We had a 928 battery explode here in Houston near me a few years ago in a beautiful low miles '91. The car hasn't run right since, so we're thinking the computer might be damaged, but haven't swapped it out with one of mine yet.

 

Oh, god. I am just thankful then that nothing happened with mine. She seems to idle fine.

I tested the old charger. I measured above 14V with a multimeter straight at the connection. I tried one of mine (a new one) and that only puts out 1.6V. Maybe it really was just the charger.

Anyway, people, look at your chargers and your battery voltage!

 

FYI, the cen-tech trickle chargers are Harbor Freight's old, cheap line (~$10 for the little "float" charger), and this is not the first time I've heard of them cooking a battery.

I hadn't heard of one exploding, but I had heard of them boiling cells dry.

I can give good reviews for the Battery Tender brand float chargers.
I just sold a motorcycle after not riding for the last 2.5 years, and the battery was a few years older yet. The tender had been plugged in the whole time, and the bike fired up with no issues starting.

 

Yep, that's the one!

 

My experience is that if you need a trickle charger you have a drain somewhere in the system. Find it and fix it. I have owned many “collector” cars such as my 928 , and belonged to the associated forums. Many owners seem to use trickle chargers on their cars that get driven infrequently. I have never had the need as long as I start and run the car at least once a month. Trickle or battery tenders seem like a band-aid to me. Long way for me to throw in my opinion that you should not add a new charger to the mix. See if you need one first, and if so, try to track down the drain and repair.

 

Get a C-Tek or Battery Tender brand charger. They're 4-8x the cost of the one you posted, and now you know why. I have a C-Tek on one 997 but everything else I maintenance charge is on Battery Tender brand chargers - 2 dirt bikes, 2 batteries in a wake boat, 2 928s, another 997, a 914, a Cayenne Turbo, and a backup generator with electric start.

Never had an issue with any of them, and my AGM batteries last a ridiculously long time (6-12 years). All of them are connected directly to the battery with the quick connect plugs, with the exception of the Cayenne that's connected to the under hood jump posts.

 

That can vary quite a bit depending on the car. One month is probably good for a 928, but much longer there is enough drain from the electronics on board (computers, alarm etc..) to bring a battery close to a critical level. The early cars the analog clock will drain a battery in a couple of months (winter storage).
Either way it's really good practice to keep batteries in tip-top shape and that's done with a maintainer. Even my daily drivers go on a maintainer if they are going to sit over a long weekend. I typically get 10-12+ years out of a battery.

The more modern the car, the more a maintainer is needed between drives. Some cars with all systems working as intended, can drain a battery in as little as two weeks.

 

Totally agree with hacker on this. Just because a battery doesn't fail to start the car after a couple weeks doesn't mean it isn't being havily discharged between those start cycles, which will over time limit it's charge carrying capacity. At some point, the plates will only take a surface charge which makes it look full but as soon as it has a load on it, there's no power to supply.

 

I concur that the battery tender was over charging the battery.
I've switched my collection to this one, note the 25watt also works well.
I buy the ones w/ the display so I can see exactly how many amps it's supplying and they also are a great battery testing tool.

The old "dumb" battery tenders, many are floaters as mentioned above give too many amps when the battery doesn't need it, had one damage a battery in my '85 (blue 928) resulting in me having to use baking soda to neutralize some acid and respray part of the battery box lid (it was an old "Battery Tender" brand, like someone said the HF ones are total junk and have fried more than one ATV battey's before I got wise.

I now buy the AGM batteries as less chance of acid damage (no out gassing too) and use these smart tenders, via the jump post under the hood.

https://www.batterysaver.com/store/p78/2365-lcd.html

 

Alright, I have one more likely big thing to add here: This was an Interstate vented-style battery that was not vented! At this point, I think what happened, was that gasses accumulated outside the battery, there was a spark during cranking, which resulted in the battery igniting also on the inside. If you install one of these vented style batteries, make sure to vent them, otherwise, you are making a little bomb in there.

 

A CTEK trickle charger was used by the Houston owner that had the explosion.

 

At this point, I am really sure the lack of venting (together with the happy charger) were the culprits. Vent these batteries, people. A $3 hose could save you from big issues!

 

It doesn't help that on both my 86.5's I was never sure exactly where to vent the battery vent tube. I could only see three options and none seemed "correct":

1. a slit-like hole which is too small for the tube,
2. one of the two holes under the battery which I don't know how I could use without potentially pinching the vent tube, and
3. the rubber grommet where the battery wires exit the compartment - this is what I end up using but inevitably manage to rip the rubber boot part when doing so.

Where do folks vent their battery vent tubes?

 

Worse case scenario, you could always drill a hole. Just watch what's on the other side. I will see what I can do. Just ordered the venting tubes and attachment.

 

Exactly, there is no "hole" provided for the vent tube, you would need to drill one and paint the bare metal.
That is why I switched over to the AGM, however I've been running the standard "Wet" batteries for over 20 years in my 928's w/o venting issues or explosions, only acid damage from over charging from the Floater type "maintainers".
As stated the "dumb," floater type are just an on/off and do not provide a metered current supply, if they state 1.2 amps per say, that is what you get each time they turn on 1.2 amps whether needed or not, boiling off the water, and over heating the battery.