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 > Battery Discharge Test

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frank60

Maine

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Posted: 08/02/12 01:25pm Link  |  Print  |  Notify Moderator

I have one 90ah deep cycle battery in my Class B with a Parallax 7345 45amp converter/charger.

I just bought a Champion 1800/1400 watt to charge my batteries while dry camping. However, I have never been dry camping, so I decided to discharge the battery to 50% in my driveway. Then I will hook up the shore power cord to the generator and see how long it takes to recharge the battery to 13.8 volts which is what it is fully charged.


While discharging the battery, I have been using a DC lamp fixture to monitor the voltage instead of directly off the battery (pita to get to). With nothing on/running I'm getting a reading of 12.38 volts. If I turn on a combination of DC lights, cpap machine & tv I get readings from 12.19v down to 11.51v. ie: 3 lights, cpap & tv gives me a reading of 11.51. One light on and I get a reading of 12.19v. I should note that I have been running the cpap&tv for several hours.

Everything I've read said not to discharge the battery more than 50%, which according to a chart I have is 12.06v.

My question are: 1) At what point do you determine the battery is 50% discharged, When nothing is running and the reading is 12.06v or less? or 2)when things are running?.

I've learned alot regarding this subject from this forum and I want to thankyou all in advance.

Frank

skipnchar

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Posted: 08/02/12 02:06pm Link  |  Print  |  Notify Moderator

The only way to get an accurate reading with a volt meter is to have the battery completely at rest (nothing going in and nothing going out) for about five or six hours. when you charge the battery you will also get a surface charge that will test MUCH higher than the at rest level. this surface charge will burn off rather quickly if the battery is put into use because it's not REALLY there in the first place. Many smart chargers have a means of gauging the percent of charge which is probably more accurate than a stand alone voltage meter because it takes into account the surface charge.


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pulsar

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Posted: 08/02/12 05:48pm Link  |  Print  |  Notify Moderator

Moved from Forum Technical Support to Tech Issues.

RoyB

King George, VA

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Posted: 08/02/12 07:59pm Link  |  Print  |  Notify Moderator

Consider this brochure report from Progressive Dynamics on how long it will take to re-charge deep cycle batteries using different charging voltages.

"Progressive Dynamics ran this test on the amount of time it took a PD9155 (55-amp) converter/charger set to three different output voltages to recharge a 125 AH (Amp Hour) battery after it was fully discharged to 10.5-volts.

14.4-VOLTS (Boost Mode) – Returned the battery to 90% of full charge in approximately 3-hours. The battery reached full charge in approximately 11 hours.

13.6-VOLTS (Normal Mode) – Required 40-hours to return the battery to 90% of full charge and 78-hours to reach full charge.

13.2-VOLTS (Storage Mode) – Required 60-hours to return the battery to 90% of full charge and 100-hours to reach full charge."

If you converter/charger only puts out 13.6VDC then I suspect you will need 40-hours of charging to get to the 90% charge state and 78 hours to reach full charge. No short cuts here - believe me...

It may take a couple of days to re-charge your batteries if all you have available is 13.6VDC charging voltage from your on-board converter/charger unit.

Using 14.4VDC smart-mode charging techniques should re-charge your batteries in a 2-3 hours generator run time... This can be accomplished with a smart-mode 45 AMP or larger on-board converter/charger or a Black and Decker VEC1093DBD stand alone 40AMP smart mode portable battery charger running from a generator connected to your trailer as noted in your post.


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Normk

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Posted: 08/02/12 08:12pm Link  |  Print  |  Notify Moderator

>>My question are: 1) At what point do you determine the battery is 50% discharged, When nothing is running and the reading is 12.06v or less? or 2)when things are running?.
<<

Test with no load. Think of it this way: You are wishing to determine the voltage as an indication of the battery electrolyte specific gravity. If you test with a load, you will read the battery voltage (at the state of charge in effect) but minus the amount which the voltage is pulled down by the load applied. If you were to close your battery isolation switch and crank the engine during the voltage reading, the heavy load applied by the starter would pull the battery voltage down to 10.5 volts or less with completely charged batteries.

The batteries might be 100% charged (12.6 - 12.8 volts) but the voltage idicated during cranking would only indicate dead batteries (10.5 volts) which would be incorrect because of the load applied.

Does that help?

MEXICOWANDERER

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Posted: 08/02/12 08:12pm Link  |  Print  |  Notify Moderator

skipnchar X2

RoyB

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Posted: 08/02/12 08:13pm Link  |  Print  |  Notify Moderator

This is a handy state of charge chart for your deep cycle batteries..



mena661

Southern California

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Posted: 08/02/12 10:02pm Link  |  Print  |  Notify Moderator

mexbungalows wrote:

skipnchar X2
X3


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red31

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Posted: 08/03/12 06:13am Link  |  Print  |  Notify Moderator

And here's a chart that shows 12.24 volt = 50% at rest


DryCamper11

Hartford

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Posted: 08/03/12 08:18am Link  |  Print  |  Notify Moderator

Measuring state of charge SOC with a voltmeter is really hard. You need to wait hours, disconnect the battery to completely remove all the loads and have an accurate meter. A decent temperature compensated hydrometer is a better way to measure it. Basically, it's not easy to measure. It takes some care to get a number anywhere close to correct.

RoyB wrote:

Consider this brochure report from Progressive Dynamics on how long it will take to re-charge deep cycle batteries using different charging voltages.

"14.4-VOLTS (Boost Mode) – Returned the battery to 90% of full charge in approximately 3-hours. The battery reached full charge in approximately 11 hours."


Most chargers won't let you control whether they are in boost mode. Even the PD, which does have a boost mode manual override, will only stay in that mode for 4 hours if started manually and 8 hours if it decided on its own to enter boost.


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