Some questions about using a 12-volt-to-110-volt power inverter on board a vehicle.

You expected less?

TOM K8ERV Montrose Colo

 

Let's see. Two handheld and two computers, all lying on the seat besides you. And you're driving?

 

I knew a guy that could eat a Hoagie while steering with his stomach....

 

For what it's worth, I have two mobiles set up with an secondary battery system. Both make use of battery isolator's. Both are fed with #2 awg welding lead on the positive side and body for the negative return. A good source is Alan's website for references. The battery difference between the two is that one makes use of dual Costco golf cart batteries (vented) and the other is a sealed AGM (used cell phone site backup power). The cheaper route with good power is the golf cart batteries but there are precautions that go with flooded batteries. Safety first.
I agree that laptops can work off power adapters better than an inverter. I do this with digital comm's using the secondary battery bank for the transceivers and powering the laptop via cigarette power plug adapter. I do have a 1700 watt inverter purchased from a RV supply store that works well but rarely uses it. It is a modified sine wave type that does not produce noise. As mentioned earlier the cheaper ones will most likely be very noisy RF wise. You can't go wrong with a true sine wave inverter of quality. Both secondary power systems have a battery booster in line to insure 13.8vdc is supplied to the transceivers. The newer rigs have a low tolerance for low supply voltage. All DC power cables use the same quick disconnects to allow configuration to various operating needs. Like I said earlier, take a look at Alan's site for many good references.

 

Everyone here agrees on the sine-wave noise problem of inverters. The other problem is the amount of power drawn is not without significant loss. When you convert 11.5 to 14 VDC to 110-128 VAC, and look at the power input versus the power output, you will find that a loss of 40 percent is not uncommon. This is usually dissipated as heat, which explains the large heatsinks ( metal fins ) and fans on inverters. That said, you should be certain that the 12 VDC input leads are rated for sufficient current and that the vehicle alternator is running all the time so as not to end up with a dead vehicle battery. That also means taking care not to overheat the vehicle, with best cooling using the vehicle radiator at speed- not the auxilliary electric fan. If audible noise is a problem, grassy areas can help- yet also be a potential fire hazard under the vehicle. Your air-conditioner can give you clues about how long you want to use your alternator at idle. It has a separate 'radiator' type system- not as big a problem in winter as summer. Don't forget that those portable batteries should never get frozen nor overheat.

 

I agree. If the inverter is designed correctly (who know if they are, as no schematics are available), and the load is about 80% of the maximum, then ≈80% is possible using a modified syne wave design. Pure syne wave units are always inferior efficiency wise, and 40% is closer to reality.

All this sounds rather poor, doesn't it? Here is something to mull over. What do you think the efficiency was (wattage in vs. wattage out) of an old dynamotor, like those used on pre-transistorized, commercial mobile transceivers"?

 

I'm not sure what the efficiency was but a modern equivalent should be very efficient. A typical motor can get easily above 80% efficient. Special purpose, and very expensive, motors can get 98% or more. Large industrial generators can also achieve 98% or more efficiency. Something small and inexpensive should still be able to be above 80% efficient. Put the two together and you'd get a range of something like 65% on the low end to 95% or so on the high end.

Here's something to mull over as well. If a modern dynamotor can be so efficient then why don't we see more of them for sale? Especially when the output could or should be far "cleaner". I have a few guesses. One is weight, a dynamotor that would be large enough to run even a laptop would likely be quite heavy. Another guess is that a dynamotor would take too long to get started to be useful as an uninterruptible power supply. The dynamotor could be kept spinning so that there is no delay but then even at 95% efficiency that's a lot of power wasted unless the power supply is lost very often and for long periods. Another guess is that few people demand a true sine wave output, and those that do can get it from an electronic device and not have to deal with the size and weight of a dynamotor. There's also the advantage of millisecond response times to start, but at the cost of efficiency in conversion.

 

Let us not forget, Kurt, that in this case, the "motor" is DC powered. Even if we drive the motor with electronics (brushless design), getting 80% isn't a cheap exercise!

 

Okay then, what do you consider a good range for the efficiency of a modern dynamotor? Would 70% to 75% be a good range for the motor? Is a 90% efficient generator out of bounds for you? Let's do some math.

0.70 x 0.90 = 0.63
0.75 x 0.90 = 0.675

That doesn't change my low end estimate much. For someone that wanted to turn a DC power supply into a pure sine wave AC power supply I imagine that a modern dynamotor could compete very well with a modern solid state inverter on efficiency. Other matters, like costs, maintenance needs, size, weight, and so on can complicate the calculation very quickly, and therefore make a dynamotor impractical.

 

It has been years since I tested a dynamotor. If memory serves, the only surplus military ones (nominal 6 volt DC input) ran about 50%. I suspect that aircraft ones were a bit better, if for no other reason than the higher input voltage.

Of course, we could talk about the old vibrator supplies??