987. Help! Disconnected battery under the bonnet. Sadly I disconnected the earth.

Hoping someone can help. I had (presumably) an electrical fault with the car parked in heavy rain in London. The car went haywire with the rear spoiler popping up, alarm going off and the hood tried to open itself breaking the strut that opens one side of the hood. Bad. Given it was pissing down with rain still, and the noise disrupting neighbours, I managed to get the hood closed and decided to disconnect the battery. I closed the hood. I had assumed that the hood release by the drivers door was a cable, not electric. I think I foolishly disconnected the earth. This means that there appears no way to open the bonnet, even with the fuse box trick. Can anyone shed some light on what to do next ?

Actually - on thinking about it, the fuse box trick should still work. Sadly too I keep my battery boost starter kit under the bonnet too. Will buy another and use the fuse box trick.

This scenario is a recurring theme, it’s one of my least liked features of these cars. I believe somebody was able to overcome this exact same scenario through the lighter port, try a search. In a worst case scenario, you can go through the fender liner and locate a cable release if memory serves correctly it’s in the left front.

If you don't have wheel-locks, or if you do and you have the key and it's not locked in the trunk - removing the left side wheel and wheel liner might be the quickest and easiest way to open the frunk. A battery- booster will ONLY work if it outputs voltage without seeing voltage. MANY of them don't - they look for at least some minimal voltage at the clamps before allowing the boost function to work and provide voltage. They don't want people starting fires or flash-welding the clamps together when not connected to a battery. It's a safety thing. Some of them with this "feature" do have a manual over-ride. My battery booster wouldn't work with the fusebox connection, but a motorcycle battery and a set of motorcycle sized jumpers did work.... those are now in my garage - the battery on a battery maintainer.

One of the reasons I'm looking for a deal on the NOCO GENIUS10 charger is that, in addition to being a very modern 10A charger, it can also be used as a 12VDC 10A power supply. So as long as you have the black lead connected to a good chassis ground, the fuse box method should work even when the starter battery is disconnected.

Thanks. I've ordered a jumper battery but hopefully it doesn't have the safety check you mention. I'm guessing you could fool it into thinking there is a battery by putting a 9V battery with a resistor in parallel. That way the jumper kit will think there is a voltage, but the resistor will stop it frying the battery.

The NOCO five has the same exact feature, I love mine. In the scenario we are discussing, a battery removed from a different vehicle brought close to the Porsche using plain leads should do the same thing as forcing a charger to send voltage.

Made me look, made me look! 😂😂

While 'Force Mode' might do the trick, I'm talking about '12V Supply Mode' in the Genius 10 manual (which seems missing on the 5). It's not clear if they will behave the same way in this scenario. It depends on how Force Mode is implemented and if it just blindly feeds 12V@5Amps or if there is any current limiting protection logic applied.

That feature is missing on the five, after all they have to have a reason besides power to entice you to buy the 10. However, unless I’m not understanding it correctly, it sure seems to me like it would accomplish the same thing. If I get bored this winter, I will hook it up to my meter and activate force mode to see if it works. The way it reads to me is after you choose force mode you choose any one of its available charge options to force. Force mode only stays in effect for a given time. Whereas the mode you are talking about, I believe continuously provides power until you tell it not to maybe that’s the difference.

Won't know without trying it. I'm thinking it just forces the underlying charge mode profile selected before you press+hold for 5 sec. But the current flow might initially be limited to too few amps to be useful for this purpose. It is trying to detect if a battery (dead) is there, but IDK what else it does.

I've decided I'm going to try the 12V battery jump starter kit across the red fuse box pin and earth of the car. I'll add a 9V battery in parallel too to simulate a flat battery. 12V across a 9V battery for a minute or so probably won't hurt anything too much (while 12V supplied to car) and a 9V is probably a good proxy for a flattish car battery. Wish me luck!

OK - I tried this. Sadly the jump start kit didn't work. I even tried a small applying a small amount of voltage between to simulate a dead battery. Oddly the battery jump pack is supplying 16V when no current is flowing. I struggle to understand why this doesn't work. Next step is I will try a real battery connected to the red pin tomorrow. I hope that works. Finding th cable under the wheel arch is a nightmare as I have a lock nut on each wheel and of course the key to those is also under the bonnet in the frunk. Really poor design IMHO. A much more user friendly design would have been a cable pull under the dash in the car.

The real battery works, at least on mine. I have a 30Ah motorcycle battery on constant maintenance charge, with a set of motorcycle sized jumper cables sitting in my garage ready for use. Since I've been prepared like this, and not too long ago replaced the battery - of course I've had no need to use them.

Some very good news. I am now into the bonnet/frunk. I tried 12V across the red pin and this didn't work. Next step was going to be a fuse port or the wheel arch to get the cable. I had an auto-electrition come and help onsite. This is remarkable. He went in with a screwdriver between the windshield and frunk to get power to the battery connector. Very impressive as in all of the posts on here and other sites, I've never heard of that solution. Worked instantly. Also good news is that the rear electrics are working again so will take the controller unit out and slow dry it and remove any water in the area, then dry the interior fully with hot AC. It may be recoverable! If that's the case, then after cleaning drainage pipes and fixing the hood lifters and I'll be back to good again. Thank you all for you help!

Don't slow dry the module, drying is when the corrosion happens. Use alcohol (70+% rubbing, or even "Dry Gas" Methanol.) If you can open the module use a q-tip to clean as much as you can. Next time: Open a Computer UPS and pull the battery from that. Or for $25 you can pick up a "Lighting Backup Battery" (Same thing) at Lowes or Home Depot. A couple of alligator clips and away you go.

Thanks. I actually immersed the board in rubbing alchol and will leave it in for a few hours. I lightly cleaned it while submerged with a soft brush bristles. By air dry, I mean just take it out of the rubbing alchol and allow it to dry for an hour of three at room temperature. No guarantee it will work, but given the cost of replacement, it's worth a try. I'm at that stage where tossing up insurance claim for repair of controller and hood, or doing it myself. One of the big problem is my insurer has already told me the car will likely be judged a write off and the book value is several thousand pounds below my replacement cost. Sure, I can haggle, but still not ideal. Also claiming will take away my no claims bonus. Interestingly sometimes when they write off a car you can buy your own car back as scrapped. It then has a special registration class and has higher premiums, but at least you get your car and some money back.

US Insurance usually requires they pay "replacement" costs.

Yes, I think the problem in the UK is that there are three agencies that provide lowball book values for cars. There is probably some horrid crony capitalist rubbing shoulders between insurers, politicians and insurance regulators . It's hard to explain it any other way, and it wouldn't be a first. All three tend to be significantly below the price you'd expect to pay buying on AutoTrader, even after haggling. For example my Boxster 's current replacement value would be £9-11k, however the book value is about £6.5k. When you look on AutoTrader, there is nothing at £6.5k or close, but about a dozen similar ones in the £10-14k range. Boxster 3.4L 2006 987 72500miles.

Just since it was discussed in this thread, Amazon Black Friday sale has the NOCO Genius 10 for $79.96 (normally $99.96).