AGM Batt under the bonnet,good or bad

Submitted: Saturday, Feb 04, 2012 at 16:26
ThreadID: 91655 Views:4504 Replies:13 FollowUps:3
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As some of you now i was in the hunt for a Aux Batt and ended up getting a AGM Powersonic which i needed in a hurry as we were leaving the next day to spend a couple of days in Mildura,now whole there the BIL had to get a starting Batt for he's Cruiser so we went out to the TJM store and whole there i told the guys i had just fitted this Batt under the bonnet and he went on to say that it won't last 12 mths because of the heat factor,so have i done my self in here and brought the wrong Batt or not,Batt Experts need to reply to put my mind a rest.cheer's GREENDOG
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Reply By: Jeff D - Saturday, Feb 04, 2012 at 16:35

Saturday, Feb 04, 2012 at 16:35
I'm not a battery expert by any stretch of the imagination, but speaking from experience I would have to say that an AGM under the bonnet is not a good choice.

I made the same mistake 4 years ago and only just got 12 months out the battery. I replaced it with a 2nd starting battery as an auxilliary and it's still going strong.

Jeff D
AnswerID: 476863

Reply By: Member - Captain (WA) - Saturday, Feb 04, 2012 at 17:02

Saturday, Feb 04, 2012 at 17:02
While an AGM is a good battery, it is sensitive to heat. While some have managed to get a decent life from an underbonnet AGM, it can have as much to do with location. A 3.0TD GU has its aux battery next to the turbo, a poor choice. An 80/100/200 landcruiser has it behind the headlight and more in the airflow to help keep temps down and more chance to get a better life.

Typically its the loss of electrolyte that causes the issues. An AGM (VRLA) battery cannot have any electrolyte added if it is released, unlike a wetcell battery that can easily be topped up with distilled water. But if the wetcell battery is not topped up when required, then it too will have a short life.

I run AGM's in my camper, where temps are not an issue, as I believe they are a much better design of battery. But under the bonnet I run wetcells as I can easily replace the eletrolyte by simply topping up with distilled water whenever required.

It is possible to achieve an acceptable battery life from an AGM under the bonnet, but unfortunately there are many who have not been able to do so.


AnswerID: 476866

Reply By: olcoolone - Saturday, Feb 04, 2012 at 18:07

Saturday, Feb 04, 2012 at 18:07
In theory not a good idea.... in real life yes you can.

Under bonnet temps are not as high as you may thing, try 45 to 60 degC.

Most batteries under bonnet will be around the 50 degC.... to overcome some issues you can insulate the battery and duct cool air to it.

We have had all types of batteries fail within 18 months and we have had all types of batteries last for years.

The biggest thing is correct charge rates for the battery type and conditions.

Is you can run aux batteries outside the engine bay...... in our 200 series we have 3 x 120 amp agm mounted in the back.
AnswerID: 476871

Follow Up By: olcoolone - Saturday, Feb 04, 2012 at 18:09

Saturday, Feb 04, 2012 at 18:09
And about Powersonic..... they are good quality and what we use and sell.
FollowupID: 752016

Reply By: Member - Rob D (NSW) - Saturday, Feb 04, 2012 at 19:03

Saturday, Feb 04, 2012 at 19:03
I have had an AGM in the engine bay of my 100 series LC for over 4 years (over 100,000 km) and have not had an ounce of trouble. It is located back near the firewall on the passengers side. It is a Deep River brand as far as I remember.

I will be replacing it with another AGM battery when it fails.
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AnswerID: 476874

Reply By: Member - Graham N (SA) - Saturday, Feb 04, 2012 at 20:18

Saturday, Feb 04, 2012 at 20:18
I've had two consecutive AGM batteries under the bonnet as second batteries both failed after twelve months replaced the last one with a deep Cycle battery (slow learner) six months ago, now the alternator has gone cactus (was it old age, keeping up with the AGM's or some other cause) . The sellers of the batteries now don't recommend them under the bonnet.


AnswerID: 476885

Reply By: Member - Des Lexic - Saturday, Feb 04, 2012 at 21:22

Saturday, Feb 04, 2012 at 21:22
I have had my AGM mounted under the bonnet just behind the RH head light and it's been there 6 years now and still going strong.
AnswerID: 476899

Reply By: Member - Phil G (SA) - Sunday, Feb 05, 2012 at 00:17

Sunday, Feb 05, 2012 at 00:17
Have a look at the spec sheet for your battery:
Powersonic Spec sheet

Under the charging section, you will notice that the initial current on the 92Ah battery cannot be more than 25 amps. And the charging voltages are at 20 degrees C. If charging at higher temps, it will want lower voltages.

So, you need a way of limiting current to an AGM battery if you want a normal life. In many installations the suboptimal wiring does this nicely!
AnswerID: 476913

Reply By: The Bantam - Sunday, Feb 05, 2012 at 01:42

Sunday, Feb 05, 2012 at 01:42
Any battery will last longer and perform better in a cooler place.

