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Old 05-04-2015, 04:18 PM   #1
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Voltage @ 50% remaining charge?

So I am adding a voltmeter to my jayco. I want to paste a battery percentage charge chart somewhere in the camper. I realize specific gravity is the better test, but I will use this for a quick eyeball estimate.

My understanding is that we never want to fall below 50% remaining charge to prolong battery life. So, looking for a good chart to print, I find lots of variation... For comparison purposes, the following show 50% when voltage is...

The 12volt Side of Life (Part 1) : 12.06v
http://www.mmbalmainauto.com.au/PDF/..._batteries.pdf 12.20v
Battery “State Of Charge” Chart : 12.10v
Battery Metering | Otherpower: 12.20v
Measuring State-of-charge - Battery University: 12.24v
Low Tech RVing: How Much Juice In Your Batteries?: 12.18v

So..... at which voltage level to YOU feel is closest to 50%??
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Old 05-04-2015, 04:22 PM   #2
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I use this one
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Old 05-04-2015, 04:57 PM   #3
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Voltage @ 50% remaining charge?

Quote:
Originally Posted by Owen Stubbs View Post
snip
My understanding is that we never want to fall below 50% remaining charge to prolong battery life. So, looking for a good chart to print, I find lots of variation... snip...

So..... at which voltage level to YOU feel is closest to 50%??
It's a battery life issue. As far as I know, there is no sudden death if the batteries ever drop to, or a bit below, 50%. You just want to plan on not going that 'low' or lower on a regular basis if it can be helped.

Bottom line, my take is that it's a judgement call to be made by the person that will be buying the batteries.
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Old 05-04-2015, 05:13 PM   #4
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In addition to the expert's disagreement as to the proper 50% discharge voltage level, complicating things further is that the voltage measurement should be made under no-load and also after the battery has rested a bit.

I use 12.1 volts as the cutoff point, but I seldom get that low.
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Old 05-04-2015, 08:54 PM   #5
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This is the chart I use. I have an alarm from MTR Designs so I don't have to guess and constantly check a volt meter or charge controller. It begins blinking yellow when I am within .25 volts of my set alarm (12.2 volts) and then it sounds an alarm when it gets to 12.2. Here is the link if you are interested:

Battery Failsafe Devices
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Old 05-04-2015, 09:12 PM   #6
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I just use 12.0VDC to tell me I am probably close enough to 50% charge state and stop using my batteries until they get re-charged again...

12.0VDC is easy to remember for me....

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Old 05-05-2015, 06:19 AM   #7
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When converting my new Designer from 12v batteries to two 6 volt golf cart batteries I wanted to know the battery manufacturers opinion on low discharge levels. Having had a 36v (six battery) golf cart for years with Trojan batteries I wrote to Trojan asking about the voltage level to be considered minimum for no damage to their T-105 batteries that I now have. Here is their response from October 2010:

Below is Trojan Battery's response:
We never want you to discharge your batteries below a 20% state of charge (80% depth of discharge). For a 36 volt system of batteries this would mean you never want to go below about 35 volts. The voltage of 34.9 volts is an open circuit voltage, which means the batteries would be at rest. For your 12 volt RV system, a 20% state of charge would be an open circuit voltage of 11.6 volts. Thanks.
If you have additional comments or questions, please contact me at the information below.
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Old 05-05-2015, 06:43 AM   #8
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Quote:
Originally Posted by edatlanta View Post

Below is Trojan Battery's response:
We never want you to discharge your batteries below a 20% state of charge (80% depth of discharge). For a 36 volt system of batteries this would mean you never want to go below about 35 volts. The voltage of 34.9 volts is an open circuit voltage, which means the batteries would be at rest. For your 12 volt RV system, a 20% state of charge would be an open circuit voltage of 11.6 volts. Thanks.
If you have additional comments or questions, please contact me at the information below.
Sincerely,
Stacey Delzeit
Product Engineer
Trojan Battery Company
sdelzeit@trojanbattery.com
678-518-7378
Here is the Trojan Battery Companies Life-Cycle chart (Copied from the TROJAN Battery web site) for the T-105. Sure you can take it to the 20% level but it will have a major effect on the Life of the batteries. Note the Trojan response did not include the word REPEATED occurrences.. If you use the 50% limit, you could be rewarded with 500 extra days of use.

Just my thoughts,

Don
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