Using a tired AGM deep cycle

**Ranga** · 3rd July 2019, 05:24 PM

Under the suspicion that my 120Ah AGM in my slide-on camper was at end of life (~8 years old, accidentally abused a couple of times, but otherwise always float charged using solar 98% of the time), I've replaced it with a new 120Ah AGM deep cycle. However, due to other circumstances, this new battery has become the vehicle auxiliary. After I left the old AGM sitting in the shed for ~6 months, I was surprised that it's still showing ~12.6 on the multi-meter. I'm not sure if it's still good or not, but I suspect this voltage would drop quickly if I used it on it's own (I really should do an amp draw test). However, I was considering putting it back in the camper, to be connected to the new vehicle auxiliary battery.

The question is, if this old AGM is indeed as tired as I think it is, if I put it in the camper would it 'fight' against the new AGM in the vehicle, i.e. would the new AGM be used up trying to keep the old AGM charged?

**loanrangie** · 3rd July 2019, 05:37 PM

I'm no expert but I think it would pull the new battery down.

**austastar** · 3rd July 2019, 05:39 PM

Hi,
What I think I would do (as I have a Bluetooth Battery Monitor).
Monitor each battery for 24 hours with the same load, then if the graphs are reasonably similar, recharge and parallel them for the same 24 hour test and then again if similar, continue the test another 24 hours and compare the slope and final voltage.
Then make an informed decision.
Cheers

**drivesafe** · 3rd July 2019, 05:48 PM

Hi Ranga, first charge the old AGM, and then do a GENTLE load test with something like a blinker globe ( 21w ) and leave it on the battery for a few hours.

Monitor the battery voltage and see if it drops quickly or see if the voltage drops at first and then stabilises and then there is little to no voltage drop after a few hours.

If the voltage drops quickly and keeps dropping, it’s good for use as an anchor but not much else.

If after a few hours, it holds its voltage, then just wire it up in parallel with your new battery and all should be fine.

**Aaron IIA** · 3rd July 2019, 08:07 PM

If it only dropped to 12.6V after sitting unused for six months, I don't see how it could flatten the other auxiliary battery.

Aaron

**drivesafe** · 3rd July 2019, 08:26 PM

Hi Aaron, the problem with measuring the voltage on a battery with no load applied, which is called a OPEN CIRCUIT voltage reading, is that a battery can be totally stuffed but shows it has a fully charged voltage reading.

You can then apply a small load, say a compressor type fridge, and the battery can go flat in as little as a few seconds.

This the reason I suggested Ranga try a simple load test to see if his battery is still usable and able to hold a charge.

**Aaron IIA** · 3rd July 2019, 08:43 PM

I don't dispute that. You essentially can have a charged battery, but with high internal resistance and very small effective capacity. My comment related to the old battery flattening the new battery when connected in parallel. In this instance, the old battery, with a settled terminal voltage of 12.6V, could only bring the good battery down to 12.6V.

Aaron

**drivesafe** · 3rd July 2019, 09:15 PM

Hi again Aaron, the problem with a battery that is stuffed but showing a fully charged voltage reading is that you have no idea how that battery will behave when connected to a good battery but there is also the bigger problem of how will it behave when trying to charge it.

The reason I suggested that Ranga first charge the battery is to see if it can be charged.

A stuffed battery can take a full charge current for hours and the battery charger never goes into float, or as soon as a charger is connected to a stuffed battery, the charge may go into float with in minutes of turning the charge on.

I would rather know if the battery is any good before connecting it to a good battery.

**Ranga** · 4th July 2019, 06:13 AM

Did a test last night with a 12V 1A light. At 2000 it was showing 12.52V, at 0600 it was showing 12.48V (according to the Bluetooth battery monitor). The battery has always charged fine on my Ctek charger, and has probably only been cycled less than 50 times.

Worth trying in the camper?

**trout1105** · 4th July 2019, 06:28 AM

Originally Posted by Ranga

Did a test last night with a 12V 1A light. At 2000 it was showing 12.52V, at 0600 it was showing 12.48V (according to the Bluetooth battery monitor). The battery has always charged fine on my Ctek charger, and has probably only been cycled less than 50 times.

Worth trying in the camper?

That sounds like the battery is still "Viable"

I recently replaced the 2x 100AH "House" batteries that I use to run my 40l and 80l when I go away with a couple of new 125AH batteries simply because they were about 4 years old and I "Suspected" that they were on their way out.
After doing a similar test as you did but with a bilge pump to transfer water I found that there wasn't much wrong with the old 100AH batteries after all.
In hindsight I should have tested the old batteries and saved myself a few $Hundred But looking on the bright side I have now got a couple of 100AH batteries as spares

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