volt meter v. hydrometer

[MMulcahy]

I recently charged my coach batteries and let them sit for 24 hours. Reading 12.6v. Feeling really good. Used a hydrometer (bulb type) in each cell and all were showing closer to 12.0. Not feeling so good. I'll admit I haven't maintained the batteries though they were not much short of water in each cell. Batteries were made in 6/15. Pretty intense travel since then, though mostly with electrical hookup.

Do I take the hydrometer reading over the volt meter?

Thanks for your feedback.

[ScottBrownstein]

Always, but make sure it is a decent hydrometer, a lot of them are kinda junky. If it's the plastic kind with the floating balls, not so cool.

[pgandw]

Both cheap voltmeters and cheap hydrometers leave something to be desired in the calibration department.

I use voltmeters so I don't have to worry about opening the battery vents except to check water levels once a year. When I installed a small digital voltmeter in the camper, I calibrated it against my Sears voltmeter which was advertised as plus/minus 0.5% - which is +/- 0.06 volts - and much better than most cheap meters. The other advantage of a voltmeter is that I can see what the converter output is.

Based on Scott's chart, I could easily be off as much as 20% on the state of charge. But I'm pretty sure I'm very close to right on. I've checked numerous AA and AAA batteries, and I have read the converter output in various charge modes and all read correctly.

just my thoughts and experiences
Fred W
2014 Rockwood A122 A-frame
2008 Hyundai Entourage minivan

[ScottBrownstein]

While you can buy as many cheap hydrometers as voltmeters, the problem with using voltage as an indication of SOC is neither about precision or accuracy of the measurement. It is about battery chemistry. Specific gravity is pretty much the only way to get a handle on SOC regardless of the history or current usage of a wet cell. You can be charging, discharging, idle...it doesn't affect SG. It does, however, have a big effect on the SOC to voltage measurement calibration. Most say that the cells need to be left totally at rest for at least 4 and sometimes 24 hours before a reliable voltage to SOC prediction can be made. Sure it's a lot easier, but it really isn't very accurate.

If you want to know SOC at all times, get a battery monitor like a Trimetric or Victron. These units "count" charge going out of and back into a battery bank and are far more accurate. However, they do not take into account the aging of the battery. If you tell them you have 400 AH capacity, 5 years from now they will still assume that.

[Steveboe]

Battery's like to be used.
Try draining the battery by leaving lights on and then recharging it again. This will reactivate the Lead/Acid plates and they should improve. Each cell, fully charged will produce 2.2 volts. A fully charged 12 volt battery will show 13.2 or more.

[clintbonnie73]

Originally Posted by ScottBrownstein

While you can buy as many cheap hydrometers as voltmeters, the problem with using voltage as an indication of SOC is neither about precision or accuracy of the measurement. It is about battery chemistry. Specific gravity is pretty much the only way to get a handle on SOC regardless of the history or current usage of a wet cell. You can be charging, discharging, idle...it doesn't affect SG. It does, however, have a big effect on the SOC to voltage measurement calibration. Most say that the cells need to be left totally at rest for at least 4 and sometimes 24 hours before a reliable voltage to SOC prediction can be made. Sure it's a lot easier, but it really isn't very accurate.

Excellent advice.. X 2
I just checked my batteries last week after giving them a good charging.. I was reading 12.9 volts.. great you would think.. but then I checked the specific gravity of each cell and it was 25 % of a full charge.. Hydrometer is the only way to go !
Time for new batteries, which by the way are only 8 months old..(Interstate RV batteries, 12v)

[Bluepill]

Good idea to equalize the batteries then let them rest for 24 hours before making a cross-check of SG vs. Voltage

[pgandw]

Originally Posted by clintbonnie73

Excellent advice.. X 2
I just checked my batteries last week after giving them a good charging.. I was reading 12.9 volts.. great you would think.. but then I checked the specific gravity of each cell and it was 25 % of a full charge.. Hydrometer is the only way to go !
Time for new batteries, which by the way are only 8 months old..(Interstate RV batteries, 12v)

If the 12.9V was after 24 hours after disconnecting the charger, your meter reads high. I suspect your specific gravity reading is likewise low, unless your batteries are in really poor shape.

For all the flaws of using voltage to read batteries, it's what a multi-mode battery charger or converter does.

