# newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v bat

Hi all,

Been reading up on solar sytems etc. I'm going to put in a small system in my Baja house, but I'm not sure of my needs yet.
From what I've read a 120w 120vac bulb would use 1amp on a 120vac system and would use 10 amp in a 12vdc system. A 12v battery with 35ahr would run this bulb for only 3.5 hours ...and less because of the efficeincy of the inverter. Is this correct?

• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
Re: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v

You're on the right track. However, fully discharging a battery will lead it to an early death, so it's good practice to not discharge the battery below 50% state of charge, and typically not below 80% SOC (= using just 20% of capacity) on a regular basis. Using the 50% rule and assuming a 90% inverter efficiency, you could run a 120 W 120 VAC bulb for 35 Ah x 50% / (10 A / 90%) = 17.5 Ah / 11.1 A = 1.58 hours.

This is a good example of why standard incandescent lamps should be replaced with compact fluorescent lamps (CFLs). A 14 W CFL puts out as much light as a 60 W incandescent. So, the same battery could power two 14 W CFLs instaed on one 120 W incandescent bulb for ~6.77 hours, and require a smaller (cheaper?) inverter.

Six-packs of mini-spiral CFLs are available at Home Depot for ~\$10. DC CFLs are also available, but they're typically fairly expensive.

HTH,
Jim / crewzer
• Solar Expert Posts: 7,224 ✭✭✭✭
Re: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v
pmaru77 wrote:
Hi all,

Been reading up on solar sytems etc. I'm going to put in a small system in my Baja house, but I'm not sure of my needs yet.
From what I've read a 120w 120vac bulb would use 1amp on a 120vac system and would use 10 amp in a 12vdc system. A 12v battery with 35ahr would run this bulb for only 3.5 hours ...and less because of the efficeincy of the inverter. Is this correct?

Just about. Assume 30 useable amp-hours (50% discharge). That's 360watt hours, spend it anyway you like.
You could get a 12v LED bulb and run direct. and skip inverter losses.
(some expensive ones here, but good descriptions http://www.ccrane.com/lights/led-light-bulbs/index.aspx )

So for 120V via an 80% efficient inverter 360WH gets 2.4 hours
360WH : 80% = 288WH remaining / 120V = 2.4hrs

Camping, I take a small inverter, and hook many strings of LED xmas lights on it, runs all night on a single COSCO deep cycle battery.
http://www.sbainc.com/PStuff/060828-BM2006/images/img_4153.jpg panel is on right edge of light strings
the photo is of a more elebrate setup, with lots of EL wire, strung up on a 45' tall tower with sign, 2 batteries, and 2 panels ran it all night for a week.
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,

• Solar Expert Posts: 10,311 ✭✭✭✭
Re: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v bat

you are correct as far as the inverter efficiency goes. if an inverter would have a 90% efficiency then it needs 10% more power to produce what you are using. now this efficiency will vary with different makes and models and will also vary with the inputted power to the same inverter. in other words the efficiency changes throughout the inputted power range and usually the peak efficiency is what a company will list.
as to the battery, that would be wrong. you would need roughly 10-15% more for a standard lead acid battery due to charging efficiencies and you cannot excede the 50% dod point. around the 15% point would mean another 5ah of current for a 35ah battery, but because of the 50% dod i'd say a good 85-90ah battery would be needed to do what you describe.
Re: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v bat

OK, another newbie question - you talk about a 35 or 65 or 90 AH battery - what exactly does this mean? I see many ratings for my Trojans: Capacity in minutes at 25 amps, capacity in minutes at 75 amps, cranking performance (I guess not a factor), 5 Hr AH rate, 20 hr AH rate. So, what do these really mean, and which do I use (if I know approx. how many amps per hour my trailer accessories consume)? The batteries in question are Trojan (2 ea, in parallel) SCS225. Thanks,

John F
LV, NV
• Solar Expert Posts: 7,224 ✭✭✭✭
Re: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v
barshnik wrote:
OK, another newbie question - you talk about a 35 or 65 or 90 AH battery - what exactly does this mean?

