Is this typical for a 75 AMP HR Battery?

[VisualMonster]

So, I have a 75AMP HR (walmart) battery which I believe is 900 watt hours of juice... using this equation (amp = watt / volt).

Here are the specs for it:

I understand that I shouldnt deplete the battery beyond 50% or so.

Last night I had my laptop and another device drawing about 70 watts continuously for about 5-6 hours.  Towards the very end of that, I noticed that the charge light on my laptop turned off.  Reason for this is that my inverter shuts off when the battery drops to 11v.  Should my battery have depleted this quickly?  I'm guessing I used about 400 watt hours, where's the other 500???

I bought this battery about 3 months ago, do you think it's too late to add a 2nd battery (so that I get 150 AMP HRS)?

I charge this with 90 watts of HF panels...

[The Crazy Noob]

The way I understand it is that you are using an inverter to drive your laptop etc.? If so: try to hook your laptop directly to the battery. The shorter the chain, the less you loose so you think you drew 400Whr but you probably drew more (like 600Whr).

Also: you may try to freshen up your battery by trying a desulfator on it, Jerry here has plans for one. Even if yor battery did not sit uncharged it might have been left uncharged before you got it.

link to plans for desulfator: http://www.fieldlines.com/story/2004/2/12/23130/7578 (the original plan is offline, but the rotated one by Hank is still online)

[willib]

i'm not sure but drawing 5A for 5 to 6 hours is not too bad for one battery

and that is with a charge of 14V

as the voltage drops you would be drawing more current , to still supply the 70W

at 13v the draw would be 5.4A

at 12v you would be drawing 5.8A

at 11v you would draw 6.4A

[willib]

i'm also not sure how they rate the Amphours but im sure the voltage will be below optimum at the end of 75Ahrs

thats 12.5 A for 6 hours at rated 75Ahrs

in my opinion your battery is in its prime

[veewee77]

Unless you already had that battery, don't waste any more money on those "deep cycle" trolling batteries.

Those are not made to go the distance as "traction batteries" or other true deep-cycle batteries are.

Those "deep cycle/marine batteries" are like a half-breed between a starting battery and a deep cycle.

How many people do you know that actually use those for trolling and have to buy a new one every year. . .?  There is a reasoon for that. . . They could use "traction batteries" and with proper maintenance could use them for several years, even up to 5 or 6 depending on their maintenance routines.

Doug

[scottsAI]

Hello VisualMonster,

Most batteries are rated with a 20 hour discharge rate.

75 / 20 = 3.9 a load.

If discharged faster at like 5 hr rate the capacity drops a lot.

For example one battery I was just looking at was rated 12v 80aH (20hr) 60aH (5hr)

The 5 hour rate the capacity drops by 25%!!

The efficiency of the inverter may claim 88%, at full load.

Measure the inverter current while on and no load, most likely 0.2 to 0.5a.

The efficiency with a light load is maybe 60-70%.

So, lets go back to your situation.

900wH * 0.75 * 0.6 = 405wH right where you thought you were!

Mystery solved.

Two things you can do to make this better, as already suggested directly connect the laptop to battery if it can run on 12v (many can), Or get a 12v adapter. I use a device like this:

http://cgi.ebay.com/Universal-Car-Notebook-laptop-Power-Charger-adaptor-X05_W0QQitemZ280028109142QQi
hZ018QQcategoryZ60252QQrdZ1QQcmdZViewItem?hash=item280028109142

I have not used that unit. Check if works with yours. (searched: on laptop adapter car)

Of course it does not power the other unnamed item. 12V version available? Like a 12v CF lamp?

Secondly remove the battery from the laptop (if it will work, some won't) the laptop always tries to charge the battery, several watts are wasted by this.

If all else fails get a smaller inverter, efficiency should go up. Read the data sheet to confirm. Like 100 to 150w.

Getting a second battery will help, I use golf cart batteries, they are $53 ea at Costco, need two for 12v.

