Battery Power : LUSENET : TimeBomb 2000 (Y2000) : One Thread

Can someone help me convert amphours into watts? Have a genarator but don't want to run it all the time, would like to set up a battery bank to use at night to run a few appl's [ref, and a few light's]. Sam's has a golf cart battery with 210 amphours for 42.00 [6 volt].How many would it take to store enough power for one night of ref and some lights.

-- James Jones (, January 24, 1999


Not an answer, but a follow-up question. The 500 watt inverters I've seen on the retail floor that sell for about $70 say that the output is modified sine wave AC. I presume that light bulbs don't care about the nature of the AC, so long as it's AC. What devices require modified sine wave output and what devices would require true sine wave output?

-- Zorro (a@b.c), January 24, 1999.

Can't help directly on the watthours. However, what is important is how long it will take to charge a bank of those batteries from a specific amount of discharge.

If you're running a generator, it needs to be able to handle the load of charging the batteries, and maybe other appliances, too.

We have a 4KW generator and a 2.5KW sine wave inverter. The inverter takes care of all the charging for us whether from the grid or the generator. (It's a Trace brand).

Another note when figuring your charge time is that you want to avoid discharging your batteries below 50% as that will decrease the life of the battery. It's even better to not discharge below 75%. This applies to Lead-acid batteries. NiCd batteries should frequently be completely discharged, but require more power to recharge than a lead acid battery.

As for those 500 Watt "mofified sine wave" inverters you talked about. There are two things to note: First, almost assuredly it is very inefficient. Probably below 80%. Remember how much power you're losing if 20% or 30% is going to heat.

Second, some appliances are sensitive to power. Incandescent light bulbs are usually ok, but small battery charges - such as those for power tools will burn up on a modified sine wave. The rechargers on laptop computers may be affected too, but I haven't heard that they are as sensitive.

Also with a modified sine wave, most stereo equipment will have an audible hum that will drive you nuts.

That's why we opted for a "True sine wave" (sic) inverter. It costs lots more ($2000), but I can run any appliance including my lap top and stereo from it.


-- Jollyprez (, January 24, 1999.

Jolly, Thanks for the info. For anyone else reading this thread, the archived articles in this forum contain an interesting Sine Wave v. Modified Sine Wave discussion.

-- Zorro (2@2.2), January 24, 1999.

OK, that 210 AmpHr rating is at a 20 hour discharge rate. So, you can expect to pull 10.5 amps out of the battery pack for 20 hours. (210/20 = 10.5) At which time the battery would be totally discharged which is not good. In practice most inverters will disconnect themselves before the battery gets below 25% or so, which is still not good.

Simple relationships:

Watts is Volts x Amps WattHours is Watts x time (in hours) AmpHours is Amps x time (in hours)

In general, anything with a motor will be much happier with a true sine wave inverter. Motors will run cooler, start easier and take les power when run from a true sine inverter.

There are no hard and fast rules here - to estimate how large a battery bank you need, you will have to tabulate exactly which appliances you want to run, how much power they draw, and for how long they will run. (For example, a 100W light bulb for 5 hours is 500 Watthours at 120VAC. This will translate roughly to 46Amphrs at the 12VDC battery pack (two of the 6V batteries in series) assuming 90% efficiency in the inverter.) Remember that you can only use about half of the battery pack amp hour rating. Batteries can be connected in parallel to realize a larger capacity system. Make sur you use large enough wire - that 100W bulb will draw about 9.3 Amps into the inverter. If you don't know what this all means, or where to look it up, find an electrician experienced with 'renewable energy' systems. Failure to exercise proper care here can result in fire or even an explosion hazard. (Batteries can explode under some circumstances.)


-- John Kuklewicz (, January 24, 1999.

A few thoughts (from one who has read up a bit on the subject, but is NOT an "expert", not even 10% of an "expert"):

210 amp hours x 6 volts = 1260 watt hours x inverter efficiency (which may be on the order of 80 to 95 per cent) minus the above mentioned point that it is not good to drain lead-acid batteries any where near completely.

Check out the "Juice Page" at

It has many good tips and links on the subject.

