Longevity of AGM batteries

Well after such a great series of response about my Power Steering Pump 80 series why not ask another question.

Oh before I ask run a search doing it now - Cool no previous questions on this topic.

How many years will an AGM battery last and/or is there a way to tell when it is going? The reason I ask is that I checked the service log in the car over the weekend and found that I installed the 100AH Full River battery back in 06 and I noticed a slight cracking around the case at the pos terminal. The battery is still taking charge and works great (at least it did on our last trip for Wiluna last year). But this year we are doing the Ningaloo thing again and we park the car on the beach for around 2 weeks (start it to launch the boat and collect water) and use a 100W solar system to keep the fridge, computers, kids toys, coffee grinder and other mandatory items running. So this poses a different usage curve on the battery and I do not want to be without my coffee grinder and oh the fridge.

So is there a rule, test or check that I should do?
David (DM) & Michelle (MM)
Always working not enough travelling!

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Reply By: GrumpyOldFart - Wednesday, Jun 08, 2011 at 22:51

Wednesday, Jun 08, 2011 at 22:51

Derek should jump in here


here is a link to his web site that explains testing AGM batteries

Testing AGM batteries

Very well written and simple to do

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AnswerID: 456832

Follow Up By: Richard W (NSW) - Thursday, Jun 09, 2011 at 05:51

Thursday, Jun 09, 2011 at 05:51

Thanks for the link.
Will try it myself as I have 2 AGM 100AH batteries.
FollowupID: 729975

Reply By: ABR - SIDEWINDER - Wednesday, Jun 08, 2011 at 23:00

Wednesday, Jun 08, 2011 at 23:00
Hi David

Glad to see you are still using my 100W Bi-Fold solar panel.

I recommend you load test the battery and see if it is still in a serviceable condition. 5 years for a AGMM mounted under the bonnet is good service and if it fails the load test please replace it.

Email me if you have any questions.


Derek from ABR.
AnswerID: 456836

Follow Up By: Richard W (NSW) - Thursday, Jun 09, 2011 at 05:53

Thursday, Jun 09, 2011 at 05:53

Appreciate your testing procedure that Dru posted the link to
FollowupID: 729976

Reply By: Member - Mark E (VIC) - Thursday, Jun 09, 2011 at 00:09

Thursday, Jun 09, 2011 at 00:09

Whilst I'm not great on the technical side of testing, I have a Fullriver 120AH battery that I have had for 7 years and it seems to be going strong.

I do however look after it, not discharge it too much and charge it with the Rotronics dual battery system as well as giving it a regular top up with the CTEK 240v charger.

This battery has outlasted any others that I have had over the past 20 years. I would not hesitate to buy another, provided the qualtiy hasn't slipped.


AnswerID: 456842

Reply By: Peter_n_Margaret - Thursday, Jun 09, 2011 at 06:00

Thursday, Jun 09, 2011 at 06:00
How long AGMs (or any other battery lasts) depends more about how it is treated than how old it is.
It is also important to use the correct battery for the aplication. There are several types of AGM batterys - for standby use, cranking use and cycling applications.

See if you can find "DCcyclelife" pdf on the Fullriver website.
It describes the relationship between depth of discharge and the number of cycles. The deeper the depth of discharge, the less cycles the battery will last. This assumes that the battery is properly charged of course.
Deeper discharge does not reduce the total input/output of AGMs as much as some think though and working them harder also means there are less to carry around and they are cheaper to replace, even though they are replaced more often.

OKA196 Motorhome
AnswerID: 456844

Follow Up By: ingo57 - Thursday, Jun 09, 2011 at 16:50

Thursday, Jun 09, 2011 at 16:50
Purchased 90Ah AGM from Derek in 05, still holds a full charge out of vehicle when not in use and works perfect. However did notice this easter that voltage dropped a little quicker than I remember (whilst stationary) but still ran the Engel and camp lights for 3 days without needing a charge.

FollowupID: 730012

Reply By: Battery Value Pty Ltd - Thursday, Jun 09, 2011 at 09:31

Thursday, Jun 09, 2011 at 09:31
Hi David & Michelle,

yes, a load test would reveal the residual capacity of your battery.

And if it comes to replacing your battery, the following might be useful.
The cost-of-extracted Ah over the full life span of a typical deep cycle AGM battery can be derived from the manufacturer's data.

A battery is either at the end of its 'float' life (up to 10 years), or at the end of its 'cycle' life.
Cycle life depends on how deeply the battery is discharged before it gets re-charged to 100%.
Looking at a typical AGM 'deep cycle' design you get the following depth of discharge DOD[%] versus total extractable Ah versus cost of extracted Ah:
parameters: 100Ah capacity, $200 cost of battery, discharge during the night, full recharge during the day, not exceeding the 25A charging rate:


The cheapest Ah can be had at a nightly DOD of 14% because the float life becomes the limiting factor at shallower DOD.

And not much difference in the cost between 14 and 30% DOD. This 14~30Ah figure coincides with the typical nightly Ah consumption in a mobile setup with compressor fridge and some TV/lights AND coffee grinder, making the 90~125Ah deep cycle AGM battery so popular.

It's also interesting that the cost of every Ah extracted from the battery doesn't change much between 50 and 90% DOD.
And the cost in the higher region of DOD is only 1.5 times higher compared to the cheaper end, making a second battery in parallel appear uneconomical (2 times dearer).

Still, wiring another identical battery in parallel, gives one major benefit:
Twice the max allowable charging rate (50A instead of 25A), so that alternator bulk charging is now acceptable because it won't shorten battery life.

cheers, Peter
AnswerID: 456860

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