Inexpensive voltmeter - Page 7 - Fiberglass RV


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Old 12-20-2012, 01:46 PM   #85
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A solar battery maintainer (or tender) is a battery maintainer (or tender) POWERED BY a small solar panel. The charging voltage is under 2V as is the case with grid-powered maintainers or tenders. I have no experience with solar-powered charge-maintaining devices. This is not an endorsement of any device, real or imaginary, made mention of by other than myself, to include but not limited to solar battery.

jack

I guess I had a brain f... I read solar battery as in some sort of in HD tv antenna type of thing. I guess I should wake up a bit more before trying to read.
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Old 12-20-2012, 01:51 PM   #86
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Old 12-20-2012, 04:00 PM   #87
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Ferget the voltmeter (even the $2.50 variety) as a major current draw. There is something called "Surface Charge" that will allow a battery to show a higher voltage right after charging that will quickly drop with even the slighest load an/or time passage. This is more noticed on older batteries that have become sulfated from excessive use.

I can run my TV, DVD player, a hand full of lights, water pump and what ever else I need for two nights and not see than much drop on a fully charged battery

I think it might be time to investigate battery condition as well as your charging system.

All this talk about current draw from the voltmeter is 99% theory and stems back to the days of needing vacuum tube voltmeters to read voltages in electronic circuits. Not really a concern with what RV's are using.

BTW: The voltminders I install are usually hard wired to stay on all the time and are disconnected only when stored for extended periods.
I bought a cheap illuminated DC volt meter from eBay from China a couple of years ago and hard wired it into my trailer system and mounted it on the wall in the kitchen. Although the current draw I measured was less than 10 milliamps (using an American made multimeter that I picked up for my job as a semiconductor laser diode researcher in the late 1980s), I installed an off/on switch at the meter to make sure that there would be no current draw unless I wanted to see the reading. Since I prefer to run off-grid using solar power when possible, every bit of current is important to me. The voltage meter readings agree with all of my several multimeters I have around the house.

Although it is not a comprehensive system, you can tell the state of your battery from the voltage level using the little chart found on this website: http://www.marxrv.com/12volt/12volt.htm - I don’t use the colour chart, but instead the black and white one 3/4 down the page. I have reproduced a portion of that chart and stuck it on the wall beside the volt meter in our trailer, so that I don’t have to actually learn the readings.

This works very well for me for running our LED lights, TV and DVD player, which is about all we have that is electric other than our propane leak monitor (which I also have on a switch so that we can turn it on only when the furnace is running). I have used Chinese electronics for many years and, although some of the mechanical construction can be flimsy, the quality of the electronics is about the same as similar items made elsewhere.

You can see the meter mounted in a plastic box with the switch on it in the “before” picture of my “straightening T moulding” thread, attached.

Hope that helps,
Rick G
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Old 12-20-2012, 04:25 PM   #88
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Trying to charge 2 dissimilar batterys in parallel usually results in neither getting a decent charge. As the voltage will usually rise more quickly in one battery, it will start shutting down the charger before the lower battery is charged.
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I don't believe that's true. When charging in parallel they both charge to the same voltage level. One cannot be of a higher voltage than the other. The charging current will be different, yes. I can't see anyway one will be undercharged while the other is fully charged.
Perhaps Bob could have expressed the situation more clearly, but I do understand the point.

The voltage you measure at the battery terminals is the sum of the voltage resulting from the state of battery charge, plus the voltage driving current through the battery's internal resistance.
Two different-sized batteries of the same design, connected in parallel, will get different charging current because even if they start at the same state of charge (so the same resting zero-current voltage) because they have different internal resistance.
The batteries will reach a different state of charge, with the same externally applied voltage, unless the internal resistances are perfectly proportioned to the capacities... so yes, one may be undercharged (or one will be overcharge) when the charger shuts down based on the response of charging current to charging voltage.

If the batteries are connected by different lengths or sizes of cable, external resistance will be different as well, further complicating the situation and making it more likely that the batteries will get unevenly charged.
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Old 12-20-2012, 04:38 PM   #89
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Originally Posted by Brian B-P View Post
Perhaps Bob could have expressed the situation more clearly, but I do understand the point.

