Battery clarification

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

ThreadID: 85478 Views:2825 Replies:8 FollowUps:13

This Thread has been Archived

Battery Power

In this article we explain why you might need an additional power source independent of the starting battery installed into your vehicle and we look in detail at types of batteries,

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.

Back Expand Un-Read 0 Moderator Share

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

Member
My Profile Send Message

AnswerID: 450559

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

FollowupID: 723017

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

Member
My Profile Send Message

FollowupID: 723020

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.

Regards

Derek from ABR

Member
My Profile My Position Send Message

FollowupID: 723064

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.

Image Could Not Be Found

Member
My Profile My Position Send Message

FollowupID: 723065

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

Thursday, Apr 07, 2011 at 09:04

So how many AMPs per HR are your fridges drawing by your reconing ? Keep on taking your batt down to 11.7 and it,ll be [insertword########] very soon, as an example , having run a Waeco 80 lt as fridge /freezer + a 15 lt Engle as drinks fridge for the last 8years ,summer /winter from ambient 5c through to 48c the Waeco averages 63amp hrs per 24 hrs and the Engle 43amp hrs per 24hrs .

AnswerID: 450560

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

Thursday, Apr 07, 2011 at 09:09

The idea is to not run it down that far hence my post and concern!

When they are both running around 5amps a hour

My point is I don't even get 24 hours I only get 12 hours

FollowupID: 723016

Follow Up By: Battery Value Pty Ltd - Thursday, Apr 07, 2011 at 09:37

Thursday, Apr 07, 2011 at 09:37

Hello Troll81,

presuming the fridges cycle on/off for 50% of the 12 hours at night, that's 6hrs @ 5A, or 30Ah.

Because the low voltage alarm goes off first when there's max current flowing, it would have started screaming while 5A were being drawn. Looking up the discharge curves of a 100Ah battery, that would indicate around a depth of discharge of 75%.

So your total battery capacity available from this 'do-it-all-wonder' is 40Ah.

How often approx has the battery been cycled (discharged more than 30% and charged again) during its lifetime?

What's the weight of the battery, if known?

Note that typical 100Ah deep cycle AGM batteries weigh around 30kg.

cheers, Peter

FollowupID: 723019

Follow Up By: Member - Troll 81 (QLD) - Thursday, Apr 07, 2011 at 09:44

Thursday, Apr 07, 2011 at 09:44

Peter,

The battery alarm is not on the fridge and only measure volts it's connected straight to battery and is totally independent front the fridges.

During our last trip..2 weeks away the battery got drained over night for at least 6 of those nights.

Specs of my battery can be found here Battery

FollowupID: 723021

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

AnswerID: 450563

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.

FollowupID: 723028

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.

FollowupID: 723029

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.

FollowupID: 723038

Follow Up By: Alllan B (Sunshine Coast) - Thursday, Apr 07, 2011 at 14:02

Thursday, Apr 07, 2011 at 14:02

1000>>100 litres??.... that's killer algae Mike. LOL But a good simile nevertheless.

Cheers
Allan

Member
My Profile My Blog My Position Send Message

FollowupID: 723044

Reply By: Notso - Thursday, Apr 07, 2011 at 13:02

Thursday, Apr 07, 2011 at 13:02

Have you thought about the possibility that the charger isn't taking the battery to full charge?

I know it's a three stage, but maybe worth a check on just what state of charge the battery is in before you start discharging.

AnswerID: 450574

Follow Up By: Member - Troll 81 (QLD) - Thursday, Apr 07, 2011 at 13:48

Thursday, Apr 07, 2011 at 13:48

Never thought about the charger side of things but will check it out.

Thanks

FollowupID: 723041

Reply By: Gronk - Thursday, Apr 07, 2011 at 13:56

Thursday, Apr 07, 2011 at 13:56

As you have found out......only a real world test can determine what system suits a particular circumstance...

And in your case....even if the battery is 100%.....you either need to reduce the load....or increase the battery capacity..

You are doing it tough now with a battery that is suspect, but I would think it would be marginal even with a new battery..

I have 210 a/h of batteries, but have never worked out how many amps they will provide over any given ammount of time........but thru using them while camping, I have worked out how many days they will give me while still staying at or above 12V.....that then determines when or how I will charge them ( usually at home after a typical 2 days away for a weekend )

AnswerID: 450578

Reply By: gbc - Thursday, Apr 07, 2011 at 18:34

Thursday, Apr 07, 2011 at 18:34

I'm guessing it's a supercharge allrounder? Performance is about commensurate with what you paid for it.
Get a proper deep cycle battery (or two) and pay the right amount of money (about twice what you paid) and your battery performance will about double.
I learned the long way that 12v is all about quality.

AnswerID: 450602

Reply By: Ianw - Thursday, Apr 07, 2011 at 23:38

Thursday, Apr 07, 2011 at 23:38

Grab one of these little meters. Will show current volts, amps, watts, and also shows Ahrs (total), Watthours, Peak amps, Mininum volts and peak watts.

power meter

Put anderson plugs on both ends, insert in power lead and it will tell you eveything you want to know.

Reverse it and put it on battery charger output wires and it will tell you everything thats going into battery.

I have run a Waeco 40 with this system and proven that it can run for 72 hours on 28 Ahrs ! (In winter) Only had a 50 Ahr battery and was still going strong when I disconnected it. Amayzingly enough, when charging the battery it recorded about 32 Ahrs to full charge. (over 2 days). Exactly what one would expect.

Ian

AnswerID: 450634

Follow Up By: Member - Troll 81 (QLD) - Friday, Apr 08, 2011 at 08:40

Friday, Apr 08, 2011 at 08:40

Great idea Ian I will add it to my shopping list. It's certainly much cheaper than some of the other automotive options out there

FollowupID: 723122

Reply By: Member - Troll 81 (QLD) - Friday, Apr 08, 2011 at 08:43

Friday, Apr 08, 2011 at 08:43

I started my testing at 3pm yesterday with a fully charged battery with my 2 fridges hooked up. This morning 17 hours later it's sitting on 12.30V so the battery might be okay in the end and I might have had other things running off it as well that I can't remember.

This was done in my garage so no hot temp and no opening and closing of the fridges. If I can get that out in the real world I will be happy...by the sun comes up the solar panel will kick in and nothing should get damaged.

Thanks for all the input

AnswerID: 450658