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Old 12-01-2020, 10:51 AM   #1
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Battery Information

I currently have six six volt batteries in my Motorhome. I know you need to have the batteries basically at the same voltage level. my batteries are the traditional water fill type

My question is what voltage level difference is allowable before changing the set out.

Also should you be taking a voltage check on the new batteries before you purchase them.

After purchase should they be individually full charge before hooking them up together so you have a balanced system

Thanks for your time reading this and thanks for your thought, much appreciated
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Old 12-01-2020, 11:38 AM   #2
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They should all hold 12.6 or 12.7 volts, depending on brand.

Charge them for a day, all hooked up together.

Next day, remove the 3 + to - jumpers and measure the voltage of each 6 volt battery. Keep notes. It should be over 6.4 volts.

On day 3, measure the voltage again. All should be the same and at 6.3 to 6.4 volts.

If you find one lower then the others, it and its sister need to go. Never change one battery that's series paired up to another.

If they are all the same age, change them all.
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Old 12-01-2020, 11:41 AM   #3
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Quote:
Originally Posted by Gordeno View Post
My question is what voltage level difference is allowable before changing the set out.

Voltage is a poor measure of battery condition. Said another way, a deeply discharged brand new high end AGM battery will read low voltage, but once charged, is an excellent battery.


Batteries need to be fully charged and then load tested or their SG checked if wet call to get any indication of condition.
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Old 12-01-2020, 11:45 AM   #4
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Lead acid batteries basically have three voltage profiles. There is a "charging" profile, a discharging profile, and a static profile. Using voltage alone to determine viability is not reliable. Lead acid batteries are usually replaced when the actual amp hour capacity is down to 80% of rated capacity.

For 12 volt batteries, charging voltage profile depends on battery design and charger capability. It reaches up above 14.4 volts.

Discharging profile reaches down into the low 11 volt range. The greater the current draw the lower the voltage. In the good (bad) old days, automotive engine starters were designed to work at about 9 volts while cranking.

Static voltage (not charging or discharging for 4 hours) ranges from 13.0 to 11.4 volts. It is used to determine "state of charge" not battery life. 12.7 volts is fully charged. 11.4 is discharged. 6 volt battery voltages are half that.

Automotive lead acid batteries are tested at high current for a short period. They must provide necessary "cranking amps".

Deep draw RV house batteries are tested with a 20% discharge current for a long period of time. So a 100 amp hour deep draw battery would be tested at 20 amps and it should work for 5 hours. (Usually a shorter time is used and the resulting static voltage is measured to calculate when the battery is below 80% of storage capacity.) Battery manufacturers may recommend optimum methods for their battery design. A 20 hour test is also used.

Owners often approximate the 20% discharge method and accept lower accuracy. Sometimes they measure individual battery voltage while under heavy load in a battery bank and compare them. They also may charge weak batteries individually to see if they can get them up to others in the bank.

6 volt batteries in series should have the same voltage within a couple of tenths of a volt under all conditions. If one is weak, they are usually both replaced since the other is likely to fail soon. Some people try to recover the weak battery using various techniques.

I wish you good luck and happy trails ahead!

Battery University https://batteryuniversity.com/

How does the Lead Acid Battery Work? https://batteryuniversity.com/learn/...ased_batteries

Charging lead acid batteries https://batteryuniversity.com/learn/...d_acid_battery

AGM https://batteryuniversity.com/learn/..._glass_mat_agm

How to Charge and When to Charge? https://batteryuniversity.com/learn/...o_charge_table

How to Store Batteries https://batteryuniversity.com/learn/...tore_batteries

Summary of Doís and Doníts https://batteryuniversity.com/learn/..._battery_table

and much much more...
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Old 12-01-2020, 04:04 PM   #5
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"Also should you be taking a voltage check on the new batteries before you purchase them."

I did when I replaced 8 6V GCs this spring.
Fortunately all had same very recent mfg date and all read w/in +/- 0.1V
I just hooked them up and turned the charger on but would have charged in pairs if any were much lower.
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Old 12-02-2020, 08:54 AM   #6
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By the time terminal voltage indicates battery merit, things have gone pretty far south in terms of battery condition.

A better way would be to do a capacity test. All the batteries are given a known good full charge, then discharged at a standard rate (usually C/20) and their performance will conclusively dictate their serviceability. This is simple to do and removes all doubt.

Mark B.
Albuquerque, NM
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Old 12-03-2020, 10:21 AM   #7
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Battery capacity testing

Quote:
Originally Posted by Mark_K5LXP View Post
By the time terminal voltage indicates battery merit, things have gone pretty far south in terms of battery condition.

A better way would be to do a capacity test. All the batteries are given a known good full charge, then discharged at a standard rate (usually C/20) and their performance will conclusively dictate their serviceability. This is simple to do and removes all doubt.

Mark B.
Albuquerque, NM
You reference capacity test, is this a simple load test with like a schmaucker load tester, ( I have one of these.) I read someplace that a simple battery load tester was not reliable on deep cell batteries.

what type do you use?

Thanks for your response.
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Old 12-04-2020, 08:41 AM   #8
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Quote:
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I read someplace that a simple battery load tester was not reliable on deep cell batteries.
True.

Quote:
what type do you use?
$50 400 watt AC inverter, outlet strip, incandescent light bulbs. Or, any other house or external load you can apply and control. Digital voltmeter, and a clock.

Step one, charge the bank. It must be 100% charged as evidenced by the charger completing the requisite charge cycles, verifying specific gravity or manually. Even better is to run an equalize cycle if the charger has that option.

Connect the inverter/load to the batteries. There should be no other loads connected.

I pick a combination of light bulb wattage that gets me the appropriate current draw from the batteries as drawn by the inverter. So if you have 220 Ah batteries, divide that by 20 (for the 20 hour rate), = 11 amps.

Turn the inverter/load on and start the clock. Run the test until the batteries reach 10.5V. How many hours the test runs is your net capacity. New/good batteries will run for 20 hours, usually a bit more. 15 hours would be 75%, 10 hours 50%, etc. I consider "good" to be greater than 80%. Less than 80% and it's battery shopping time. 50% or less is failed.

Cold batteries will exhibit less capacity so factor that into the result. Some battery data sheets will provide that derating but others don't. Just so you're not surprised if your batteries come up short when they're cold (<77F). Below is a Trojan derating chart.

As a bonus, what I do is with this freshly dead battery bank I plug in the RV and let the converter bring them back up. Gives me an idea how long that takes and verifies the converter goes through the proper cycles.

Mark B.
Albuquerque, NM
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