voltage cutoff for my car radio/battery

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kheston
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voltage cutoff for my car radio/battery

Post by kheston » Fri Oct 08, 2004 8:33 pm

I have an old VW that often sits for weeks without being driven. The clock/memory 12V radio feed (yellow wire) seems to be a bit of a hog. When I disconnect it (pull out the fuse), I can start the car after leaving it sitting for a month or two. However, when I leave it connected, I'm lucky if it starts after a week of sitting.<p>I've considered just hooking the yellow wire to my accessories ciruit so it goes off with the ignition, but this would become annoying after driving the car (when I do) for a couple of days in a row. I've also considered just installing an in-line switch for easy disconnection, but where's the fun in that?<p>What I'd really like, is a circuit that would maintain power to the clock/memory for a while, but depending upon battery voltage, would cut power before the battery got too weak to crank the motor. The other power wire (red) would be available to trigger the circuit to re-engage when I turned the key on. Possible?
Kurt - SF Bay

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Re: voltage cutoff for my car radio/battery

Post by Mike » Sat Oct 09, 2004 9:28 am

Sounds like a bad radio.<p>You could always put about $50 into a new radio

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Re: voltage cutoff for my car radio/battery

Post by k7elp60 » Sat Oct 09, 2004 12:11 pm

The older clocks in the cars were mechanical and used set of switch contacts to energize a solenoid to wind the spring. I have heard of cases where the contacts would stick and cause excessive current draw.
It is also possible that there is a problem in the charging system and the battery is not getting fully charged, or it is possible the battery has lost a good share of it's capacity.<p>It is possible to build a voltage sensitive switch with a comparator, that will turn of the radio feed wire when the battery voltage drops to certain point. The cutoff point may be difficult to find even thou the amount of remaining battery capacity is directly related to the battery open circuit voltage, the amount of charge required to start the engine may be hard to determine.

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kheston
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Re: voltage cutoff for my car radio/battery

Post by kheston » Wed Oct 13, 2004 8:34 pm

With some experimentation, I will be able to figure out the minimum voltage required to start the car.<p>The radio is a late model Sony, the battery relatively new, and the alternator is charging it just fine. Can anyone propose a cutoff circuit to get me started?
Kurt - SF Bay

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haklesup
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Re: voltage cutoff for my car radio/battery

Post by haklesup » Thu Oct 14, 2004 1:27 pm

Just because the battrery is not that old, does not mean that it is still good. Just a few good deep discharge events can reduce the capacity of a car battery while still having enough charge to start when recently charged.<p>Use an ammeter to measure the current into the yellow wire while the car is off. You can also repeat this measurement with the pos or neg battery lead which will tell you total load in case you are missing something else. One car I looked at had a fog lamp relay but the lamps were gone and the owner left the switch on which caused the relay to be energized continuously<p>Loads of only a few mA should be tolerated by the battery for a very long time unless the battery capacity is diminished by being allowed to remain dead too long (especially in cold weather).<p>If you have a load of more than 100mA then you should locate the source (or confirm it is indeed the battery backup, yellow wire)<p>A low voltage cutoff circuit could work but a poorly designed one (using a low resistance coil in a relay for example)could end up being an even bigger drain.<p>Frankly, if you are not going to be using the car that often but still want the radio memory to be good when you are using it, A toggle switch on the yellow wire would be the simplest solution. Just leave it on if you plan to use the car soon and turn it off if you are storing it for a while.<p>IN any case, comparing the actual drain to the rated capacity of your battery should yeild an estimate of how long it should last. If it is much shorter, the battery is probably borderline dead. If you store it often, consider a battery cutoff switch or a trickle charger (permanantly installed under the hood)<p>Take it ot a service station and have them do a Battery Load test to get a feel for how good it is.<p>[ October 15, 2004: Message edited by: haklesup ]</p>

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Re: voltage cutoff for my car radio/battery