The monitor systems measure voltage across a very small resistance resistor to determine current in and out of the batteries. The accuracy of that resistor and the resistors inside the device determine the accuracy of the monitor in measuring current. For the monitor, it's not as critical as errors in charge current are the same errors in discharge current. As was mentioned, what is critical is the input total AH of the battery bank, since the state of charge is calculated from that.

All that number stuff aside - are you really going to shut down the heater in the middle of a 30 degree night because your monitor or other device says your batteries are down to 50%? In my case, I'm going to let it run and try to figure out whether the batteries are salvageable in the morning. A happy (and warm) wife makes for a good life - and is worth the cost of replacing the batteries after 4 years instead of 5-6 years.

Fred W
2014 Rockwood A122 A-frame
2008 Hyundai Entourage minivan
camping Colorado and adjacent states one weekend at a time

[ScottBrownstein]

Doubt that they would measure 12.9 after a long rest, if so the meter is undoubtedly wrong. However, he didn't say if it was a digital volt meter, in which case they are generally pretty accurate. My bet is that they were recently charged and that voltage will continue to drop as the surface charge dissipates.

Of course you aren't going to shut down the heat, although in my case, I have started the generator of a couple of hours in that situation. Converters are fundamentally constant voltage devices, but it is my bet that their programming is based on measuring the current that results from a known voltage level and deciding to down shift their output to normal level from boost and eventually go into storage mode.

Lead acid batteries have been around for a long time but it is still amazing how much about them is still not that well known. A low SG indicates that the plates are still sulfated (either reversible or permanent) and, if permanent, while the voltage may be there, it will not remain when current is drawn. You are right, it is either 5 years or 3 or 4, depending on lots of factors. Either way if you amortize them they don't cost that much.

[pgandw]

Scott - have to disagree on most meters.

The numerous claims on this forum of 12.9V fully charged batteries brought out the electrical engineer in me. I finally came to understand that not everybody who was seeing 12.9V was getting the reading with a surface charge still present. Then my cheapie digital multimeter broke, and I went to buy another.

I did not find anything I liked (had all the ranges I wanted, including currents) at Lowe's and Home Depot for what I thought was a reasonable price. So I went to Sears and spotted several $15-$30 models that had the ranges I wanted. Started checking accuracy and discovered many were +/- 1.5% or higher on DC voltages. This means the 4th digit is totally fabricated, and the 3rd digit can be off by 1 or 2. Which isn't really good enough for determining battery state of charge. I found a meter which advertised 0.5% on DC voltages, which means the 3rd digit is good (assuming meter meets specs).

I also noted that very few reports with voltages report low readings, which tells me the meters are biased to read high. I did check my new meter against several batteries to see if it was reading high, and compared it to my trusty old analog meter.

As you point out, specific gravity indirectly measures the concentration of sulfuric acid in the water. Just like voltage, very small differences in specific gravity can mean significant differences in state of charge, so accuracy of the device is important.

And as you say, there are more legends than facts about batteries floating around.

Fred W
2014 Rockwood A122 A-frame
2008 Hyundai Entourage minivan

[D. J. Schuler]

I have to agree about the good volt meter - 12.73 +/- .5% = 12.793/12.666, +/- 1.5% = 12.920/12.539. The hygrometer test is good at the temperature range it is designed for.

[clintbonnie73]

Originally Posted by D. J. Schuler

I have to agree about the good volt meter - 12.73 +/- .5% = 12.793/12.666, +/- 1.5% = 12.920/12.539. The hygrometer test is good at the temperature range it is designed for.

Found this info on Hydrometer readings..:

"Any density measurement is affected by temperature, and most hydrometers are calibrated to read properly only at a normal operating temperature of 77 F. For example: at an electrolyte temperature of 125 F (maximum for battery operation) a hydrometer reads 16 points low; at a 26 F, the reading is 16 points high"

So I have been charging my Interstate 10 month old newer RV batteries with my converter for the last 4 days. Disconnected converter and voltage reads 12.9.. Great uh..? Well with the outside temperature at 60 *, most of my cells have 25% charge on them using my hydrometer ( which I just checked with my Truck battery which checked 100% charge).. Time to get new batteries.. Voltage readings IMO, don't really give the whole picture of the condition the battery..