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Old 09-19-2019, 06:51 PM   #1
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How to test the 12 volt battery

I am getting ready to take our TT down south this winter. We will be boondocking once in a while and I need to know if our 12 volt battery is in good shape, or should I plan to replace it.

What is a simple test I can do with my voltmeter?

Thanks.
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Old 09-19-2019, 06:53 PM   #2
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Put the meters red lead on the positive cable of battery. And the black lead on the negative. That will tell you how many volts are in battery
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Old 09-19-2019, 06:54 PM   #3
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It is tough to determine capacity with a voltmeter. You can remove it an take it to an auto parts or battery store and have it load tested.
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Old 09-19-2019, 07:02 PM   #4
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Originally Posted by dodgemark View Post
Put the meters red lead on the positive cable of battery. And the black lead on the negative. That will tell you how many volts are in battery
If only it were that easy. Everyone tells me different. Fully charged should read 12 to 12.5 volts. Turn all lights on for 5 minutes. Test now while everything is on. Should be 9 volts or more.

And then people tell me complete opposite. Confusing.
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Old 09-19-2019, 07:22 PM   #5
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I installed a voltage meter and continually monitor, charging when necessary, so I generally know the health of my batteries. If it does not hold a charge, then I replace it.
I have two 6 volt batteries and they have been perfect. But anything less than 12 volts is not good.

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Old 09-19-2019, 07:31 PM   #6
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Battery...

If you're gonna be boondocking some, and if the battery has a couple of years (+?) age on it, I'd suggest getting a new one. There are all kinds of suggestions of "what's best" -- 2 (or more) batteries or one high capacity, "golf cart" batteries vs. "regular deep cycle" etc. and etc.. If you're a regular with Walmart stuff, a fairly good choice is their group 29 deep cycle -- it's usually a reasonable price, an easy swap ('though heavy as heck to lift!) and performs very well under "light" boondocking, especially if you'll have a generator or, at least, quality/capable solar panel(s) to assist at times(?). My $.02
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Old 09-19-2019, 07:44 PM   #7
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It is tough to determine capacity with a voltmeter. You can remove it an take it to an auto parts or battery store and have it load tested.
Unfortunately "Load Testing" like they do at Auto Parts stores only tells one if the battery is capable of starting an engine.

Battery Stores will more likely put the battery on a charger for 24 hours and after it's fully charged will check with both voltmeter AND Hydrometer.

Checking cells for close to even Specific Gravity readings is a better measure of a battery's condition.

Short of putting a known load on a deep cycle battery and measuring the amount of time it takes for it to reach a minimum voltage, there's no real way to test a deep cycle battery for capacity.

A hydrometer with built in thermometer so temp correction can be made is the best way to test a deep cycle lead acid battery.

AGM's are a different kettle of fish. Voltmeter or "battery impedance tester" are about all you can use to check condition but still no measure of capacity.
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Old 09-19-2019, 07:47 PM   #8
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The only way to truly test it is to to put a know load on it for a certain number of hours and then let it sit for an hour and check the voltage at that point. A load test done by an auto parts store is close to worthless for a deep discharge battery.

For example, if you have an 80AH battery put a 10A load on it for 3 hours. That's 30AH. Measure the voltage on it at that point and compare the reading to a SOC chart.
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Old 09-19-2019, 09:37 PM   #9
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https://www.harborfreight.com/100-am...ter-61747.html
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Old 09-19-2019, 09:42 PM   #10
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endorse the Harbor Freight load tester ABOVE to get a good picture of battery health... I have had one for years... don't use it much but when you need it, it still works
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Old 09-20-2019, 02:59 AM   #11
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If only it were that easy. Everyone tells me different. Fully charged should read 12 to 12.5 volts. Turn all lights on for 5 minutes. Test now while everything is on. Should be 9 volts or more.



And then people tell me complete opposite. Confusing.


Like others said. To see the health of the battery itís best to put a load on battery. And not many people have the load tester, so itís easy to take it to parts store and have them load test it. Voltmeter is just to see what you have available in battery. If camper is going to sit unplugged from shore power in storage, unhook batteryís or shut off disconnect, the propane detector will will kill a battery in a short time is left connected
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Old 09-20-2019, 03:03 AM   #12
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endorse the Harbor Freight load tester ABOVE to get a good picture of battery health... I have had one for years... don't use it much but when you need it, it still works


Thatís a great little load tester. Have to just remember to kno how to use it properly. Have seen people hold load switch on too long and smoke the tester. But itís a great tool to have
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Old 09-20-2019, 06:00 AM   #13
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If you will be boondocking a good deal , what is needed is a battery gas gauge that tells you how full the batt is .

These are called SCO (state of charge) meters.

These are about $150 and do need to be installed , easy , an included shunt must be installed with the meter..

This goes between the house batt ground cable and the ground.

It will measure the charge going in from any source , as well as the load being drawn and show a percentage of the charge at that instant.

This is the only way one can monitor the battery to not discharge it too much , which can ruin any battery.

A volt meter does not work and any device with no shunt can not work.
TriMetric Battery Monitor - - Bogart Engineering


www.bogartengineering.com › products › trimetrics

○Measures battery % full, based on amp hour measurements for accurate information on state of charge (SOC). This method is more accurate than monitors ...



