Which voltage readings are correct?

[overdoc]

So i have a recently installed diy off grid 24V solar system comprised of 4 230W panels, a 60amp charge controller, a 3000W inverter/charger and 8 6V batteries wired in series/parallel. One set of batteries is new and the other set was 90%. I am having a hard time understanding why my voltage readings from charge controller, inverter and meter are all over the place. An example...this morning under load the CC reads 60% (24.6V),the inverter says 83% (25V) and my voltmeter reads 24.8V which is mid-way between the two. Which one should i trust for an accurate reading of SOC? The voltmeter?
Anyone experience this?
thanks

[littleharbor]

Voltage readings, especially under load are a poor indicator of state of charge. Once the load is removed the voltage should rebound to a more realistic number. The gold standard for finding state of charge is a hydrometer. A shunt based monitoring system, properly calibrated, is another way to estimate SOC.
If your batteries are healthy you can remove all loads and charging sources, let batteries sit 4-6 hours and then voltage will indicate approximate SOC.

[overdoc]

Understood however since my system is new i thought i could keep an eye on things in real-time so that the batteries do not go below 50% charge. Letting them sit for a long period kind of defeats the purpose. If i understand correctly then the load reading underestimates the actual voltage of the bank so dropping to 50% should not worry me too much?

[Bala]

The difference in Volt meter readings is due to quality and calibration and probably how many digits they read to.

Meters like Fluke brand can be calibrated but it costs to have it done and quality multi meters are not cheap.

If one of the meters is good quality you could use use that, if they are all cheap brand items, either pick the most convenient or average them all.

As suggested, V readings are not the best for SOC but you can learn to understand quite well where your SOC is at using it.

[overdoc]

I agree. I think that my meter is relatively accurate as i have a second meter that gives the same readings. I believe my issue is my CC reading under load. It goes from 100% to the 80s in no time while my meter and inverter readings are in the 90s. The subsequent drop in voltage is much more gradual. I think that taking a measurement without load will provide a better reflection of the actual state. my only concern was going below 50% which i cannot explain given my load especially with new batteries.

[chrisski]

I've got the shunt based battery monitoring system I will install on mine, Victron BMC-712. This will tell me the most accurate state of the batteries at any point in time. Basically the shunt gets installed as close to the battery as possible between the battery and the ground. This is what I'm going to use to decide when to turn my generator on.

The Hydrometer would be more accurate, but like you said, it's hard to stop power production to measure 32 battery cells.

IMO, using a decent battery shunted monitor will keep you from buying a new set of batteries for as long as possible. I'm told I could get one much cheaper than the Victron, but I'm not told specifically which manufacturer and model that cheaper quality monitor is.

How Many Amp Hours are your batteries? For what I'm putting in my RV I'm going with 12 Volts and 400 Amp Hours of batteries (Four Batteries 6 V each in 2S2P) with 600 watts of solar panels for a 1000 watt inverter and if I upgrade to 24 VDC, It'll be 24 VDC and 400 AMP Hours of batteries (8 Batteries 6 Volts Each in 4S2P) with 1200 watts of Solar Panels.

[overdoc]

I read about putting in a shunt meter. Still debating. My batteries are 8 6V 230Ah wired in 4S2P for 24V 460Ah with a 3000W I-C. I have 1kW of panels. I now think the 3000W is overkill however i was concerned about the 1hp jet pump that i just replaced for a 1/6hp piston pump that is terrific.

[chrisski]

Keep in mind I have not built my system yet, have designed it for the last two months purchased parts, and am only waiting for the temps to cool to assemble the system.

Although nor related to a battery monitor, this video here made me realize why a battery monitor with a shunt would work so well for determining charging strength.

https://www.youtube.com/watch?time_c...ature=emb_logo

Until I saw that, I thought amperage would be the same wherever I measured it along the circuit, but boy was I wrong. I thought about buying both those multimeters to replicate the experiment, but then I saw that would have been about $700. Ans that was before Purchasing the power supply in the video.

[Bala]

I have followed this forum and another before it for 13 years, that had the same main contributors for most of the time. Recently a poster made the comment that hydrometer testing under load was not accurate. That is the only person in all that time to make that claim, I believe testing with a hydrometer is accurate under load. Learning to take accurate temp compensated hydrometer readings is necessary. You can use those readings to understand better what you V readings are telling you.

I only use V as a SOC measure but I have been living with my system for over 13 years so based on our usage and the weather I pretty much know how its going.

Info on shunt based SOC I have gleaned over the years is that the battery capacity is diminishing over time. So a shunt based system is not set and forget, you need way to recalibrate 100% as you lose capacity.

[littleharbor]

[QUOTE=chrisski;n418666]
I'm told I could get one much cheaper than the Victron, but I'm not told specifically which manufacturer and model that cheaper quality monitor is.



Look at the Bogart Tri-metric. They have more functions than most will ever need in the advanced settings but are pretty straightforward to install and the basic level one mode works well for all needed info. You can even use it on two separate battery banks if needed.