How do I calculate the inverter / batteries / Solar panel thingy?

[MoonTimber]

 

John T Long retired EE so no warranty, Im rusty on this stuff

 

Cool! I'm getting schooled by an EE!

 

Normally I'd have to pay for that.

[MoonTimber]

Actually, the elephant in the room is the existing generator and the old converter.

 

I would bring the batteries up to par (matched set as above), a good 3/4 stage charger (progressive Dynamics - PD9280) and your inverter (as Above)

 

Chances are the old MH came with an old charger. You get up, fire off the genny, make your coffee / breakfast (and charge the batteries auto). Do your whatever, fire it up for supper. I would get a battery monitor to use the batteries between 40 (50 is better) % and 90%. You can ad the solar portion at a later date. Maximize what you have (generator), The batteries and the rest of the improvements will still be usable with the solar.

I completely missed this comment yesterday.

One of the existing batteries is just for the motor. If you drain the living quarters it will still be fully charged, so I think I should leave that one alone. The rest is as you've described. It has a battery monitor built in but the previous owner told me it's junk and I can't rely on it. He said the same for the propane monitor - he would go outside and look at the guage on the tank.

 

I should probably track down a monitor. What keyword am I plugging into Amazon?

[skp51443]

For battery monitoring I really lean to a $25 multi-meter that reads 1/100 of a volt, it has many uses aside from battery monitoring.

 

If you want a true battery monitor try trimetric but I'm not seeing it on Amazon

 

http://www.bogartengineering.com/

[MoonTimber]

For battery monitoring I really lean to a $25 multi-meter that reads 1/100 of a volt, it has many uses aside from battery monitoring.

 

If you want a true battery monitor try trimetric but I'm not seeing it on Amazon

 

http://www.bogartengineering.com/

I have several multimeters. Can I just unplug a light bulb and see what voltage I'm getting in the living quarters, or do I need to go out to the battery terminal in the engine compartment?

I think I'm incorrectly assuming that you look for 12V, since I know a dying battery will show 11V or less. But that seems pretty inaccurate. How do you know when you are at 50%?

I'll go find one of my multimeters when I get a lunch break.

[Rif]

I'm afraid it's not that simple. Using voltage to determine the charge level requires removing all loads and letting the battery rest for some period of time. The test should be done at the battery itself because any wire run will cause a voltage drop. Any battery at 11 volts when at rest is already a dead battery. In fact, batteries should never be allowed to drop below 50 percent capacity. That would be somewhere in the 12.1 volt area when the battery is at rest.

 

I suggest you read some of the very well written discussions about 12V systems. Mark has one at The 12V Side of Life.

[MoonTimber]

Another factor to add is that today, very few power supplies are transformers. I would venture that less than 1% and that might be a stretch. Today, most everything is a "solid state" or "switching" PS. In the old days, you took a slab of iron, and wrapped copper or aluminum around it (very crude description). A 12V / 80A supply would weigh in at 30 some pounds. Today, we take some triacs (fast silicon switches) use some small core torads, chop up the 120 AC power, modulate the pulse, filter it and 12 VDC comes out. And the thing weighs in at 7.5 pounds - 1/2 of which is case and fans.

 

Your 140 watt computer supply would weigh 10 lbs in the old days and 8 oz today??

 

It's the chopping of the harmonics and such that Stan spoke above that kills PS's today. I had the same inverter as RIF, had the same problems as RIF, and did the same thing as RIF.

 

Fan replaced, Microwave replaced. 2 coffee pots replaced, couple of 120 volt light dimmers replaced.

 

 

ON Edit - Also by a Master Electrician, later an EE that dealt in BIG Power (120,000 volt stuff) and also did 1000 hp DC drives, 800 hp variable freq drives and way to many major production electronic stuff.

 

I think this may have unfuzzied it for me. When I think of a transformer / power supply, I think of my fifth grade science class where we wrapped a bunch of copper around some nails and stuck a big capacitor or two somewhere in there. You're telling me my laptop power supply is not the same gizmo.

 

I finally put on my glasses and picked up my laptop power supply. My laptop is a portable workstation. The power supply is a honker and weighs at least one pound. It's not 140W. It's 170W!!! It says 170W 20V. Output 20V @ 8.5A.