If your camper has only one batt that is seldom used , a SOC would be overkill.

Instead charge the batt , charging the voltage will be 13.5+ and resting the batt should be 12.6 -12.8.

With no load on the discharged battery resting should be above 12.2 ,then recharged.

There is a HUGE difference between batts for starting and deep cycle (golf cart) batts used for the house loads.

Only use a rated deep cycle batt for lights, water pump etc., never a starting batt.
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Old 09-20-2019, 06:19 AM   #14
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Ok. This is memory and I am old.

The HF type fixed load tester isnít much and itís 1950ís technology. Useless? No, but itís only a 100 amp/200-300 amp CCA battery tester. They came out with carbon pile testers that would vary the load to 5-600 amps(ie-1000-1200 CCA battery). These were out in the 80ís. The main problem with these load testers is state of charge and having experience using them.
Electronic testers are the way to go. The first one I used(90ís) cost over $1000 and I griped about it....God I learned to love it.

One can buy a cheap version for like $30 these days...think amazon.
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Old 09-20-2019, 07:02 AM   #15
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Wet acid batteries should operate between 12.0 and 12.6 volts on a good voltmeter. 12.0 is a little under 50%. 12.6-7 is fully charged.

When charging they are at a higher voltage. 13.1 is a trickle. Over 14 is bulk for a low battery.

After charging they will show a higher voltage for several hours.

I have an installed BM2 from Amazon. Costs $30. Transmits to you cellphone. Records data. So you can study your usage to determine when charging is needed. Installs in minutes.

Looking at the recorded voltages you can see how the battery is doing and decide when you need a new or bigger one.

The more expensive soc meters display amps remaining and usage amps. Really nice.

I bought the BM2 as it is cheap! Enough for us.

If we had a big solar system and boondock a lot the soc would be nice. We run minimal dc stuff not plugged in. If I had big users like a residental fridge or microwave, I.e. a big inverter then the soc makes sense. The manufacturer should have installed one.

To answer the question. Charge your battery for 24 hours(converter). Turn the battery disconnect off. Wait 4 hours and read the voltmeter. Less than 12.6 indicates battery issues. Use the rv overnight with minimal draw. Read the meter in the morning. Below 12.4 and your battery may have seen better days.

Replace it with 6 volt batteries from Costco. Measure first! Battleborn are the best if you do not need a second mortgage to get them.
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Old 09-20-2019, 10:29 AM   #16
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The problem with older batteries is that they will show 12.6V after fully charging them. However, their capacity is low so it doesn't take as much discharge current to deplete them. That's why only checking the voltage is a poor test.

I have a Victron BMV-712. They are around $200 and you can connect to them through Bluetooth. All it takes to pay for itself is to prevent you from wiping out a couple Trojan golf cart batteries. You don't need a huge solar system to take advantage of their usefulness.
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Old 09-20-2019, 10:54 AM   #17
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Teaters like this really only work for testing small starting batteries.

Larger deep cycle batteries will test OK but won't "keep the furnace running overnight".

The load they apply is merely designed to remove any surface charge remaining after charging.
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Old 09-20-2019, 12:19 PM   #18
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A load test is best way. I took mine to Wal Mart. They had a computerized tester. Thorough test
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Old 09-20-2019, 12:22 PM   #19
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Unfortunately "Load Testing" like they do at Auto Parts stores only tells one if the battery is capable of starting an engine.

Battery Stores will more likely put the battery on a charger for 24 hours and after it's fully charged will check with both voltmeter AND Hydrometer.

Checking cells for close to even Specific Gravity readings is a better measure of a battery's condition.

Short of putting a known load on a deep cycle battery and measuring the amount of time it takes for it to reach a minimum voltage, there's no real way to test a deep cycle battery for capacity.

A hydrometer with built in thermometer so temp correction can be made is the best way to test a deep cycle lead acid battery.

AGM's are a different kettle of fish. Voltmeter or "battery impedance tester" are about all you can use to check condition but still no measure of capacity.
I agree... the only good way to check the condition of your battery (wet cells) is to use a hydrometer.. It checks each cell individually.. Go to Oreillys Auto parts or Napa and get:
https://www.oreillyauto.com/detail/b...ometers+&pos=0
Be sure to charge battery up to full charge before checking...
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Old 09-20-2019, 12:31 PM   #20
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When we first bought our TT 2.5 years ago, on the first trip we ended up with problems holding charge when boondocking (which we didn't/don't do much). We'd be dangerously low after only one night of conservative power usage. While trying to diagnose the problem, I brought the battery into 3 auto parts stores who did their fancy test and told me the battery was fine - including at an Interstate store (it's an Interstate SRM-24). I then finally got the bright idea of buying a hydrometer and tested the cells, and 3 of the 6 were dead. Bought a new battery and problem solved. The moral of the story is, as others have said, a conventional load test won't tell you the full story.

In the middle of this, at the recommendation of a long-time full-timer friend, bought the Bogart Engineering Trimetric TM-2030 that a previous poster recommended. PRICELESS to have a way to tell what the discharge rate is and % full indications at any time. I installed it while we were on the road and double-sticky taped the display to the wall next to our bed. Check it every night when we're off shore power to be sure the outside light or other loads aren't mistakenly left on. Enjoy!
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