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Old 10-16-2019, 10:41 PM   #1
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1976 Argosy 22
cranbrook , British Columbia
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Incorrect Battery Readings

This fall we were on a six week trip down south. I started to notice some odd readings on our Trimetric battery meter. On some cold nights with the furnace, water pump and lights running in the morning the meter showed 12.2 volts but still had a battery life of 70% which did not seem right. We have a Honda generator so I charged up the battery based on the voltage reading for the rest of the trip. The battery is an 12v agm with 65 amp hrs and is 4 years old now. When I took the battery out for the winter I put it on the charger and at full charge got 13v, then I hooked up a 12v fan motor which was drawing 3.5 amps and checked the voltage every hour. After 7 hours I was down to 12.1 volts, I found a formula to calculate battery amp hours which is - current draw in amps x run time = Amp Hrs. So 3.5 amps x 7 hrs = 24.5 amp hrs. Not sure if this is normal, but it looks like the battery has lost 16 amp hrs over the last 4 yrs and I need to adjust the setting on the Trimetric meter to 49 amp hrs in the spring to get the correct battery life reading to match the voltage reading. Hope this is helpful if you are like me and is always wondering how much juice is left in the battery.
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Old 10-17-2019, 07:26 AM   #2
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Take a look at this chart showing state of charge versus temperature. The batteries are fairly sensitive to temperature. You may find that the battery in those cold nights had more charge than you think.

http://sail-delmarva.blogspot.com/20...s-voltage.html

"Temperature. In the summer they run higher, in the winter, lower. Also, if the battery is heated from the charging process, it can be warmer than the air temperature. As you can see, there is a 0.6 volt range between summer and winter, depending on where the batteries are located. In the winter many electronics will cut of around 50% state of charge due to minimum voltage requirements. Thus, the usable power in the winter is much less. Fortunately, solar charges more efficiently, though for fewer hours and at a lower sun angle. No fans, but more light and heating appliance load."
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Old 10-17-2019, 07:38 AM   #3
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Hi

If you are running a single 65 AH battery and doing a lot of off grid camping, 4 years is not a bad lifetime for a battery. Most manufacturers consider a battery "done" when it is at 80% of capacity. Yours is a bit past that.

Many Airstream furnaces pull enough that on a cold night they will take down a pair of fully charged batteries. Getting a night out of one is actually doing pretty good.

If the Trimetric does not have a temperature probe on it, it will indeed be a bit off when the batteries get quite cold.

Bob
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Old 10-17-2019, 09:51 AM   #4
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Quote:
Originally Posted by uncle_bob View Post
Hi

If you are running a single 65 AH battery and doing a lot of off grid camping, 4 years is not a bad lifetime for a battery. Most manufacturers consider a battery "done" when it is at 80% of capacity. Yours is a bit past that.

Many Airstream furnaces pull enough that on a cold night they will take down a pair of fully charged batteries. Getting a night out of one is actually doing pretty good.

If the Trimetric does not have a temperature probe on it, it will indeed be a bit off when the batteries get quite cold.

Bob


Bob, are you aware of any more efficient furnace blower motors that would drop the amp draw? Also, are there any more efficient inverters (as compared to the stock Airstream) that also would enable the same?
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Old 10-18-2019, 08:20 AM   #5
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Bob, are you aware of any more efficient furnace blower motors that would drop the amp draw? Also, are there any more efficient inverters (as compared to the stock Airstream) that also would enable the same?
Hi

Well, the stock Alde in my 2017 does not pull anything near what the older furnaces did. It has both a pump and a blower for the propane. Combined they are in the "I don't notice it" range running over night ( so maybe 10AH to 20AH).

The idle current draw on an inverter apparently isn't something they worry about. Indeed if you look at the spec sheets and then start measuring devices. The device typically pulls more than the spec says it should. Mention this to the manufacturer (any of them) and back comes mumble .... mumble ... mumble...

From the numbers I've seen on inverters, something in the tenth to two tenths of an amp is pretty typical.

Bob
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