 

One of the great things about posting in an RV forum is the high odds of getting responses from well educated experienced people.

[MoonTimber]

. . . The only "fancier" one would have a automatic start to the genny, but added complexity. The charger that came in the rig is probably a old magnetek and was designed to float your battery until cooked charger. The newer ones, fairly heavy power to the battery (85% charged), ramp (85% to 95% charged), and then float to 100%.

 

What your looking for is that bulk rate charge 40 to 90%. That does not mean that they should always stay at the 90% charge level. Always bring them back up to 100% before and after your boondocking runs (maybe at home, or a FHU campground.

 

This whole exercise is really a numbers game. It all gets back to the energy audit and the meter that Stan mentioned will be doing that on a continual basis.

 

You just went over my head. Could you rephrase that for the novice in me?

[MoonTimber]

OK, so I've been convinced on the Inverter. The last two inverters I purchased have lasted me for nearly 20 years, so it seems to me it's better to spend a little more for something I'll be able to use ten years from now. I am putting off buying a solar panel so I can buy the Xantrex 806-1210 PROwatt 1000 SW Inverter for $240. I can't foresee us needing more power than that with the limited RVing we're planning to do. While I really wanted the option of charging up my batteries without bothering any boondocking neighbors, I guess for our first few trips I'll just have to bring along a bale of hay to put in front ot the generator!

 

The next bit to tackle though is the batteries. I want to have an inverter and batteries ready for the first time we leave the driveway.

 

I think I want to go as inexpensive as possible on the batteries. As I've mentioned before, we're only going boondocking once a month for a few short days. I think I need to follow everyone's links and figure out a good way to monitor the battery power so they don't get fried, but I should stick with inexpensive marine batteries. The battery that's in the camper now is a walmart special - an Everstart Deep Cycle 690 Marine Cranking amps, 101Ah @1A. The previous owner just bought it three months ago, so it's pretty new. I looked it up on Walmart's site and it only costs $100.

 

How many of these batteries do I need to get 2 hours of computer time on those two laptops with 170W 20A power supplies, assuming they draw as much as they claim?

[MoonTimber]

Side question:

 

I have three unused UPS devices with good batteries. APC Back-UPS XS 1300. According to APC, they will keep a 100W load running for 1 hour 9 minutes.

 

If I know I'm just going for a weekend, it makes sense to me to charge these up at home and bring them along. But if we go for a week long trip, are these worth the space they take up in the RV?

In other words, if I hook them up tho the inverter and charge them via the generator, do they waste too much energy getting charged up to be worth using in the RV?

 

Conversely, these have a pair of 12V 8A batteries on the inside. Would it be better to cannibalize the batteries and add them to the RV battery bank? I have to imagine that the UPS circuitry is eating a decent bit of power. Is the additional power from these batteries too small to be worthwhile? It would give me six of these batteries to borrow them from my unused UPS devices, but they are about half as small as a mtorcycle battery.

[wa_desert_rat]

 

Side question:

 

I have three unused UPS devices with good batteries. APC Back-UPS XS 1300. According to APC, they will keep a 100W load running for 1 hour 9 minutes.

.................... are these worth the space they take up in the RV?

 

I don't think they'd be worth the trouble.

 

However, if you got one of these little buggers http://www.amazon.com/Antigravity-Batteries-Micro-Start-Starter-Personal/dp/B00FDYYK4A/ref=sr_1_6?s=automotive&ie=UTF8&qid=1425418201&sr=1-6&keywords=lithium+jump+starter and charged that up at home it would probably run your laptops all by itself for a weekend.

 

I have one of these and take it on every trip just to have an extra source of power for jump starting the RV or charging cell phones or whatever. They are remarkable!

 

WDR

[wa_desert_rat]

As far as batteries go, you're better off with 2 six-volt deep cycle golf cart batteries than you would be with any 12vdc battery in that size. If you can fit them (they're taller than a standard car battery) they're much better. A little bit of a PITA to install because you have to wire them up together and then wire them to the RV but worth it.

