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Old 09-19-2013, 06:07 PM   #1
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Battery voltage drop under load?

I have a pair of 12 volt deep discharge batteries wired in parallel that drop from 12.6 to 11.6 volts at their terminals while supplying about 140 amps to an inverter to run a microwave for 20 seconds. The batteries are of unknown age and rated for 550 CCA. Is this a normal voltage drop for deep discharge batteries with a 70 amp load, or are they showing signs of old age? What would be the expected voltage drop if I had two 6 volt golf cart batteries in series with this load?
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Old 09-19-2013, 06:19 PM   #2
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TLI too little information, what are the make and amp hours of the batteries. Usually you see CCA expressed for a starting or dual purpose i.e. marine battery. These are not built for sustained loads and if you are unsure of their pedigree it would be worth your while to have them tested. One thing to remember is that they will default to the weaker of the two batteries. Heavier duty batteries will show less drop.
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Old 09-19-2013, 07:14 PM   #3
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Sounds normal to me as long as the voltage goes back up when the draw is removed.
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Old 09-19-2013, 07:24 PM   #4
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I have a trimetric battery monitor, my microwave pulls 125 amps, my 4 golf cart batteries drop .5 volt while the microwave is on, but returns to the previous voltage or .1 of a volt less when complete.

If your voltage returns to normal then I would not worry about it. But if the microwave after 20 seconds draws the batteries to less than 50% state of charge (SOC), you probably need new batteries. Remember, you should never let your batteries get below 50% SOC which about 12.0 volts.

Clean the terminals, give the batteries a full charge, maybe equalize them if your charger has that mode. Then try it again.

See if your charger is a 3 stage charger. After first bulk charging to around 80%, the charger changes to an absorption charge that increases the voltage to around 14.4 volts for a while while the current drops as the battery tops off, then holds the battery topped off with a float charge.

It is the absortion charge that is needed to top off the batteries. If it does not do it, 80% charge is about all your going to get into the batteries.
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Old 09-19-2013, 07:37 PM   #5
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Any battery wita a CCA rating is not a true deep cycle battery. At best, it is a marine battery, which is a hybrid between a true deep cycle and a starting battery. If you do end up replacing them, consider going with a pair of 6 volt golf cart batteries wired in series.

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Old 09-19-2013, 07:54 PM   #6
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Of the batteries substantially recover soon after the heavy load period and otherwise are performing ok, then I don't see a problem.
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Old 09-20-2013, 05:08 PM   #7
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Thanks, folks. Here is a little more info.

The converter is a Progressive Dynamics PD4600, which goes to 14.4 volts in “Boost Mode”, 13.6 volts in charge mode and13.2 volts in storage mode. I have not tried the 16 volt equalization, but I have run a 24 hour high frequency desulphate cycle on both batteries. The battery terminals are all very clean. The battery bank does not fully recover from the discharge, but does return to a little below the starting voltage after several minutes. The operating voltage of 11.6 volts is well below the supposed 50% level, if this is a concern.

The main problem is that my Xantrex inverter does not like the one volt battery drop and the half volt wiring drop. The AC output drops to 107 vAC. The microwave seems to take a little longer to heat things than when using shore power at 120 vAC and one of the GFCI outlets sometimes rattles loudly for some reason I do not yet understand.

The batteries are from Les Schwab and labeled “RV Deep Cycle”. No amp hour rating. When I asked at the local Les Schwab dealership about age and specifications for the batteries, the clerk told me that they do not have specifications, and if I want to know about electricity I should be talking to an electrician. Lots of help and good service from the dealer.

Thank you very much for your help!
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Old 09-20-2013, 05:50 PM   #8
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Johnson Control makes the battery, and the Les web site has NO information (big red flag). the ignorance/stupidity of the employee should be brought to the attention of management and I would want to contact them by phone and ask for answers. You can write JC provide size and identifications and they should provide what you have. General inquiries about batteries | Johnson Controls Inc.
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Old 09-20-2013, 06:37 PM   #9
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Quote:
Originally Posted by mgscott4 View Post
Remember, you should never let your batteries get below 50% SOC which about 12.0 volts. .....
WRONG and dangerous to battery life.
Approximate state-of-charge v. Open circuit voltage 12v
100% 75% 50% 25% 0%
12.65 12.45 12.24 12.06 11.89
As can be seen, a value of 12.0V would be less than a 25% SOC and well into wreck your battery life territory.
Source: Measuring State-of-charge - Battery University
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Old 09-20-2013, 06:58 PM   #10
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Quote:
Originally Posted by NormD View Post
Thanks, folks. Here is a little more info.

