Capacitor to smooth-out crank voltage drop ..#2

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Blaser

Joined Jan 29, 2024
1
You could assist thousands of boaters that have the same problem - the GPS quits when you start the engine, and restarting WILL cause you to lose your desired location above the fish you are trying to catch. If I add a sealed lead acid battery, instead of a capacitor, will the single Shottky diode inline with the 12v allow the battery to be charged when the main 12v is in a charging state? What value diode should I use?


Mod: Link to old thread.
https://forum.allaboutcircuits.com/...th-out-crank-voltage-drop.174073/post-1567754

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Reloadron

Joined Jan 15, 2015
7,517
You could assist thousands of boaters that have the same problem - the GPS quits when you start the engine, and restarting WILL cause you to lose your desired location above the fish you are trying to catch. If I add a sealed lead acid battery, instead of a capacitor, will the single Shottky diode inline with the 12v allow the battery to be charged when the main 12v is in a charging state? What value diode should I use?
View attachment 313924
The last post was November 20th 2020. You would fare better starting your own thread, As drawn your diode will drop about 0.7 volts or less with a Schottky diode. A Schottky diode will give about a 0.35 voltage drop. Consider the effect on the charge voltage or run voltage. Again, I would start a new thread and in detail explain your objective.

Ron
 

MisterBill2

Joined Jan 23, 2018
18,491
The long answer is YES, BUT! A separate battery will hold up the voltage to the GPS system very well, it it happens to be correctly separated. That is, a separated negative side as well as a separated positive side, so that only the GPS operating power flows in the wires and conductors between that separate battery and the GPS package.
And the one really big question is just how much of a voltage drop can the GPS system endure without any undesired results. The backup battery must constantly hold at least that voltage to avoid the problem. With cranking the engine there may also be a burst of noise from the starter motor, that can get thru the filters between the battery and the GPS system equipment.
 

Reloadron

Joined Jan 15, 2015
7,517
Thank you moderators for straightening things out.

Back to the fish? Yes, I would just get another battery like a 7 or 8 AH SLA battery and power the GPS and anything else critical off that battery allowing it to charge of the regular system. I would also find out, as was mentioned, if ACC disconnects when starting engine(s).

Ron
 

MisterBill2

Joined Jan 23, 2018
18,491
One thing to be aware of is that the connections at the battery are critical, as well as the connections to the power distribution segment of the system. The starter motor is usually by far the largest current draw of any engine driven system, and so it will produce the largest voltage drop. So a bit of diagnostic work can be useful in solving problems.
 

prairiemystic

Joined Jun 5, 2018
307
Sometimes the (engine block) starter ground, the battery (-) and vehicle electronics ground, (car) chassis ground are poor connections so the voltage drop can be excessive or worse (appear negative) during cranking. I'm not sure how boaters deal with the fiberglass chassis, it must use a daisy chain scheme?

OP should look for an "ideal diode" board as these have no appreciable voltage drop, allowing the backup battery to charge from the main system. They are also used for motorhomes, RVs, caravans when using an aux battery as "diode isolators".
Only danger is charging current can go sky high if the aux battery is dead and a 5Ah battery can get say 20A which is too much.
 
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