# voltage dependent timed relay

#### sdowney717

Joined Jul 18, 2012
705
This would be for a DC setup such as on a boat to help manage a starting bank and a multi bank battery charger.
Right now the boat charger charges every bank all the time forever. But a starter bank only needs charging if the battery is down below a voltage threshold of say 12.45v. Idea would be detect that battery voltage, then close the relay to the charger output for the starter battery bank if it falls as that shows the battery needs charging, have it run for a period of selectable time, then turn off and remonitor the starter battery.

does such a device already exist? I think I have burned up the starter battery as the charger just slowly cooks off the water, and the battery was sealed so cant add any water. Your car does not have to be plugged into a charger all the time, so why do that for the boat starter battery, it just gets it used up. Of course battery sellers want you to buy a new battery every couple of years... This idea here is an opportunity, I dont think anyone has such a thing yet, likely it would sell. You dont keep your car plugged in all the time on a charger, if you have to do that, the battery is bad.

Joined Jan 15, 2015
6,583
A good charging system should manage the battery charge rate of a sealed lead acid battery. The battery should not get cooked at all. Yes, you can monitor a battery voltage and do whatever you want when battery volts goes below or above a set point but part of the problem is on a 12 volt system a normal 12 volt SLA Battery when fully charged is about 12.6 to 12.8 volts and is typically charged at anything above 2.15 volts per cell, normally about 13 volts or a little above. Anyway if the system is cooking batteries something is wrong. Once a 12 volt battery reaches charge the system should only "maintain" the battery and not cook it.

A simple Google of Voltage Monitor Relay should give you plenty of hits as to what you seem to be asking for. With that it will need a large slave relay to connect or disconnect your battery bank. They run from cheap off the boat from China to about $100 USD for an Eaton version. So yes, you can get an off the shelf turn key solution. Ron Thread Starter #### sdowney717 Joined Jul 18, 2012 705 A good charging system should manage the battery charge rate of a sealed lead acid battery. The battery should not get cooked at all. Yes, you can monitor a battery voltage and do whatever you want when battery volts goes below or above a set point but part of the problem is on a 12 volt system a normal 12 volt SLA Battery when fully charged is about 12.6 to 12.8 volts and is typically charged at anything above 2.15 volts per cell, normally about 13 volts or a little above. Anyway if the system is cooking batteries something is wrong. Once a 12 volt battery reaches charge the system should only "maintain" the battery and not cook it. A simple Google of Voltage Monitor Relay should give you plenty of hits as to what you seem to be asking for. With that it will need a large slave relay to connect or disconnect your battery bank. They run from cheap off the boat from China to about$100 USD for an Eaton version. So yes, you can get an off the shelf turn key solution.

Ron
I dont think you understand the issue here. It has to do with charging unnecessarily, the chargers never turn off their outputs.
The status quo, the way it has always been done, favors quicker battery destruction. I looked for voltage relays, seems all I can find are for AC use, and the few DC ones are all the ACR type that turn off below a certain voltage not on.

Also a high amp disconnect relay is not needed, the only amps to be disconnected is the charger wire from the charger, your not disconnecting the bank itself from the boat, no need to interrupt starter sized currents..

#### LowQCab

Joined Nov 6, 2012
2,037
I suspect that You are experiencing some other type of problem.
Stop guessing, it will cost You a lot of Money in the long-run.

Test your Charger's Battery Voltage after it has been powered for at least 2-Days.
If the Battery Voltage is ~13.6 to ~13.7-Volts, it is operating perfectly,

Lead-Acid-Batteries will start to develop Plate-Sufation,
which will ruin the Battery,
if the Battery is allowed to sit for long periods at less than ~13.5-Volts.

To actually do any damage to the Battery by Over-Charging,
the Charging-Voltage must be maintained over ~14-Volts for weeks on end.

If You want to design and build a useful Circuit for something ........
There is a Voltage Regulator specifically designed for Lead-Acid Battery-Maintenance .......
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Joined Jan 15, 2015
6,583
I dont think you understand the issue here. It has to do with charging unnecessarily, the chargers never turn off their outputs.
No I do understand and don't believe a work around is a solution. My thinking is something is not right with the charging system. A charging system when working properly will not overcharge or cook a battery. My last boat was decades ago and was a '63 Chris Craft Cavalier express with twin engines and twin batteries in addition to a larger battery deep discharge type for lights and cabin stuff. As mentioned above a charging system running at 14 volts will not cook a battery. I suggest you look at your charging syatem voltage at your battery terminals.

Yes, if you want you can get a voltage monitor which will take your batteries offline if that is what you really want but that becomes a work around. When your system was designed at build time it should have been designed so no work around was needed. If you want a relay type voltage monitor you can buy a turn key off the shelf solution already built, I mentioned that.

Ron

#### sdowney717

Joined Jul 18, 2012
705
The Victron 3 bank 2- amp charger drops the final volts to 13.2 on the 4th stage, less likely to cook batteries. I may end up replacing it.

Joined Jan 15, 2015
6,583
That should work. During the NE Ohio winters I leave my bike in the garage with a 1 Amp battery tender. It keeps the battery fully charged and every now and then I start the bike and let it run a little.

Ron

#### LowQCab

Joined Nov 6, 2012
2,037

This is what all the Battery Manufacturers that I am aware of recommend.

The Voltages are important though,
13.7-Volts is the perfect average "Float-Charge", or "Maintenance-Charge",
that ideal will only vary by approximately +/- 0.1-Volts,
over a ~100F Battery-Core-Temperature Range.
( Slightly higher Voltage is needed when very cold, and vise-versa).

Gel-Cell-Batteries need a few tenths more Voltage than Wet-Cell-Batteries.
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