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I thought I would put this in a follow up post. You can make this as simple or as elaborate as you would like.
The simple way is what I posted with the battery wired in parallel with the load, and a diode to isolate them from the starting circuit. This way has a somewhat serious problem, namely a power source that is always on. No problem if nothing ever fails, but as we all know boats are destined to have shock and vibration failures.
The more elaborate way, is to have a switch to "engage" the battery. It can be a simple panel switch, or an active circuit that is fully automatic. You could call it ummm well battery backup
After that one could consider the sensed voltage of the load where it would automaticaly begin supplying power. Throw in some current limiting to keep everything nice and safe, and you got something real dandy. Marketable even. You could call it the Hull Truth Battery Backup
Sea Dad,
I wanted to mention that the battery should be a rechargeable type.
My circuit also has an extra diode to prevent the backup battery from backfeeding the main 12V battery electrical system. I still like your switch for safety.
I'll try and draw the schematic and post it tomorrow.
Birdman,
I found the dis-charge specs on the battery.
It is capable of 2.5 amps @ 11.5 volts for aprox. 8 min. More than enough. Try the link below.
Sorry that I have not been able to add some inputs to this subject, especially after stirring the pot that got it started. It has been one of those weeks for me and absolutely no time at all. I have just looked at some of the recent posts and would like to offer a few comments.
Sea-dad, lots of good inputs from you. Congrats. I especially like your recommendation for the Shottky diodes. They offer a major advantage over the PN junction silicons; that forward voltage drop problem with the PN's is only half that value with the Schots.
One thing that I personally think is wrong is everybody going with ANOTHER battery. Anyone that has more than 1 battery on the boat can accomplish the same result with an existing battery, a couple of Schot diodes, a single pole switch and a couple of resistors.
BW23, replace that 7.5 ohm, half watt resistor with at least a 2 watt resistor. It only takes a quarter amp to burn it up.
Still pressed for time, I will try to get back on the subject next week.
Gil, put down the fishing pole and expand on your thoughts, the fish will circle back around if you chum.
I have 3 batteries, 2 crankers and 1 deep cycle house battery all nestled in the console. So I dont have gear drop outs while cranking issues, but if I can help I am more than willing too, because my last boat most definitely had this problem (I just never bothered to do anything about it).
You have tweaked my curiosity about the resistors, and diodes. So like I said give the pole a rest and fess up.
Sea Dad & Gil, http://www.newmarpower.com/pdf/Manual-NS-12-20.pdf
Take a minute and check out the Newmar site. Their product/design are nearly the same as ours except they disconnect the main 12V battery and connect their backup battery for clean/stable voltage when the ignition key is turned.
I wonder if the electronics are shutting down because of low voltage or noisy voltage?
Very informative thread and good ideas and great follow through on information for providing some back up battery protection for small current draw applications.
Birdman, for your complement of equipment I would suggest a more complete solution. That is an additional battery dedicated just to your electronics. I have a similar complement and installed a battery in the cabin under the wiring console that is dedicated to electronics. It is an Absorbtive Glass Matt (
AGM) battery that is closed sealed and does not emit gasses as it is charged. Batteries that start the motors are totally separate except that the negative (ground wires are cdonnected). To charge the electronics battery, I have a high capacity solenoid connected to each motor battery and the electronics battery. A three way (dual throw in EE speak) switch located at the helm selects between connecting the electronics battery to
1. The main motor
2. The trolling motor
3. Nothing, the electronics battery is running without any charge source. In most cases I can go hours with the electronics battery not being charged by either motor and this lets me avoid overcharging it.
This system takes a little bit of attention from the driver to make sure you have the switch in the correct position depending on which motor is running and the charge state of the electronics motor (monitored by the voltage of the battery) but is the most flexible approach I have come up with. Never have a glitch when a motor starts and always have a charged battery for all of the electronics.
reelife,
Thanks, but I don't want to 1. Add another battery (already have 3). 2. "Protect" ALL my electroncs. I just need to "protect" my chartplotter. It's the only unit with any problem.
I have been dinking around with solutions to this engine starting problem for quite a few years, both on my boat and friends' boats. Sometimes the straight capacitor clamp circuit works and sometimes it doesn't. For the sake of "keeping it simple", the capacitor circuit is the best way to go as long as you don't push it beyond it's practical capacity like trying to hold 12 volts at 15 Amps for 3 seconds. In most cases, the capacitor serves to filter out the horrible, transient spikes that occur from the starting solenoid and starter rotor when the 12 volts are applied, and which last for fractions of a second. Based upon Wayne's success with that over-simplified circuit that I suggested, it appears that his equipment is more sensitive to the starting transients than the absolute drop in battery voltage. As Sea-dad pointed out, the capacitor is going to lose a lot of it's charge feeding back through the fuse and wiring, which it does, but it still held the equipment voltage stable enough to prevent shut-down.
Most of the newer electronic gear have pretty good voltage range characteristics. I believe my Garmin 2006c, gsd20 and 182c work down to about 8 volts. However, my old LMS350A and Interphase Loran were terrible and that is where my dinking started.
Sea-dad's circuit shown in his post of 11-03-03, 06:23, looks to be an excellent approach to using either a capacitor clamp or a battery clamp, whichever works for you. The Schottky diode pack is the key. I had tried similar circuits in the past but was forced to use the typical PN silicon diodes that have an inherent forward voltage drop of .6 to .8 volts; too much drop for a lot of circuits. In other words, under normal conditions, 12.5 volts feeding terminal 3 would result in 11.9 volts at terminal 2, and that is with a fully charge battery. The Schottkys only drop half of that voltage.
If someone wishes to try the circuit, I would suggest an IRC 47CTQ020-1 Schottky which can be obtained from digi-key for $2.43. Biggest problem there is minimum order. The diode is rated at 20V reverse voltage and max forward current of 40 Amps, 20 Amps per diode since there are 2 in the pack. The resistor value is OK at 10 ohms, but I would increase the wattage to at least 10 watts since if you want to use a really big capacitor, you could burn out the 2 watt during the charge cycle. I suggested a 4700 microfarad originally and if you want to increase the size, no problem. You can parallel capacitors to increase the overall, ie, another 4700 in parallel increases the overall to 9400 microfarads.
If you prefer a battery source rather than the capacitor, just take out the capacitor and resistor and connect the clamp-battery plus to terminal 1 of the Schottky pack and minus to ground. You should also plan on using a single pole switch mounted somewhere to turn off the clamp-battery. If not, you will always have the clamp-bat voltage feeding the equipment when your boat-12v is off.
I would use a single Schottky for each piece of equipment and the clamp- battery plus connected to terminal 1 of each. The boat 12 volts obviously connects to terminal 3 of each Schottky. I don't think you really need a fuse between the boat 12 and terminal 3, but I would connect terminal 2 to the line side of the regular equipment fuse.
Anyway, that is enough for now. Will try to get back later with the procedure for using both of your cranking batteries to supply your electronics. Really the same as above, but the external clamp-battery is replaced by the voltage from your second battery. Boy is that going to stirr up some comments.
Hey Birdy, they are still isolated and it ain't the same as ALL on the BAT Switch.
Maintain Voltage Drop Capacitors ??? 
