Battery drain f/ programmable tach

[AgentX]

Hi guys--

Would like to use a particular electronic tach which, alas, only comes in a programmable model. These need constant-on battery power to hold their settings, including critical stuff for me like number of pulses per rev.

I am using an antigravity 12v/4ah battery on my kickstart-only bike, plus a capacitor which is integrated with the reg/rect. I was averse to anything that might drain the battery, due to the sensitivity of the Antigravities to excessive discharge, but after some thought...this wouldn't be a big deal, would it? I'm guessing the drain would be next to nothing. Any input?

 

[Sonreir]

Usually something requiring an always-on feature will have a very minimal drain. Less than 100 mA is the norm and often as low as 20mA. This is something you can measure with a multimeter.

Assuming 100mA, that gets you about 10 hours per AhEq, or a bit less than two days. Best to keep it in a charger.

 

[AgentX]

Thanks for helping put that in concrete terms, Sonreir; I don't think that's gonna work for me, alas. Which sucks. All I want is a simple 3"-3.5" tach that'll work with my single. I was a dumbass and bought a nice clean VDO unit and mount but it will only read half my true revs.

Maybe I should look at an old Harley unit or something. If I had a spare $300, I'd get a Procycle (autometer) race gauge, but, umm...$300. VDO makes one that'll do the job but is only 2". yadda yadda.

There's a German company called MMB I'm considering; they make an 80mm unit which looks like it might fit the bill.

 

[teazer]

I realize this is going to sound off the wall nuts, but why not change teh ignition to wasted spark arrangement to fire every engine revolution. Tacho will instantly read twice as high. I have no idea what your ignition looks like to know if that's the dumbest idea I had all week, but one of them has to be.

Not a lot of snowmobiles where you are but some of them run off the AC and as long as yours generates 6 pulses per rpm, that would work too.

Check with teh manufacturer spec sheet to see the current draw of the one you have/like to be sure. Two days to flat doesn't sound right just to hold settings. A watch battery should be able to do that for 2 years without replacement - just to hold them in volatile memory can't take much more power than running a watch or your desktop PC.

 

[Sonreir]

It was conjecture. Without knowing the true current draw of the tachometer, we can only guess.

If it's 10mA instead of 100mA, then you get 16 days instead of two.

 

[AgentX]

I realize this is going to sound off the wall nuts, but why not change teh ignition to wasted spark arrangement to fire every engine revolution. Tacho will instantly read twice as high. I have no idea what your ignition looks like to know if that's the dumbest idea I had all week, but one of them has to be.

Not a lot of snowmobiles where you are but some of them run off the AC and as long as yours generates 6 pulses per rpm, that would work too.

Check with teh manufacturer spec sheet to see the current draw of the one you have/like to be sure. Two days to flat doesn't sound right just to hold settings. A watch battery should be able to do that for 2 years without replacement - just to hold them in volatile memory can't take much more power than running a watch or your desktop PC.

Alas it is already a wasted spark ignition.
Current tach has 4, 6, 8 cyl modes, none of which read correctly.

Programmable tach i was looking at defaults to .5 pulses/rev, which sucks.

Whatever the drain is, though, if it is measured in days or weeks rather than months, I am not ready to deal with that. I need a simpler setup. Plus I don't want to run a new wire.

Worst that happens is I use the smaller vdo unit in the end, which would be fine I am sure.