Post by jollyrgr » Sat Oct 16, 2004 1:01 pm

Not only is it possible to do what you requested it has already been done for you. There are battery cut off circuits made to do exactly what you want. In fact there are (or were) batteries with an automatic cut-off circuit built into them. DieHard (Sears) had batteries with this feature but I have not seen them advertized for some time. Battery cut-off devices seem to be very popular with RVs. You might need to see an RV store or at least that section of the automoive stores to find this device. <p>Some of the older battery cut-off devices used a circuit box to control a relay that would disconnect the battery once the power failed. A button mounted on the dash or in the glove box would reset the circuit and allow you to start the car. In other words you had a circuit box that went under the dash, a push button switch that mounted somewhere inside the car, and a relay under the hood near the battery. Now everything appears to be in one neat little package.<p>First, make sure the battery is not bad. As others have said a battery should be able to handle a load of a few mA for quite some time. If an automotive battery has been taken to a completely dead state consider it trashed. Any car battery five or more years old is one that is living on borrowed time no matter what the warranties say.<p>Now to what you want to do; cut the battery off at a certain voltage drop. There are devices that do this already. There are several different styles but all work the same more or less. One is called the "Battery Brain". It is about the size of a large relay and has a companion remote for the keychain. You connect this device in series with the positive lead of your car battery. If your lights are left on or something drains the battery, such as the radio in your case, the device disconnects the battery voltage to the car. When you discover this you simply hit a button on the remote to reconnect the battery. Forget the remote or have it on another keychain? No problem. Lift the hood and press a button on the Battery Brain. Here is the website to the battery brain:<p>http://www.batterybrain.com/<p>This is just one example of a number of these devices. There is a slightly more advanced version of this device called PRIORITY START. Instead of needing a remote control or going under the hood it senses a change in the load and automatically resets itself. For instance the battery has been drained by the radio at a constant 100mA for ten days and the Priority Start has disconnected the battery. When you open the door and the dome light goes on the Priority Start senses a change in the load and automatically resets. If the dome light was not enough to reset the device, hitting the brakes or turning the key forward should reset it and allow you to start your car. Here is the Priority Start website:
http://www.prioritystart.com/<p>Numerous web sites sell both the Battery Brain and the Priority Start. Of course there are always the local vendors and eBay.
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k7elp60
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Re: voltage cutoff for my car radio/battery

Post by k7elp60 » Sat Oct 16, 2004 5:15 pm

Kurt,
If you want to build your own circuit, I be happy to share a circuit I developed several years ago that monitors the 12V battery voltage and disconnects the load when the battery voltage drops to the cutoff point. This point is adjustable with a trimpot. The limitation is that the load that is shut off can not exceed about 15 amps.
Let me know.
Ned

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kheston
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Re: voltage cutoff for my car radio/battery

Post by kheston » Sun Oct 17, 2004 8:13 am

I plan to place the circuit in line with the radio only. It won't draw anywhere near 15 amps.<p>It'd be great if you sent me your circuit design.<p>Thanks!
Kurt - SF Bay

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Re: voltage cutoff for my car radio/battery

Post by k7elp60 » Tue Oct 19, 2004 3:46 pm

Kurt, here is the info and the circuit. It was developed about 5 years ago. Even though the relay specified has 30A contacts, you can use any 12V relay that has a contact rating at or over your requirements.<p>12 Volt DC Load Control and Battery Protection
The purpose of this circuit is to be able to connect a 30A load to a 12 volt battery and to automatically disconnect the load if the battery becomes discharged to the battery cutoff voltage(approximately 11.5 volts). A 30 amp contact relay was chosen as tests conclude that the power lost with the relay and the relay drive circuits was less than with a power mosfet.
For example if the power mosfet has an on resistance of .02 ohms the power loss at 30 Amps would be .02 X 900=18W. Measurements showed that the contact resistance of the relay was .003 ohms. Using the same formula for power, .003 X 900=2.7W. The power consumed by the relay driver circuit is very close to .055 amps (55Ma).<p>Circuit operation<p>With the circuit energized, operating S1 applies 12V to the coil of K1. The of contacts of K1 applies power to the load. As soon as S1 is operated, C3 starts to charge through R6. When the voltage across C3 reaches the reference voltage of U1(approximately 7.0V), U2B (open collector) goes low. This shorts R3 and reduces the relay coil voltage to approximately 6V. This is the continuous coil voltage for K1. Trimpot R5 adjusts this voltage. When the applied voltage decreases to approximately 11.5 volts, U2A (open collector) goes high and turns off Q1, which causes K1 to deenergize. This trip voltage is adjusted by R7.

