Testing the Ah capacity of a Leisure Battery

[StuartO]

A man came to do the annual test/service of our domestic alarm system today and using a special tester, he found our alarm system's AGM battery to have dropped below an acceptable Ah capacity.  When a battery drops below 65% of its nominal Ah capacity they replace them.

 

The test instrument, as illustrated below, applies a pulsing load test which simulates a 20 hour discharge test (without discharging the battery significantly) and tells you how much AH capacity remains.  It can test automotive batteries up to 150 Ah capacity so I asked him to test my old Exide G80 battery, taken off the MH at 10 years old, to see what it could still do.

 

The battery had been charged a couple of times in storage but not for several months, yet it was still showing a voltage of 12.88 volts, so apparently was still in some sort of usable condition.  The discharge test however revealed that it had only 13 AH capacity rermaining, of the nominal 80 Ah, so on the basis of the test result it deserves a decent burial.

 

The ACT 612 test machine is a calibrated professional instrument and presumably, since these machines are widely used, gives an accurate result.  The maker's website talks vaguely about some sort of pulsing load system but otherwise keeps their technology to themselves.  An internet search on "battery load tester" revealed all sorts of testers, varying in price from £30 up to several thousands.  The ACT 612 sells for £220 to £300.

 

Do motorhome dealers use this sort of technology to test leisure batteries and if not why not?  On the face of it this is a useful way of testing whether a leisure battery needs replacing.  Does anyone know?

[Robbo]

Stuart - When I had a couple of 80Ah gel batteries they always showed well over 13 volts when fully charged. Perhaps my inboard control panel wasn't calibrated properly. I never tested the batteries with a multimeter as they were located under the front seats and difficult to access. The batteries appeared to be still going strong when I sold the van 10 years later.

 

All I'm saying is, if you don't know the state of charge of your gel battery when the test was made (apart from battery voltage), then don't assume it is knackered. Suggest you do your own drop test before discarding to the recycling bin. Hope I'm not teaching grandmother to suck eggs, haha !!!

 

 

[StuartO]

Not chucking it away yet - it's already on charge.

[John52]

This is a great opportunity for you to establish how accurate the tester is Stuart. Charge your battery and see how long it lasts at a known current drain. B-)

[andytw]

You could re-charge it fully. Then leave it to 'rest' for a while, say a few hours.

 

Then connect a 50w headlamp across it and time how long it takes to get to half charged at say 12.1 volts.

The lamp will draw 4 Amps.

 

The Ah could then be worked out as :-

 

Time Hrs x 4 Amps x 2 = Ah

 

So suppose it got down to 1/2 charged in 4.5 hours then the capacity of the battery would be :-

 

4.5 x 4 x 2 = 36 Ah

[Vernon B]

This may be of general interest. I've just replaced my leisure battery with a 90AH Varta battery from the very helpful people at Manbat in Shrewsbury. I have a control panel in the Frankia which has an integral AH meter. After being installed and charged for 24 hours the meter was only reading 72AH. On contacting Manbat I was told that Varta batteries only build up their AH capacity over a number of discharge/charge cycles - and I had that corroborated by ringing the manufactures directly. As with so many things associated with M/Hs time will tell!

 

Fascinatingly - the chap at Manbat told me they have no means of checking the AH!!!

 

V

 

 

[veletron]

Hi

 

I suspect the 'AH Meter' in your Frankia is an 'AH-Counter' - eg something that counts total AH into the battery (eg when charging via mains, solar or driving) and AH removed from the battery while in use.

 

If that was the case, there would usually be A reset button to zero the counted AH ?

 

Any display of calculated total AH remaining via some built in algorithm will be wildly inaccurate. There are simply too many variables.

 

I have a similar meter retro-fitted to my van (NASA BM100), which endeavours to give an indication of the batteries state of charge irrespective of current load (using an inbuilt algorithm). The only thing I tell it is the total capacity of the battery bank. The state-of-charge indicator is at best 10-20% out. The AH-Counter is accurate at counting AH but assumes 100% battery efficiency (which can never happen). That said, its still a good indicator of remaining charge - or how much was picked up via solar/driving/mains charging.

 

Nigel

[aandncaravan]

Vernon, Batteries do take a few charge discharge cycles to reach full capacity.

Additionally some Motorhome Power Charger/Controller units also take time to 'learn' the battery capacity. They will monitor the power put into the battery and also as it's drawn out, so can calculate the battery capacity. This can take a couple of weeks, depending on usage, with some Motorhomes to build up an accurate picture.

