Battery Voltage Monitor

[Camreal]

Built this little battery monitor a while back. I picked up the readout at a flea market for $4 and mounted it in a small box. The switch on the side turns the display on/off. Sure makes for a quick and easy check of the battery.

[time2roll]

I just want to see voltage above 14 at some point in the day. 14.1 or 14.111 does not matter to me.

Even if I had a generator with me I don't see the need for more than one digit.

[jrnymn7]

Agreed, and yet at the end of the day, even '3 decimal place' voltage meters are basically useless for monitoring day to day usage, compared to an accurate amp hour meter, as you pointed out. And as you well know, to get an accurate soc from a voltage reading, one must turn off all loads and let the bank sit until it has recovered from the Peukert effect. My conclusion is voltage readings are quite helpful in determining overall battery health, but not for establishing an accurate soc of an in-service bank.

[MEXICOWANDERER]

jrnymn7

4-1/2 digit voltage GAUGES are now available that exhibit as an example 12.689 volts. What is gained is accuracy if the gauge is made correctly. Percentage error and least significant plus and minus variances (typically three or four numbers) becomes a moot point because this occurs in the thousandth of a volt place. This eliminates rounding off errors, for instance a meter showing 14.1 volts instead of 14.0 volts. This is particularly important when verifying the accuracy of kWh meters.

I've lost track of the number of kWh and ampere hour meters that I found to have significant error. I consider error in excess of 10% to make an amp hour meter next to worthless. An accurate 4-1/2 digit volt gauge and a precision Manganin shunt is a formidable tool for verifying accuracy of a day in and day out meter.

[jrnymn7]

At the risk of sounding like I'm nit-picking, it's important to distinguish between 'digits' and 'places'. Digits simply refer to how many numbers appear on the display; which could be hundreds, tens, tenths, hundredths, etc.

A 'four digit meter', for example, may give you voltage to 4 digits, but only to within one tenth of a volt, while another 'four digit meter' may give you voltage to within 1000th of a volt:

999.9v vs. 9.999v ... both are four digit meters, but the decimal >> place << is what really matters.

For truly useful battery voltage monitoring, one requires a four digit "two (decimal) place" meter:

99.99v

A difference of only 00.09v can be the difference between 60% soc and 52% soc. And on a rather large bank, a difference of 50 or 60 Amp Hours.

The link Pnichols posted is a good example of a four digit, two place meter.

[tenbear]

abideejay wrote:

tenbear wrote:
Here is my power monitoring panel. Voltage and current for both AC and DC.

Source for those meters? I'm particularly interested in the AC meter!

I bought all these meters on eBay.
Here is a link to the AC meter.

[pnichols]

Here's the four-place digital voltmeter I mounted right on the dash of our motorhome. I plugged it into a 12V receptacle in the coach area to make sure it monitors the coach batteries and not the engine starting battery. I didn't build it in - just stuck in on the dash with Velcro tape. I also have a three-place digital ammeter right next to it that montors current into (+ reading), or out of (- reading), the coach batteries:

http://www.voltminder.com/

[abideejay]

tenbear wrote:
Here is my power monitoring panel. Voltage and current for both AC and DC.

Source for those meters? I'm particularly interested in the AC meter!

[jrnymn7]

There's a big difference between 12.51v and 12.59v... and any volt meter that only goes to one decimal point is pretty useless, in my (not so humble?) opinion. My Trimetric fits that bill, unfortunately. It rounds up to 12.6 at 12.51/12.52. On a 430Ah bank, that's a difference of about 35Ah's, and an extra 45 minutes of charging with a 60 amper, at that particular soc. Fortunately, I didn't install the Trimetric for reading voltages.

[time2roll]

BCI (battery Council International) capacity testing RC (Reserve Capacity) is the minutes a battery can give 25 amps down to 10.5 volts. This is where I get the 25 amps. Note the charts that say 10.5 is fully discharged will be very close to 12.06 as 50%.

Yes the Trojan numbers seem to be a bit lower than the generic charts for resting voltage. This is what you should follow if you have Trojan Batteries and ignore the rest.

http://www.trojanbattery.com/tech-support/battery-maintenance

I looked for a US Battery chart to no avail.

[waynefi]

smkettner wrote:
The biggest difference is one chart is a disconnected battery rested 48 hours and the other chart is a battery in moderate use (up to 25 amps).

Resting 12.24 is 50%
Under load 12.06 is 50% (see 12v side of life)


JMHO

Actually, the charts I was looking at all claimed to be open circuit. For example, the Trojan web site lists 12.1 at 50% open circuit.

For loaded, the voltage at 1 amp would have to be quite different from 25 amps, and voltage at 25 would depend on the size of the battery.

JMHO

[MEXICOWANDERER]

This is where a good AMP HOUR meter is useful. No quibbling over voltage numbers, resting or working, temperature, etc. Wake up to a meter reading of -75 amp hours, and you intuitively know you'll have to operate a forty ampere charger more than 2 hours to replace most of the extracted energy.

[time2roll]

waynefi wrote:
Those are convenient little monitors, but how exactly do you interpret the reading?

I've seen several charts that give voltage for a given SOC, but they don't agree with each other. I've seen voltage for 50% SOC listed as anything from 12.06 up to 12.4.

Those are open circuit voltages, but a wired in meter is likely to be reading with, at a minimum, the fridge, the CO monitor, and probably the standby mode of the TV/stereo drawing power. So what voltage means "time to recharge"?

The biggest difference is one chart is a disconnected battery rested 48 hours and the other chart is a battery in moderate use (up to 25 amps).

Resting 12.24 is 50%
Under load 12.06 is 50% (see 12v side of life)

I primarily use the voltmeter to see my batteries are above 14 volts at some point during the day indicating solar is working enough to get me through to the next day.

Otherwise it is a bit of fuzzy interpretation. Generally if you are less than 12.3 it is a good time to recharge when convenient. Below 12.0 is time to charge right away.
JMHO

[tenbear]

There is no magic number at which battery life is shortened. The 50% is a compromise and is a variable depending on battery type, temperature and who knows what else. I recharge when convenient, usually about 12.3v with the fridge on.

[waynefi]

Those are convenient little monitors, but how exactly do you interpret the reading?

I've seen several charts that give voltage for a given SOC, but they don't agree with each other. I've seen voltage for 50% SOC listed as anything from 12.06 up to 12.4.

Those are open circuit voltages, but a wired in meter is likely to be reading with, at a minimum, the fridge, the CO monitor, and probably the standby mode of the TV/stereo drawing power. So what voltage means "time to recharge"?

[tenbear]

Here is my power monitoring panel. Voltage and current for both AC and DC.