The voltage you measure at the battery terminals is the sum of the voltage resulting from the state of battery charge, plus the voltage driving current through the battery's internal resistance.
Two different-sized batteries of the same design, connected in parallel, will get different charging current because even if they start at the same state of charge (so the same resting zero-current voltage) because they have different internal resistance.
The batteries will reach a different state of charge, with the same externally applied voltage, unless the internal resistances are perfectly proportioned to the capacities... so yes, one may be undercharged (or one will be overcharge) when the charger shuts down based on the response of charging current to charging voltage.

If the batteries are connected by different lengths or sizes of cable, external resistance will be different as well, further complicating the situation and making it more likely that the batteries will get unevenly charged.

In all things electric there are small differences, the problem is discerning those that make a practical difference and those that the difference so small it doesn't matter. The internal resistances and wire resistances are there, but they really matter in this case? Probably not.
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Old 12-20-2012, 04:46 PM   #90
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The internal resistances and wire resistances are there, but they really matter in this case? Probably not.
At trickle charge rates, I agree that the internal resistance won't be a big deal.

Charge a battery at high rate, note the voltage at the battery terminals while charging, disconnect the charger, and note the battery's voltage: the difference in the two voltages is due to charge current and internal resistance. A couple of volts of difference is normal - is that significant? Make your own assessment.
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Old 12-20-2012, 05:20 PM   #91
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At trickle charge rates, I agree that the internal resistance won't be a big deal.

Charge a battery at high rate, note the voltage at the battery terminals while charging, disconnect the charger, and note the battery's voltage: the difference in the two voltages is due to charge current and internal resistance. A couple of volts of difference is normal - is that significant? Make your own assessment.
Hi Brian. Glad to see you have rejoined the group. Perhaps you mean a couple of tenths of a volt?? Or maybe I am missing something?? Raz
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Old 12-20-2012, 06:51 PM   #92
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Hi Brian...Perhaps you mean a couple of tenths of a volt?? Or maybe I am missing something?? Raz
By the end of the charge cycle, charging voltage is well over 14V (depending on charge rate; Trojan recommends 14.8V). A fully charged battery with no current flowing is under 13V (ideally 12.73V according to Trojan). Roughly, a couple of volts, but much less if "trickle" charging at a lower current, and thus lower applied voltage.

Automotive observation: I have one of those plug-into-lighter-socket voltmeters (which I am now told are useless trash, but magically work anyway ). When I am driving, I routinely see 13.2 to over 14 V, but after the engine is stopped (and thus the alternator stops charging), I see the 12-point-something V of a fully charged battery. Even if the meter is wildly inaccurate, it is quite repeatable, and the difference in voltage reading is believable. With nothing else on the lighter socket circuit, this is not the effect of voltage drop due to current loss in the socket's circuit.

The driving example is from my Focus. The Toyota's charging system has never failed me, and thus doesn't need monitoring

For those who like physical analogies...
I have small air pumps with gauges. With the digital gauges (which don't bounce like a needle) I notice that if I am pumping up a tire (like charging a battery) and I stop the pump when the pressure is - for example - 32 psi, the pressure then shows 30 psi. The 2 psi difference between what the pump (charger) was putting out and the pressure (charge voltage) in the tire (battery) is what was being dropped in the hose (cables, internal battery components) due to resistance to airflow (current).
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Old 12-20-2012, 07:09 PM   #93
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By the end of the charge cycle, charging voltage is well over 14V (depending on charge rate; Trojan recommends 14.8V). A fully charged battery with no current flowing is under 13V (ideally 12.73V according to Trojan). Roughly, a couple of volts, but much less if "trickle" charging at a lower current, and thus lower applied voltage.
Part of the charging process results in something called surface charge which gives a false initial voltage. Let a charged battery sit for a few hours and it will drop to about 12.6-12.8 volts. Perhaps that's what we are talking about here?
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Old 12-20-2012, 07:18 PM   #94
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Part of the charging process results in something called surface charge which gives a false initial voltage.
Yes, surface charge is part of it - but only a part. This part of the discussion started from the challenges of charging parallel batteries, and both batteries accumulate surface charge.
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