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Andrew asked what the electricity in my mileage logs was for...

If anyone can remember my rants about not being able to FAS at night because my battery would run down, I have been experimenting to "solve" the problem. I have bought a 1.25A trickle charger. and am now using it to charge my battery overnight when I get home. Here are my quick and dirty calculations about the economics.

Typically, I have to recharge after every second or third nightime FAS. This way I don't have to NICE-on coast at night. Here are the numbers:

When I FAS P&G, I typically get 60+ mpg on my commute home from work. When I Nice-on coast, I get 50-. Using those numbers with my 23.5 mile commute, it works out to 0.0783 gallons saved per trip. At $2.00 per gallon, that is about $0.157 saved per trip, so that would be $0.314 per required recharge.

The charger draws 0.2A when it is charging, and (effectively) nothing when the battery is fully charged. That works out to 22w per hour of operation. I have not done an exhaustive study of how long it takes to fully charge the battery, but I leave it on for about 8 hours, so lets say that is how long it takes. That means I use 1760w (probably really less than 1500w) of electricity to charge my bettery. I am paying a marginal rate for $0.11 per kwh, so that is $0.165 per charge. Thus, I am saving $0.149 per charger use. In order to pay for the charger ($30), I will have to use it 200 times as long as gas is $2.00. If we get back to $4.00 gas, I will be saving $0.463 per use, and we are down to 65 uses to recoup the investment.

Of course that only counts the energy saving. My battery will probably last longer with a fuller state of charge, and the aggrivation of not having to jump start when I forget is, as they say, priceless. :)

Larry

Very nice. I'm actually partway through making a homebrew version of the same. I had a 12v 1000mA wall-wart adapter on hand already, so the only challenge is making the connections.

It draws about 5W from the wall, so it's less than 1/4 the power of yours. Time will tell if it's worth anything.

Nice plan!

... The charger draws 0.2A when it is charging, and (effectively) nothing when the battery is fully charged. That works out to 22w per hour of operation. ...

Is your charger core a transformer (heavy), or a switching converter (light)? If the former, the actual power is usually less than (volt x amps). An actual power measurement, such as from the Kill-A-Watt meter mentioned in an older thread, will be more realistic.

BTW, 'watts per hour' is redundant. The 'per (time unit)' is already included in 'watts'. The mass media makes this mistake on a regular basis, and "watts per hour" and "watts per day" makes me pull my hair (or what is left of it) out.

-- Dean, electrical engineer

BTW, 'watts per hour' is redundant. The 'per (time unit)' is already included in 'watts'. The mass media makes this mistake on a regular basis, and "watts per hour" and "watts per day" makes me pull my hair (or what is left of it) out.

-- Dean, electrical engineer

Whoops, failure to do dimensional analysis. Why else would units of energy have to be expressed in watt-hours (watt-days).

Larry, who fails to state his degree major, due to acute embarrasment!
:)

Is your charger core a transformer (heavy), or a switching converter (light)? If the former, the actual power is usually less than (volt x amps). An actual power measurement, such as from the Kill-A-Watt meter mentioned in an older thread, will be more realistic.
-- Dean

It is a transformer type. And I am sure you are right about the power (and current draw). The charger didn't have a label stating the current draw, and the Schumacher phone droid, er.. customer service repreresentative said it was 0.2 A at full charge and almost nothing when the battery is full. I wanted to estimate highest bound for home electricity use to avoid optimistically predicting my savings. My number should be a "pessimistic" number.

Larry

Mine measures 5w on the kill-a-watt meter. All night long (12 hours) would be 60 WH daily.

I've used it 2 nights now and haven't registered a difference yet.

Mine measures 5w on the kill-a-watt meter. All night long (12 hours) would be 60 WH daily.
Whoa! That's really cheap! Really makes it worth while.

I've used it 2 nights now and haven't registered a difference yet.[/QUOTE]

A difference in what? How much FAS P&G do you do at night? About an hour and 30 minutes of FAS P&G at night will drop my voltage to the 11.3 range. I suspect that if you start charging from that point, you will be drawing more than 5w.

Larry

That is cheap, but I'm not sure it's enough to be effective.

I only drive for about 30 minutes. I've been as low as 11.3 / 11.2 several times in the last week, while FASing with headlights. Lights off and it pops up to 11.5 or so.

Park at 11.5/11.6V, charge overnight, leave at 11.8V. I'm hoping it will gradually raise the baseline charge above 12 - haven't seen that in a while except from the alternator. It's unclear how much the temperature drop overnight changes things, either. It was right at freezing this morning.

That is cheap, but I'm not sure it's enough to be effective.

I only drive for about 30 minutes. I've been as low as 11.3 / 11.2 several times in the last week, while FASing with headlights. Lights off and it pops up to 11.5 or so.

Park at 11.5/11.6V, charge overnight, leave at 11.8V. I'm hoping it will gradually raise the baseline charge above 12 - haven't seen that in a while except from the alternator. It's unclear how much the temperature drop overnight changes things, either. It was right at freezing this morning.

Yeah. Judging from your post, I am throwing a lot more watts at my battery at night, but I am at 12.0 or 12.1 in the morning, and the charger is saying that is is no longer charging. The way I figure it, I am putting between 150 and 200 watt hours into the battery each time I charge it. Maybe less if the charger cuts off early.

Larry