it, then the wire will get hot and/or a fuse will blow.

I know my wire from the TV to the TM is fused at 30 AMPS under the hood. So it is not possible to exceed 30 amps draw.

So total current draw as measured in the TM is:

fridge
converter overhead
charging current to batteries
booster overhead
booster conversion ( to bump voltage up then current must increase)
resistance of all the wire in between

The sum of all this can not exceed the fuse size of the hot wire back to the Bargman.

So the interesting number becomes the booster overhead and the booster conversion. I'm pretty sure that is a very visible number.

What am I missing?

[ShrimpBurrito]

Wayne / PopBeavers - I don't think you're missing anything. The charging circuit under the hood of my TV is also fused at 30A. The input on my new charger is also fused at 30A by the manufacturer. So that means the charger will never pull more than 30A, although the actual pre-set limit is probably closer to 25A. Thus, the fuse in my TV should never blow under normal operating conditions.

As Bill said in different words, all the charger does is pull a high current (up to 30A) at a low voltage (in your case 10v), and outputs it to the TM battery at a lower current (say 20A, which is its max output) and higher voltage (14.2v or whatever). This enables you to charge the TM batteries at a voltage higher than the TV battery. This is physically impossible without such a charger.

The TV or TM wiring does not have to be upgraded. The TV circuit is fused at 30A, so it should be able to handle 30A. The TM has similarly sized wiring, so it too should be able to handle that much current. Does that mean the voltage will drop? Yes. But the charger does not care. If the voltage drops, it draws more current, up to its rated max input.

FWIW, you could essentially set up the exact same thing if you installed an inverter in the TM. The inverter would be connected to the charging circuit from the TV (just like my charger is now), and output 120VAC. You could then plug your on-board TM factory converter into the inverter, and the converter would also charge the batteries in the same way it charges the batteries when you plug the TM into the wall at home or camp. Most inverters can accept voltages down to 10.5-11v, so like my charger, it will simply draw more current to support the AC current drawn by the converter.

My charger is more efficient than that setup, however. While it supposedly is 95% efficient, an inverter is probably only ~85% efficient, and there will also be additional loss in the converter. And when the sole purpose is to maximize charging current to the batteries, those combined losses translate into a fair amount of energy loss.

Dave

I think my confusion was that since the voltage is dropping due to the length of the wire and/or a small sized wire, then any attempt to draw more current (amps) would run into the same problem.

I guess there is a voltage drop problem, but not an amp drop problem.

The only limitation is to not go over any fuse limit.

I took 3 years of electric shop in HS plus several courses in college before I changed majors. You would think I would remember more of this stuff, but that was a long time ago. Before the PC was invented.

[Wavery]

Personally, I don't care if the TV's engine only uses a thimble full of fuel an hour.

Leaving an engine idling for a long time without actually being behind the wheel and seeing what is going on, is risky business (at best).

An idling engine can over-heat. I don't care that your car has dual-electric fans or whatever else it has........stuff happens........ If you over-heat the engine (or anything else goes wrong) at the camp-site, you have a real problem on your hands. In some cases, the vehicle may have to be towed (a long way) to a shop (warranty or no warranty) and you may end up camping longer than you planned or trying to arrange to have your camper towed home......

I just think it's a bad idea. Who needs that kinda grief? If my Honda Generator dies.....I throw it in the back of the truck and take it home.

[ShrimpBurrito]

I think the chance that your TV's engine would overheat in the winter is virtually zero, and that is the time when most folks use more battery power.

In the summer, maybe. But a peak in on the temp gauge every 15 min or so should substantially reduce that risk as well.

Using the charger for this purpose isn't ideal. It isn't what I bought it for. But for someone who does not have the capacity or desire to carry a genny or solar panels, I think it's a realistic option for short periods of operation.

Dave

[Wavery]

Originally Posted by ShrimpBurrito

I think the chance that your TV's engine would overheat in the winter is virtually zero, and that is the time when most folks use more battery power.

In the summer, maybe. But a peak in on the temp gauge every 15 min or so should substantially reduce that risk as well.

Using the charger for this purpose isn't ideal. It isn't what I bought it for. But for someone who does not have the capacity or desire to carry a genny or solar panels, I think it's a realistic option for short periods of operation.

Dave

Actually, the outside air temp has little to do with an idling vehicle over-heating. It could be 10* below zero. It's more to do with ........."Stuff" happening. There are so many things that could fail on a vehicle at any time. If you're behind the wheel, you can turn the engine off within seconds. If a relay, water pump, hose, belt and/or dozens of other things go wrong and the owner doesn't come back for 15 minutes, the engine could be toast........

I'm not saying that it's a BIG risk....it's not....I'm just saying......"I wouldn't do it".....then that's just me........having my background.....I've seen too much of this kinda thing.


Most of my life has been about risk management. If I have a choice.....I take the one with the least risk.......not just talking financial either.......it's more stress than $.

[Bill]

Originally Posted by harveyrv

Personally, I don't care if the TV's engine only uses a thimble full of fuel an hour. I just think it's a bad idea. Who needs that kinda grief? If my Honda Generator dies.....I throw it in the back of the truck and take it home.

Wayne -

I agree 100%. My point was simply that if you want to recharge your TM battery by running the tow vehicle - and many people do - then it doesn't cost much. But unless you use a setup like Dave / Shrimp Burrito has installed, you won't get much charge, either.

Bill

[Wavery]

Originally Posted by Bill

Wayne -

I agree 100%. My point was simply that if you want to recharge your TM battery by running the tow vehicle - and many people do - then it doesn't cost much. But unless you use a setup like Dave / Shrimp Burrito has installed, you won't get much charge, either.

Bill

This is true and it can be done that way......not using this device would be virtually a useless waste of time. However, the only way that I would do it would be to sit in the vehicle and read a book or take my laptop and a movie....leaving an idling vehicle (as I have seen people do).......scary in my book...