Community Forums

GaryKD · 11-30-2010, 06:08 AM

Hi TQ60,
Your last post mentions
"fully charged voltage will be the same regardless of degradation so leaving the battery sit to later measure the voltage is of little value."

Coming right off a charger, will a fully charged battery have a surface charge? Shouldn't the surface charge be allowed to dissipate so an accurate reading can be obtained?

RJay · 11-30-2010, 08:02 AM

Hi Gary,
This might be of interest.

TQ60 · 12-04-2010, 12:14 PM

Forgot about the Surface or float charge in my longwinded post.

Let me explain the statement below.

"fully charged voltage will be the same regardless of degradation so leaving the battery sit to later measure the voltage is of little value."

All other things equal, either measured during floating conditions, or after a period of time where the surface or float charge settles down to the idle state after the charger is removed, the voltage measurement ONLY identifies the "state of charge" of a battery, and this is limited to the overall condition of the battery, IE all cells are "functioning".

In other words, if none of the cells in a battery are open or shorted they will have a voltage measurement.

If the chemical reactions and status of all of the materials are such that the cell is at its maximum charge level it will measure a specified voltage for the given battery type.

Now we consider the capacity of any battery, a small capacity (70 amp hour) AGM battery will have the exact same basic chemical structures as a larger one (225 amp hour), the difference being the SIZE of the parts.

So the voltage readings on both batteries should be very close to the same as long as both are at the same "state of charge" (50%, 100% etc).

Next is we add in age degradation, as batteries age and get used, both cause wear and tear on the chemicals and structures in the battery. The BASIC result is the capacity of the battery is reduced, usually by the erosion of the plates inside the cells.

As the plates get smaller the CAPACITY of the battery gets smaller.

So just a measuremnt of the voltage alone can ONLY assist in estimating the state of charge of the battery.

So if one is testing their system to evaluate the performance, they could use the voltage measured to determine if the system is ready for further evaluation.

If the voltage is made under a floating or charging status, then the voltage alone may not indicate the battery state of charge, the charging current needs to be known.

If the charger is removed, and there are no loads on the battery then it can be allowed to stand for a few hours to have the surface or float charge dissipate, then the voltage measured to confirm state of charge.

If we knew that the rate and time of charge should have the battery fully charged and the measured voltage was 12.4 then we could guess that either the rate of charge was too low, or that there are problems in the battery, and take the next step.

Measuring the cells via the vent caps would show the bad cell if there was ONE, or confim the rate of charge.

There are many variables related to the care and feeding of battery systems.

We must understand how they work and how to do the basig engineering to determine what the battery system is capable of before we simply accuse the battery system of being defective.

Once we determine there is a possible problem we need to correctly measure the voltages, loads and charge voltage/currents to determine what is broken and then make repairs as needed to insure the system performs as designed.

To confirm if a battery is GOOD, one needs to have a set load for a measured time, this confirms amps output for given time, this amp hour rating.

Other measuremets can quickly determine a bad or suspect battery, they CANNOT claim certianty that a battery is GOOD (meets output specification) with any other type of test.

I hope this long post helps.

rvfun · 03-05-2011, 08:00 AM

I have 4 AGM lifeline 4D 12volt batteries wired in parallel. I'd like to test individually. I suspect one or more is weak, but I would like to confirm before purchasing all 4.

Is the following a good way to do this: Reconfigure the wiring so that only one battery is connected to the motorhome. Then run my refrigerator for X number of hours while monitoring that battery's voltage. Then do that for the other 3 batteries. Maybe I will be luckly and find only one is weak.

I would appreciate any comments on this approach.

Thx, Bob...

az bound · 03-05-2011, 09:47 AM

I think it would be much easier to charge all batteries, then allow them to stand with no load for about an hour, then test each one independently with a volt meter. Or you could remove them and have them load tested.

wa8yxm · 03-05-2011, 09:59 AM

I have a collection of batteries here, ABM, Flooded wet cell and maintenance free.

People on the forums talk about the long life of AGM's..

Well. the AGM's I have are the newest batteries (At least this week) and the deadest batteries, in my coach..

Sure they last longer.... Even the manufacturer's web page says they do not last as long as other types. (read the number of cycles specified)

GaryKD · 03-05-2011, 10:03 AM

Hi rvfun:
Consider purchasing a battery load tester. One can Google "battery load tester" to find quite a few choices. I have an Actron CP7614 130 AMP load tester. Go to http://www.actron.com/product_detail.php?pid=16312 and take a look. 130 AMPs is the minimum I'd consider.

One does not use a load tester very often, but it really comes in handy when it is needed. For me, it is more handy for camping buddies than for myself.

TQ60 · 03-05-2011, 10:18 AM

A load tester is a good thing when properly applied.

