BATTERY LIFE

larglo · Post by **larglo** » Fri Sep 18, 2009 5:01 pm

Hi all,

Once again I'd like to ask you kind folks for some help to understanding house batteries and their life span before having to recharge when using an inverter for 110 v power.

(group 29 battery installed in original battery housing under the top stair into the cabin. 2 group 29 deep cycle batteries hooked together in parallel (pos to pos, neg to neg) - yielding approx 250 amp hours of juice to a house system that would max out at 25 amp hours)

This was written by a fellow Trojan site member. As I understand this correctly, having 25 amp hours,,means that if I total up all the amps from all my components and divide that total into 25,,,that is the amount of hours they will run until recharge the batteries is required?

Exp;

Fridge.......1.3 amps
A/C...........5.8

Total..............
7.1 amps [25 divided by 7.1 = 3.52 (hrs)?]

Can I look forward to 3.52 hours running continuously , which is very unlikely.

Thanks for your help,,,,again!

Larry

jav · Post by **jav** » Fri Sep 18, 2009 7:42 pm

No time to go too deep but your missing some conversions.

Batterie are rated for current at 12 volts so unless the current you listed is also at 12 volts- you'd need to reclaculate the inverter current draw and then do the math.

If you expect your batteries to last more than a season or two-you should not draw them down more than 50% and even that number presumes they are true deep cycle batterires and not starting or combination batteries.

Also understand that many appliances can draw 3-5 times more current when starting so allow some reserve for that as well.

g36 · Post by **g36** » Sat Sep 19, 2009 5:27 pm

if you are using the ac amps from the appliance name plate you will be sad to see that the draw using the invertor and the loss it has converting from dc will be alot more than what you think. i might be wrong but my numbers are at the boat but my small extra refrig i use sometimes also has the 1.2 amps at 120v ac but using my current clamp and checking the dc at the battery when its running is more in the neighborhood of 6-7 amps dc. jav is correct that motors will have thier high current draw when it starts. you could borrow a dc amp clamp from someone, it makes this kind of thing a piece of cake to see how many amps you are pulling out of your batteries when you turn something on. i dont beleive you be able to run a/c from the 1500 watt invertor you currently have. if it starts its really going to run your batteries down. even running a microwave from a 1500 watt invertor the amp draw on the batteries will be high, way more than the fridge.. you might notice if your invertor has any indicators on it that shows how hard its working. a voltage or amp meter on it. you should also take into account ANY other dc draw that you may use.

Peter · Post by **Peter** » Sat Sep 19, 2009 8:55 pm

may I refer you to the following reference; Boatowner's Mechanical and Electrical Manual, by Nigel Calder.

This is an excellent easy to understand manual/reference book that I have found to be very, very informative. It starts right from scratch, explaining the difference between automotive, deep cycle Marine, starting, house, absorbed glass mat [AGM] batteries etc. their differences in capability and design, weaknesses and strengths. It walks you through a typical power boats electrical system, its design, and will teach you how to answer all the questions you have about modifying your boats battery banks. If you spend three evenings reading the first three chapters [1; Establishing a Balanced Battery Powered Electrical System, 2; Electrical Systems for Energy Intensive Boats, 3; Maintaining and Troubleshooting a Battery-powered Electrical system] you will be able to design the system that best fits your needs and understand it completely.

This is a very easily obtained reference and is almost as valuable as Chapman's Piloting and Seamanship

[If you don't want to spend the 50 bucks, put it on your Christmas, birthday, or other gift list, because it really is a necessity to have as a reference for an older boat. Mine has paid for itself several times just this year]

randyp · Post by **randyp** » Tue Sep 22, 2009 11:50 am

Larry,

There's a lot of very good advice from these other members here, but if I understand your question, and just assume that you are a newbie to the wonders of electricity, this may help. Your question was how long would your battery last without using the inverter.

My advice was based on the amp draw of 12 VOLT appliance - not a 110V inverter. The amp draws you showed were for your 110Volt appliances, I think (Fridge and AC). My numbers showed what a 250 amp hour deep cycle house bank would be capable of if the total house draw of all the 12V appliances was 25 amps. Bascially they would last 10 hours (probably less since they would poop out before they got totally drained).

But you're looking at using a 110 Volt AC inverter drawing off your 12 volt dc battery. What is the wattage of you inverter? The 7.1 amps you showed, if they are AC would yield 781 watts of power, using the trusty formula of "WAVE" (Watts = Amps x Voltage). That seems really low for these 2 big drain AC appliances.

I agree with the others that your 1500 watt inverter would be really taxed, especially at start up as the motor for your A/C compressor and fridge will have a very high drain at start and until the thermostat kicks in at the desired cooling temp.

You should not try to run your A/C off a battery-invertor set up. The fridge may be a different story, but I think you will need at least 250 amp hour set up or more to use the inverter reliably for more than a very short time. My math here may be wrong, but I THINK they 1500 AC invertor would draw 13.6 amps of AC power at 110 volts. Converting that to the required DC amperage at 12 volts is where my head is starting to explode - I don't know the answer, but I do know it's "A LOT".