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Old 05-24-2023, 07:05 PM   #1
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Load Test Procedure

I have (2) standard dealer issue 'marine' 12v batteries in our travel trailer that are only a few months old. We are in our third week of a lengthy trip and today our Lippert Ground Control system error'ed out with a 'low voltage' message. This was after a 6-hour drive. After plugging the 7-way back into the truck, I had enough juice to complete leveling. I have 380w of solar and the truck providing a charge while driving. The 12v Magic Chef fridge was running while in transit. It was a semi-cloudy day, but this hasn't happened prior.

After connecting to shore power, the GoPower solar controller display shows the batteries at 100%, and getting 13.7v, so charger is working. (We don't boondock and I've never seen my solar controller state anything but '100%').

I would be surprised if one or both of the batteries was bad, but just in case, I ran up to Harbor Freight and purchased one of their 6/12v load testers.

I was going to disconnect from SP and remove the negative cable from each battery, and then test each one individually.

Is this the proper way to do it?

Thanks in advance.
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Old 05-24-2023, 08:52 PM   #2
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Should be instructions with the load tester.

Turned off the inverter (if you have one) and the battery cut off, disconnected the batteries and tested one at a time.
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Old 05-25-2023, 07:50 AM   #3
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It sounds like what you really need is a shunt type battery monitor.
https://www.amazon.com/Hillman-Group...dp/B002YGY7F2/

As far as the load tester, I agree with the previous post - follow the instructions included with it.
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Old 05-25-2023, 08:00 AM   #4
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Is it a load tester that has a needle for display, 2 clamps like booster cables and a test button? If so, I've had one for years and use it quite a bit. I always unhook everything from the battery. That way I know I am testing only the battery and run zero risk of damaging anything.
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Old 05-25-2023, 08:28 AM   #5
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I recently had something similar happen with my battery. it showed fully charged and everything ran fine, but would not pull more than 15 or so amps. it failed a load test even though it was "fully charged" and everything showed it was. im assuming that your batteries are a wet type with removable caps for the cells. if so, you will want to get familiar with how to water them.
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Old 05-25-2023, 08:33 AM   #6
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This load tester (I have one) is designed for starting batteries and measures the voltage drop when a massive load is placed on the battery as when starting the engine. Tells you nothing about the capacity or condition of a deep cycle battery which is optimized for low amperage draws.

So-called "marine" 12v batteries are normally not deep cycle batteries but rather "dual purpose" batteries designed to supply enough amps to start the engine and also supply house battery power. Fun fact: If "cold cranking amps" are listed anywhere on any battery is ain't a deep cycle but a starting (or "dual") battery.

Your refrigerator is probably sucking the life out of your batteries which may be indifferently charged. Solar won't power anything only slowly charge the battery which is concurrently being discharged by the refrigerator so the charge is even slower. The charge from the tow vehicle is similarly affected.

A pair of real deep cycle batteries -- e.g. - two 6v golf cart batteries in series -- will provide about twice the power you have now. And there's the expensive Lithium option as well.

An amphour meter ("shunt type") will provide a good fuel gauge.

Battery problems became rampant a couple of years when RV manufacturers started fitting cheaper and easier to install 12v or inverter refrigerators rather than "proper" but more expensive propane/AC models. Solar battery charging was one of the "solutions" but these refrigerators really require shorepower (or massive batteries -- Lipos) which seems to be how most of us camp these days.

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Old 05-25-2023, 10:25 AM   #7
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Originally Posted by Chuck_S View Post
Your refrigerator is probably sucking the life out of your batteries which may be indifferently charged. Solar won't power anything only slowly charge the battery which is concurrently being discharged by the refrigerator so the charge is even slower. The charge from the tow vehicle is similarly affected.

-- Chuck
It is completely impossible for a battery to be charging and concurrently discharging. That's not how solar works - at all.

The solar panels must supply all of the electrical loads on the dc buss first. Any power left over from the solar panels after that will then go into the battery charging it.

If the load on the DC buss exceeds the capacity of the solar panels, then both the solar panels and the battery will share the load.
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Old 05-25-2023, 11:26 AM   #8
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Originally Posted by Chuck_S View Post
This load tester (I have one) is designed for starting batteries and measures the voltage drop when a massive load is placed on the battery as when starting the engine. Tells you nothing about the capacity or condition of a deep cycle battery which is optimized for low amperage draws.
Absolutely true.