The amount of energy stored in a battery.
[edit - hypothetical values, for educational use, your mileage may vary ]
A motorcycle battery has 30AH
A car battery has 500AH
A large deep cycle battery has 1,000AH

AH = Amp Hours It can put out (car battery)
500Amps for 1 hour,
1 amp for 500hours
It's a standardized rating system, so that you know what size battery you are buying.
In actuality, you only get about 80% of that rating, and it varies at the speed you withdraw it.
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,

• Super Moderators Posts: 26,666 admin
Re: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v

I think the typical 12 volt car battery is only rated for around 75-120 amp*hours (weight of ~50-70 lbs). And because a car battery is designed for starting, it should only be discharged by about 20% (not the 50% or more for batteries designed for storage)...

500 AH car battery would be pretty rare I would think. 1,000 amp*hours is a very big storage battery (for 12 volts worth of cells--that would be around 600 lbs).

Here is a pretty good link that explains 12 volt power systems in terms of RV's (recreational vehicles)... Might answer many of your questions.

http://www.ccis.com/home/mnemeth/12volt/12volt.htm page 1

Page 2 includes solar chargers and inverters...

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
Re: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v bat
you talk about a 35 or 65 or 90 AH battery - what exactly does this mean?  I see many ratings for my Trojans:  Capacity in minutes at 25 amps, capacity in minutes at 75 amps, cranking performance (I guess not a factor), 5 Hr AH rate, 20 hr AH rate.  So, what do these really mean, and which do I use (if I know approx. how many amps per hour my trailer accessories consume)?  The batteries in question are Trojan (2 ea, in parallel) SCS225.
John,

GREAT question! Unfortunately, the answer is fairly complicated. Let’s use your Trojan SCS225 batteries as an example.

http://www.trojanbattery.com/Products/ProductSpec.aspx?Name=SCS225

For deep-cycle applications, the important specs are voltage, reserve capacity (RC) and 20 hour AH capacity. Your batteries are nominal 12 V models. The RC spec is 225 and the 20 hr rate capacity is 130 Ah.

RC is a specification for how long (in minutes) a healthy and fully charged battery will sustain a 25 A discharge at 77 degrees F until the battery cell voltage drops to 1.75 V. For a 12 V battery with six cells, that’s 10.5 V, at which point the battery is considered fully discharged. Continued discharge may permanently damage the battery. So, under the conditions described, each of your batteries can sustain a 25 amp load for 225 minutes, or 3-3/4 hours.

To me this spec is useful because it is load based (25 A) and it allows direct comparison of batteries’ RC specs.

The 20 hour rate spec defines how many Amp-hours (Ah) the battery can deliver at 77 degrees F when under a constant-current load that will cause the cell voltage to drop to 1.75 V (10.5 V for a six-cell 12 V battery) in 20 hours, at which point the battery is considered fully discharged. Continued discharge may permanently damage the battery. So, under the conditions described, each of your batteries can sustain a 130 Ah / 20 hours = 6.5 amp load for 20 hours.

I don’t care for this spec because it is time based. I’d rather see this spec based on a constant load of 5 A, as this would allow direct comparison between different batteries for the same anticipated load.

You may see other battery specifications such as CA, CCA, and MCA. These generally apply to starting batteries (“SLI” batteries) and to hybrids (Marine/RV) used for both starting and deep-cycle applications, but they are of little practical use in just deep-cycle applications.

It’s not a good idea to fully discharge a battery, as that treatment will substantially shorten its useful life. Although Trojan allows for a maximum 80% discharge, 50% is a more common rule-of-thumb. For daily discharges, a 15% - 20% discharge limit is typical. No matter what, it’s good practice to recharge your batteries as soon as possible after each discharge.

