Battery clarification

Submitted: Thursday, Apr 07, 2011 at 08:35

ThreadID: 85478 Views:3297 Replies:8 FollowUps:13

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Member - Troll 81 (QLD)

One for the experts please:
Yesterday I went and got my battery tested at the local battery place and the guy said it tested 100% fine under load. Reason for this is that I somehow drain a fully charged battery in 12 hours right down to 11.7v. I worked on the calculations that if both my fridges run nonstop for 12 hours it should roughly use 60amps...but they don’t and off course cycle on and off. The guy at the shop told me that the label might say 105amp but in reality because it’s a hybrid type battery (Starting and Cranking ) that number should more be like 70 usable amps.

If it is really more like 70amps then I could understand why it might be flat if I run lights/stereo/fridges/otherbits during the night.

I am going to run my fridges again and double test everything to make sure of my fridge current draws.

Battery is a Allrounder 105 amp around 2 years old.

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Reply By: Robin Miller - Thursday, Apr 07, 2011 at 09:04

Thursday, Apr 07, 2011 at 09:04

70 probably is about right Troll, for most people the term usable means what you can repeatedly cycle the battery down to without affecting its normal life cycle.

You should still be able to get the 100 out of it over a day.

11.7 volts can still mean your battery has maybe a 1/3rd of its charge left.

As per recent thread 85440 - how/when you measure the 11.7v is a factor as well.

Robin Miller

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Follow Up By: Member - Troll 81 (QLD) - Thursday, Apr 07, 2011 at 09:11

Thursday, Apr 07, 2011 at 09:11

Agreed but the fridges cycle and I should not be using 60amps in 12 hours especially at night...In my case I had the battery charged up with my 3 stage charged by 7pm....night time was nice and cool and by the morning 7am my battery monitor is screaming it's head off.

Something is wrong

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Follow Up By: Robin Miller - Thursday, Apr 07, 2011 at 09:39

Thursday, Apr 07, 2011 at 09:39

A practical & useful test I have done Troll , is to use a headlight bulb (usually 55w) and just stick it across the battery as a test overnight.

This is about 4 amps and represents a typical load overnight.

Much easier to get to real indication this way than the vagaries of fridges and their duty cycle.

Robin Miller

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Follow Up By: ABR - SIDEWINDER - Thursday, Apr 07, 2011 at 16:59

Thursday, Apr 07, 2011 at 16:59

The load test done by the battery shop will not show battery capacity.

I agree with Robin.

I use a 50W Halogen downlight to load test. They draw 4A and should run for approx 24 hrs on a fully charged 105aH battery. If the battery drops to below 11.5V during the test or way before 24 hrs it needs replacing.

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Derek from ABR

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Follow Up By: ABR - SIDEWINDER - Thursday, Apr 07, 2011 at 17:05

Thursday, Apr 07, 2011 at 17:05

These 50W down lights are great for all types of testing, here is the test unit I built to test chargers, isolators, breakers etc.

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Reply By: Alloy c/t - Thursday, Apr 07, 2011 at 09:04

Reply By: oldtrack123 - Thursday, Apr 07, 2011 at 09:56

Thursday, Apr 07, 2011 at 09:56

Hi Troll
A load test & a capacity test are two totally different things.
A true capacity test can only be done by discharging the battery over time carefully measuring both amps & volts
The capacity will also vary with the rate of discharge.
Using the 20hr rate is the normal way to test
For a 100amphr that would be 5amps
A test such as Robin has suggested is one way you could check.
Batteries do loose capacity over their life cycle, even though they may show fully charged by voltage.
All that means is that their REMAINING capacity is fully charged
It could even be only 10amphrs

Peter

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Follow Up By: Mike DiD - Thursday, Apr 07, 2011 at 11:50

Thursday, Apr 07, 2011 at 11:50

I agree.

My 3 year old battery has no trouble starting a Diesel, so I know it would perform well on a discharge current test.

But if I leave the AM/FM radio on for 15 minutes, it can't turn the engine over, so it know it has much less capacity than when new.

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Follow Up By: Mike DiD - Thursday, Apr 07, 2011 at 11:56

Thursday, Apr 07, 2011 at 11:56

Say you had a watertank that was 1000 litres and 10 metres high, the pressure would be 15 psi.

Algae etc grows on the sides so it only has 100 litres capacity - but the pressure is still 15 psi.

It'll still provide the same flow as when new - but for a much shorter time.

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Follow Up By: dbish - Thursday, Apr 07, 2011 at 13:07

Thursday, Apr 07, 2011 at 13:07

Hj Troll81, I use a N70ZZ battery for an ocasional 12V diving compressor it draws 25Amp. I have a 6 yr old battery it gives 2 Hrs diving before compressor starts to slow down. Have a 11Yr old battery it will give about 1Hr diving. Both batterys when charged give the same hydrometer reading. So one obviously only has half the capacity, these are not deep cycle batterys. The 11 yr battery is an Exide the 6yr is a Sprint $95 made in the Philipines battery.

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