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Premium Member
1,257 Posts
Discussion Starter #1 (Edited)
Deep Cycle Absorbant Glass Mat (AGM) batteries have been around since the mid 1980's, originally developed in the United States for aircraft applications. Basically, these AGM type batteries are an advance on the traditional lead acid type, in which 95% of the sulphuric acid is suspended in microfibre glass matting sandwiched between the internal metal plates. This sandwich effectively reinforces the structure building in shock resistence and tipability, whilst supporting plate integrety during long drain periods (a chemical process known as Sulphation, which causes the plates to weaken and buckle). Further refinements have been made down the years, allowing thinner plates to be used, reducing internal resistance and resulting in higher power output for any given size.

For the first time, Starter Batteries are able to offer some of the advantages of the traditional Deep Cycle Gel and Deep Cycle Electrolyte type Leisure Batteries (which are not suitable for starting, or boosting) so some of the applications and sizes, of lower capacity, began to disappear from stockists' shelves. I'm talking about those 45 and 65 Amp hour sizes that are fitted under the front seats of the T3, that are 175mm tall...

AGM and Gel batteries are VRLA (Valve Regulated Lead Acid) batteries, where oxygen and hydrogen gases are trapped and recombined to form water, reducing fluid loss. The main difference between the two types is that a silicone polymer is added to form the gel in a Gel battery creating extra resistance within the cells, hence a leisure battery is unable to cope with extreme current loads (such as starting an engine).

... Prefering to keep as much additional weight as possible between the axles, I purchased and sited a Reimo battery base plate under the front passenger seat of my T4 with the intention of buying a small leisure battery. Perhaps foolishly, I then set about my quest for a suitable Gel battery to fit. No longer is it possible to order a Gel battery from a VW dealer (at least in the UK), the part number supercedes to a normal car battery - a weedy 36 Amp hour one, at that!. The prospect of having my living quarters filled with flamable sulphurous fumes (T3 batteries vented through the floor via a plastic tube) did not appeal, so I tired of my endeavours and abandoned that area of my 2001 camper build...

It is worth noting at this stage that some T4 convertors fitted the additional battery under the bonnet, next to the main one (turned through 90 degrees), and others concealed it within the living area, eating into valuable cupboard space.

... Half-hearted attempts to find a suitable battery (that would fit under the seat) persistently drew blanks but, in the meantime, I had been making do with the Van's main battery - the biggest size (017) that would fit - and a small solar charger, to ease it's pain. Two batteries, in seven years, later (one Bosch, one Motaquip), my search for another starter battery resumed.

A Deep Cycle starter battery sneaked onto the aftermarket in the autumn of 2005, yet they had been fitted to many ranges (including the T5) long before then. It was generally believed that these AGM batteries were for leisure and marine - and aviation, and motabilty, and motorbike - use only - but, hold on, how do you start a boat or aircraft? Companies such as Lifeline (the pioneers of this technology) have been supplying the American Marine and Recreational Vehicle industry with Deep Cycle AGM starter batteries for two decades!

High capacity AGM batteries can cope with:

  • plug-in Sat-Nav, DVD, games consoles, phone chargers (not to mention interior lighting, fridges and cool boxes, microwaves and kettles (!)- the real battery killers;
on top of in-built

  • electronically controlled active safety systems (antilock brakes and brake assist, stability control, active suspension, distance control and parking sensors)
  • electrically assisted steering, engine (and transmission) management and emission control
  • comfort packages (climate control, power windows, central locking)
  • security devices
Some 3,500 to 5,000 Watts, all at once!

Now, on offer are the sizes, post layouts and clamping arrangements, which happen to be compatible with those fitted to VW Group products. Thinking in tangents, as one generally does, this rocks my boat on so many levels (!) ...
  • Camper Vans, Day Vans and Surf Vans (obviously!)
  • Classic cars (modern classics, at least) that are laid up for weeks and months on end - like my GTI - also, bringing a whole new dimension of ICE to the scene? Finally, Lancia Delta Integrale owners have a credible solution to their battery woes...
  • Motorsport - even if resiting the battery in the boot (in a box) may not be a consideration, running two up / two down 100watt Cibies (as well as 100watt headlamps) - and interior navigation lights - may be?
  • Any car which currently has a standard type starter battery fitted under a seat or in the boot (Beetle, a number of BMW's, Mercedes, Volvo...and, urm, the T3)
  • Any car that has an over reliance on the vehicle electrical systems for basic safety functions - electric power steering, and basic information on the dash display, for instance (the small Citroen C3 springs to mind - I had a scary moment in one)
So, who can supply these wonder batteries?

Original equipment AGM batteries from:
  1. 100 size - Length 278mm, Width 175mm, Height 190mm
  2. 017 size - Length 353mm, Width 175mm, Height 190mm
as fitted to T5, Touran, Touareg and Pheaton and the Audi range, Bentley, BMW, Mercedes

Varta ULTRA dynamic range (guaranteed for 3 years)
  1. 570901 076 12v 70Ah - Cold starting current 760amps
  2. 595901 085 12v 95Ah - Cold starting current 850amps
Bosch High-Tec (Bosch S6) Range (guaranteed for 3 years)

  1. S6001 12V 70Ah
  2. S6002 12V 95Ah
Bosch claim that these big AGM batteries will replace any battery from 66Ah and up (provided it is not a Japanese style battery!) but please double check your battery space before you commit. Varta claims that their range is able to cope with 5,000Watts of power consumption.