 

This would be more than your cheap Walmart "deep cycle" battery but you won't regret it. Try Sam's club for the prices.

 

WDR

[MoonTimber]

 

Kill-A-Watt meter.

 

One thing you might consider is purchasing 12v cords for your laptops, printer, etc. It's not terribly efficient converting DC (battery bank) to AC (inverter w/overhead) then back to DC again (laptop cord).

 

One additional strategy that might prove helpful... once you are on solar.. is to be aware of "when" you are recharging. If your solar array is sufficient to fully recharge your battery bank on a daily basis, you 'may' have surplus energy production before the sun goes down. It is ideal to use that surplus production time to recharge your devices or perform other more energy intensive tasks.

I just ordered the Kill-A-Watt meter. I can see a lot of fun uses for that!

[wa_desert_rat]

You know, for years I used two panels (35-watts each but you would probably want more) with no controller at all; they came with two clamps (like jumper cables) and I just connect them both to the plus and minus terminals of the batteries.

 

Then I watched the voltage with a digital voltmeter. If they hit 14.5vdc I disconnected them; the batteries were good enough. If they got down to 12.2vdc then I connected them because it was time to charge 'em up.

 

I tied the panels to the front hitch of the trailer which was right next to the battery (that was handy). And I would go out periodically and aim them towards the sun. It interupted my nap and reading time but what the heck.

 

Aiming panels at the sun really maximizes their ability to put power back into a battery bank. Remember to tilt them appropriately too.

 

WDR

[wa_desert_rat]

This is long and probably overkill but I wrote it and, by golly, I'm gonna post it!

 

First of all, a decent $50 digital voltmeter from Amazon is all you really need most of the time; especially if you're just weekending. It will tell you most of what you want to know about what voltages your batteries are at. There are things like "surface charge" (the charge remaining in the battery after the charger is turned off... that isn't really usable energy) to deal with but you don't have to be an EE for this.

 

Secondly. Your engine driven alternator is probably charging both batteries through an "isolator" (a couple big diodes). That's typical.

 

But your shore power charger - which is typically left on all the time - is possibly damaging your batteries. If it's just a trickle charger (and most before about 2006 or 2007 were) then you really should consider changing it. Fortunately they are not very expensive. Many of us are moving to 3 or 4-stage "smart" chargers like those made by Progressive Dynamics. This is the one I put in my old 1970s Streamline 21' travel trailer (which I use for fishing trips locally as a good place to nap): http://www.amazon.com/Progressive-Dynamics-PD9245CV-Converter-Charge/dp/B000GANZZ6/ref=sr_1_2?s=electronics&ie=UTF8&qid=1425415917&sr=1-2&keywords=progressive+dynamics.

 

You can buy and install this stuff over time, you know. My PD converter/charger in my motor home was my first upgrade. Took about 2 hours to install. Just keep track of which wires go to the batteries and where you plug into the circuit for shore power. A new one connects just like an old one. That 45-amp version (with the "Wizard" pendant (very imporant to get that)) is about $150.

 

Now you won't burn up a battery by leaving the charger plugged in all the time.

 

If you want to do an "energy audit" you will want to at least consider getting one of the Bogart Engineering TriMetric units mentioned by Stanley. They are a little complicated in their design but the idea is that it measures the voltage lost across a very low-resistance "shunt" in series with the ground lead of the house battery. The very low resistance is there so it won't turn all your battery's energy into heat. But by using that "shunt" the TriMetric unit can keep track of ALL the energy going into and out of that battery and total it up in a way that you can tell whether you are slowly discharging that battery or keeping it happily charged. About $150: http://www.altestore.com/store/Meters-Communications-Site-Analysis/Meters-Battery-Monitors/Ammeters-Voltmeters-Battery-Monitors/Bogart-Engineering-TM-2030RV-F-TriMetric-Battery-Monitor-wFuse-Holder/p11263/?gclid=CKOJ1OWKjcQCFQeUfgodpEAA-A

 

You will also need a shunt. Get the 500A version since it doesn't take much to suck 100A from a battery (that's only 10-Amps of and what a microwave will suck down). About $25: http://www.altestore.com/store/Meters-Communications-Site-Analysis/Shunts/500A-50mV-DC-Shunt-for-Current-Monitoring-Meters/p4638/

 

So... now you have a way to keep your battery charged safely (without damage) and a way to calculate just how much power you actually use during a weekend of camping. When you are sure you battery is fully charged (turn off the charger, wait 30 minutes, and measure the voltage... if it's 12.7 or more you're good... or use a hygrometer) you can tell the TriMetric unit that it is and from then on it will keep track of all the charging and discharging. Even if you charge it up with an outside charger or a generator or a solar panel or a wind generator it will know and track it.

