reading voltage with a multimeter? - Fiberglass RV


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Old 09-02-2018, 09:18 PM   #1
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reading voltage with a multimeter?

I've started using a multimeter to know when my battery has been on the charger long enough, since my rather basic charger doesn't tell me very clearly when I've got a maximum charge. But I find that if I disconnect the battery from the charger, then immediately take a reading with the multimeter, I get a higher reading than I will 30 minutes later, and I'll get a still lower one an hour later. I assume this has to do with the temperature of the battery fluid during the charge. If I'm looking for a full charge in my 12 volt battery (12.7 or 12.8 volts, as I understand it), am I okay to keep it on the charger until I get a a reading of 13 volts or higher immediately after disconnecting the battery from the charger? If I take it off the charger around 12.7 or so, it keeps dropping down to 12.05 or 12.1 as it cools. Or is this 12.05 or 12.1 reading perfectly fine for a cooled battery?
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Old 09-03-2018, 04:23 AM   #2
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The numbers you're reading right after charging are a bit false. The battery reading an hour or so after charging is the number you're looking for. Don't remember the tech name for that but it...levels out for a truer reading. There will be a few of our electrical gurus along soon for the techy info .
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Old 09-03-2018, 05:44 AM   #3
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Surface charge.

See the paragraph just after the "State of Charge Voltage Charge" here: The 12volt Side of Life (Part 1)

Other suggested reading:
http://usbattery.com/info-center/care-and-maintenance/
https://www.solar-electric.com/deep-...ttery-faq.html
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Old 09-03-2018, 06:26 AM   #4
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Resting Voltage

Richard, the Resting Voltage, i.e., voltage measured after a fully charged 12 volt wet lead acid battery has been disconnected (not charging or discharging) for 12 to 24 hours should be at least 12.6 volts. If the charger can't get the battery's Resting Voltage above 12.6 volts, take the battery to an auto parts store and have it tested.
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Old 09-03-2018, 06:42 AM   #5
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Originally Posted by John in Michigan View Post
Richard, the Resting Voltage, i.e., voltage measured after a fully charged 12 volt wet lead acid battery has been disconnected (not charging or discharging) for 12 to 24 hours should be at least 12.6 volts. If the charger can't get the battery's Resting Voltage above 12.6 volts, take the battery to an auto parts store and have it tested.
And if the battery is OK, then check your charger. If you believe "HandyBob" then most people are not even fully charging their batteries in the first place. You mentioned a "basic charger" so perhaps you are not using the battery in the most efficient way.

Note that some people take issue with some of Bob's claims. And charging a battery to only 90% is not ideal, but it is not a huge deal in the greater scheme of things.

BTW, you might want to buy a hydrometer also and check the per-cell voltage. Its all in the reading I linked to.
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Old 09-03-2018, 08:26 AM   #6
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Thanks for the helpful info. In the past, I'd always just charged my battery for 9 hours at 10 amps, because that's what the seller told me to do, given the battery's rating. But there seems a big difference between charging after a winter's complete dis-use and "topping up" before a heading off for yet another weekend in the summer. Earlier this summer, the electrolyte started bubbling up and out after only 7.5 hours of charging. Hence, I started using the multimeter. But I've only recently become aware of the different readings I get in relation to the time the battery has been resting after charging. Thanks for helping me sort it out. That's a helpful link, Gordon. So I guess I'm okay to charge up to a "false" reading (that is, a reading taken right after disconnecting from the charger) of 13+ volts if the true voltage will settle out at 12.7 or 12.8. Correct? PS. I just checked the voltage after the battery sat overnight, and it is lower still. So obviously the short 2.5 hour charge I gave it yesterday evening wasn't enough, even though the immediately-after-charging reading was "acceptable" (had it been TRUE!) I'm pretty sure the battery and charger are up to speed -- just a slow operator....
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Old 09-03-2018, 09:21 AM   #7
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BTW, you might want to buy a hydrometer also and check the per-cell voltage. Its all in the reading I linked to.[/QUOTE]

A hydrometer does not read Voltage. It shows the level of acidity in the electrolyte which is an indicator of charge level. Its best purpose is to see if one or more cells is going bad.
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Old 09-03-2018, 09:59 AM   #8
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Originally Posted by Wayne Collins View Post
...
A hydrometer does not read Voltage. It shows the level of acidity in the electrolyte which is an indicator of charge level. Its best purpose is to see if one or more cells is going bad.
You are quite correct, I got very sloppy in my message composition.

