Installed alternator cut off a few Questions.

[bbjsw10]

Well I installed an alternator cut off tonight. I want to know will it be safe to run my charger on the 2 amp trickle setting over night when I get home. I want to try running with it shut off as much as possible. I believe this is going to be a great fuel saver it is amazing how much an alternator draws down a lil Metro motor.

I also made a circuit board that monitors voltage and turns on led at 11.8v. I also installed a group 24 deep cycle battery. I just need to know how to Safely charge it now. And does running without alternator then turning on during same trip cause a major drop in FE because of added battery drain?

[bbjsw10]

Doax asked me about the monitor I made for this. Here is a schematic for this. Schematics Depot (tm) - 12v "high" or "low" battery monitor

[NiHaoMike]

Originally Posted by bbjsw10

Doax asked me about the monitor I made for this. Here is a schematic for this. Schematics Depot (tm) - 12v "high" or "low" battery monitor

That will not work. It is a Wheatstone bridge which means that one side will always be higher than the other or both sides will be equal regardless of supply voltage. But replace one resistor with a zener diode and it will work as intended.

[Daox]

This is the schematic that I found while surfing. I was wondering if I could replace the LM339 with an LM311 (sorry, I'm clueless as to what the differences are even after reading the datasheets) since I have some of them laying around from an old project.

Low Voltage Alarm

[Daox]

I think I might be able to answer your questions too. I don't think the 2 amp trickle charger will be enough, but it depends on how long its on the charger and how long you use the car. A car uses roughly 20-30 amps while running. We'll use 30 to be more conservative. Now, if you have a 30 minute drive to work, thats 1 hour per day of constant 30A use or 30 Ah. Now you're at home and you have to regain that 30Ah. At 2A per hour, you'd need to have it on the charger for 15 hours. Thats pushing it awful close. I don't know about you, but I don't have 15 hours a day that my car could be plugged in. If your trips are shorter you may be fine. Alternatively, you could also get an ammeter and see how much your car actually uses. Keep in mind headlight, defroster, radio usage is going to majorly effect it. I'd recommend a faster charger.

If the battery runs too low and you have to kick the alternator back in I'm sure its gonna take its toll. I would think that the alternator would be working as hard as possible to recharge the battery.

[NiHaoMike]

You can modify a cheap PC power supply into a charger good for about 15A or so.
13.8 V / 15 A from a PC Power Supply
Also include a backflow prevention diode, output voltage adjustment (tweak for about 14.4v at the battery) and a current limit circuit. If you like, use a circuit to automatically start a timer when the voltage rises above 14.3v and drop the output voltage to 13.8v after a few hours.

[MetroMPG]

I agree with Tim: a 2A charger probably isn't going to be enough, but then that's a guess, not knowing how long you're actually using the car each day.

If you're forced to switch the alternator back on after drawing the battery too low, your mileage will plummet. I made that mistake once, and went from being able to achieve something like 75 mpg (driving with load) to just 50ish mpg with the alternator struggling to recharge the flat battery and run the car.

If you ever do find yourself in that situation, disconnect the additional battery so you're only recharging one. (I'm assuming you added one in parallel, rather than replaced the regular starting batt.)

[MetroMPG]

Oh, one more thing... to replace 30 amp hours:

At 2A per hour, you'd need to have it on the charger for 15 hours.

Nominally, yes. Except that doesn't account for charging losses. I've read on the EVDL that "retail" grade 12v battery chargers aren't particularly efficient. At 70% charging efficiency (W.A.G.), you'd actually need more like 21.4 hours to recharge.

[Daox]

Originally Posted by MetroMPG

If you're forced to switch the alternator back on after drawing the battery too low, your mileage will plummet. I made that mistake once, and went from being able to achieve something like 75 mpg (driving with load) to just 50ish mpg with the alternator struggling to recharge the flat battery and run the car.

Well, being as his system is fancier than yours, he could charge for a little while and shut it back down. My question on this would be, is it bad to constantly run a battery at say 50% capacity and keep it around there?

[bbjsw10]

Originally Posted by NiHaoMike

That will not work. It is a Wheatstone bridge which means that one side will always be higher than the other or both sides will be equal regardless of supply voltage. But replace one resistor with a zener diode and it will work as intended.

I have been using that for almost 4 months now works like a charm, no diodes needed.

[MetroMPG]

Originally Posted by Daox

My question on this would be, is it bad to constantly run a battery at say 50% capacity and keep it around there?

Depends on how long you're talking about.

Lead acid batteries start to degrade through sulfation (causing lost capacity) when left partially charged. They shouldn't be left partially discharged for more than a day.