The previouys poster has rasied an issue that many fail to grasp or simply ignore......many deep cycle AGM batteries have a very limited maximum initial charge the order of 20 ish amps....this alone will account for premature death of many batteries, add an elivated operating temperature and the tolerance to overcharge will be much worse.

In addition lots of people install deep cycle batteries and then expect to charge them at a high charge rate.........almost every deep cycle battery will neither tolerate high dischage rates nor high charge rates.

There are also some specific heat related issues that effect som AGM batteries, such as thermal runaway....

Personally I would be running a flooded battery in vehicles, because I am not convinced of the benifits of either AGM or GELL in these situations.

Personaly I use a marine battery in both cranking and aux positions...and that would be half the price of AGM.

I like the Supercharge, Seamaster is a sealed maintenence free battery that has a a pretty solid following......what A lot of people don't know is that once out of waranty, the top sticker can be removed and the vent valves can be unscrewed and the battery can be topped up like a normal screw top battery...ya just have to be carefull not to over fill.

the going rate for a Seamaster Gold in N70 size is arround $200.

no commercial connection.

AnswerID: 476916

Follow Up By: The Bantam - Sunday, Feb 05, 2012 at 01:49

Sunday, Feb 05, 2012 at 01:49
Oh BTW.... 50C is quite hot for a battery....look at the spec sheets for a few sealed batteries and 50C will be outside the recomended operating or storage temperature range.

If you look at where most vehicles have the original battery fitted, it will be at the front of the engine bay. mostly behind the headlight opening.

Unfortunately, many second batteries are fitted furrther back in the engine bay where it is far less cool.

FollowupID: 752060

Reply By: joeblogs - Sunday, Feb 05, 2012 at 08:32

Sunday, Feb 05, 2012 at 08:32

I have had an AGM ( Fullriver )battery in my GU patrol for the last 6 years and it is still going strong.
I did add some heat shielding foam around the battery when initially installed and I will put another AGM battery in when it comes to replacing the original
AnswerID: 476939

Reply By: Peter_n_Margaret - Sunday, Feb 05, 2012 at 08:34

Sunday, Feb 05, 2012 at 08:34
I am using a Fullriver 120Ah HGL AGM as a crank battery in the OKA.
It has been there over 2 years now and looks like giving better performance than anything I have used previously.
Not an ''under bonnet'' spot, of course.

OKA196 Motorhome
AnswerID: 476940

Reply By: Member - Rob D (NSW) - Sunday, Feb 05, 2012 at 10:12

Sunday, Feb 05, 2012 at 10:12
Lifeline Batteries have a Mil Spec 95 AH battery with an operating temperature range from -45C to +65.5C ($446 at Batteries Direct).

Most of the Lifeline batteries have a +50C maximum operating temperature. These come with a 5 year warranty.

Lifeline are one of the most expensive batteries, but if you look at the specifications of plate thickness, construction etc, you are getting what you pay for.

I have no connection with any supplier.

If you relax at a faster pace you can get more relaxation in for a given time.
Regards Rob

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AnswerID: 476954

Reply By: GREENDOG ! - Sunday, Feb 05, 2012 at 17:16

Sunday, Feb 05, 2012 at 17:16
Thanks every one for all your replys,just hought i'd ask he question,well just after se how we go with it.cheer's GD
AnswerID: 477015

Follow Up By: Member - Grant- Sunday, Feb 05, 2012 at 18:14

Sunday, Feb 05, 2012 at 18:14
Hello Greendog.
I made some enquiries and did some research regarding auxillary batteries. Century have an AGM and don't recommend placing it under the bonnet. I researched others and most companies who I contacted don't recommend it. I didn't contact panasonic however.
I would have bought an Optima AGM if I could fit it under the bonnet, but don't have room. I contacted Optima and they stated because of the way they are made, (different to other AGM batteries) they can cope with the extra heat.
I had no choice but to get another wet cell battery, and have put the same brand in the trailer to assist the battery management system and alternator.
I would look at making a heat shield and you may be pleasantly surprised.
FollowupID: 752191

Reply By: Nomadic Navara - Tuesday, Feb 07, 2012 at 22:43

Tuesday, Feb 07, 2012 at 22:43
Most vehicles these days have the starting battery in a compartment that is insulated a bit from the main engine compartment. The batteries run much cooler in that compartment.

A lot of vehicles these days have Ca-Ca (calcium-calcium) cranking batteries. The charging system for these batteries is set to a much higher voltage than for other batteries. If you replace a Ca-Ca battery with any other type you will boil it dry. What type of battery is fitted as the original battery in a Cruiser? If the original battery was a Ca-Ca battery then get your AGM battery out of there before you do any more damage to it.
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AnswerID: 477254

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