Part Label Part Description Qty
C1 100pf ceramic 1
C2 100uf 25V electrolytic 1
C3 4.7uf 50V electrolytic 1
D1 1N4001 1
F1 1/2 amp. agc fuse 1
J1 5 Pin Chassis Din connector 1
K1 Potter & Brumfield KUHP-11D51-12 1
Q1 2N2907A 1
R1,R4 5.1K, 1/4W 2
R2 680, ohm 1/4W 1
R3 1.2K, 1/4W 1
R5 5K, 10Turn trimpot 1
R6 470K, 1/4W 1
R7 100K , 10Turn trimpot 1
R8 4.7K, 1/4W 1
R9 68 ohm, 1/4W 1
S1 SPDT toggle switch 1

U1 LM723CN Regulator 1
U2A, U2B LM2903N Dual Comparator 1

Image

k7elp60
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Re: voltage cutoff for my car radio/battery

Post by k7elp60 » Tue Oct 19, 2004 3:50 pm

Well the schematic didn't come thru. Here is the URL for the schematic.
http://icon.anyboard.net/cgi-bin/i2i?cm ... CUTOFF.JPG

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haklesup
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Re: voltage cutoff for my car radio/battery

Post by haklesup » Tue Oct 19, 2004 4:39 pm

If you use this circuit, you should replace the relay with a FET or select a relay with the highest coil resistance you can find (and still closes at 12V).<p>While K7elp60 needed a low contact resistance for a high current load, you do not. The relay coil is powered whenever the load is powered. The net current your battery now needs to supply is that of your radio backup and the relay coil (and associated circuits) making it die all that much sooner. A relay coil current could be greater than the current used by the yellow wire (don't want to make it worse)<p>A FET will take no (gate) current to turn on thus the cutoff circuit only needs enough current to power the ICs U1 and U2 and the voltage divider formed by R9 and R7 (~12uA).

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Re: voltage cutoff for my car radio/battery

Post by k7elp60 » Fri Oct 22, 2004 7:51 am

Perhaps a little more info is needed in regards to the circuit I posted.
When it was developed the selection of power mosfets with a low Rds was very limited, and tests I conducted showed that once a relay was energized and the energizing voltage was reduced to 1/2 half the contact resistance didn't change. Thus by reducing the energizing voltage the power consumption was reduced. The circuit shown with the relay specified draws very close to 55mA on a continuous basis. Depending on the size of the 12V battery this may be less than the self discharge current. In this case after a 100 hours the battery has lost 5.5 Amp/hours.
I agree with haklesup in the the relay contacts can be replaced with a fet. Since the contacts have to be a high side switch. If a N channel is used then a voltage pump has to be added to raise the gate voltage above the source or the load voltage. With a normal N channel this is about eight volts, or about a + 22 volts with a 14 volt battery, and the cofiguration of the comparator U2A has to be changed to go low to turn off the mosfet.
I am working on a ciruit to use a n channel power mosfet, I will post the schematic and parts list when I complete all the tests.
I tried to get a p channel circuit to work, but was unsucessful. If anyone has a circuit diagram using a p channel I would appreciate it.
I still think the n channel is the way to go as searching for a p channel with a low rds was not very good.
To be as efficient as possible a power mosfet with a very low rds would be best, even thou Kurt's requirement is less the 10A, there is still a voltage drop across the drain/source junction.

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