Dual battery problem - F'ed AGM already

[nathan96]

Hi all Can you help me diagnose this problem its a 100AH AGM from southsidebatteries.com.au. I'll post a long version and short version

SHORT
with engine running i get approx 14V across the leads in the tray that run to the aux battery.

However when i connect these leads to the flat aux battery it reads 11.9V across the terminals. Shouldn't this read 14V (i.e. alternator is charging the aux battery

LONG
ON a recent Birdsville trip a circuit breaker got acidently opened meaning the AUX battery wasn't receiving and charge from the alternator whilst driving.

This was only noticed when the fridge started throwing a low voltage fault light a few days later. It was deeply discharged at about 11.8V

I put it on the ctek when I get home and 24+ hours later I get a green light, all charged up and good to go

Fast forward to this weekend just gone, Overnight the fridge flattens the battery to 11.9V (unusual because the battery usually last 2-3night before needing a charge). So I go for an hour drive to get some charge in but the battery is still at 11.9V. Trouble shooting time

With engine running
Voltage across starting batt is approx 14V
voltage on aux side of redarc is approx 14v
voltage across aux battery is 11.9V
weird!. I thought it would also be approx 14V (maybe voltage drop?)

So disconnect aux battery and measure voltage across cables that run to aux battery, approx 14V! so no voltage drop.

connect aux battery again and were back to 11.9V. So, this is what i dont understand and need help with.

Also with the engine running and fridge connected to the aux battery it cant run. Shouldn't the fridge be powered by the engine alternator while the engine is running? (i.e. power comes to aux battery and some bypasses it and moves onto the fridge?)

 

[3ltflogga]

Hi nathan - check your voltages with engine running - engine off and compare as variations occur durng charging etc. Make sure you dont dis/connect leads when things are running.

At a guess, that v drop may be telling you the battery is dying (not taking charge - ie it sucks up that 14v trying to charge your aux) so if you can get a discharge test done on that battery...even if it is new they can sometimes never recover from a good draining (not that 11.8 is drained, i get my century's down to 10.4....

 

[nathan96]

AUX battery engine off and isolated is 11.9V same as when connected and engine on.

 

[Old.Tony]

Sounds to me like there's an internal issue in the AGM. If it were less than 6 months old I'd be asking the supplier to test it for me - I wouldn't say anything else, just "it doesn't hold a charge, can you test it". They have equipment that will cycle the battery over and determine if there's a fault, and they'll replace it if there is.

If the battery is ok then I'd disconnect the fridge and see what the battery is doing under charge. I'd also have the AGM battery completely disconnected and check the battery terminal voltage while it's not being charged and while it's being charged. Is there something wrong with the fridge, or the fridge wiring that's causing a problem? Need to check that too.

When you run the fridge from the same source as the one you're charging the battery from, the fridge will "see" whatever voltage appears at the end of the cable, which in this case is the 11.9V that the aux battery is pulling the voltage down to. You're right that the battery and the fridge share the power coming from the alternator, but it's done at the voltage that is on the end of that cable.

This is why I invert my power and use a charger. My fridge (Engel) auto-switches over to 240V while the inverter is running, taking that load off the battery, allowing the battery charger to simply top the battery up.

As for your AGM - it should be able to handle a reasonable amount of physical punishment. They don't cost a bundle for nothing! However they are sensitive to excessive charging voltages (don't whack them on a solar regulator, I've seen those puppies pump out 16V). Don't cycle an AGM below about 50% either, as the sulfation will be caught in the glass mat and destroy the cell.

The glass mat is like cotton wool that's actually made of glass fibre. It's designed to absorb and hold the battery acid (electrolyte) suspended between the positive and negative plates inside the battery. Damage that, and your battery is gone.

That said - the car alternator is not really capable of charging the AGM too high, so I'd not be too concerned about that.

So - take it out, get it tested and see what they say, but I'd check that fridge too.

 

[ljacko78]

silly question - you have a good earth contact point?

 

[nathan96]

Well batterys f'ed

Hopefully its just sulfanated and can be rescued by a ctek recondition. But because the battery has only been discharged to 12V once (hardly a hard life) in its 18 months. I reckon its dropped a cell or something.

So much for AGM being the bees dick. Probably go down the marine battery route this time

 

[Bosshog]

Well batterys f'ed

Hopefully its just sulfanated and can be rescued by a ctek recondition. But because the battery has only been discharged to 12V once (hardly a hard life) in its 18 months. I reckon its dropped a cell or something.

So much for AGM being the bees dick. Probably go down the marine battery route this time

I dropped a cell doing the Simo in September. The corregations kill batteries.

 

[nathan96]

Was that an AGM

I reckon that's what happened to mine.

Next battery I get I'll make sure the plates are well supported

 

[Bosshog]

Was that an AGM

I reckon that's what happened to mine.

Next battery I get I'll make sure the plates are well supported

No just a run of the mill deep cycle

 

[Old.Tony]

Nathan, where are you at mate, we can give it a recon cycle with my CTek if you like.

 

[nathan96]

Cheers Tony, but I'm in SA

I've got it on recon at my mechanic to see if I can save it.

Didn't realise my location wasn't showing, updated it now

 

[Bosshog]

Cheers Tony, but I'm in SA

I've got it on recon at my mechanic to see if I can save it.

Didn't realise my location wasn't showing, updated it now

Nathan

I recommend taking it to Tim at Belair Road Auto Electrics. He is involved in the 4WD scene and knows his stuff.

Bosshog