 

However, the figures it comes up with will only be accurate if the type of power unit described above, controls/'sees' ALL power going in/out of the battery.

If an Inverter is withdrawing power directly from the batteries that the Charger/Controller is 'unaware' of then it may calculate there is more capacity in the battery than there actually is.

 

Likewise if a Solar solution is charging the battery directly, i.e. not going through the Elektroblock as intended by the Builder/Electronics manufacturer then the Power unit may be unaware of all the charge going into the battery and calculate there is fewer Ah than there actually is.

 

Provided there is no power directly into or taken out of the battery on your vehicle, the AH will be a pretty accurate guide with a new battery once it has learnt your usage.

 

 

[StuartO]

I understand that Schaudt Electrobloks monitor current flow in and out of a leisure battery and can use that to calculate the Ah status - and on my MH that seems to provide a useful indicator.

 

But I'm still not much wiser about how these different sorts of battery tester work.  Load testers (ie testers which apply a calibrated load and use the voltage response to extrapolate to the big picture) are simple enough to relate to schoolboy physics but how do these more expensive testers "simulate a full 20 hour discharge" in a few seconds?

 

[aandncaravan]

StuartO - 2017-01-18 11:04 AMI'm still not much wiser about how these different sorts of battery tester work.  Load testers (ie testers which apply a calibrated load and use the voltage response to extrapolate to the big picture) are simple enough to relate to schoolboy physics but how do these more expensive testers "simulate a full 20 hour discharge" in a few seconds?

Stuart, You and me both.Derek Uzzell recently posted that he had to replace the Starter battery on his Skoda because at 6 years it was showing sign of losing charge after standing a few days.He replaced it and all was well. He fully charged the old battery and then let it stand. It reached full charge in an acceptable manner, but after standing a short while had lost a lot of capacity.If he had taken that 'rubbish' battery to a shop for testing after charging, it would have almost certainly passed an old fashioned load test where they just check if the battery will deliver x amps for x seconds. Just as it would start his car engine when fully charged. Clearly a battery that is rapidly losing charge, regardless of any test result, is fit only for scrap and why the usual Car battery test has always been flawed.I have not come across the testers that "simulate a full 20 hour discharge" in a few seconds so don't know how they work, but it sounds to me like marketing 'talk'.We have used the 'Resistance meters' costing up to £360. Ours was a Sealey, I think £160??On these you type in the claimed 'capacity' from the label and then it estimates how many amps the tested battery MIGHT be able to deliver.There are variations on this where the software can use Amp hours as the base. Because the functioning of the tester is based around changing resistance it is limited in it's accuracy. While quite good on Starter batteries we have seen such varying readings for habitation batteries, especially those not of conventional construction, that we found they couldn't be trusted.One G80 Gel battery that 'passed' with 75Ah capability, would last less than 30 minutes on a couple of Halogen bulbs after being fully charged. We charged/discharged it 3 times because we trusted the Meter more than our own eyes. When our meter was dropped, I didn't get another.I think the easy, reliable way to test a battery is remove it from the vehicle. Charge it up for 48 hours, leave it 2 weeks then do a standard discharge test to around 50%.Leaving it 2 weeks after charging before beginning the test is important. As noted above in Dereks case, if he had carried out a discharge test straight after charging he would have most likely got a good result?A good modern battery will stay close to full charge for 3 months (note that Banner and most budget batteries have a higher self discharge rate and can be down to 'Sulphation' voltage inside 2 months). Our favourite Varta LFD's/Bosch L5's will still be close to full after 6 months. So all new batteries, even those with the highest self discharge, should survive being left 2 weeks with almost no voltage drop or change in capacity. Any drop in voltage at all is a dying battery, but the standard discharge test will show how bad/good.

[StuartO]

This new type of tester was being used to check standby alarm system batteries, which are AGM batteries not that disimilar from AGM motorcycle batteries, except the terminals are small 5 amp ones.  They are used to maintain alarm function when the mains power fails, otherwise they just sit there, presumably on trickle charge.

 

My alarm battery (of unkown age, we moved in a year ago) was showing 40 Ah, which was less than the 45 Ah which represents 65% of full nominal capacity.  He recommended replacement so his test result sold me a £50 battery.  Nice guy, no suspicion that he was manipulating anything but clearly he was taking the test result a face value, whatever that was worth.

 

 Is it possible to simulate a full 20 hour discharge test in a matter of seconds and if so how?  It would be nice to know.  The manufacturer's website doesn't give any detail at all:

 

https://www.actmeters.com/products/battery-testing/act-612-6v12v-lead-acid-intelligent-battery-tester/