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  #1  
Old 12-02-2015, 05:38 PM
Islandguy Male Islandguy is offline
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Default Battery help please

Good Day, I have a battery backup system, but no panels yet. I have a 24v system, 8 golf cart batteries being charged by a 25 amp charger/power supply and a 1500 watt inverter. We haven't got much load, one small freezer, computer, TV and a few LED lights. The other day in a power outage my volt meter showed the batteries were down to 24.8 volts, 80% of my battery left after only a few hours. My meter showed a 3 amps load of 120v . I gota admit I haven't got amp hours all figured out and what I should get out of these batteries but I does seem I should get more then a few hours out of 500 amp hour worth of batteries. Does this sound right? Thanks
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Old 12-02-2015, 11:05 PM
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I will assume 3 amps @ 120 v.a.c. If this is true, the 24 volt battery is seeing a 15 amp draw from the inverter plus a small bit of losses..

You should never draw the battery below that 20% mark. (Usually ~25% of your total amp hour rating)

Assuming 500 aHr. reserve..
25% of that is 125 aHr. This is the amount that you can use safely. At a 15 amp draw, you should get ~8 hrs. of usage..

Most golf cart batteries are ~110 aHr. Connecting them in series (4 batteries) increases voltage BUT the aHr. rating remains the same as one single battery. Assuming a parallel/series connection to achieve 24 volts, I'll figure somewhere around 220 aHr. of reserve. (8-6 volt golf cart batteries.. 4 connected in series.. and the two sets paralleled)

220 aHr.. 25%.. 55 aHr. of useful reserve.. 15 amp draw.. ~3 hrs. of "safe" usage..

This explains your short run time..
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Old 12-03-2015, 10:49 AM
wywhitewolf Male wywhitewolf is offline
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I think 12Vman has about summed up the problem. To really know we'd need to know just what batteries you have. With everything you have listed for load I think you're underestimating your usage.

I am wondering though, did you take that voltage reading with the batteries under load? If so voltage readings are only an accurate measurement of SOC if they have been resting for at least 4 hours. Get a good hydrometer and learn how to properly take readings to see read your SOC. Only after doing lots of specific gravity, load, and voltage comparisons can you use active voltage readings to determine the SOC of your batteries.

WWW

Last edited by wywhitewolf; 12-03-2015 at 10:57 AM.
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Old 12-03-2015, 04:32 PM
Islandguy Male Islandguy is offline
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Quote:
Originally Posted by wywhitewolf View Post
I think 12Vman has about summed up the problem. To really know we'd need to know just what batteries you have. With everything you have listed for load I think you're underestimating your usage.

I am wondering though, did you take that voltage reading with the batteries under load? If so voltage readings are only an accurate measurement of SOC if they have been resting for at least 4 hours. Get a good hydrometer and learn how to properly take readings to see read your SOC. Only after doing lots of specific gravity, load, and voltage comparisons can you use active voltage readings to determine the SOC of your batteries.

WWW
When you refer the SOC, in theory, wouldn't I just shut down the inverter when my volt meter reads 24.80 (80%)? your saying I'm not getting a accurate reading from my meter with a load on the batteries and to check every cell with a hydrometer after 4 hours? I must be missing something. Sorry, I'm obviously new at this.
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Old 12-03-2015, 06:02 PM
Islandguy Male Islandguy is offline
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Quote:
Originally Posted by 12vman View Post
I will assume 3 amps @ 120 v.a.c. If this is true, the 24 volt battery is seeing a 15 amp draw from the inverter plus a small bit of losses..

You should never draw the battery below that 20% mark. (Usually ~25% of your total amp hour rating)

Assuming 500 aHr. reserve..
25% of that is 125 aHr. This is the amount that you can use safely. At a 15 amp draw, you should get ~8 hrs. of usage..

Most golf cart batteries are ~110 aHr. Connecting them in series (4 batteries) increases voltage BUT the aHr. rating remains the same as one single battery. Assuming a parallel/series connection to achieve 24 volts, I'll figure somewhere around 220 aHr. of reserve. (8-6 volt golf cart batteries.. 4 connected in series.. and the two sets paralleled)





220 aHr.. 25%.. 55 aHr. of useful reserve.. 15 amp draw.. ~3 hrs. of "safe" usage..

This explains your short run time..
OK, thanks, you've assessed my system just fine to be able to answer my question without going into allot of things that I will need to know, but later. Now it seems I will learn to live with what I have, the batteries are ok. The system worked just fine in the three outages in the last month, except what I thought way too short a battery time. Thanks, you guys hit the nail on the head.


Edit...But wait, my batteries are 220 amp hr, I dug out the paper work. So I should be able to run at least 6 or 7 hours drawing 15 amps. Thats better

Last edited by Islandguy; 12-03-2015 at 06:53 PM.
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Old 12-03-2015, 07:38 PM
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Do you have a link to the exact battery that you are using?
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Old 12-03-2015, 08:14 PM
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This is a great read with lots of information. Try to understand how the load on a battery affects the aHr. reserve. The harder you pull, the less reserve they actually have..

