Convert watts to DC amps formula

SteveG · 02-16-2008, 10:19 AM

I found this formula on the Xantrex web site HERE to convert watts to actual DC amps: (Watts /12 volts) x 1.1 = actual DC amps. The question is what is the "1.1"? Is that the inverter's internal loss which makes it the "actual" part?

SteveG · 02-16-2008, 10:19 AM

I found this formula on the Xantrex web site HERE to convert watts to actual DC amps: (Watts /12 volts) x 1.1 = actual DC amps. The question is what is the "1.1"? Is that the inverter's internal loss which makes it the "actual" part?

MrTransistor · 02-16-2008, 11:38 AM

Hi Steve,
Correct, internal losses. Most inverters typically run around 90% efficiency.

SteveG · 02-17-2008, 04:45 PM

Thanks Mark. I'am using the formula to try to understand the total amp hours that three 105 amp batteries will support: how long I can watch a TV, run a lamp, run the lap top... So wanted to be sure I had a grip on the formula.

BryanL · 02-18-2008, 08:12 PM

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">total amp hours that three 105 amp batteries will support </div></BLOCKQUOTE>
Keep in mind that you should only run your batteries down 50% so the 3 batteries will have only 150 AH usable at 12v or 1,800 watt hours.

Also keep in mind that battery capacity can vary by 15% or more from cycle to cycle no matter how well you think you have them charged between cycles.

Temperature can also make a difference in available capacity on this order.

The AH rating is probably a 20 hour rating. That means it is determined with a load of about 5 amps. If your load pulls twice that current from each battery, its available capacity will drop by 15% or so. (and if you draw only half, you'll gain that much).

A long steady draw is also going to reduce available capacity faster than short intermittent uses.

In other words, trying to get anything other than a ballpark estimate of how long a battery will last is likely a fool's errand. Your best guide is actual experience. If your batteries won't last as long as you want without going below 12v after resting (no charging or current draw) for a half hour, then you need more battery or better energy conservation. If you don't run your batteries down to 12.4v resting between charges as a normal thing, then you have more battery than you need.

Mikeso · 02-22-2008, 09:36 AM

Somewhat on the subject, when we bought our coach new in '05, we had the dealer install new batteries (they were Interstates). We can run the TV or other fairly minor appliacnes but when it comes to brewing up a pot of coffee, it'll start but won't finish. Do other's have the same problem? Are there a different batteries I need to install to get more out of them? When we had our Luxor, we pulled the 3 12volts out and put in two 6 volts with much better usability. Unfortunately the battery tray on the Journey is too small to fit 6 volt batteries into.
Mike

MrTransistor · 02-22-2008, 10:23 AM

Hi Mike,
I believe the battery trays on your Journey are the same as on our Horizon. Our Journey was an 03. If its capacity your after, then this squeezes the maximum into the tray size. Expensive, but there are many advantages to AGM's. No limit on charge current (a big plus), completely maintenance free, and higher Amp./Hour capacity for the weight. We have a ProSign 2.0 installed which charges at 100 Amperes and are getting ready to parallel a second unit for a total of 200 Amperes charge current. You can't do that with flooded lead acid type batteries.

These batteries make an excellent source for the inverter and we can run the Microwave for 10 or 15 minutes with the refrigerator, computer, and TV all at the same time with no problems.

I also replaced the original 000 cabling from the batteries to the inverter with 0000. I wanted 250MCM but couldn't find any locally. When I get the second ProSine installed, I'll ad a picture in the Photos section.

Batteries Lifeline GPL-4C 6 Volt 220 Amp./Hour AGM

Mikeso · 02-22-2008, 10:26 AM

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by MrTransistor:
Hi Mike,
I believe the battery trays on your Journey are the same as on our Horizon. Our Journey was an 03. If its capacity your after, then this squeezes the maximum into the tray size. Expensive, but there are many advantages to AGM's. No limit on charge current (a big plus), completely maintenance free, and higher Amp./Hour capacity for the weight. We have a ProSign 2.0 installed which charges at 100 Amperes and are getting ready to parallel a second unit for a total of 200 Amperes charge current. You can't do that with flooded lead acid type batteries.

These batteries make an excellent source for the inverter and we can run the Microwave for 10 or 15 minutes with the refrigerator, computer, and TV all at the same time with no problems.

I also replaced the original 000 cabling from the batteries to the inverter with 0000. I wanted 250MCM but couldn't find any locally. When I get the second ProSine installed, I'll ad a picture in the Photos section.

Batteries Lifeline GPL-4C 6 Volt 220 Amp./Hour AGM

</div></BLOCKQUOTE>

Mark,
Thank you, great tip. I knew there had to be a fix.
Mike

SCVJeff · 02-22-2008, 02:51 PM

I just replaced the 3ea. GP-29's with 4ea. U-2200 6v, and they fit with zero room to spare on the sides, and a pair of 2x4's in the front, just like Mark's. The top tray is rated at 250lb, and you'll be loading it to 252lb.

Pusherman · 02-22-2008, 03:54 PM

Jeff, what brand AGM? Any pics?

On edit, probably looks just like Mark's install.

MrTransistor · 02-27-2008, 07:11 PM

Just finished installing the parallel ProSign. I currently have them set to each charge at 75 Amps. I'll see later on if upping to 100 amps. each is needed.

Sammie · 02-27-2008, 08:00 PM

Nice job Mark. Why did you use Prosine instead of the Xantrex everyone seems to be talking about? I now have a Dimensions Inverter/Charger that I'm thinking about changing out.

Thanks, Steve

MrTransistor · 02-28-2008, 10:21 AM

Pure sine wave instead of modified sine. The Winnebago supplied inverter that came with our coach was of the modified sine type. Just another way to say square wave. We had installed the ProSine on our Journey and moved it over when we bought the Horizon. Since I carry a spare any way, I thought why not have it working for us instead of being just extra weight.

NoMoreAZ · 03-01-2008, 02:52 PM

Sammie,

The Prosine you referred to is made by Xantrex. You may be thinking of the RS2000 or RS3000 which many folks use. The Prosine has a smaller form factor.

MrTransistor · 03-01-2008, 07:48 PM

ProSine used to be its own company (Stat Power I believe) before Xantrex bought them out. They also bought Trace and Heart. Yes, Xantrex does make their own pure sine wave inverters, but we bought our first ProSine before it became a Xantrex product. I've looked at other products trying to get more for less but the Prosine 2.0 with its 100 Amp. charging circuit, the included remote and temperature sensor, and the extensive operation programming menu makes it hard to beat. Outback Power Systems X2012MT looks very good because it seems to be very rugged and it's sealed. Except for price and weight it would be my next choice.

Xantrex does have extremely good service!

Sammie · 03-01-2008, 08:26 PM

Thanks Guys, I'm still in my research stage while testing my batteries and Dimensions Inverter/Charger. Its always a learning experience and doing it yourself is the only way to learn.

Thanks, Steve (Sammie)