Battery information Trickle charger for my winter stored 914, Battery recommendation 1.25 Amp or 4. Amp better?? Options

[TJB/914]

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Looking for a battery trickle charger recommendation.
1.25 Amp or 4. Amp best for winter storage on my 914?
Price range?

Tom in cold Michigan.

[chmillman]

Thought this might be interesting to relate.

When I bought the car this summer, the battery was already 5-6 years old, it still seemed in fair condition, but I said what the heck and went ahead and got a new one anyway - 52 AH sealed lead-acid (original spec is 45 AH I think).

Later I installed a new audio head unit (Blaupunkt) and also a subwoofer (Panasonic). Both of these want a direct always-on +12V connection, plus a connection to +12V ignition on. In principle, they only then can be on when the ignition is on (internal solid-state switch), the always-on 12V connection keeps the internal clock and other stuff in memory in the head unit – I guess because they are too cheap to have a battery or capacitor protected memory in the thing. Not sure why the subwoofer needs this. The +12V always-on - which I assume is also the main power input - I wired directly from the input side of the fuse panel and through my own 15A fuse.

Anyway, recently I hadn’t driven the car for about three weeks and when I got in to start it, it turned over, but much slower than usual. Hmm, something is draining the battery more than normal...

So I got out my trusty original Fluke 77 (10 years younger the car) and measured the current flow between the battery negative terminal and ground with the ignition off. It read 60 milliamps. Doesn’t seem like a lot... Then I pulled the fuse on my line going to the audio components, and the current draw dropped to 8.5 milliamps. What’s pulling that? The original VDO clock. Disconnected the clock and the current draw dropped to 0. So the audio components alone are drawing around 52 mA – even when turned off.

Doing some math – let’s say one would not like the battery to go down below about 50% of full charge. That means a discharge of about 26 AH. If the discharge rate is 60 mA/h, it takes 433 hours to get down to 50% charge. Divide that by 24 and you get…18 days. So in a little over two weeks, half the battery charge is gone. A lead-acid battery also has a self-discharge rate – which varies depending on temperature, battery style and age – but let’s say 5% per month. In 2 weeks, that’s 2.5% of the 52 AH, about 1.3 AH, so relatively negligible, but still there. But in a month, the 60 mA pull plus the self-discharge will drain the battery completely.

So, I ended up putting in a relay so that when the ignition is off, the audio units get no power at all. Only the clock is running with its 8.5 mA pull. That will pull a fully-charged battery down to 50% in about 3 months (including a 15% self-discharge). So, yeah, a trickle charger is a good idea if you are not going to drive the car for awhile.