Thanks, Wind and Sun..

http://www.solar-electric.com/deep-c...ttery-faq.html
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Old 12-04-2015, 12:44 PM
Islandguy Male Islandguy is offline
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The 8 batteries I have are new, Costco, Interstate 6 volt golf cart batteries

https://www.google.com/search?q=inte...vnlsZMw5vaM%3A
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Old 12-04-2015, 01:05 PM
wywhitewolf Male wywhitewolf is offline
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Quote:
Originally Posted by Islandguy View Post
When you refer the SOC, in theory, wouldn't I just shut down the inverter when my volt meter reads 24.80 (80%)? your saying I'm not getting a accurate reading from my meter with a load on the batteries ...
Correct that voltage alone is not an accurate indicator of SOC when the batteries are under load. Only Specific gravity is. If you wish to use voltage as the SOC indicator you to keep track of voltage/load/SG readings. Over time you can compare back to them so that you only need to take voltage and load reading to determine SOC.

A trimetric battery monitor would be easier and better though.


and to check every cell with a hydrometer after 4 hours? I must be missing something. Sorry, I'm obviously new at this...

You don't need to wait 4 hours to take SG readings. They are accurate under load. You also only need to read 1 or 2 cells to get a representation of all cells that are in a series. When you do your monthly equalization you should take readings of all cells and at that time determine which ones to use.
Seems like my post may have confused you more than helped. May I suggest http://batteryuniversity.com/ as reading material on batteries.
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Old 12-04-2015, 01:40 PM
Islandguy Male Islandguy is offline
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Here is the battery monitor I got... http://www.so-bfd.com/ I couldn't use
the trimetric monitor as the 230 foot run from my pump house/engineroom to the house required bigger wire than I could afford too use. For this "Failsafe" device all I needed was Cat 5 wire. So your saying my monitor is not giving a true reading. I have a digital volt meter at the batteries, which I cant say I checked when I shut down the inverter (took the load off the batteries). So when the monitor alarm goes off at 24.80 volts (80%), I need to check the batteries with the hydrometer?
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Old 12-04-2015, 03:26 PM
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Quote:
Originally Posted by Islandguy View Post
The 8 batteries I have are new, Costco, Interstate 6 volt golf cart batteries

https://www.google.com/search?q=inte...vnlsZMw5vaM%3A
Are these the guys?

http://www.interstatebatteries.com/p...911-2147384903

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Old 12-04-2015, 03:53 PM
Islandguy Male Islandguy is offline
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"item # 850284 outrageously dependable group size GC2" This is all it says on the batteries and receipt and nothing comes up when I search Costco's site, which I don't guite get.
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Old 12-05-2015, 05:57 PM
Islandguy Male Islandguy is offline
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What I'll do is, next week I'll run the system, being aware of the amps used until the SOC meter alarm goes off at 24.8 volts and then see what the battery voltage is, load off, I'll also check the SOC with the charger off before the test. Anything additional to check or add?
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Old 12-08-2015, 10:29 PM
JeepHammer Male JeepHammer is offline
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I might not be understanding the first post correctly,
But I can't believe there is any freezer that will draw only 3 Amps.

The draw might have been 3 Amps when you had the alarm/looked at the meter,
But I would watch that meter when the freezer kicked on, and keep looking for the 'RUN' current draw.
Start up can easily be 4 times the Run draw.

A 1,000 or 1,200 Watt draw wouldn't take long to get the end result you saw...

3 Amps/120 Volts is 360 Watts, plus at least 10% more for inverter inefficiency,
Close to 400 watts right there...
Add in that freezer compressor and you are burning through power faster than you might have imagined...

The good news is, the freezer eventually kicks off!
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Old 12-09-2015, 03:55 PM
Islandguy Male Islandguy is offline
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Quote:
Originally Posted by JeepHammer View Post
I might not be understanding the first post correctly,
But I can't believe there is any freezer that will draw only 3 Amps.

The draw might have been 3 Amps when you had the alarm/looked at the meter,
But I would watch that meter when the freezer kicked on, and keep looking for the 'RUN' current draw.
Start up can easily be 4 times the Run draw.

A 1,000 or 1,200 Watt draw wouldn't take long to get the end result you saw...

3 Amps/120 Volts is 360 Watts, plus at least 10% more for inverter inefficiency,
Close to 400 watts right there...
Add in that freezer compressor and you are burning through power faster than you might have imagined...


The good news is, the freezer eventually kicks off!
I think what I need too do is run a controlled experiment to find out just what I'm using without any loads that cycle on and off. So if I have no loads that cycle that I might not notice, I should be able to run at least 6 or 7 hours using 3 amps of 120v? (With 24v, 500+- AH.)
If I understood wywhitewolf correctly my alarm/volt meter might not be giving a correct SOC reading with a load, althougt it seems if that was the case why have such a meter?
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