The converter is a Progressive Dynamics PD4600, which goes to 14.4 volts in “Boost Mode”, 13.6 volts in charge mode and13.2 volts in storage mode. I have not tried the 16 volt equalization, but I have run a 24 hour high frequency desulphate cycle on both batteries. The battery terminals are all very clean. The battery bank does not fully recover from the discharge, but does return to a little below the starting voltage after several minutes. The operating voltage of 11.6 volts is well below the supposed 50% level, if this is a concern.

The main problem is that my Xantrex inverter does not like the one volt battery drop and the half volt wiring drop. The AC output drops to 107 vAC. The microwave seems to take a little longer to heat things than when using shore power at 120 vAC and one of the GFCI outlets sometimes rattles loudly for some reason I do not yet understand.

The batteries are from Les Schwab and labeled “RV Deep Cycle”. No amp hour rating. When I asked at the local Les Schwab dealership about age and specifications for the batteries, the clerk told me that they do not have specifications, and if I want to know about electricity I should be talking to an electrician. Lots of help and good service from the dealer.

Thank you very much for your help!
Note that if your Xantrex is a MSW (modified sine wave inverter) readings of 107 volts are PERFECTLY normal as they are not measuring RMS voltage off the square wave generated. To see the TRUE voltage output from a MSW unit you have to have a Volt meter with an RMS setting or you will measure significantly less voltage on standard meters.
If you have a Xantrex pure sine however...you DO have a problem.

EDIT...oh yeah...FWIW if you have wet cell deep cycles from either EastPenn or Johnson...you can figure pretty close to these amp hours (20 hour rating) regardless of the label pasted on the case:
G24 75ah
G27 90
G31 105
8D 250

Drop these ratings about 10ah each if ANYWHERE on the battery label it also says something like Deep Cycle /Dual Purpose since you don't have a true deep cycle, but a hybrid.
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Old 09-20-2013, 07:22 PM   #11
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What you describe is normal. It just so happens I did some updating to my lighting last week (adding more LED strips to the existing old floresent fixtures). After completed, I turned both 12v LED TVs on, antenna booster, 8 overhead fixtures, 4 bath lights, 7 reading lights, Fantasic Vent on high and ran the furnace. (All lights are LED).

Batteries fully charged and voltage was 12.6v. With everything on, voltage went to 11.9. I walked away for an hour, when I came back it was still at 11.9. Turned everything off and voltage went back to 12.5. I have two seven year old 6v GC2 Batteries (Trojan T105). I was rather surprised it dropped that much voltage, even though the amp draw wasn't that much. Certainly nothing compared to the amps you drew with through the inverter to the microwave.

As others have said, it's the recovery that matters.
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Old 09-20-2013, 09:46 PM   #12
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Voltage drop upon high load is NORMAL!

Now, what load and how much drop?

Depends on a few things, battery design (resistance and capacity) as well as the intent of the battery, float service battery may not perform like starting battery for high current loads.

Other things matter as well, these are the main things with the battery.

Contact the battery manufacturer to see if you can get a data sheet.

140 amps on any single string is a great amount and the internal resistance will force a drop in the output voltage.

Wire size and connector type make huge difference as little resistance at high loads is enough to add enough drop in voltage to make it not work correctly.
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Old 09-21-2013, 05:01 AM   #13
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Quote:
Originally Posted by camaraderie View Post
WRONG and dangerous to battery life.
Approximate state-of-charge v. Open circuit voltage 12v
100% 75% 50% 25% 0%
12.65 12.45 12.24 12.06 11.89
As can be seen, a value of 12.0V would be less than a 25% SOC and well into wreck your battery life territory.
Source: Measuring State-of-charge - Battery University
Wrong and it is OK to take a deep cycle battery down to 12.0 open circuit volts.
Your looking at a chart for starting batteries not deep cycle. Deep cycle batteries usually have a higher specific gravity close to 1.33 at full charge rather than 1.265 for starting batteries. This raises the SOC on the chart so that 50% SOC is around 12.0 volts as correctly stated earlier. I think 12.06 volts to be exact. However, the only way to really tell the exact SOC is to use a hydrometer to measure the specific gravity.
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Old 09-21-2013, 05:32 AM   #14
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You have a good converter. It has a three mode charging system that will fully charge your batteries.

I know you say your terminals are very clean, but it is all about resistance and voltage drop. Sometimes you can't see the corrosion until the connector is disconnected. Using an accurate meter, measure the voltage on the battery post, then measure the voltage on the connector or ring lug, if there any difference in voltage at all, even .1 volt, the connection needs to be cleaned. Clean the terminals on the Xantrex inverter and your ground connection too.

I clean mine at least twice a year, usually 3 or 4 times. But I am full timing and using my batteries everyday.

It can't hurt, and it doesn't cost anything.
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