Complete Discharge of AGM Battery

Submitted: Sunday, Apr 16, 2006 at 10:57
ThreadID: 32928 Views:7906 Replies:7 FollowUps:18
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Happy Easter to all.
I have an Optima 75AH AGM Battery and 3 Stage Smart Charger fitted to my campervan. After a recent trip, the 3 way Dometic Fridge
was accidently switched to " DC " instead of "Off" ( I must start wearing my specs)
and was left like this for the next four days. Of course by this stage the battery was completely discharged and barely registered on a volt meter.

When connected to 240V the onboard charger did not even start up so I disconnected it and connected up a 2.5 Amp Arlec charger(a meter on the front showing up to 6 Amps which I don't understand) which repeatedly switched on for about 30 seconds and switched off for about 30 seconds while showing on the meter a charge rate of 5 Amps. This went on for about 4 hours at which stage the meter showed about 4.5 Amps. I went to bed and left it connected. About 8 hours later the meter showed 3 Amps so I disconnected the Arlec and reconnected the onboard charger which now switched on and began charging. About a further 8 hours on the onboard charger indicated that the battery was fully charged.

The battery is about 2 1/2 years old and never been discharged below about 50%
before this and has only done about 10 full cycles(it has mostly been hooked up to 240v at Caravan Parks and at home and in "conditioning" mode according to the charger.
I am of the understanding that this type of battery can be discharged to about 20% without really harming it, however in a fortnight I will be away for a week at an unpowered camp up along the Murray where I will be depending on my campervan battery to at least power some lights for the entire week, so I ask........

1. Why didn't the "Smart" charger commence charging the battery?
2. How much damage has been done as the battery was completely "sucked dry"?
3. What is the best way to "test it out" before my trip?
4. Should I replace it with a new one to be on the safe side?
5. How can the 2.5 Amp Arlec charger charge at up to 6 Amps?

Any advice would be greatly appreciated.
Cheers,
Keith
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Reply By: MartyB - Sunday, Apr 16, 2006 at 12:07

Sunday, Apr 16, 2006 at 12:07
Keith,
The only real way to see if the battery is ok is to try it.
Load it up with all the lights etc you will be using when away. Run these for about twice as long as you would when camping & see how it goes.
If it handles this for a week then you have no problems.

from Marty.
AnswerID: 167224

Reply By: Derek from Affordable Batteries & Radiators - Sunday, Apr 16, 2006 at 12:20

Sunday, Apr 16, 2006 at 12:20
Hi Ozikeith

The Smart charger would have done the job but would have taken a fair while to bring the voltage up. When a battery is deeply discharged it will only pull very low amps and then build up and start to draw charge. If you had left it on it would have come up. The Arlec would have been cutting out by thermal overload as it was trying to charge as it is not a smart charger. The smart charger 'Thinks' the low amp draw is 'Float' stage and would take a while to 'Bulk' charge again.

As you have said that battery is fairly new you should be quite safe to charge it up and use it again. I would cycle it a few times down to 10 volts just to make sure.

Test it with a spot light of 50 or 100 watts and divide it by 12 to give you an amp hour draw. IE: 4.2 amps / h and 8.4 amps per hour.

I would only replac it if after 2 or 3 cycles it does not give you at least 60 amp hours use.

The Arlec is not very accurate and the 6 amps is possibly RMS 3 amps (-) and 3 amps (+)

Last of all I would suggest that you buy a battery protector. $79.95 and it will cut off power when the battery drops down to 10.5 volts.

Regards Derek.
AnswerID: 167226

Follow Up By: Sand Man (SA) - Sunday, Apr 16, 2006 at 14:50

Sunday, Apr 16, 2006 at 14:50
Derek offers some good advice, but cut off at 10.5 volts is still too low.

10.5 volts equals zero voltage in battery. (dead flat)

I can suggest a cheap product from Projecta called the Battery Discharge Protector (ODP500) which cuts out at 11.6 volts, or approximately 25%. (75% discharged)

This is about as far as you should drain the battery if you want reasonable life out of it.

The Projecta products are available from KMart, Repco, etc.
The Battery Discharge Protector costs about $30.00 and comes with a ciggy style plug and socket on the input and output leads. I replaced these connectors with Merit type plug and socket, to suit the connectors in my vehicle and because they are superior connectors.
Bill


I'm diagonally parked in a parallel Universe!