 

At this point you will be able to determine just what you'll need when it comes to solar.

 

The solar charge controller is a very improtant part of what you'll be installing. Now you probably think of charging as a low current being applied to the battery to keep it cahrged or a very large current applied for a short time to get it charged when it's low. That's the old way. Now we use pulses of current. It's called Pulse Width Modulation (PWM) and all modern "smart" chargers use that method. But chargers that are labeled PWM are now out of date and we prefer a new type that uses digital logic to carefully monitor the battery's condition of charge and maximize the ability of a solar array to affect that charge. It's called Maximum Power Point Tracking and it can add 30% to the efficiency of a solar array. So over a weekend of camping you'd get much more energy from a panel feeding an MPPT charge controller than a standared PWM controller.

 

I think that the controller is so important that I actually paid as much for my controller, all by itself, than I did for my panels!

 

I mentioned in an earlier post that panels have a "nominal" votlage rating. That is, they're typically rated for 12vdc or 24vdc. This is a nominal number because it does not reflect the actual voltage output of the panels. Since you cannot charge a battery unless you connect a voltage source that's higher than the voltage of that battery, a 12vdc panel will typically be around 17vdc and a 24vdc panel will be around 36vdc.

 

So having said that I'll deal with 12vdc and 24vdc and drop the "nominal" part.

 

The big market for solar panels is in houses and buildings and because for the same power a high voltage will cause less current in the wiring than a low voltage that means that the wiring can be smaller for long runs and small wiring is cheaper than big wiring. So the demand right now is 24vdc panels which cost about half what a 12vdc panel will cost. Plus the wires cost less, too.

 

But with an MPPT charge controller you can use a 24vdc panel - or even two of them wired in series (which would be 48vdc) and STILL charge your 12vdc house battery. So to save money, try to get 24vdc panels. Two in series doubles the voltage but the current remains the same; two in parallel doubles the current but the voltage remains the same. Until about 2 or 3 years ago the standard RV solar system was two, three or four 12vdc panels wired in parallel wired to a 12vdc PWM charger.

 

My own system is a pair of 12vdc panels wired in parallel and then those wired in series with two 24vdc panels feeding an MPPT controller about 80volts in full sunlight at 8amps for about 640 watts of power.

 

Once you know what you need, then don't skimp on the details.

 

And park in the sun...

 

WDR

[wa_desert_rat]

You know the problem with engineers, right? Ask us what time it is and before we tell you that we have to describe designing and building a watch.

 

WDR

[Jack Mayer]

I've stayed out of this discussion until now because there was no sense repeating things. You have been getting good advice.

 

However, if you do upgrade your battery bank to at least two new 6 volt batteries, improve your inverter as recommended and boondock more than just a day at a time then I would say the next thing to do is to spend the money on the Trimetric battery monitor. You will learn more about boondocking successfully by looking at that than by any other single thing. In fact, if you reorganize your battery compartment as part of the battery upgrade - build new cables, etc - then that might be the time to fit in the shunt - EVEN IF you don't put in the Trimetric yet. The shunt is not going to cost you that much, and while you are building cables is the time to add it as well.

 

If you never boondock more than over one or two nights max, and do not use more items then you have described, then you can continue to use voltage to monitor your bank. That is not very good, as has been discussed, but for such light use you could get away with it. But it really will not teach you much.

[Yarome]

the next thing to do is to spend the money on the Trimetric battery monitor. You will learn more about boondocking successfully by looking at that than by any other single thing. In fact, if you reorganize your battery compartment as part of the battery upgrade - build new cables, etc - then that might be the time to fit in the shunt - EVEN IF you don't put in the Trimetric yet. The shunt is not going to cost you that much, and while you are building cables is the time to add it as well.