A "load tester" as you describe has only two benefits when used on Deep Cycle batteries. First is to remove surface charge so voltage reading now gives a more accurate SOC. Second is to warm your hands on a cold day.

A hydrometer is best for testing a Deep Cycle batteries condition.

Testing a Deep Cycle battery's actual capacity requires that a known load be attached to a fully charged battery and measure time until discharged to 10 volts.

The fixed load current and time to discharge are then used to calculate capacity.

(Load is generally one that should discharge battery over 20 hours.)

Forget testing at auto parts stores if you want to know the actual condition of a deep cycle battery. They use "push button" load testers so they don't have to hire trained techs.
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Old 05-25-2023, 02:04 PM   #9
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N

You are sort of flying while blind.

Your solar on its best day can produce 100 ah

You tv typically adds only single digits of charge.

On a hot day your fridge likely uses over 100ah in a day.

Your batteries store likely less than 100 ah.

Using the furnace uses a ton of power.

I suspect you cannot check usage with out a battery monitor.

A single SOK 205 ah battery with the built in monitor might be a good buy. $1020 when I last looked. More than double what you have.

Or do nothing. Often your system will be fine. A day or few per year not having much battery capacity is no end of the world.

Our rv would not power the front lift jacks with a dead battery. However, the generator makes it work in seconds.
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Old 05-25-2023, 03:51 PM   #10
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Thanks for all the replies.

Simple fix: I opened up my battery boxes to perform the load test this morning and the first thing I noticed was one of the positive cables was extremely loose. Tightened it down - and all the others - and all is well. A 6-hour drive again today and at 100% when I set up 'camp' and used the auto level system. I have lock bars on the boxes that are a moderate PITA to take off/put on so I put it off 'til this morning.

Lesson learned - I should have simply checked out the connections first. I think subconsciously I just wanted to buy a new tool.....
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Old 05-25-2023, 05:19 PM   #11
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Failure mode of batteries

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Originally Posted by IsleDog View Post
I recently had something similar happen with my battery. it showed fully charged and everything ran fine, but would not pull more than 15 or so amps. it failed a load test even though it was "fully charged" and everything showed it was. im assuming that your batteries are a wet type with removable caps for the cells. if so, you will want to get familiar with how to water them.
One of the primary failure modes of batteries--all types, Alkaline, NiMH, Flooded Lead Acid, AGM, all the Lithium types, even the old Carbon-Zinc ones, is an increase in the internal resistance. Any of the battery types is best modeled as a perfect voltage source in series with a resistor. See this article.

As a battery ages, the resistance increases. If you measure the voltage with a high-resistance voltmeter, it will appear perfect. But if you put a load in it, the voltage divides across the internal resistance and external load resistance. That's exactly what those load testers do.
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Old 05-25-2023, 05:24 PM   #12
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Hmm....

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Originally Posted by NavyLCDR View Post
It is completely impossible for a battery to be charging and concurrently discharging. That's not how solar works - at all.

The solar panels must supply all of the electrical loads on the dc buss first. Any power left over from the solar panels after that will then go into the battery charging it.

If the load on the DC buss exceeds the capacity of the solar panels, then both the solar panels and the battery will share the load.
Hmm... I would have said that any amount that the solar panel can raise the voltage on the loaded system will serve to charge the battery.

Suppose you have a loaded circuit with solar disconnected and the bus voltage is 12.0 volts. The open circuit voltage from the solar is 13 volts. You connect the solar to the bus and the bus voltage rises to 12.1 volts. In that scenario the battery is charging, right? It was discharging before the solar was connected and is charging after the solar is connected.
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Old 05-26-2023, 12:14 PM   #13
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Hmm... I would have said that any amount that the solar panel can raise the voltage on the loaded system will serve to charge the battery.

Suppose you have a loaded circuit with solar disconnected and the bus voltage is 12.0 volts. The open circuit voltage from the solar is 13 volts. You connect the solar to the bus and the bus voltage rises to 12.1 volts. In that scenario the battery is charging, right? It was discharging before the solar was connected and is charging after the solar is connected.
No. The battery was supplying enough load to pull its voltage down to 12 volts. When the solar panels came online, they picked up some of the load, so now the load on the DC buss is enough to pull both battery and solar voltage down to 12.1 volts. The battery is still discharging, but at a slower rate.

The batteries will only start charging when the load in the DC buss decreases enough so that the solar panels can provide the entire load and the solar voltage then rises above the open circuit voltage of the battery.
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