250SE Battery disconnect switch
I like to fit a battery disconnect switch to all my cars. It ensures the batteries don’t run down and prevents fires. Fires are a real risk from cars with old wiring. I had put one of those cheap on terminal switches on the 250SE when I got it a few years ago. Of late it had been playing up and was difficult to reconnect each time. I also wanted to close the bonnet on the car when connected to the trickle charger, but the alligator clips were too large.
Most of the battery disconnect switch models available online were quite ugly. They also required holes to be drilled to mount them. I wanted to come up with something that is:
- Completely reversible, with no holes or modifications to the car.
- Unobtrusive
- Provision for a quick connect for my battery charger
- Added security
I figured that I could create a housing for a switch that would sit next to the battery. Summit racing had one online that is operated by a key and could be mounted inside a small housing. I also bought a Ctek comfort indicator which is a charge port for the Ctek chargers that also incorporates a battery charge indicator. I didn’t want to cut the factory battery cable, so I crimped another. The plan was to connect the new short one from ground to the switch, then the factory one from switch to battery.
The first step was to drill the necessary holes in the housing. I purchased a housing normally used for computer parts. It is plastic, but it is not near the hot engine so should be ok. The housing came from ebay. I needed holes for the Ctek adaptor, the actual switch and the two battery cables.
The components needed to be fitted in order. First the switch, then the Ctek indicator then the battery cables. The ground cable for the Ctek indicator can connect to the battery side ground post. the positive side needs to connect to the battery.
The unit looked reasonably neat once it was fully assembled. 
I had originally planned to mount the unit black side out. However it worked much better the other way.
Overall I am pretty happy with how it turned out. It would have been better if I could have attached it to the side of the battery, but the mounting post was in the way. The good thing is that the solution is completely reversible. The key operation will provide a little added security, although a dedicated thief could bypass it with their own ground cable.
If I find this works well for me, I will build a similar battery disconnect switch for the DS.
4 Responses
[…] My conclusion is that if the battery is otherwise good, but has suffered a major discharge, this recondition mode can make a big difference. If the battery is otherwise on its way out, it really has no impact. Before I replace this battery I want to research AGM batteries to see if that will work better in this car. 2-3 years does not seem enough in my book. I also want to add a better disconnect switch and connection for the CTEK charger. This would be similar to the 250SE. […]
[…] At some point I plan some upgrades to the wiring around the battery. There are all sorts of auxilliary wires coming off the positive terminal. Some of them have in-line fuses, but not all of them. I plan to fit a bused fuse box with a single wire from the positive terminal to feed the headlight relays, radio and cooling fan. I also plan to add a relay for the driving lights. Basically that would mean a fuse for radio, fan, low beam, high beam and driving lights. The fuse box would then feed the existing relays that can’t be seen to the left of the battery. While I am there I will also fit a better disconnect and Ctek charger port like I did for the 250SE. […]
[…] was also an opportunity to fit an attachment for my Ctek battery charger. I didn’t do a full disconnect switch like on the 250SE but the attachment makes it easier to connect the charger. For some reason […]
[…] had recently noticed that the battery on the 250SE was not in good health. The battery disconnect switch I built a few years ago has a battery health indicator included. Even when connected to the […]