ANY AND ALL methods used to test batteries where the test is done in seconds can only indicate a BAD battery.

Let me explain, my earlier posts I describe a water container and filling it, then placing sand in same container and refilling again.

If you have a battery of any type that is "good", (no shorted or open cells), but is worn out or has sulfated cells, a short load test still may indicate good as the battery still can output the current, just for a shorter time.

The only true way to test a battery as good is to have a fully charged battery, place a known load on the battery and measure the time it takes to the discharge voltage.

Check the manufacturer data sheet to confirm the actual capacity of the battery per the load, different loads equal different actual capacity, a labeled 100 amp hour battery is usually at the 8 ro 20 hour rate depending on manufacturer, so if load is more that 13 amps on the battery you will not get the labeled capacity on the 100 amp example.

AGM batteries are designed to last 10 years in "float" service, this is where they provide backup power and not are the primary power source.

They also have a limited amout of charge cycles, usually 1000 such cycles, so if you are fully charging the battery plant and discharging more than 50% or so then calculate what 1000 cycles are and that would be close to the life expectancy of the battery plant.

Next factor in how much time the batteries will be above 90 degrees as they do not like heat.

Wanabee FTer · 03-05-2011, 11:10 AM

The way I tested my battery bank was to fully charge all the batteries, then I immediately disconnected all of them. I then let them sit for 24 hours with no load before testing them. Reason being is one bad battery will pull all the others in that bank down. . A hydrometer test quickly showed me where the problem was, 2 batteries had one weak cell each. I suspect those 2 batteries caused my 3rd newer battery to not be able to charge all the way up, so I replaced all 3.

rvfun · 03-05-2011, 02:42 PM

The battery capacity is 210 amp-hrs at the 20 hours rate. That's about 10 amps. I think my fridge is about 5 or 6. Possibly add some lights. Hours of discharge at 8 amps is 22 hours, at 15 amps is 11 hours and at 25 amps is 6.5 hours.

I'll go out to the mh and connect only one battery and see how long it lasts checking voltage every so often. Then I'll repeat for each battery. Hopefully, one battery will be shot, but I am expecting all to be shot. $475 X 4.

Thanks for the discussion. Bob

TQ60 · 03-06-2011, 12:56 PM

When batteries are in parallel a single shorted cell in one battery can discharge the rest, so if the battery plant (all of the installed batteries) seems to be bad, and not that old, then one could have bit the dust.

Harbor freight has 100 amp load testers for about $20.00, these are quick tests to see if a battery is BAD, so turn off the battery system switch, then remove one lead from each battery and test each with the load tester.

Also have a digital volt meter, also very little money.

If you have a helper with a watch, test each for say 20 seconds or so, making sure to allow the tester to cool down.

Make note of the battery with the highest and lowest readings as these may be your best and worst.

Now reconnect either one, and find a constant load, I would use every light and other constant load, the ise box is a cycleing unit and may not be the best.

Measure the time it takes to drop exactly 1 volt, still not an exact measurement but you are looking for a battery that is "different".

So say the better battery takes 35 minutes to drop 1 volt and the other takes 20, then one is a lot better than the other one.

Next do the others, depending on how they measure you will have an idea of how they all compare.

If you want to know the exact capacity or exactly how much capacity th batteries have, then you wil need to know what the load is, use the ampmeter function and turn on lights until 5 to 10 amps are "on".

Then with a fully charged battery measure how long it takes to reach 10.5 volts which is concidered "dead", now you can calculate the amp/hours of the battery under test.

Repeat for all batteries, replace the bad ones, add up the scores and you know what you have.

Write down the 1 hour times and what was used to make the test, (which lights, starting voltage, etc), then when you retest sometime later you have a reference point.

Wanabee FTer · 03-06-2011, 02:30 PM

All good info, but I would never run any DC battery down to 10.5V on purpose.
Running a battery bank below 12.1 to 12.2V will significantly shorted battery life.

tims52ford · 03-06-2011, 02:43 PM

remove the battery cables from all batteries, if you know someone who can load test them, that's thw way to go. as far as, replacing just one bad battery, that's not the way to go.

TQ60 · 03-07-2011, 01:01 AM

The operational range of lead acid batteries bottoms out at 1.75 volts per cell, so the design of the battery is such that 10.5 volts is concidered "normal" and simpley fully discharged.

But most folks do not like going there, all batteries have a limited amount of cycles, so doing a test down to 10.5 will not damage a battery, but it will do some wear.

That is why in describing the test I used a single volt for a testing measurement, it allows a "constant" to be applied to all batteries under testing to determine which one9s) are bad.

If one needed to know exactly what the battery plant tested as then a single run to 10.5 could still be a valid measurement while under a controlled measurement.