Battery specs and performance can get complicated when temperature or different loads get thrown into the equation. Figure on a 1% loss of capacity for every degree C drop below 25 C (77 F). In other words, a fully charged battery may be able to deliver only ~75% of its rated capacity when ambient temps are at 32 F (0 C).

Varying loads throw the capacity numbers for a real loop. For example, the SCS225 RC spec of 225 suggests that the battery’s capacity is 25 A x 3-3/4 hours = 93.75 Ah. But, the 20 Hr spec says its capacity is 130 Ah.

WTF?

Welcome to Peukert’s Law, which basically states that a battery’s capacity is temporarily increased as discharge current is decreased, and that capacity is temporarily decreased as discharge current is increased. Related topics are Peukert’s Equation and a battery’s Peukert Number. You can research these topics in my previous posts and/or on the Internet. Once you’ve mastered the concept, you’ll no doubt be the center of attraction at just about any bar that I can think of.  :evil:

What it really means is that a battery’s capacity is not a constant. Instead, battery with a 20 hour rating of 130 Ah will deliver 130 A for far less than one hour, and it can deliver 1 A for a bit longer than 130 hours.

As a practical matter, the Peukert Number for your batteries, based on their RC and 20 Hr specs, is 1.24. This suggests that your two well-maintained and healthy batteries, which together appear to be 20 hr rated for 260 Ah, could sustain a 5 A load for ~65.57 hours (100% discharge), indicating a load-normalized capacity of ~328 Ah at 77F (25 C).

In other words, that’s a fine pair of batteries you have. Aren’t you glad you asked?

HTH,
Jim / crewzer
Re: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v bat

Thanks for taking the time for the explainations, crewzer/Jim, and the link provided by BB.  Between them, I'm just a little less confused than before.  I'll have to re-read everything a few more times (my mind is in 'bulk' mode right now, not even approaching regulation) before I can absolutely fascinate everyone at the bar.

I got a cheap digital 20A current meter off eBay, will install it when it arrives, then will be able to accurately measure each of my loads, estimate time usage, and arrive at a run-time based at (for winter camping) 32 degrees and a absolute minimum 50% SOC.  I'm anticipating my max drain at any one time to be around 16 amps for short periods of time (furnace fan + water pump + lights + LP leak detector) and max charge current of 8.6 A (the highest I saw reported by the TriStar).

BTW, new TriStar is installed, custom program created for the Trojans (including their .028 temperature compensation) according to charge specs on their website, and is working.  It did take 1.5 days for them to finally reach 'float' mode, at which time I considered them fully charged.  Measured SG of each cell, and they are close to spec (although more variation than they recommend, but I'll give them a few more discharge/charge cycles before I consider having the distributer check them under warranty.)

Armed with all my new knowledge, I'll be able to explain in detail to my wife why we must freeze our a**es off sleeping without the furnace after a couple of overcast days and batteries at 50% SOC <g>.

Sorry, didn't mean to hijack the thread, but I've really been wondering how all these battery ratings work...

John F
LV, NV
Re: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v bat

Thanks all .... for the feedback. Hijack thread? This thread could only go so far anyways.
• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
Re: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v bat

John.

Great news!

You're right to be concerned about the heater's fan.... I think they typically draw 4 A or so (at 12 V). An RV'ing trick is to replace the cheesy standard mechanical thermostat with digital set-back model. The thermostat could be set back from, say, 11 PM 'till 5 AM, which would allow the heat to run in the evening, to provide some toasty warm ambiance in the morning, and save you ~30 Ah per night (plus the propane).

There are several basic Hunter digital models that can be used, and I seem to recall that they're ~\$20 - \$25 at Wal-Mart. Check on-line for their models, user manuals, and wiring diagrams. They run on their own little batteries...