 

X2 on the Trimetric, but as also stated.. one or two nights a couple of times a year probably does not warrant the expense.

 

Along the lines of building new cables... along with the shunt, I might recommend adding in a terminal post on your ground line and small bus bar on your positive line at the same time. It's fairly inexpensive and lays the 'backbone' for future upgrades to your system without the expense of having to re-cable at a future date.

[oldjohnt]

Moon dude, you stated "an Everstart Deep Cycle 690 Marine Cranking amps, 101Ah @1A."

 

And then you asked.....

 

"How many of these batteries do I need to get 2 hours of computer time on those two laptops with 170W 20A power supplies, assuming they draw as much as they claim?"

 

I can only base my answer on the data you provided you know, so here goes BASED ON WHAT YOU GAVE US, NOT what may be reality so NO WARRANTY

 

If the two laptops each required 170 watts and you used both for 2 hours.......That's 340 watt hours x 2 computers = 680 watt hours

If you supplied all that via an inverter fed from a 12 volt battery (NOT allowing for inefficiency and losses and using 12 volts as nominal), 680 watt hours/12 volts = 57 Amp Hours

 

You state the battery has 101 Amp Hours of energy storage (but that's at the 1 amp rate!!!!!) so in theory you wouldn't get 2 hours before the batteries were low plus as their voltage decreases the power supplies can have problems I WOULDNT WANT TO RUN MUCH OVER ONE HOUR LET ALONE TWO

 

AGAIN those quasi deep cycle 12 volt RV/Marine batteries are NOT the same as true deep cycle golf cart batteries. They need to crank a huge marine engine and then resort to running a trolling motor. They need to do both jobs, cranking and trolling.

 

Before I spent $100 on another 12 volt RV/Marine Battery, Id go spend around $200 on two 6 volt true deep cycle golf cart batteries. SaMs has them or Costco if you don't want to pay more for Trojans.

 

Once more, if you had at least two 6 volt golf cart batteries,,,,,,,,,,,200 Watts of Solar,,,,,,,,,,,a 1000 watt or more PSW Inverter,,,,,,,,,,and a Smart 3/4 Stage Charger (or Combo Inverter/Charger) to replace on old noisy heat generating converter/charger, I as an engineer would feel better BUT ITS YOUR MONEY AND YOUR CHOICE NOT OURS

 

Hope my old engineers ramblings have helped and remember Im long retired and RUSTY on this stuff so listen to those more current in the field of engineering NOT me

 

John T

[Daveh]

Moon Timber, the cranking amps on your battery is not the number that is typically looked at for determining capacity. From what I can see online your battery has a a rating of about 100 Ah @ 20 hour rating . two of them will give you about 200 Ah. Only 50% of the batteries capacity should be used so that gives you 100 Ah. The golf cart batteries will get you about the same each battery is a 6 volt with a rating of approx 210 Ah. If you get two of them that is 420 but you need to divide by 2 since you will be wiring these two 6 volt batteries in series to make one 12 volt battery. So for $100 you can add another marine battery and for about $200, probably less, you will get approx 210 Ah of Sam's Club or Costco golf cart batteries. Your capacity will be about the same. either way. 100 Ah @ 20 hr rating. The Walmart battery you have is mixed use and is not likely to last as long as the golf cart batteries, as John describes above. Also, you take some risk that your existing battery is not as good as represented by the previous owner or has been drawn down below 50% (maybe far below) and has been damaged.

[MoonTimber]

Moon dude, you stated "an Everstart Deep Cycle 690 Marine Cranking amps, 101Ah @1A."

 

And then you asked.....

 

"How many of these batteries do I need to get 2 hours of computer time on those two laptops with 170W 20A power supplies, assuming they draw as much as they claim?"