Specifically, the hydrometer measures specific gravity

That correlates with per-cell voltage but is certainly not the same thing. Oh, don't forget the effect of temperature and compensate for it when using a hydrometer. And you are also correct that the prime reason for using one, in addition to judging state of charge, is to find any individual cells that are (or are going) bad. Thats really what I meant to express.. checking each cell. Anyway, thats why I provided the links.. they explain it all better than I do, especially when I get lazy.
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Old 09-03-2018, 04:12 PM   #9
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Yes, I have a hydrometer (and the holes in my jeans to prove it!). It always seemed like such a lengthy and laborious process, ultimately to figure out only that it was time to replace the battery. Part of the reason I've been trying to take the voltage as I'm charging the battery has been to keep from over-charging the battery, AND to make sure I had the voltage as close to 100% as possible before hitting the road. But this recently-discovered concept of "surface charge" complicates that, evidently. Anyway, I charged it up for another 4.5 hours this morning, and added to the 2.5 yesterday, that's 7 hours. Last time, the electrolyte started overflowing at 7.5 hours of charging. No telling WHAT the voltage would be tomorrow when the "surface charge" factor has settled down, as I'll be on the road with the fridge running on 12 volt. Post-Labour Day Scamping, here we come!! Thanks for your good assistance to all!
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Old 09-03-2018, 04:29 PM   #10
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Originally Posted by Richard Davis View Post
Yes, I have a hydrometer (and the holes in my jeans to prove it!). ,,,
Yes, a hydrometer is not a thing I like to use often.

I think you have yet to mention the specifics of your charger. If you really want to do it right and get the most out of your battery, then a good multi-stage charger is a must. Charging current must start out high, and then in the late stage, the voltage goes higher and the current drops. And the charger should have a remote temperature sensor that goes at the battery, and a program to automatically adjust the charging based on the that data also. A good battery manufacturer will provide the specs for charging at each stage and also temperature compensation value. A really good charger will allow you to adjust those parameters. Also an occasional over-charge should be done for regular wet cell batteries (but usually not advised for AGM).
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Old 09-03-2018, 05:32 PM   #11
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If you top up the water in your battery, and then charge it, it will overflow. Just put enough in to cover the plates, then charge, then top up.
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Old 09-03-2018, 09:47 PM   #12
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That's good to know, Glenn. Most likely, when I had the overflow previously, I had topped up the fluid level before charging. Thought one was supposed to! When I "top up," I always leave about a centimeter/half inch of the split tube showing above the electrolyte. So far with this battery, I haven't needed to do much topping up, in spite of lots of checking both before and after charging. Chances are, they're going to put me in a home somewhere before I ever figure out batteries....
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Old 09-03-2018, 11:25 PM   #13
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Go here:


https://www.trojanbattery.com/tech-s...y-maintenance/


for everything you need to know about batteries.
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Old 09-08-2018, 11:08 AM   #14
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Knowledge is power.....spend an hour reading.....The 12 volt side of life.....
just google that and start reading.....batteries are the penance you must pay for owning an RV.

I also recommend that you invest in a good quality battery charger....cheap low grade chargers destroy batteries.
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Old 09-08-2018, 11:50 AM   #15
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Originally Posted by gordon2 View Post
Did you mean 'State of Charge Voltage Chart'?
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Old 09-08-2018, 11:52 AM   #16
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Originally Posted by Lloyd (aka Santa) Coltman View Post
Did you mean 'State of Charge Voltage Chart'?
Yes, but one should read it all.
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Old 09-08-2018, 12:02 PM   #17
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Agreed, there is a Wealth of Knowledge there!
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Old 09-08-2018, 12:39 PM   #18
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Battery Care Made Easier

VDC Electronics makes several versions of what they call a Battery MINDer (their spelling). They seem to work very well and keep your battery both charged and in good condition.

If you use one of these it's a good idea to check water levels anyway, which should never fall below the top of the plates.

If you add water to a battery make sure it is distilled water. Tap water will work but usually contains minerals that will shorten battery life.
If a cell needs water it is best to add it before you finish charging, as charging tends to mix the solution. Never add water and allow the battery to just sit if it's going to be exposed to freezing temperature. The water is likely sit on top of the solution and when it freezes crack the battery casing and/or damage the tops of the plates and separators.
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Old 09-09-2018, 08:40 AM   #19
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Another source for information on all things electrical in our RV world is the excellent and free advise found in the newsletters from. RVTravel.com.

Knowledge is Power you cannot get from a storage battery !!
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Old 09-09-2018, 08:57 AM   #20
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Quote:
Originally Posted by Glenn Baglo View Post
Go here:


https://www.trojanbattery.com/tech-s...y-maintenance/


for everything you need to know about batteries.
A good source, as is Battery University.
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