Hi all.


Our cars are often subject to spurious electronic fault messages and electric malfunctions. Some of the most typical are malfunctioning of automatic window lowering and false ABS messages.
These faults are very often caused by too low battery voltage. This again are caused by different factors some of they being:
- Infrequent driving.
- Short trips
- Start of cold soaked engines
- Use of high electric consumers (heated windows, power seats and cab, heated seats and ventilation fans)
-And last but not least: Too low charging voltage AT THE BATTERY.


Due to the long wire from the front alternator to the rear battery, the voltage will drop to a value too low to ever get your battery fully charged. Measuring at my own car this was 13,98 volts. The ideal would be 14,4 volts this again depending on temperature and battery type.
One solution would be a new regulator measuring the voltage and temperature AT THE BATTERY, but this would be complicated.


The easy solution is a modern high quality electronic battery charger. This can be wired to your battery via a quick disconnect plug placed in the trunk so you don't have to get at the battery.
Used now and then you will always have a fully charged battery and avoid lots of problems. It will also prolong your battery life.

 

Very few owners seem to check their battery cell water levels on a regular basis. Checking them at least quarterly (more often during the hot summer months) and topping them up as required goes a long way towards extending battery life. My 2005 S-Type 3.0 (built in May 2005 and purchased by me in December 2008 at 18,000 miles) still runs perfectly on its original factory battery. I attribute that to driving the car at least three or four times per week and keeping the battery cells topped up on a regular basis....

 

AGM batteries, although costly, can provide a lot of benefits as well. No topping off or losing electrolyte. Faster charging and deeper discharge without permanent reduction in capacity. Very little self discharge during storage. Coupled with a battery tender it would make a pretty robust system (assuming all connections are clean and tight).
I am suprised that the previous poster measured 13.8 at the battery, but that should be enough to charge a healthy battery on a long enough trip. What is the voltage at the alternator? Maybe there is some resistance in the circuit from bad connections. Also, was the 13.8 at idle, or 1,500 rpm?

 

Long ago and far away I maintained a number of VERY large lead/acid battery strings and charging systems and the 'float' charge was 12.6v and the 'charge' voltage was 13.6v. I have always assumed that that was the best match to the physics of lead/acid batteries. Automotive systems seem to charge at a little above that, probably because of the high current starting draw.

AGM batteries provide an additional benefit in that they have a substantially longer life. I just replaced the AGM battery in the wife's Miata and it was 18 years old.

 

18 years. That's remarkable.

Adding to the list ... AGMs almost never outgas, and when they do it's minimal compared to a flooded cell. When equipped with a vent tube, it's more than likely been put there by a lawyer, rather than an engineer.

If we want to get fancy, the charging numbers might be bumped by, say 0.1 volts compared to a flooded cell, but there's no real need. In practice, just a drop-in replacement. (The charging system in my MG -- generator and mechanical regulator -- keeps an AGM perfectly happy.)

Now: if they could just get the price down a bit more ...

 

I went to my local Jaguar dealer in Newcastle to ask about AGM batteries, I was told to use only lead acid type which the car was designed for. The cost of a replacement was about £170 + VAT. However if I bought there and then they would wave the VAT......I was also told tat the AGM battery was not designed for the XK8's Circa 1999 charging system. According to the dealer the AGM battery only takes a charge when it needs it, so is not compatible with the older style of continuous charge.
When I bought the car I was told the battery was new and it was quite expensive. However I could not find a vent pipe on it to connect to the old system. On examining the newer battery there seems to be a very small opening at each side level with where the cells are located, I would imagine that is where the breather hole is?
Just my humble ramblings,
Cheers,
ArtyH

 

If you don't mind my $0.02 (USD) ...

I believe what you're describing are vent ports (breathers). IAC, it is really important to vent a traditional battery when it's confined as it is in our cars. The OEM battery comes with a vent tube which exits down through the floor of the battery box.

It's not like the thing will explode without venting, but it could happen under some (rare) circumstances.

FWIW ...

 

B.S. The AGM Batteries work just fine. AGM Batteries are a Form of Lead Acid Battery.

AGM batteries differ from flooded lead acid batteries in that the electrolyte is held in the glass mats, as opposed to freely flooding the plates. Very thin glass fibers are woven into a mat to increase surface area enough to hold sufficient electrolyte on the cells for their lifetime. The fibers that compose the fine glass mat do not absorb nor are they affected by the acidic electrolyte. These mats are wrung out 2–5% after being soaked in acids, prior to manufacture completion and sealing

 

How interesting considering that AGM was the OEM battery type in all of the early Miata's and those have been on the market since the 1990 model. I cannot envision that the alternator functioned THAT much differently in a 1990 Miata than a 1999+ XK8...

 

Arty, dealer doesn't know what he's talking about....probably didn't have an AGM in stock to sell you at highly inflated price. Dennis07, a three-stage charger will go high-current 14.2v or so as a bulk charge on a battery run down to about 80% or more. Then, as the battery charges, it's ability to accept charge lessens. The charger drops the current and voltage down to 13.6 +/- to slowly get more charge into the cells. Once the current flow becomes much less, the charger drops into float mode at 12.6 or so to maintain the charge. With the system you describe, there is no danger of the batteries gassing and, if time is not a factor, is a 'safer' way to charge without maintenance worries about topping up the lost water. Boaters with large battery banks like the "bulk" charging stage so they can recharge quicker. By the way, quality voltage regulators and battery chargers on boats do measure temperature and voltage at the battery and compensate their output accordingly. As the OP stated, that would be optimum for our cars, but probably expensive overkill.
Rant over.

 

Mike,

Amen on your view of the dealer.

Charging regimens remain at least partly a black art. As a discussion topic ... it can be right up there with motor oils.

In practice, it seems we can get many years of service out of the (necessarily) imprecise charging that the car's electrical system does. And it's a good thing too, because that's the only care most batteries are going to have.

My reference to "0.1 volts or so" had to do with AGMs typically having a slightly higher rest voltage than a flooded cell at any given state-of-charge (% of full charge) and temperature. I forget why this is so.

 

Agreed, an AGM should work just fine.

Judging from the late model forums, dealers are seeing a lot of complaints with the batteries on the late model cars. In response, Jaguar has AGM batteries as standard now on some of the newest models. They even offer a Jaguar relabeled CTEK battery maintainer as a late model XK 'accessory' as evidence of the issues.

Since they have AGMs on some cars now, I am theorizing some dealers might have considered swapping them in under warranty on cars not so equipped. Jaguar had issued a bulletin not too long ago saying that this is not an acceptable practice. I suspect the reason behind this is more $ than technical issues.