12v * 225ah = 2700wH your load is now within the 20hr discharge rate! Heck of an improvement!

Have fun,

Scott.

[VisualMonster]

Thanks for the responses, I forgot that actual AMP Hrs are much less the faster you deplete the battery.  That explains it.  Also, when i said I used 70 watts for 5-6 hours, the kill a watt meter was reporting 64w, but I averaged it up to 70 to account for inverter losses (maybe I shouldve estimated even higher).  I'm using a 100w inverter (I think it's pretty efficient since it doesn't get THAT hot and doesn't use a fan).

Switching to DC is probably a good idea.  I already have some DC lighting to use, just need a DC charger for laptop (I'll check ebay and Fry's for this).  Anyone know of a good place to get a 20 foot cable to run DC from outside to my computer room?  Less than 100w would be running through this.  It needs to be white so that it isn't visible against the white wall.

Also, I measured my laptop with and without the laptop battery using a kill-a-watt meter and didn't find any noticable differences with power usage.

While idling, it uses 15-16watts without the laptop battery and also 15-16 watts with the laptop battery.  Maybe over the course of a full day I'd see a difference in kwh's used.  If anyone is curious, I'm using a Gateway MX6440 laptop (15" widescreen).

- VM

[scottsAI]

Hello VisualMonster,

Looks like you have a good laptop, low stand by numbers, was that with the LCD active?

Excellent you have a Kill-O-watt meter to verify and make the best choices.

Nothing like having real data!

Please put a fuse on any wires you run from the battery! Lets be safe! You know this, just saying it to be safe for the others that may read this.

Check the efficiency of the inverter, I have found the small ones not to be so good.

Put a 60w bulb you verified with watt meter, then use inverter and measure it's current and voltage. I bet you can do this, after all you have a watt meter! You or me may be surprised. I noticed the not warm part:-)

You have done a good job observing. Must not be your first time to be messing with this stuff?

Have fun,

Scott.

[ghurd]

All good info.

But I think you should get more battery.

You are losing a lot of the 90W PV potential because the battery gets to regulation fast, but it is just surface charge.

A pair of 200AH 6V's is a huge step up, but another 75AH is still a very large step up.

I wouldn't worry about adding a new battery to that one.

Depends on the wallet, $100 or $35.  Being a cheap SOB, I'd go with another $35 this time.  Then a pair of 6V next time.

G-

[maker of toys]

for moving 100W at 12V a distance of 20 ft, on the cheap, I'd use 10-2 romex . . . get the 10-2UF type (grey) and paint it.  the UF doesn't use paper as filler, and will stand up to ultraviolet radiation and other insults much better than the white/yellow kind. latex paint will stick better than spray paint, particularly if there is flexing going on.

 (lmost any hardware store will stock various sizes of romex for something less than a buck a foot.  get the largest diameter conductors you can afford.)

  Solder appropriate connectors to the appropriate ends. if you can, crimp them first with a good crimper for a good mechanical joint.  Don't, under any circumstances, trust wire-nuts in an exposed application.  Heat shrink tubing may be helpfull as you dress the ends. . . .

-Dan

[VisualMonster]

Yes, the laptop numbers includes the LCD, but set to the lowest brightness.   The CPU scales down to 787mhz (and back up to 1.8ghz when needed) which also lowers the power usage.  I was amazed when I saw how little power it draws, especially for a 15" widescreen.

I'll try comparing efficency differences between my cheapy 100w & even more cheapy 400w inverter... though I have a feeling in my case the 100w will win.

[scottsAI]

Hello maker of toys,

When I saw 10awg, wow, must be tooo big for only 100w load 20' away.

So I went to a trusty wire calculator:

http://www.powerstream.com/Wire_Size.htm

Any better ones out there?

Yep, loose 0.349 volts with 8.5a load at 20'. Power loss is 2.91%

General recommendation is to keep power losses to less than 2% in the wiring. With 64w it will be.

As you can see, the wire gets thick quickly and costly.

