So last week the battery went totally flat as someone who shall remain nameless forgot to plug in the cable to the CTEK charger. This battery is only 12 months old, has had very little use and always been on a CTEK charger when laid up. It went totally flat after only 2 weeks. In somewhat of a haste I ordered a new VARTA G14 017 019 AGM unit at a whopping £179 ($229) as this seems to be the only VARTA that is specific for my XKR (https://www.ebay.co.uk/itm/G14-Varta...72.m2749.l2649).

Anyway, much to my surprise the "dead" battery has risen Lazarus-like after 48 hours of gentle charging (I thought after going totally dead it would remain so) and it looks like there is still life in it although whether it is to be trusted is another matter.

So to my question. If I don't immediately fit the new battery how long should it remain good for assuming I periodically charge it?

 

Put it on the charger and it could last for years.
I would have it checked for load, simple to do, take it to the place you got your battery if you don't have one.
I keep three on the once in a while cars and charge them for a few hours once a week, when I remember. LOL

 

My understanding* is that the damage is not caused by a battery just going dead flat, but by being left dead flat for a period of time. If you caught it early enough then there's a good chance it will survive, as Wayne suggests.

The AGM batteries do seem to be more durable for the extra cost. I replaced the one dated 08/2012 in my wife's X5 last year (my misdiagnosis for bad starting). It still charges properly and holds for weeks.


*Disclaimer:- I only got as far as A-level chemistry.

 

^^ I concur that the AGM batteries seem to be more durable and hold up longer between drive cycles. I have had one (a Bosch) in my wife's XK8 since July 2018 and one (a Champion) in my Outback 3.6R since August 2019. Both of those vehicles can comfortably sit for two weeks and then start right up with no battery issues (although I try not to let any of our four vehicles sit for more than a week without being driven)....

 

I'd say check the ignition key flap to see if it sticks open and causes the battery to discharge. You should be able to leave your car alone for a few weeks and be able to restart it after that, assuming a healthy battery. The alarm system apparently has a deep sleep mode that kicks in after some time and disables listening for the fobs, all in the hope of saving the battery. There are several techniques to isolate these "parasitic drains" and get to what is causing the battery to drain so quickly. The other interpretation is that the old battery was nearing the end of its life and had to be replaced.

Separately, AGM batteries are a good choice in the sense that they do not require the acid level to be adjusted every so often (adding distilled water to that fill line to compensate for evaporation). They are sold as "maintenance free". They are also on the higher range of quality (and cost) so tend to last longer. I followed @Jon89 lead for a Bosch battery a few months back after my old battery just died, and some gremlins went away instantly.

For your old battery, even if it shows good signs of holding a charge, I'd say it is hard to trust it at this point. Here in the US, batteries usually come with a core charge that gets reimbursed as the old battery is traded in, so keeping an old battery around is less common. Check your paperwork for the new battery, maybe you can get some money back if you turn the old one in.

Best of luck, keep us posted.

 

So I went ahead and fitted the new AGM battery today. This had a resting voltage of 12.97 24 hours after fully charging (not connected to car). The 1 year old battery in contrast was 12.74 24 hours after fully charging (connected to car) so not dead or dying. Ill put a DVM across it after a week and fortnight and see what it gets down to.

 

The first battery in my car (I think - I bought the car used) lasted around 5 years and I replaced it in 2007. The second battery lasted 12 years and was still working but I didn't want to press my luck any further and replaced it in 2019. When the parts guy at Napa went to grab my old battery as a core he was a little astonished when he say the 2007 sticker.

My car sits during the winter (4-5 months) and always has a tender on it. The trunk is a better environment for battery instead of the engine compartment due to the cooler temps so batteries should last awhile if taken care of.

 

Worrasf,

That 12.74 reading on the old battery is pretty encouraging. I would base a decision about trusting it on how well it holds that charge over your test period, and not worry about the discharge event if the numbers look good.

 

Exactly my thoughts. Beginning to wish I'd not splurged all that moolah on the VARTA