 

I can only base my answer on the data you provided you know, so here goes BASED ON WHAT YOU GAVE US, NOT what may be reality so NO WARRANTY

 

If the two laptops each required 170 watts and you used both for 2 hours.......That's 340 watt hours x 2 computers = 680 watt hours

If you supplied all that via an inverter fed from a 12 volt battery (NOT allowing for inefficiency and losses and using 12 volts as nominal), 680 watt hours/12 volts = 57 Amp Hours

 

You state the battery has 101 Amp Hours of energy storage (but that's at the 1 amp rate!!!!!) so in theory you wouldn't get 2 hours before the batteries were low plus as their voltage decreases the power supplies can have problems I WOULDNT WANT TO RUN MUCH OVER ONE HOUR LET ALONE TWO

 

AGAIN those quasi deep cycle 12 volt RV/Marine batteries are NOT the same as true deep cycle golf cart batteries. They need to crank a huge marine engine and then resort to running a trolling motor. They need to do both jobs, cranking and trolling.

 

Before I spent $100 on another 12 volt RV/Marine Battery, Id go spend around $200 on two 6 volt true deep cycle golf cart batteries. SaMs has them or Costco if you don't want to pay more for Trojans.

 

Once more, if you had at least two 6 volt golf cart batteries,,,,,,,,,,,200 Watts of Solar,,,,,,,,,,,a 1000 watt or more PSW Inverter,,,,,,,,,,and a Smart 3/4 Stage Charger (or Combo Inverter/Charger) to replace on old noisy heat generating converter/charger, I as an engineer would feel better BUT ITS YOUR MONEY AND YOUR CHOICE NOT OURS

 

Hope my old engineers ramblings have helped and remember Im long retired and RUSTY on this stuff so listen to those more current in the field of engineering NOT me

 

John T

Your formula was what I was looking for. Are you saying that if you ignore energy lost in the system, one Walmart battery should supply about 1 hour of power for two laptops without frying the battery? If that's the case, then one more Walmart battery would be enough for us to get started.

 

But are you also saying that the golf cart batteries are about $100 each? I thought they were a lot more expensive than that. If they are only $100 each then I can stop at Sam's club next week and pick up a pair. But how long would that allow me to run two laptops?

[sushidog]

My local Sams has GC-2 (6v golf cart batteries) starting at around $85 each.

 

Chip

[MoonTimber]

Moon Timber, the cranking amps on your battery is not the number that is typically looked at for determining capacity. From what I can see online your battery has a a rating of about 100 Ah @ 20 hour rating . two of them will give you about 200 Ah. Only 50% of the batteries capacity should be used so that gives you 100 Ah. The golf cart batteries will get you about the same each battery is a 6 volt with a rating of approx 210 Ah. If you get two of them that is 420 but you need to divide by 2 since you will be wiring these two 6 volt batteries in series to make one 12 volt battery. So for $100 you can add another marine battery and for about $200, probably less, you will get approx 210 Ah of Sam's Club or Costco golf cart batteries. Your capacity will be about the same. either way. 100 Ah @ 20 hr rating. The Walmart battery you have is mixed use and is not likely to last as long as the golf cart batteries, as John describes above. Also, you take some risk that your existing battery is not as good as represented by the previous owner or has been drawn down below 50% (maybe far below) and has been damaged.

 

Based on what you're saying I think it makes more sense to get one more Walmart battery for now and see what our actual needs are after we do a few trips. I can be pretty careful about not draining the battery once I understand how to measure it. For our initial weekend boondocking trips we should really spend more time being in "away from civilization" with our kids than being online doing work anyway, so a one to two hour time limit might be a good thing.

 

We're talking about driving from our home in Sacramento to Florida later this year (October?) to visit my parents. It's going to take us about three weeks roundtrip. I will definatetly need to put in two or three days of remote work or my boss won't let me go. For that trip I'll need to do some of the things everyone has recommended. Before that trip comes I will have to get my hands on at least two of the golf cart batteries. If I can afford it I'll try and get at least two solar panels. I'll use a switch for the batteries so that we use up the golf cart batteries before tapping into the walmart batteries. When we go on that trip we will stop in at an RV park every few days to charge up all the batteries and take advantage of campground WiFi to get some of our work done. My wife will want those stops anyway.

 

But I don't think I need to do all of that for us to start going on weekedn trips in the next few months. I think I need a few things everyone has recommended that I never thought ot - like a decent battery meter, the power usage meter to do an audit, and the pure sine inverter. Then I think for our first trip we can get away with just one more Walmart battery.