As suggested above, it seems that AC motors really do like real sine wave juice. While "true sine wave" inverters provide only an approximation of real sine waves, they would seem to be better than mere "modified sine wave" inverters.

AC motors require a large surge of current to get started. Most inverters can provide such a surge, but keep it in mind.

I suggest not using your big batteries for lights. Save the big batteries for those apps for which alternatives are not readily available. There are plenty of alternatives for lights. Plus: if things get really bad for a while, it may not be prudent for your neighbors to be seeing your house well lit up apparently with standard lights while they are freezing in the dim light of candles or in the dark.

As you will find in links from the "Juice page", there are batteries that can handle a deep discharge better, and can recharge faster, than the "golf cart" batteries. Gel Cells for one. They are, of course, more expensive.

As far as the refrigerator is concerned, (as opposed to a freezer), you might consider treating it as an "ice box" by filling the "freezer" portion of it with as much ice/water as conveniently fits. When running the generator, you can freeze the water, when the generator is not running, that ice will melt, thereby cooling the rest of the refrigerator.

Also, keep in mind that your refrigerator probably would not be running its motor full time. If it does, you might want to invest in a more efficient one.

In any case, if you do pursue the invertor/battery bank approach, take to heart the safety cautions mentioned above and on the "Juice page". You will also find safety related tips on the Trace web site along with all their marketing stuff.

FWIW, I looked into inverter/battery bank possibilities, primarily for my oil burner and circulation pump, and because of the dampness of my basement and the hassle of being sure that what I might choose to do complied with town electrical codes, I am now looking into another option: a device called a UPS (uniterruptible power supply) which is designed mainly for powering computers, but which looks adequate for my purpose, is self contained, and does not seem to tangle with any local electrical codes. Now I need to see if it is permitted to insert a plug and outlet in the ciruit to the oil burner instead of installing a "transfer switch". :-)

As someone mentioned in another post: it is not rocket science, but it also is not a no brainer!

Do take care!


-- Jerry B (, January 25, 1999.

Just another note or two.

Though I've done minimal electrical wiring, I'm not an expert on this subject, either. All that I have learned was in conjunction with converting a bus to a motor home, and my new system at home.

The model inverter I purchased (Trace 2512) had *all* the instructions and diagrams to install a system that was to National Code.

It includes instructions for hooking to the grid, separate circuit breaker panel(s), battery, and generator hook-up.

In addition, it acts as an UPS by tranferring in less than a second if power goes out.

I'm telling you all this to let you know that it isn't too hard, if you follow the instructions that come with the unit.


-- Jollyprez (, January 25, 1999.

Wear safety goggles when working with batteries. And only store them outside or in a well ventilated area. I have had a car battery explode in my face - the explosion wasn't much - about like a modest punch - but I got battery acid all over my face. Good thing I was wearing industrial safety glasses at the time. And then bathed in baking soda water about a dozen times. And my shirt was covered in burnt and bleached spots - fell apart when washed with baking soda. So be careful.

-- Paul Davis (, January 25, 1999.

Hi James, I won't go into too much detail here...looks like a lotta other folks provided you with some good stuff. I'm going to say that your fridge will use 3500 watt hours per day and throw in another 500 watt hours for lighting. That gives a nice round figure of 4000 watt hours. Divide that 4000 by 12 volts and you get 333 amp hours of battery power used to provide that fridge and those lights with juice. Because you shouldn't discharge your batteries more than 50%, you double that ...that makes a total of 666 amp hours of total battery capacity needed. Every 2-6volt golf cart batteries makes a 210 amp hour/ 12 volt bank. 3 of these 2-battery banks would equal 630 amp hours @ 12 volts...close enough for this estimate. Bottom line...6 of those golf cart batteries would do it nicely. This is assuming that your fridge is relatively efficient ('95 and newer and under 20cf should be OK) and you keep the lighting to a minimum. And don't forget to size the inverter to take the fridge's compressor motor surge...something around 1500 watts should run it and some lights simultaneously. Hope that helps answer some questions for ya! Roy Four Winds Renewable Energy Co.

-- Roy @ Four Winds (, January 25, 1999.

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