HTH,
Jim / crewzer
Re: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v bat

Jim/crewzer,

This is a TINY camper, everything including the thermostat is in arms reach - so we've always used the 'manual' setback method. The cheapo thermostat only goes down to 55 degrees, though, and we'd be quite comfortable sleeping at 40 or so - I'll check and see if the little electronic set-backs go lower... Thanks for the tip.

John F
LV, NV
Re: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v

Or, pull the cover off, have a good look and you might see a little "travel limiting" arm, or post, that if removed, may allow the therm to be turned lower. Did that years ago for another situation and also to the similar thermostat I have attached to the inside of my "chest fridge", which centers at 33F. In fact, with that one, I even turned the mercury bulb end for end, so it could directly operate the relay that controls the compressor.
Wayne
• Solar Expert Posts: 202 ✭✭✭✭✭
Re: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v bat

Pmaru77;

you've got some great answers to your question, but I'll add my 2 cents worth. This is an exerp from my blog at: http://solarjohn.blogspot.com

A 10-amp load on a single 100-Ah battery is a significant load. However, a 10-amp load on a battery bank consisting of ten 100-Ah batteries connected in parallel is a light load. In the case of ten batteries connected in parallel, the load current is equally divided between each battery, or one amp per battery. The size of the load hasn’t changed, but the load on each battery is ten times lighter than it was for a single battery.

Keeping in mind that we shouldn’t completely drain the battery, a single battery under a ten-ampere load can be expected to last 8.12 hours. From this, one might expect a bank of ten batteries to last ten times as long, or 81.2 hours. Actually, according to Peukert’s Law, the ten-battery bank should last 162 hours. This is important to keep in mind as you design your solar photovoltaic system. By increasing the battery bank size by a factor of ten, the power available increases by twenty times. By over-sizing the battery bank, the efficiency of the system has been greatly increased.

John
• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
Re: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v bat
By increasing the battery bank size by a factor of ten, the power available increases by twenty times.

Solarjohn,

Peukert’s Law cover’s a usefully wide range of discharge applications, but it does fall apart at the extremes (very high discharge or very low discharge). Accordingly, the statement above is just not correct.

Let’s looks at a couple of examples. The Concorde PVX-1040T’s 8 hour capacity is 93 Ah (11.625 A discharge current). Wiring 11 of these batteries in parallel and spreading the same total load (@ ~1.06 A/battery) would increase each battery’s capacity to ~120 Ah, or 1,320 Ah for the total bank. For an 11-fold increase in bank size, that’s a 14.2 fold increase in useable energy (not power, BTW).

http://www.concordebattery.com/xtender_main.php

Similarly, the MK 8A8D battery is rated at 50 A for 230 minutes and 5 A for 3,040 minutes. So, wiring ten of these batteries in parallel (increasing the battery bank size by a factor of ten) and distributing the 50 A load amongst them (5 A per battery) would result in a capacity increase of 3,040 / 230 = ~13.2-fold.

http://www.mkbattery.com/images/lagm.pdf

There’s a big difference between 20 X and 13 X or 14 X. Peukert’s Law is indeed useful, but it has its limits. Although the Peukert Exponent for the battery behavior you've described is a not unreasonable 1.30, and it would work reasonably well over a narrow range (i.e. 25 A vs 5 A), it doesn't work well over a broad range, i.e., 10:1, or when one parameter is a particularly high or low current. For example, the capacity curve goes fairly linear for most batteries when the discharge current falls below ~5 A. Accordingly, and with all due for your intent, your efforts and commitment, I suggest that your blog on this subject is in need of considerable correction.

Alternately, I'd be interrested in learning about a lead-acid battery with published specs that behaves as you've suggested. For example, is there a battery that is rated at 100 Ah when discharging at 10 A and is also rated at 200 Ah when discharging at 1 A? Or, 100 Ah at 5 A and 200 Ah at 0.5 A?

