AGM batteries - some information

Hi Craig

I'm not aware of any AGMs recommended for under-bonnet use, AGMs are not very tolerant of high temperatures.

The ones I have tested so far are all suitable for cyclic or standby use.

A fully charged AGM will have a terminal voltage of around 12V6 - it may read higher on your digital multimeter but put a small load on it for 2 minutes and it will drop to 12V6ish.

13V8 is the "float charge" voltage for lead acid batteries.

NB. Cheap digital multimeters ($10) are often inaccurate - I have one which reads 14V on a 12V battery! Even expensive ones should be calibrated every year.

Mike Harding

Hi Mike, interesting read and if you carryout similar tests on wet cell batteries, the results are about the same.

I’m with Sand Man on not taking batteries down to 9v.

You can and if you have even the smallest of loads, once the battery gets to around the 11v mark and this backs what you were saying about how little stored capacity there is bellow 11v, the battery will get to 9v in a very short time and ANY battery taken down that low, particularly while under load, will be irrevocably damaged.

When using a fridge with a 10.5 volt cut-out, this cut-out will most likely occur while the fridge is running so that once the cut-out occurs, the battery’s voltage will actually rise.

There is a complex calculation that gives you the battery’s capacity using the terminal voltage plus the load being applied to the battery and based on the size of the battery, ambient temperature and so on, that will give you a very accurate indication of the SoC of the battery and is too make trouble for the average user to both with ( including myself )

As you posted, keeping above 11v is the simplest way to go unless you have something that cuts out automatically for you.

As to AGMs under the bonnet, both Odyssey and Optima make AGM batteries specifically for this use, and I imagine there are others.

Cheers.

I'll try again. That method for posting an image works on other forums.


Pity this forum doesnt have a preview button!

Hi Col

I haven't yet done the calculations for above 11V5 but your graph would seem to, pretty much, agree with my results, ie. below 11V5 the battery only has about 16% of capacity remaining.

Mike Harding

I'm new to second batteries and fridges. I wanted to see how the voltage correlates to the capacity of the battery. The fridge has a cut out at 10.4V, and I wanted to know what state the battery would be in at that voltage.

If the graph is correct, there isnt much value in running the battery down any less than 11.5V. Like you said, very little capacity left, with the possibility of reducing the battery life.

Its good to see some decent tests done. I note that AGMs arent the best under the bonnet because of heat. What would you recommend once my second (an N70 cranking) batteries needs replacing.

Col

Col,

At 10.4V the state of the battery is dead flat and probably cactus.

A cutout of 11.6V will leave about 20% left in the battery and is as low as you would want to go.

I have buggered two batteries by leaving the fridge running without a low battery cutout in the circuit. I now have this cutout device. (11.6V)

10.4V is way too low and will provide no protection.

>At 10.4V the state of the battery is dead flat and probably cactus.

Not correct. It's quite possible to take lead acid's down to around 9V or so but it must not be done too often and it's best if a slow recharging process, following such a deep discharge, is adopted.

Mike Harding

Mike,

One was a wet cell lead acid and one was and AGM lead acid. (Orbital)
They both ended up buggered. Wouldn't hold a charge no matter how I charged them. Tried the multi-stage 15 amp charger, tried pulsing them. In short, they were rooted.

Don't know how you get down to 9 volts mate. 10.4 volts equals 0% state of charge, or 1.7 volts per cell.

A quick Google gives the following:

Site Link

1V67 per cell - if you check the full data sheet the manufacturer specifies the battery down to 9V6 or 1V60 per cell.

Mike Harding

Yeh, I understand what you are saying Mike, but 10.4 volts equals 0% static charge, so in my book going lower still means zero charge.

Sandman: I can only quote the manufacturers specification - if you think they are wrong... I suggest you take it up with them.

Mike Harding

Doug

These batteries are AGM batteries - the difference to the others is the the spiral construction.

PeterD

disco1942
Yeh ok ......what are you on about , you lost me or did I lose you , 1 I know the post was about AGM I see they have a problem as pointed out by Mike' and for the benefit of others who may not know the Optima brand exists I put a link up for them , Is that OK ,

Doug

Hi Doug

I was just pointing out that the Optimas have a similar characteristic as the ones were the subject of the original post. They are good batteries, just helping you to understand them.

PeterD

Peter
Yep thanks for the below, I sure do understand the batteries I have under the bonnet. I have had no problems since they were installed 15 months ago , bloody expensive too but the one that does all the work is the 2nd , it runs 2X55w rotating lights,2X75w wig wags , 2 CBs, 2 headlights, Car radio, Fridge, and at times the Codan , all day ,

Have already put my support in for you

PeterD

Here is a result from another forum:
Site Link

Neville

Hi Scubadoo, that link was also an interesting read but I think you will find half the info there is based on a specific brand of battery and is not all that relevant in a standard set-up.

The recommendation to use thinner cable is a dead give-away.

Some AGMs, contrary to common beliefs, do not like high charge currents. For example ( if I remember correctly ) Fullriver batteries should not be charged with a current greater than 20% of the batteries total capacity.

In other words, a 100 A/H battery should not be charged with an input current rate greater than 20 amps. So, without spending a fortune on some sort of current regulator, how do you limit the charge current coming from your alternator?

Another mislead bit of info, also touched on by mike, is that that states a given type of AGM can be charge with the full inrush current and not be damaged.

Now this is good to know as it means the battery should have no problems when used in an automotive set-up but many people take this as meaning that the battery will take everything the alternator can put out.

Not so, for this type of battery to absorb maximum current during charging, there has to be a few other requirements met, like constant and correct temperature and charge voltage. None of which are ever available while charging these batteries in a vehicle.

Most AGM at best only charge slightly quicker than ordinary batteries when being used in a vehicle.

The main advantages of AGMs over ordinary vehicle batteries is their longer expected operating life and that they can be left for much longer periods while not in use and the disadvantage is their price.

Cheers.

Hi D-Jack, there are AGMs that make for perfect deep cycle batteries but as Mike has pointed out, you need to check the manufacture’s web sites to get the correct info, not just relating to charging but also on the intended use of ANY AGM battery.

Unlike good old wet cell batteries, cranking or deep cycle, AGMs are very use specific and you should pick the type of AGM that is stated by it’s manufacture as being suitable for your particular needs.

Cheers.

There is no perfect batt for deep cycle work !!!

Why.....because no batt likes to be deep cycled....the shallower the cycle the better !!

For a remote batt ( back of 4x4 or CT etc ) an AGM is by far the easiest and best ( apart from cost ) option !!

I go camping for 4 days ( CT with 140ah batts, Waeco 60ltr fridge, lights etc ) come home and plug in the multistage charger for approx 1 day then forget about the batts till next camping trip !!!! No watering, no spillage, no gas, nothing to worry about !!

But I never let the voltage go below 11.8 when camping !! ( although this rule could get broken if the beer was at stake !!!!! )