Problems with christmass lights

System

I have 2 truck battery's 1200 cca connected in parallel. I'm trying to run my xmass lights on them with a 400 watt inverter but cannot get anymore than 1.5 hrs from a full charge. The lights are only drawing around 170-200 watts mostly leds and mini lights. Should i be able to get longer time I thought I might get 4 hours or more??
Can someone tell what I'm doing wrong??

tallgirl

Re: Problems with christmass lights

1200 CCA is not a measure of amp hours. And while the loads don't come out to 400 watts, a 200 watt output could requre significantly more to produce. On top of that, add poor efficiency for the inverter into the mix and I don't have a problem imagining that the batteries are being run down in that amount of time.

niel

Re: Problems with christmass lights

those batteries are meant for starting and probably won't handle deep cycle use. maybe a couple of golf cart batteries would help. even the deep cycle batteries should not be drawn down past halfway so keep that in mind. what's wrong with using the utility power for the lights?

n3qik

Re: Problems with christmass lights

Bet the OP is in a RV setup and boondocking.

I would change two things. Get better battery(ies) and change to LED light strings.

System2

Re: Problems with christmass lights

The inverter shuts it down at 10.5 v. I'm just experimenting I thought if it would work, I would get a solar panel to charge the batteries in the day. The utility here have a contest here to see if you can cut 10% on your bill, I thought that might do it . What size of deep cycle batteries would I need and how many? Also I was thinking the fan on the inverter was drawing them down faster?

BB.

Re: Problems with christmass lights

Generally, for long life, you don't want to discharge a lead acid more than 50% for longer life. You can, discharge down to 20% state of charge a true deep cycle battery and it will last a few hundred cycles--As long as you recharge it back over 75% state of charge the next day (storing batteries below ~75% state of charge will cause the sulfate to harden over time--days/weeks and "age" the battery before its time).

You may not have time to get one now, but a Kill-a-Watt meter will be very helpful for both this project, and for finding power hogs in your home too.

Say you don't want too large of battery bank and you can recharge them every day (this is short term use, not recommended for long term solar)--Pick 50% use of battery bank every night. Get a pair of 225 Amp*Hour 6 volt golf cart batteries for your bank. Assume that the inverter is 80% efficient--your usable capacity is roughly:

Watt*Hours=Volts*Amps*Hours * 0.50 batt cap*0.80 invrt eff = 12 volts * 225 Amp*Hours *0.5*0.8 = 1,080 Watt*Hours

Now, the 225 Amp*Hours is based on 20 hour discharge (C/20) use... If you use a battery at more than C/5, the capacity is severly reduced (basically chemical energy and internal resistance is turned into waste heat/lost work inside the battery). So, if we assume that you will use C/5 as your maximum discharge rate, pick a T105 6 volt battery 225 AH at C/20 or 185 AH at C/5 which give us

Watt*Hours= 12 volts * 180 Amp*Hours *0.5*0.8 = 864 Watt*Hours # C/5 rate

So, assuming the battery+inverter is good for 2.5 hours of useful energy of 864 Watt*Hours, then we could run this much power every night:

Hours=Watt*Hours / Hours of use = 864 WH / 2.5 Hours = 345.6 Watts of lights

Should give you a good idea of how to figure out how much battery you will need to support XXX Watts of lights.

Note, you will need to charge the battery fully every day--deficit charging (pulling more power than you replace through charging) will kill a battery bank in weeks or even days (pulling a battery bank below 20% of capacity runs the risk of "reverse charging" a cell--batteries and their cells do not always match capacity, and there is usually one weak cell that "runs out of charge" before the rest--continued current draw will cause that one cell to "run backwards" and pretty much destroys it ability to properly recharge ever again).

Now, after all of that--how about getting a Honda eu2000i genset... You could run a 400 watt load for 2.5 hours on less than 0.2 gallons of fuel per night.

Just drain the fuel when done (use fuel stabilizer in your 5 gallon can) and you can store the genset for next year. Batteries will need charging/water/maintenance for the next year--and at best--they probably will only last you 3-4 years before they need to be replaced. The genset, properly stored, will last you much longer and is much more portable. The Honda euX000i family of "inverter"/generators is very quiet, fuel efficient, and has very clean power (voltage/frequency stable).

Also, if you are planning on using a battery bank plus inverter to save electricity on your utility bill--Remember there are losses of Eff=0.80 batt * 0.80 inverter eff * 0.80 charger eff = 0.51 overall efficiency... So, just running lights off a battery bank and recharging with your utility power will use 2x the amount of energy vs just using the wall outlet (or even using a genset).

Hope the above helps.

-Bill

niel

Re: Problems with christmass lights

what bb skipped was that at 10.5v a battery could be considered dead or 0% soc. keep in mind that would be the free standing voltage without a load or charge for a few hours that it would be dead at 10.5v. even so, that voltage would have a low state of charge in the battery as the recovered voltage after a few hours would rise a small bit depending on how deeply loaded the battery was to draw down the battery voltage. as such you do not want to depend on that feature on the inverter. you can use a battery monitor or know your loads and the time they are on to determine the soc.
as to the contest of lowering one's electric bill by 10%, i think that is too easy to achieve (at least in my opinion) as not using power and other conservation methods could easily save far more than 10%. adding pv does not save power usage as it is only another source of where the power comes from and you must see that distinction. even so, it would subtract from the utility electric bill by that much usage.