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Old 05-06-2012, 10:16 PM   #1
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Shocking Thoughts: minimum house battery voltage

Here is another newbee question: when not plugged in and using the house batteries (which in my case are four six volt deep cycle batteries connected in series/parallel to give 12 volts), what is the minimum voltage I can discharge them to before I think about firing up the generator to charge them?? I understand the concept of "deep cycle"...but I am not seeing a clear definition of minimum voltage in practice. Thanks in advance for your thoughts on this!
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Old 05-07-2012, 04:57 AM   #2
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About 12V, that is a 50% discharge (assuming all your batterys are good) continued discharge below that point will decrease the life of your batterys.
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Old 05-07-2012, 06:56 AM   #3
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Fully agree. also search for "12 Volt Side of Life" Great info on batteries. I saved it to My Documents so I could reread it several times, I am a slow learner.
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Old 05-07-2012, 09:35 AM   #4
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This chart from The 12volt Side of Life (Part 1) shows voltage vs % charge. 50% is the lowest you should draw them down before recharging. When using remember that you typically want to turn things OFF before measuring voltage. the voltage can be a bit lower than actual level when something like a furnace blower is running. When using a microwave off of an inverter the voltage drops even more while the microwave is on. Do not confuse the levels in the chart with the higher levels (13.2 to 14.6VDC) used by a charging device like a converter, inverter/charger, or engine alternator. A charging device has to use higher voltage levels in order to push energy back into the battery.

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Old 05-07-2012, 09:53 AM   #5
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When reading voltage (or specific gravity) to determine state of charge, you'll get best results by measuring after at least a half hour of no significant charging or discharging.

When the voltage gets down to 12.4 volts, start to plan on a recharge. When it gets down to 12.2 volts, put charging on the priority list. Don't let your batteries go below 12.0 volts if you want best battery life and service satisfaction.

If your batteries are sized properly for your off-grid use, you should be able to go at least 2 days before you get into the 12.2v area.

By watching how the voltage drops and recovers when loads like the furnace or water pump comes on, you can get an idea of the battery state of health. A new battery should drop voltage much with a load like this and will recover rather quickly. An old battery will groan under these minor loads and take a while to bet back to its 'idle' voltage. That sort of behavior is why you want to wait a while before assessing state of charge with voltage measures or whatnot.
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Old 05-07-2012, 08:30 PM   #6
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As usual, very good advice. Thank you all. The write-up on "the 12 volt side of life" seems well done (thank you for pointing it out...but I definitely have some reading to do!). The point regarding measuing open-circuit voltage I understand well, though it is good to be reminded of it (believe it or not, one role I play in my job is designing/building/debugging batteries - but not lead-acid rechargeables, similar concepts apply, but they are a different beast!).

A more specific QUESTION: one comment was made regarding the accuracy of the voltage meters. My (new) unit came supplied with Magnum Energy MS (pure sine wave) inverter/chargers. Inside the coach they installed the ME-MR remote control, which has a digital voltage read-out. In your experience, does this combination give a sufficiently accurate readout of battery voltage (assuming I pay attention to the low load/open circuit value), or do I need to resign myself to going outside with a volt meter to measure at the battery terminals?????
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Old 05-07-2012, 08:39 PM   #7
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I regularly see 11.9 to 11.75 first thing in the morning. My Interstate U-2200's lasted 9 years. I replaced them with the same brand/type (new model # though).
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Old 05-07-2012, 09:01 PM   #8
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I ran my starter battery down listening to satellite radio on the cab radio and it had been a cold night. My pair of 6v batteries were down to 72% (12.7volts according to my trimetric battery monitor). There was not enough juice in the house batteries to start the engine using the switch on the side of the drivers seat. I wasn't until after I paid $45 for a jump start that I realized I could have started the generator instead of waiting for the sun to recharge the house batteries. Some of us have to learn everything the expensive way. But, I'll get reimbursed by my insurance company, so it's just using up my time.
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Old 05-08-2012, 09:26 AM   #9
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re: "do I need to resign myself to going outside with a volt meter to measure at the battery terminals?????"

If you are not running any significant loads and if the connections between battery and meter are reasonably clean and in good shape, a voltmeter nearly anywhere in the circuit should be sufficient to determine state of charge. You should 'calibrate' your readings so you know the influence or bias of your meter, if any, and other potential sources of error. This is simply a matter of a tad of experience and a bit of discretion in measurement.

The reason for doubts about voltage often expressed in these forums seems based on a bias towards hydrometry and not realizing that any measure needs to be properly done and properly interpreted. You can see this bias in the enthusiasm for integrating ammeters (e.g. the Trimetric) as people like numbers, even when they don't mean much.

Barb's experience is an example. The meter said 72% but the voltage was 12.7v (which is into surface charge territory) yet it wasn't enough to start the primary engine. One key is 'cold night' and another may have been battery age. There are many factors that influence available battery energy capacity by more than 10% and that needs to be considered in interpreting battery status.

This is leading into dangerous territory, a can of worms, in that it starts to touch on the great 'deep cycle' mythos and the limitations of RV solar systems in regards to battery health, battery branding, and other topics that tend to cause flames when such things as empirical reasoning and finding a measure are brought into the discussion.
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Old 05-08-2012, 03:41 PM   #10
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The key to using a meter that is located a fair piece away from the source is that it has to have high impedence. There should be almost no current flowing through the meter read circuit so in effect it is just measuring the voltage potential. A few milliamps at most. Errors (voltage drop) are induced from the wire length when current is flowing. I use a combination digital volt/amp meter that is about 8-10ft away from the battery. It is within +/-0.1 volts of the reading at the battery. Amperage is within +/- 1 amp (200 amp / 75 mv shunt). For monitoring to get a gut feel of battery status, that is more than enough accuracy. You can easily monitor converter status, amperage output, the effects of different loads, and battery status (voltage and +/- current flow).

Dave
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