 

Does that sound off base?

[MoonTimber]

My local Sams has GC-2 (6v golf cart batteries) starting at around $85 each.

 

Chip

Gee, that's pretty cheap. I'll check my sams club before I spend money on another Walmart battery.

 

I don't want to remove the walmart battery in my RV - it's in the engine compartment and snug. Any batteries I have will have to go elsewhere, so I might as well start looking for a battery switch while I'm at it.

 

In fact, I can just leave that one for powering up everything in the RV, and use the golf cart batteries for any electronics we bring along like the laptops and TV.

[oldjohnt]

Moon states: "Your formula was what I was looking for. Are you saying that if you ignore energy lost in the system, one Walmart battery should supply about 1 hour of power for two laptops without frying the battery? If that's the case, then one more Walmart battery would be enough for us to get started."

 

MY RESPONSE: If your existing Walmart battery has 110 Amp Hrs of stored electrochemical energy, that would compute to say 10 amps for 11 hours or 5 amps for 22 hours etc. etc. and if a single laptop requires 170 watts, then in one hour it uses 170 watt hours which if divided by 12 volts = 14.66 amp hours of energy in one hour so two laptops = 29.32 amp hours per hour each so for two hours that's again 58.64 amp hours using two laptops (I rounded to 57 above) NOTE that's less then the 110 amp hrs., so you actually have more time, its just that I don't like to run them down that far and I figure things CONSERVATIVELY ON PURPOSE.

 

If you bought another walmart battery and placed it in parallel, now you have 220 AH (cost around $100)..........If you bought two new true deep cycle 6 volt golf cart batteries, that would be around $200 and you would have 205 to 230 AH BUT IN MY OPINION THAT WILL PROVIDE BETTER SERVICE OVER THE LONG HAUL..........

My EG2 Extended (their higher rated units) were around $100 and are 230 amp hrs from Sams Club (I have 4 for 460 AH), but next time I'm buying Trojans

 

YOU STATE

 

"But are you also saying that the golf cart batteries are about $100 each? I thought they were a lot more expensive than that. If they are only $100 each then I can stop at Sam's club next week and pick up a pair. But how long would that allow me to run two laptops?"

 

MY ANSWER

 

If you bought two 6 volt true deep cycle golf cart batteries (around $200) and wired them in series you would have 205 to 230 amp hrs.........but that's near what you have with two wal mart 12 volt 110 AH's in parallel.........(only $100 more since you already have one) AND IF YOUI HAVE 220 TO 230 AH, YOU CAN RUN TWICE AS LONG AS I COMPUTED ABOVE FOR A SINGLE 110 AH BATTERY ..............More Amp Hrs of stored energy = more laptop time PERIOD

 

IN RETROSPECT.............Heck if use is only occasional and money is tight Id be tempted to just buy another Wal Mart 110 and wire it in parallel with the one you already have and see how things work out......If money is less tight, Id toss the Wal Mart battery and go buy two 6 volters from Sams Club.........

 

SURE we have all made recommendations for what's best, which includes a PSW Inverter and YES I stand by all that, but its now your money and your choice........

 

I THINK WE HAVE BEAT THIS TO DEATH LOL

 

John T

[MoonTimber]

I THINK WE HAVE BEAT THIS TO DEATH LOL

 

John T

 

And I think it's been a LOT of fun beating it!

 

I'm not completely broke and the funds aren't too tight, but I do have a limited budget on what I can spend. Every dollar I spend on upgrading this RV is a dollar I can't spend on gas driving it somewhere that's going to make my wife say "Ooooohhhh that's beautiful!". And it really is an old beater, so I'm sure she'd rather spend some of my money on new curtains and sheets and wallpaper and so on. With our limited use scenario and the formula you just updated for me, I think it would be best to start off by just adding the inverter I mentioned before, one more Walmart battery and a decent battery monitor. Down the road, if we find ourselves maxing the battery usage and going on trips more often, then I think it makes sense to get two or even four of the golf cart batteries and one or two solar panels.

 

Of course in five minutes someonw may post something and I'll start this all over again!