Thick wire can be hard to come by, so the use of several thinner wires to equal has been used. Just so you know this is not up to code, How do you insure all the wires stay connected? Soldering the ends together sounds good, still not according to code. In the above example, you have two loads, lets say each is half the load, then wire two 12awg circuits, will yield the same results as the two loads on one 10awg wire. Each wire gets its own fuse, sized to the wire. Try out 100', 2.28% with 2awg wire!! 2awg wire cost more than $1/foot.

If the wire run was much longer, then using the inverter to boost the voltage for the run would be the best choice, even with the inverter losses, wire would just cost too much. Looking around here several wind gen have higher output voltage which is stepped down at the battery using transformers.

Have fun,

Scott.

[SamoaPower]

How did you determine the battery state of charge before this observation? Also, do you know your battery's ACTUAL capacity. I see many quoting battery capacity at the new rated number even after years of service and various levels of abuse. The only way to know is to measure it. It's not a constant.

Your 90w of panels may not be sufficient to maintain your battery depending on your local conditions. I wouldn't expect more than a long term average of 20 Ah per day. I see many days here that my 320w of solar only supplies 30-40 Ah (darn clouds!).

You mention a 50% drawdown. If you want reasonable lifetime, a 20% limit is better.

I agree with veewee77 about this type of battery. Most bang for the buck is the 6V golf cart type. I reccommend the Trojan T-105.

I find it more convenient to measure and think in terms of Ah rather than Wh. Most of what we use the batteries for are tolerant of the voltage over quite a range.

[maker of toys]

Hey ScottsAI-

that's a handy link you've got there. I usually go to my annotated copy of the NEC for such information. I'll try and remember to post the unit converter I use at work. . .

it is fascinating how fast I^2*R catches up to you when you're dealing with low voltages and powers, hey?  the numbers commonly bandied about for mains service is based on fire prevention, not power conservation; and are based on the ability of the wire to shed heat and not melt insulation or heat nearby flamables. the code is a discussion of the minimum effort required to be safe-- there is nothing but budget and the aggravation of dealing with teminations to prevent us from using larger wire than called out in the code.

RE NEC: I never claimed to be to code. <G> claimed it was a way to do the job cheap, with minimal budget, tools, visual impact and skills,  which seemed to be the overriding design criteria. Plus, we're talking laptop here. . . temporary wiring gets a looser code interpretation. . .<evil grin>

besides, the code is a little vague for low-voltage DC.  even if it was nominally to code, I wouldn't count my chickens until I got the local building inspector to sign off.  <G> and maybe not even then.

 I didn't feel like grabbing a calculator, so I rounded up to make a 'back of my forehead' estimate based on my 120V experience. I took the inverter's nameplate output as a starting point.  (I actually figured at 10A, and decided that less than 10% loss would be the cutoff.  in retrospect, that was probably a little glib, as I forgot to account for the low-voltage cutoff feature most inverters have.) <G> given the total power budget and the cost and appearance advantages of 10-2 UF romex vs #8 or #6 THHN and conduit, chasing 1-2 W just isn't worth the time and effort. come to that, though, if the loads were better balanced,  your multiple wires to the individual loads would work better from a loss point of view, as there would be more total wire area in 2 12awg runs than in 1 10awg conductor.

if we were trying to move 0.25 kW or more, believe me, i'd have answered much differently, and would, as you do, have favored the remote inverter option. for the viewing audience, the above discussion points up the desireablity of mounting your conversion equipment as close to the storage bank as possible.

RE soldering:  you're right- the code specifically prohibits a solder-only joint; particularly in grounding conductors, you don't want to take the chance of the join heating up from excessive current, melting the solder and going intermittant! (i've seen it happen. . . . annoying to trace in a vibratory environment.

  thus my call for a crimped and soldered connection;  <G> crimp for mechanical strength and reliablity, solder for (slightly) lower losses and environmental reliabilty.  and let's not get me started on wirenuts. . . .(ptui!)

-Dan