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Topic: 24hr Battery Drawdown Test (Read 1717 times) previous topic - next topic

24hr Battery Drawdown Test

(Moved this from another thread to start a new subject)

I decided to do a little test to determine how much 12 volt power our coach consumes while parked, with everything turned off, except the things we always leave on.  Does that make any sense?  We have two 4-year old AGM8D coach batteries, and I wanted to see how they would hold up for a 24hr period.  Our coach has been sitting for weeks in our driveway, plugged into 50 amp shore power.  I went out yesterday, at exactly 12:00 noon, and turned off my solar panels and shore power.  Batteries were at 100%.  Our residential fridge is always on so the temps were completely stabilized in both compartments, and the compressor was not running.  Our inverter kicked on immediately, but it initially showed zero amps out.  My solar controller showed a immediate -5.8 amp draw from the coach battery bank.  I have seen this number many times, and I consider it to be the baseline constant power draw for our coach.  The following items were being powered:

1.  Fridge
2.  Microwave (clock is the only thing obviously drawing power)
3.  2 LED nite lights plugged into 110V outlets (I use these for a quick visual check that my 110V system is always being powered)
4.  CO2 and propane detectors
5.  LED TV (powered, but turned off, so only the power indicator is lighted)
6.  DVR (powered, but turned off, so only the clock and panel lights are on)
7.  Two hard-wired LED dash volt meters
8.  LED engine compartment (cat and rat repeller) lights
9.  Magnum inverter remote control panel (and of course the inverter itself)
10. Blue Sky solar controller remote panel (solar panels were disconnected, but control panel still monitors battery condition)

That's all I can think of that was running.  This was a absolute best case scenario for battery power usage in our coach.  I didn't open the fridge or freezer door.  I didn't turn on any lights.  For the 24 hours the coach sat as is, and at 12:00 noon today I recorded the result.  I don't have all the fancy telemetry like Roger, but I can at least tell where the batteries ended up after 24 hours.

Re: 24hr Battery Drawdown Test

Reply #1 –
Chuck, how did you wire your solar controller to read amp draw from batteries?

Re: 24hr Battery Drawdown Test

Reply #2 –
Quote from: jcus –
Chuck, how did you wire your solar controller to read amp draw from batteries?
(Sorry - I was off editing my original post so it made sense as a new thread)

To answer your question, I didn't wire the solar controller - AM Solar in Oregon did that installation.  But I know from using the control panels that one of the things I can read on the Blue Sky panel is (coach) battery bank voltage and the sum of the amps going in and going out.  I'll show some photos later.

Re: 24hr Battery Drawdown Test

Reply #3 –
Quote from: Chuck & Jeannie –
(Sorry - I was off editing my original post so it made sense as a new thread)

To answer your question, I didn't wire the solar controller - AM Solar in Oregon did that installation.  But I know from using the control panels that one of the things I can read on the Blue Sky panel is (coach) battery bank voltage and amps (going in or going out).  I'll show some photos in my next post.
You must have the ProRemote which is a solar controller/battery monitor, very nice.

Re: 24hr Battery Drawdown Test

Reply #4 –
OK, next few posts will be some data points I recorded during the 24hr period.  I don't have any fancy equipment so it's pretty basic stuff.

Start of test:
Start battery bank voltage = 12.8
Coach battery bank voltage = 13.3
Coach battery bank at 100% SOC

As soon as I shut off the solar panels and the shore power, the inverter kicked on but showed zero output (fridge compressor off).
The Blue Sky panel showed -5.8 amp draw, which is my normal "baseline" 12V power load on our coach.
I have seen in the past that turning the inverter off drops that to about -2.0 amps, so our MS2812 inverter pulls about 4 amps in "standby".

Re: 24hr Battery Drawdown Test

Reply #6 –
At 18:30 yesterday evening, 6.5 hours into the test, got these readings:

Start battery bank voltage = 12.8  (I'm also checking the isolator function as part of this test)
Coach battery bank voltage = 12.6
Coach battery bank at 87% SOC
Blue Sky panel was showing the same -5.8 amps
But as I was standing there the fridge compressor kicked on, and Blue Sky reading jumped up to -13.0 amps.

Re: 24hr Battery Drawdown Test

Reply #7 –
At 22:00 yesterday evening, 10.0 hours into the test, got these readings:

Start battery bank voltage = 12.8  (I'm also checking the isolator function as part of this test)
Coach battery bank voltage = 12.4
Coach battery bank at 77% SOC
Blue Sky panel was showing the "normal" -5.8 amps

Re: 24hr Battery Drawdown Test

Reply #8 –
At 07:00 this morning, 19.0 hours into the test, got these readings:

Start battery bank voltage = 12.8  (I'm also checking the isolator function as part of this test)
Coach battery bank voltage = 12.1
Coach battery bank at 57% SOC
Blue Sky panel was showing the "normal" -5.8 amps

I also thought to check the "Battery Amp Hours From Full" reading, and it showed -194

Re: 24hr Battery Drawdown Test

Reply #9 –
End Of Test

At 12:00 noon today, 24.0 hours into the test, these were my final readings:

