Longevity of <span class="highlight">AGM</span> <span class="highlight">batteries</span>

David

Derek should jump in here

but

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

Testing AGM batteries

Very well written and simple to do

Dru

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.

Regards

Derek from ABR.

David,

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.

Cheers,

Mark

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.

Cheers,
Peter
OKA196 Motorhome

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:

10%...36500Ah...0.55cent/Ah
14%...51100Ah...0.40cent/Ah
30%...41400Ah...0.48cent/Ah
50%...31000Ah...0.65cent/Ah
70%...34300Ah...0.58cent/Ah
90%...31500Ah...0.63cent/Ah

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