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FollowupID: 422324

Follow Up By: Derek from Affordable Batteries & Radiators - Sunday, Apr 16, 2006 at 14:55

Sunday, Apr 16, 2006 at 14:55
I also sell the Projecta range but 3 way fridges draw a continuous 8 to 10 amps and the Projecta product goes soft and melts the plastic case.

The BP-60 is rated at 60 amps. Is adjustable if needed to cut out sooner.

My Agm's are rated to 350 cycles at 9.6 volts.

Regards Derek.
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FollowupID: 422326

Follow Up By: Derek from Affordable Batteries & Radiators - Sunday, Apr 16, 2006 at 18:00

Sunday, Apr 16, 2006 at 18:00
Hi Tink

Here is the data sheet from the manufacturer. You don't know my knowledge so back off !!



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Follow Up By: Mad Dog (Australia) - Sunday, Apr 16, 2006 at 18:27

Sunday, Apr 16, 2006 at 18:27
The cutout voltage of any good cutoff device should be customised to suit your own personal situation. All batteries will drop when under load, the more load the bigger the drop. If drawing large currents the cutoff voltage should be lower than a light load. My battery drops to eight or nine volts when the starter operates, doesn't mean the battery is flat though. You guys already know this basic stuff....why argue
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FollowupID: 422357

Follow Up By: Derek from Affordable Batteries & Radiators - Sunday, Apr 16, 2006 at 19:44

Sunday, Apr 16, 2006 at 19:44
Nice to pick and choose your lines... Tinker

'The method of choice for the wiring on your fridge is to have a mini-relay wired into the ignition circuit, so when you switch off the engine the fridge automatically switches off'

This alone is wiring nightmare running an extra wire from the ignition to the trailer ! Would be better to tie a string on you pinkie Tinker.

As for the data sheet. There is a graph with variables. The 350 cycles is a variable. The data sheet shows it. What is your problem. Why not register so we can see who you are. To be so full of it anonymously is a pain in the ...

Keep Well my friend.
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FollowupID: 422365

Follow Up By: Member - Davoe (Widgiemooltha) - Monday, Apr 17, 2006 at 02:43

Monday, Apr 17, 2006 at 02:43
What the heck is it about 12v threads that brings out all the argy bargy - Heck Nissan vs Toyota have nothing on Amp drawdowns and continous charging rates
It is the same on other forums as well - ijust dont get it
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FollowupID: 422431

Follow Up By: Jimbo - Monday, Apr 17, 2006 at 09:28

Monday, Apr 17, 2006 at 09:28
Davoe/Derek,

There was achap known as Ozi who was banned here ages ago. What he does is this.

1 He logs on under a new alias (Ozikeith) and posts a question.
2 He then logs on under another alias (The Tinker) and tries to discredit everyone.
3 Then he logs on under anither alias (Dave usually boots him before he gets this far) and puts in a post about a wonderful chap in Sydney who solved all his eletrical problems and sold him goods at a great price. The wonderful chap is infact himself.

He did the same a few days ago as MartyINEVic.

The minute I spot an electrical post from a new visitor I alert Dave. 9 out of 10 times it's this slippery character.

Cheers,

Jim.
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FollowupID: 422448

Follow Up By: ozikeith - Tuesday, Apr 18, 2006 at 23:58

Tuesday, Apr 18, 2006 at 23:58
Dear Jimbo,
I am very grateful for some very good advice from members of this forum. The questions were genuine and there was no skullduggery involved. I followed a Link to this website after I did a google search. I have never used the aliases "Ozi" or "The Tinker". Once again many thanks to all that offered helpful advice.

Jimbo, next time you "spot an electrical post from a new visitor" why not help out
if you can as isn't it logical that the new visitors or new members will be the ones asking the most questions, especially about electrical matters?

Cheers to all,
Keith
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FollowupID: 422915

Follow Up By: Jimbo - Wednesday, Apr 19, 2006 at 08:07

Wednesday, Apr 19, 2006 at 08:07
Keith,

If you are in fact an innocent victim, my apologies.

Cheers,

Jim.
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FollowupID: 422949

Follow Up By: ozikeith - Wednesday, Apr 19, 2006 at 16:31

Wednesday, Apr 19, 2006 at 16:31
Jim,
Apology accepted. I need to learn about this 12V stuff so no doubt there will be more questions in future.

