Battery charge versus remaining usable charge

[bkenobi]

This seems like a silly question, but I know it's actually complicated and may not be something anyone can really answer. That said, is there any way to know how long a set of batteries will last based on initial voltage?

I have new batteries in my 2007 NL 10-2QRR and charged them to 12.4v last week. We went on our first trip last weekend and over 4 days/3 nights we dropped to 12.3v. We were very conservative on consumables since we are new to TC's and only used the furnace, water pump, some water heater, and lights (mostly LED). I know some equipment only works down to a minimum voltage (11.5-12v I think), so I'm not sure if that means we could go 4x longer or if we would be more/less.

Any thoughts?

[bkenobi]

Right, which is what u thought but could remember. I'll have to find a DC current meter then.

[towpro]

bkenobi wrote:
Not saying it's high quality, but I have this one that I've used a couple times.

https://m.harborfreight.com/clamp-on-digital-multimeter-95683.html

From Harborfreight site:
Test AC/DC voltage, AC current, resistance, diode, insulation test, continuity

you are looking to test DC current.

[2112]

I bought one of THESE. I wrapped 5 loops of 12AWG stranded wire through it to make it a 0-20A meter. I just have to divide the reading by 5.

I plan to 3D print a case for it and power it with a 9V battery.

I compared it to a calibrated amp meter and determined it's 0.4A low at 15A. I'll accept that for a $20 meter.

[bkenobi]

Not saying it's high quality, but I have this one that I've used a couple times.

https://m.harborfreight.com/clamp-on-digital-multimeter-95683.html

[RickW]

woodhog wrote:
You need a clamp on amp meter, just clamp it around the battery wire
to see if you have a parasitic load drawing current from the batteries.

These can be bought for about $30.00 and work quite well , zero before measurement.

Current Measurement in wiring...

To understand anything in your camper's electrical system you have to
be able to measure voltage and current.

Very easy to do.

The old way is to go out after dark, disconnect the battery lead, scrape it on the terminal and look for a spark, not recommended in area of charging batteries!

Do you have an example of a $30 DC clamp on amp meter?

[bkenobi]

I have a clamp on meter actually, but didn't think it would work for DC. I also don't know if it's sense time enough. If not, I'll have to get a multiplier I suppose.

[bkenobi]

I've been meaning to order a data logging shield. Now that I know there's a parasitic load, I'll be sure to order one. I can't remember if I have a bare uno, but I do have an ESP8266 in uno for. Factor.

[2112]

bkenobi wrote:
I had 2 ideas to track voltage and was curious if others had a different approach.

1) I could buy a cheap 0-99vdv digital meter off ebay and mount it in a cabinet to see what the current charge was.
2) I could use a microcontroller (eg, arduino) to read voyage and track it over time to see if it holds correctly.

I could actually do both pretty easily. Has anyone else done either and found it useful?

I'm a little late to this party and this is slightly off topic but I found THIS to be a very useful tool. I have used mine to monitor my battery voltage while connected to a charge timer, fridge temp, temperature swings in the house, attic temps, etc. I'm currently using it to monitor the humidity in a storage shed.

If you know how to use it and design circuits around it it's well worth the $7.

[woodhog]

You need a clamp on amp meter, just clamp it around the battery wire
to see if you have a parasitic load drawing current from the batteries.

These can be bought for about $30.00 and work quite well , zero before measurement.

Current Measurement in wiring...

To understand anything in your camper's electrical system you have to
be able to measure voltage and current.

Very easy to do.

The old way is to go out after dark, disconnect the battery lead, scrape it on the terminal and look for a spark, not recommended in area of charging batteries!

[bkenobi]

The only 2 things I can think of.

1) CO detector
2) stereo

I don't think it could be the CO detector since that should be using close to what a smoke alarm uses (1 9v battery per year).

I noticed that with the stereo off I can sometimes hear static coming out the speakers over the dinette. It's quiet, but I'm sure it's there (my wife Cinfirmed I'm not crazy).

[trail-explorer]

bkenobi wrote:
When I tried to load it yesterday, the batteries were dead (9.45v). Is this expected?

No.

SInce they were discharged, there must still be something drawing power.

[trail-explorer]

Get an amphour meter to accurately monitor state of charge

[bkenobi]

I unloaded the TC a few weeks ago (2-3). At that time, it had fully charged batteries. Everything was turned off, but the battery shutoff/disconnect switch was not in the off position. The TC was too far from an outlet, so it was not plugged in. When I tried to load it yesterday, the batteries were dead (9.45v). Is this expected?

[bkenobi]

Interesting about the drop in charge from initial to real available charge. I'm wondering if that holds true for charging with the truck too? I ran the pickup for 15m and the batteries went up a bit. I haven't been in it today and it's dropped slightly (12.05 to 12.03v with only water pump and fridge on LP). I wasn't too worried about this drop, but am wondering how effective using the truck to charge vs wall vs genset. Obviously solar seems ideal but that can't run 110v appliances.