Regards,
Jim / crewzer
• Super Moderators Posts: 26,666 admin
Re: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v

The other issue with oversizing a battery bank is that there is self discharge rate and the inability for a power limited device (like Solar Panels in poor weather) to fully charge the batteries (leaving flooded cell batteries significantly discharged--weeks and months--without fully charging will reduce their life because of the hardening of sulfate deposites over time).

And, with flooded cell batteries, you should "equalize" them every so often to balance the cells and stir the acid to prevent stratification. If you have too many batteries and too little solar power, you will not be able to properly equalize the batteries.

A larger battery bank will tend to last longer (lower discharge rates and, hopefully less deep discharges)... But if the batteries last twice as many years between replacement, getting 10x as many batteries will still result in spending 5x as much for the bank's life cycle.

To a degree, from what I understand, AGM type batteries will work better (sulfation is much less of an issue and equalization is not an issue at all).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 202 ✭✭✭✭✭
Re: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v bat

Jim,

I meant to say that under certain conditions you can get twenty times more power from your battery bank than you otherwise would. Stated another way, a lightly loaded battery bank is much more efficinet than a heavily loaded one. The numbers I came up with are based on my calculations when Peukert's Law was applied to battery ah ratings and loads.

I understand that when you change the ah-rating, or the load, you change the efficiency. In my opinion, a battery bank should be significantly oversized in order to take advantage of the effects of Peukerts Law, and that's what I attempted to point out. Your own examples show a significant efficiency increase as the battery bank size is increased.

My battery examples were hypothetical, not based on any type or brand of battery. Sorry if my information was misleading or incorrect in any way.

My oversized battery bank is really helpful right now as we are without utility-supplied electricity due to an ice storm.

Bill,

I've conducted tests, and have found that my array can recharge a 50% discharged battery bank in 3 to 4 days. I believe that equalization is not a problem, as I've measured over 14 volts across the bank on occasion while charging via the array.

John

• Super Moderators Posts: 26,666 admin
Re: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v

John,

Three to four days on a battery bank (cloudy weather), I believe, is the sweet spot for battery storage. Under three days, and the battery banks are going to be stressed more, and the generator will be needed more often. More than that is a whole bunch of money, space, and maintenance issues.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
Re: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v bat

Now, one of my few remaining questions is - how do you pronounce 'Peukert' ?

But, a couple more -

1)  Assuming no discharge under, say, 50% SOC, and assuming proper liquid level maintenance, fully recharging MOST of the time, assuming a proper EQ every 5 to 10 discharge cycles, how many of these discharge/recharge cycles can I expect for the life of these Trojans?

2)  Does it hurt or help in any way to leave the batteries charging for any period of time after the TriStar indicates 'Float', or can I assume the batteries are fully (or close enough) charged when it reaches the 'Float' stage?

These questions should verify my 'Newbie' status, even though I am quite old...

John F
LV, NV
Re: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v

If all is working OK, then when the Tristar shifts into "Float" mode, that means the batteries are fully and completely charged and the TS holds them in that state using it's Float mode, without any additional significant charge going into the batteries..
Wayne
• Solar Expert Posts: 202 ✭✭✭✭✭
Re: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v bat

Thanks for the information Bill
Three to four days on a battery bank (cloudy weather), I believe, is the sweet spot for battery storage. Under three days, and the battery banks are going to be stressed more, and the generator will be needed more often. More than that is a whole bunch of money, space, and maintenance issues.

After I add two more panels to my array (soon), I'm planning to add two more batteries.  Since my current batteries are less than one year old, I think I can get away with that without serious degredation to the new ones.

One of the main purposes for my PV system is to serve as an emergency source of power for grid power outages.  Since a typical outage lasts less than twenty four hours, I can really benefit from an oversized battery bank, even though it takes 3 to 4 days to fully recharge it.

John

• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
Re: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v bat

John F,

Peukert is pronounced “Pew” (like a church bench) – kert.

Solar John,

I think we made some progress on our Peukert discussion, but I also think it’s worth a continuing dialog.