Start battery bank voltage = 12.8  (Looks like our OEM factory installed isolator is still working fine!)
Coach battery bank voltage = 12.1
Coach battery bank at 50% SOC
Blue Sky panel was showing the -13.0 amps during this reading indicating that the fridge compressor was running.
The "Battery Amp Hours From Full" reading was -240

Re: 24hr Battery Drawdown Test

Reply #10 –
My next step was to turn the solar panels back on.  High noon, clear blue sky, and clean panels (I swept them off this morning) equals best case scenario for solar production (parked in our driveway).  As soon as they came on line, the Blue Sky panel showed a "Input" of 41.8 amps, and a "Output" (to the batteries) of 45.3 amps at a initial voltage of 12.9.  Note: we have MPPT solar controllers.

The Blue Sky coach battery bank reading rose to 12.9 volts, and showed a +39.6 amps (which is the algebraic sum of amps in and amps out).  The fridge compressor cut off right after I turned the solar panels on, so the inverter was in standby, and the only draw on the batteries was the "normal" baseline -5.8 amps)

Re: 24hr Battery Drawdown Test

Reply #11 –
So I guess my question is:  What Did I Learn?

1.  Our old isolator still works!    :thumbsup:

2.  24 hours of the absolute lowest "normal" 12 volt power drain (for our coach) sucks 50% of our available coach battery bank power.

3.  Our solar panels will put out 45+ amps of charging current under ideal conditions.  (I actually saw it go over 47 amps for short periods)

4.  What else?  Looking for wisdom from the smart guys in this group.  I'm thinking another 8D battery would be nice?

Re: 24hr Battery Drawdown Test

Reply #12 –
And the same test with a kill a watt on the refer plug.  My digital volt meters seem to use more power than you would think.  Enough to unplug them from the dash cigarette ligher holes

Re: 24hr Battery Drawdown Test

Reply #13 –
Chuck. with your set-up I personally don't see an advantage to another battery. It isn't dark for 24 hrs. at a time and unless your solar can top off the 2 batteries in a day, you would only be trading down time with generator time.
 I'm not the smartest person on this power management, so this is just my opinion.

Re: 24hr Battery Drawdown Test

Reply #14 –
Think Craneman has it, if you cannot top off your 2 depleted batteries the next day by solar, adding extra batteries will just extend your non solar run time a little bit, if you are at 100% by say, 2 pm, then you could probably recharge an extra battery as well.

Re: 24hr Battery Drawdown Test

Reply #15 –
Good job Chuck.  This is really the only way to know what is going on in your coach, measure it.  I will do the same early next week and measure refrigerator use as a subset as well and report.

My experience is that our refrigerator power use is about equal to all other use in normal conditions, actually using the coach.  If that is the case you may have about 12 hrs to 50% SOC before recharge is needed.

50% SOC is the maximum discharge you should go to which is equal to 12.2 volts.  At this depth of discharge your life time cycle count is as low as it is going to get, the time to 100% recharge is very long (less likely to happen with everyday solar) and not getting to 100% recharge reduces capacity over time.

A third battery will help, more capacity means less depth of discharge, higher cycle count, maybe less time to 100% recharge.

I am sure there are other experiences out there as well.

Re: 24hr Battery Drawdown Test

Reply #16 –
Quote from: jcus –
Think Craneman has it, if you cannot top off your 2 depleted batteries the next day by solar, adding extra batteries will just extend your non solar run time a little bit, if you are at 100% by say, 2 pm, then you could probably recharge an extra battery as well.
Sounds right to me.  Since I didn't cut the panels on until noon, this was not a good test of how well they would do recharging over a full day.  85 degrees here today, and it was getting hot in the coach after lunch, so the DW wanted the A/C turned on cuz we keep a lot of food loaded in the coach, and she doesn't want it to spoil.  So I had to hook the shore power back up.

Re: 24hr Battery Drawdown Test

Reply #17 –
Roger pointed out (in a PM) that my 07:00 battery bank voltage reading (12.1) looked like a outlier.  I may have wrote down the number incorrectly - not sure.  No way to go back and check...  :facepalm:

Re: 24hr Battery Drawdown Test

Reply #18 –
Great write up Chuck! Thanks.

Re: 24hr Battery Drawdown Test

Reply #19 –
I noticed a interesting interaction between the solar charging function, and the MS2812 charger function.  As I said above, after I turned the solar panels back on, they were putting around 45 amps into the batteries (at about 13.0 volts) in the BULK charge mode.

When I hooked the shore power up, the MS2812 went into "charge" mode.  It sat there for a minute or so doing nothing, then the output jumped up to about 40 amps, but it went into FLOAT charge mode.  I am used to seeing it put out a lot higher charging rate with low batteries, but I guess it was being "fooled" by the battery voltage reading coming from the solar charger.

With both chargers running, the Blue Sky battery monitor showed about +80 amps.  (sum total of both chargers minus load)

No problem.  As long as the batteries get charged, I let the solar charger, the inverter/charger, and the alternator work it out amongst themselves.