Cheerio,
Keith
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FollowupID: 423045

Reply By: Marn - Sunday, Apr 16, 2006 at 12:36

Sunday, Apr 16, 2006 at 12:36
The arlec charger is probably rated at 2.5 amps for a 100% duty cycle. It will put out the 6 amps you talk about but this is beyond the rated current for the transformer. The thermal overload then cuts out to protect the charger from overheating and then cuts back in when things have cooled
AnswerID: 167228

Reply By: Member - Phil G (SA) - Sunday, Apr 16, 2006 at 22:21

Sunday, Apr 16, 2006 at 22:21
Keith,

One had exactly the same thing happen to an N70 battery. It read zero volts on the multimeter - basically had been dead-shorted by the fridge. Took me about a week to get it recharged. As both Derek and Tinker have alluded to, it is very hard to get charge into a totally dead flat battery because a battery in that state has a very high internal resistance.

Now, I know nothing about the way an AGM would behave, but if it were mine, I'd charge it up until it appeared to be fully charged, and then hook up say a 50watt light and attempt to measure capacity by monitoring the voltage over time. If it has little capacity, you'd need to charge it much longer.

And if you are thinking of a low voltage cutout - beware of the high amp draw of your 3-way fridge. I'd suggest that only Derek's $79 suggestion would be suitable. The Jaycar kits can be easily adjustable by varing resistors - I've used both the relay and FET types, but both are limited to 10amps max, and my guess is that at 10+amps continuous, it will fry.

Best of luck
Phil
AnswerID: 167297

Follow Up By: wheeleybin - Monday, Apr 17, 2006 at 16:53

Monday, Apr 17, 2006 at 16:53
RV Electics in SA have a special motion sensed cutout for 3 way fridges that if you accidently leave your 3 way on 12V when the vehicle stops it shuts the fridge off.
When the vehicle moves on it starts the fridge up again and its about $49 retail and built just for the job.
No stuffing around just wires into the power supply to the fridge.
Wheeley
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FollowupID: 422498

Reply By: ozikeith - Tuesday, Apr 18, 2006 at 22:20

Tuesday, Apr 18, 2006 at 22:20
Thanks to all for the helpful advice. Looks like there is still plenty of life in the battery after carrying out some of the suggested tests. I think I understand
a lot more now about battery voltages and capacity than I did a few days ago.
I will get one of those battery protectors from Derek which should stop me
ever discharging the battery like this again. The motion sensor cut-out for the fridge seems a good idea. I will have to check it out.
Cheers to all,Keith
AnswerID: 167688

Reply By: Member - Mike DID - Wednesday, Apr 19, 2006 at 18:32

Wednesday, Apr 19, 2006 at 18:32
Keith

1. Why didn't the "Smart" charger commence charging the battery?
- When a battery has been DEEPLY discharged (below 10 volts), you will damage it if you charge it using the standard 0.25C current (18 amps here. ). The SmartCharger has a protective circuit built in to protect the battery if the charger isn't able to charge at the very low rate needed (< 2amps)

2. How much damage has been done as the battery was completely "sucked dry"?
- There is no manufacturers data on the damage from discharging below 10 volts. If you discharge the battery to 0% remaining (actual voltage varies with current draw) you will get at least 300 charge-discharge cycles out of an AGM.

3. What is the best way to "test it out" before my trip?
- Connect it to a load like a light bulb that draws 3.5 amps (2 x 21watt bulbs) and see how close to 20 hours it gets. Use a Low-Voltage Cutout !!!

4. Should I replace it with a new one to be on the safe side?
- It's unlikely to fail totally if its capacity is reasonable.

5. How can the 2.5 Amp Arlec charger charge at up to 6 Amps?
- HIgh-powered electrical devices have two ratings. Continuous based on the heat it can handle. Intermittent based on electrical limits.

For those who can do-it-yourself, Oatley Electronics sell a $19 Low Voltage Cutout kit that can handle 80 amps but it will draw NO current while the load is connected (it uses a Latching Relay).

Mike
AnswerID: 167870

Follow Up By: Mad Dog (Australia) - Thursday, Apr 20, 2006 at 12:23

Thursday, Apr 20, 2006 at 12:23
Ok, it took me a little while to find that kit as its main purpose is a dual battery isolator and is described as such....lol, looks good, great value. Thanks Mike looks like I've found the cutout I need.

hey, how about the BOG DEPTH SOUNDER KIT...
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FollowupID: 423271

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