Fully charged indicator light or buzzer.

Thread Starter


Joined Dec 19, 2007
Two questions on a Homemade Weedeater/alternator/battery charger status or information display.

Assume a Weedeater engine driving a Std. 12v Automotive Alternator.
A single wire alternator that has its own regulator internal for charge control.

First question;
How am I to know when the battery is fully charged so I can shut off that noisy Weedeater engine?:mad:

A second question;
How am I to know when the battery is down far enough to start the Weedeater/charger?:confused:

This is in a small fishing boat with only an electric drop down trolling motor and one battery.:p Yup, I like to fish quietly and still be able to motor back home!


Joined Nov 6, 2005
Possibly an ammeter in line with the battery to show charge / discharge current, plus a voltmeter to show state of charge.

When you start charging, there will be significant current. As the battery reaches full charge, the current will drop to very low level.

When running the electric motor, the battery voltage will drop as the battery discharges. When it's down to around 11V it's time to recharge.

Both items should be available from car accessory shops, the voltmeter may be sold as a 'battery condition' gauge.

Thread Starter


Joined Dec 19, 2007
Well that's simple enough. I was expecting a circuit I'd have to build that had a two state led to show battery up- and below 11 volts.

The led's would be easier to see at a glance at night, but the meters would be better in the daylight. I guess I could put a light in the panel meters though.


Joined Jul 7, 2009
Lead-acid battery life is significantly impacted if you routinely discharge the battery below around 50% of a full charge.

I've attached a spreadsheet I made up about a decade ago to help me keep track of battery usage in our camping trailer. It's complicated by the fact that battery experts recommend letting the battery sit for an hour or two before measuring the voltage to estimate the charge state. I didn't record where I got the formula from, so I can't vouch for its pedigree.

Of course, if you're interested in using it, you'll need an accurate digital multimeter.

If it was me, I'd probably just use an ammeter and mentally estimate how many amp hours I had taken from the battery by roughly how long I'd been using it. And, out on a cold or rough lake, I wouldn't give a hoot how much battery life was left -- it just better last me enough to get me home after charging. :)


Thread Starter


Joined Dec 19, 2007
I guess I don't want the battery to discharge below 10.5-11 volts eh?

I don't know any details. Nothing about the Battery, or the draw on the battery from the motor or all the lights and the GPS and Fish Finder.
Somewhere back in time, I put a big wattage 1 ohm resister between the pos. terminal and the rest of the boat. Measure across that Resister, do a little math and you get the current usages. From that (if everything was stable) I could plot the time left.


Joined Jan 8, 2009
There are numerous circuits floating around based on LM3914 chip. You can pick one, modify it to add LED's and buzzers to the required level LED's and you will get what you need.


Joined Sep 2, 2006

I guess I don't want the battery to discharge below 10.5-11 volts eh?
You don't want it to discharge below 12.5V!

I don't know any details. Nothing about the Battery, [...]
Hopefully you know whether it's an SLA or a flooded battery?

Personally, I'd automate the process so that all the attention could be directed towards fishing.
Alternatively, you could make a couple of comparator circuits that just alerted you, by way of a buzzer, that action is needed (to turn charging on/off).

Your End-of-Charge voltage should be 13.8V to 14.4V (14.1V being a nice compromise with room for a little adjustment error and most batteries starts to gas at around 14.2V).

The EoC voltage is based on an ambient temperature of 25°C (77°F). compensate around -18mV/°C (-10mV/°F) - i.e. higher ambient temperatures equals lower voltage limits. Most important to avoid "boiling" the battery.
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Thread Starter


Joined Dec 19, 2007
OK, if I could get fancy, I'd have a circuit to cut off the Ignition switch.

If I had my way, I would have a starter motor clutched into the mix. When the battery voltage got low, the ignition would switch on, the starter would wind up and when the Rpm reached 1000 the starter relay would cut out.