There’s no question that I fundamentally accept Peukert’s Law. A search through the forum’s post archives will show that I’ve addressed the topic several times in the past. Rather than signaling acceptance or proof of Peukert’s Law, the purpose of my examples above was to more accurately quantify the benefit of enlarging a battery bank and/or reducing the load.

After having given this issue some additional though, there may be a practical though limited example of your claim. The Peukert Number (aka Peukert Exponent) for typical Group 27 size 12 V battery with an RC rating of 160 and a 20 hours capacity of 105 Ah is 1.291. I have no doubt that increasing the discharge current to 52.5 A (10 X the 20 hour spec current of 5.25 A) would deplete the battery in about one hour, and that the effective capacity would then be ~ 53.7 Ah.

In other words, spreading the 52.5 A load across 10 batteries wired in parallel would result in approximately a 20-fold increase in usable battery capacity. However, while Peukert’s Law works for this specific example, the argument remains flawed.

On problem is that battery capacity goes fairly linear at low discharge rates. For example, the MK 8A31 is a group 31 size AGM battery with a 20 hour rating of 105 Ah (discharge current = 5.25 A) and a 100 hour rating of 116.2 Ah (discharge current = 1.16 A). Cutting the discharge current by a factor of almost five resulted in but a small gain in total capacity (~10.7%).

Another problem is that a Peukert Number of 1.29 is not particularly good. In other words, the battery’s poor performance under high-discharge conditions (52.5 A) helped make its low-discharge (5.25 A) behavior look relatively good.

Quality deep cycle batteries have smaller Peukert Numbers. Using the MK 8A31 again as an example, its RC is 200 and its 20 hour capacity is 105 Ah, and so its Peukert Number is 1.148. Applying a 50 A load, the battery would theoretically last 1.5 hours and deliver 75.2 Ah. Cutting the load to 5 A would cause the battery to last ~21 hours and deliver 105.7 Ah. In other words, wiring ten of these batteries in parallel to distribute the 50 A load would result in a battery bank rated at 1,057 Ah, or 14 X the single battery’s capacity.

So, are there certain conditions under which you could increase a battery banks size by 10 X and yield 20 X the usable energy (not power, by the way)? The answer is obviously yes, both in theory and in practice. Unfortunately, these conditions involve other than high quality batteries, and only under very specific conditions. Accordingly, the practical benefits to increasing battery bank size and/or reducing load current, although very real, are much smaller.

So, my recommendation is that it’s fine to keep your “10 X = 20 X” example as an extreme possibility with some shock value, but that your readers should anticipate 10 X = 12 X to 14 X to be more likely when using quality batteries under likely conditions.

On a different note, it’s indeed heartening to hear that your battery bank is helping you weather the past week’s storm and aftermath. While I agree with Bill that a oversized battery bank is not a good thing (do some research on “deficit charging” or “deficit recharging”), yours sounds to be appropriately sized if your array can recharge it from a 50% SOC in three- to four days.

HTH,
Jim / crewzer
• Solar Expert Posts: 202 ✭✭✭✭✭
Re: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v bat

Jim, Thanks for the information.

Since there are practical limits to oversizing a battery bank for a given PV array size, do you see an advantage to having two or more battery banks? Once the first bank is fully charged, I could switch charging to the second battery bank. Once both are fully charged, the relatively small PV array should have no trouble providing enough energy for the float for both banks. I could switch banks manually if necessary, or I could come up with something to do it automatically. (I have an electronics background).

I happen to have about 30 - 7ah gell-cell batteries that I would like to use for my second battery bank. These were replaced while still good in the UPS units for computers where I work. And, while that’s not a terribly big bank, it might keep the motors on my alternative heater going (about 100 watts on low) for a night in the event of another power outage. What are your thoughts concerning connecting so many gell-cells in parallel? I currently have them all tied into big aluminum bus bars using 10 guage wire from each battery to the + and - bars. I saw a "monster" switch at an electronics surplus house that I think might handle the current.