At this stage, it is worth noting that the AGM battery manufacturers recommend that these batteries are vented as they still have the potential to gas - they do contain liquid acid after all, even though the majority of it is absorbed by the dense glass fibre material and they are fitted within the cabin or boot of many a luxury car...

Also, dropping below 60% of their charge capacity (i.e 7.2 Volts), too frequently, does compromise their serviceable life, but they are ideally suited to solar charging (some luxury cars have solar panels in their sunroofs!). A fridge switching off automatically at 10.5 Volts is one good indicator that the battery could do with a boost. A cosseted Deep Cycle battery should handle 300 charge cycles, three times that of a traditional battery, but who's counting?

Yet at 190mm height these offerings are too tall to sit under the T4's passenger seat - there is clearance only for a 175mm tall T3 style battery (unless you can handle your head banging the head lining - it's bad enough with swivel bases!). Certain squat seat bases, that cater for taller folk, obviously restrict this situation even more - but there are some narrower leisure batteries and smaller 20Ah motability (and motorcycle) AGM batteries that can be laid on their side. GEL batteries continue to be the favoured choice for powering up the appliances. However, I (and many T3 owners, it seems) am still on the hunt for a 175mm tall leisure battery.

Gel batteries are no longer available in this size, however, Varta offers a "dry" lead acid battery that can withstand at least 300 discharge cycles (to 60%, similar performance to the AGM car batteries)...

Varta DryMobile
  • 12V 40Ah Starting Current 0Amps. Length 210mm, Width 175mm, Height 175mm
These small lead acid leisure batteries are not suited for starting or for boosting a flat car battery, I conclude that there must be a sizeable market for a smaller (i.e, 175mm tall) high-power AGM type starter battery...

... Step up, Yuasa - famous for their motorbike batteries which have long deployed this advanced technology, who have the following applications:

Yuasa Elite range (guaranteed for 4 years)
  • 063T 12V 47Ah Cold starting current 425A - Length 207mm Width 175mm Height 175mm
  • 075T 12V 60Ah Cold starting current 590A - Length 243mm Width 175mm Height 175mm
  • 100T 12V 70Ah Cold starting current 680A - Length 278mm Width 175mm Height 190mm
  • 110T 12V 80Ah Cold starting current 720A - Length 317mm Width 175mm Height 190mm
  • 017T 12V 85Ah Cold starting current 850A - Length 354mm Width 175mm Height 190mm
At last I can enjoy my camper as I had originally planned. Of course, I will fit an AGM battery for starting; resigned to drilling yet another hole in my Van's floor, I will install for a completely independent 175mm high AGM battery (under the seat and charged by the solar panel) for the cabin lights, playstation, etc), which I can also use as a booster should I ever need it!

Why have we had to endure such a long wait for these smaller Deep-Cycle batteries?


Over the years there have been a number of high power conventional batteries:

  • Exide marketed a siamesed Starter/Leisure battery - two batteries in one.
  • Exide have now introduced a range of AGM batteries AGM700 and AGM 900
  • Unipart had the Samson range (made by Tungstone) - a range of high specification car batteries that were guaranteed for as long as you owned the vehicle (that the battery was intended for).
The latest products in this line incorporate calcium alloy plates and boast 30% more power than ordinary car batteries, from the following brands:
  • Lion (not to be confused with the Australian Lion brand of batteries or Lithium-Ion batteries)
  • Tungstone
  • Bosch S5
These are not AGM batteries, but may be worth considering as a less costly alternative starter battery...

For your nearest stockist contact


10,602 Posts
An excellent post Stephen, crammed with useful info.
The thing that I don,t understand though is the almost obsessive desire to have the battery under the seat (shared by many) or under the bonnet (also shared by many).
I don't understand why anyone would give a sock cupboard priority over what is essentially the life blood of your living quarters. If cupboard space is essential then utilise the underseat area as a cupboard! In fact this make more sense as fitting a battery under here wastes a lot of space. Fitting under the bonnet is also less than ideal due to environmental factors. Particularly in winter time.
The one consideration not mentioned in your excellent article is wiring and it's length. The shorter the cables from your battery the better as this minimises loss and maximises the performance of your accessories. Fitting the battery more central to your living quarters will achieve this. Personally I have two 110 agm batteries under my bed/seat in the centre of the van. Perfect for easy access, perfect for weight distribution and all my accessories can be wired with less than two metres of cable. Venting tubes run to the back of the van and out through the slam vents. No extra holes required.
If I were planning a conversion from scratch I would start around the location of my power source and make evrything else secondary.

255 Posts
Which AGM battery did you fit under the hood in the end? Which did you choose?

I have a separate leisure battery system connected to a solar panel. I'd like to be able to use the starter battery for a little extra, and keep an eye on the voltage.

Premium Member
1,257 Posts
Discussion Starter #4
I have fitted a Yuasa 017T battery - the biggest size that will fit under the pollen filter.

I have a Yuasa 043T to fit under the front passenger seat which will solely power the LED interior lights - both will be charged from the alternator whilst driving, and the interior battery will be charged via a Maplin solar panel when on site.
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