I’m currently using a Morningstar 20a charge controller, and will upgrade to a larger one when the size of my array exceeds its limits. Since I'll have two charge controllers, I can consider using both of them in my final configuration. (Just thinking out loud).

BTW: I don't like to rely on information from equipment suppliers, so I periodically run my own tests. I might, for example, connect a 75-watt light bulb to the inverter and see how long it takes for my battery bank to discharge to 50% SOC. I have a log book with all kinds of performance data from my system. I guess I'm just anal that way. I also have a data logger that is useful in that I don't have to take voltage readings every few minutes.

I'm still having a little trouble accepting your statement "a oversized battery bank is not a good thing", while my own experience shows that it is beneficial. Did you mean to say "oversized beyond a certain point"? Perhaps you're saying that my battery bank is not oversized.

I wish my budget would allow me to just get a bigger battery bank right now, but my next purchase will be more panels. Someday…..

John
• Super Moderators Posts: 26,666 admin
Re: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v

Here seems to be an interesting web (designs/sells battery gauge monitors) site on batteries and Peukert information...

http://www.smartgauge.co.uk/technical1.html

Also, if you wish, there is a little Excel spread sheet that lets you graph the useful battery capacity based on different ratings and Peukert values...

Warning will download a .xls spreadsheet (and on my PC) it will be directly opened by Excel (after a virus scan)... My Norton Anti-Virus says everything is OK with the spreadsheet--but for those really paranoid, you may think twice about the following link:

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 7,224 ✭✭✭✭
Re: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v
BB wrote:
http://www.smartgauge.co.uk/technical1.html
Warning will download a .xls spreadsheet (and on my PC) it will be directly opened by Excel (after a virus scan)... My Norton Anti-Virus says everything is OK with the spreadsheet--but for those really paranoid, you may think twice about the following link:

in windoze, you can RIGHT CLICK and SAVE-AS, and force it as a download, and not open it.
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,

• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
Re: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v

John,

An actual mid-day charge current of 2~3% is typically enough to bulk charge a healthy but unloaded battery bank. For example, if your “12 V” PV array can reliably supply 10 A, you should be able to recharge and maintain a 12 V x 400 Ah battery bank. Personally, I prefer a 5% minimum charge current as a part of an overall well-balanced system.

You may find a marine switch like one of these to be of interest for split-bank control:

http://www.bluesea.com/dept.asp?d_id=7459&l1=7459
http://store.solar-electric.com/basw1300amp.html

Wiring a large number of batteries in parallel is not typically recommended. However, it does work. Wiring 30 each 12 V x 7 Ah batteries in parallel will yield a “big” battery rated at 12 V x 210 Ah (not including Peukert correction). The buss bar approach is a good idea, and the main cables should be connected “diagonally” to the bars and not both the same end of the bank. For example, connect the (+) cable to the (+) buss bar at battery #1, and the (-) cable to the (-) buss bar at battery #30. This diagonal solution will help even distribute charge/discharge current to each battery.

Gel batteries, like all VRLA batteries, are sensitive to excessive voltage. The typical absorb voltage at 77 F is ~13.8 V, so check the manufacturer’s recommendation. Temperature compensation is highly recommended.

Connecting the outputs of two controllers to a single battery bank shouldn’t be an issue. They just need to be configured for the same voltage settings. Check the FAQ’s on Morningstar’s website.

“Oversized” to me means that the battery bank’s capacity is more than 40 X the available mid-day charge current (i.e., a battery bank >400 Ah would be oversized for a system with 10 A of charging current).

In a well balanced system, I think a “correctly” sized battery bank should be ~6 X average daily needs (three days of autonomy down to 50% SOC, and that the battery charge current should be between 5% and 10% of the battery capacity. But, this is a general guideline and may be adjusted based on many factors (environment, battery type, day-time loads, etc.)

HTH,
Jim / crewzer