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12V drops when engine cranks

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justfixit

New Member
Hi Guys, new to this forum. My 12v source drops on my boat when the engine cranks. This drops my GPS unit off line. I thought about putting a Cap/diode in line to fix this. I read an earlier post on a similar situation but I don't know if the components would be the same(-my-car-12v-constant-switched-live-drops-volts 12v+drop) The GPS calls for 10-20vdc, 650ma draw. Any help would be appreciated.
 

mvs sarma

Well-Known Member
perhaps it is natural as no battery has 0 ohms internal resistance .
but check whether the drop is reasonable.
if not either suspect undercharged battery or shedding in positive plates of some cells, thus reducing capacity or increasing internal resistance.

do you really need 10 to 12V?

if 5V is sufficient for working after any possible regulator in there on GPS board,
you might try out a LDO there like LM1117-5
 

alec_t

Well-Known Member
Most Helpful Member
I thought about putting a Cap/diode in line to fix this.
Not really practical; you would need something like 1 Farad capacitance to keep the volts >10V for a cranking time of 2 secs at 650mA draw.
A small auxiliary 12V battery, plus diodes, would do the job but would also need some charging arrangement. Or a DC/DC converter could provide a steady output voltage.
 

throbscottle

Well-Known Member
I know it's probably a bit too obvious, but is the battery getting old?
 

rfranzk

Member
I know it's probably a bit too obvious, but is the battery getting old?
Hello Everyone,

I suspect either defective battery or high resistance in connections. Most automotive batteries need to hold a minimum of 9.6 volts with a 300 amp load for 10-15 seconds. Depending on condition of the starter and how much current it draws the voltage should not drop below 9.6v. I would do a voltage drop on both positive and negative circuits and test the battery as starting point. Is the GPS connected directly to the battery or is it connected to a fuse box connection? Sometimes electrical accessories are disconnected in the crank mode to provide maximum current to the starter motor.
 

ronsimpson

Well-Known Member
Most Helpful Member
In most autos the electronics is disconnected during cranking.

One of my early designs was a auto computer that needed to run during cranking. (glow plug controller) Often the 12V dropped to 5V for short periods of time during cranking. Very dependent on battery age, temperature, size of wires, and the compression of the motor.
 

debe

Active Member
Had a simmilar prob with my boat, the answer was run seperate power wires direct to the battery for the GPS.
 

Mr RB

Well-Known Member
How much current does the GPS draw at 12v? Even with the backlight on it should be fairly low, maybe 50-100mA. Also the cap doesn't need to perfectly cover the voltage drop it just needs to reduce the intensity and severity of the drop to the point where the GPS electronics won't detect brownout and reset the device.

I think a big cap and a diode might be a good option. A 10000 uF 16v or 25v cap is common enough and might be enough to stop the problem. There are also those multi-Farad 12v "supercaps" you can buy for car audio use.

Also I agree with people about the wiring, it's best to have critical electronics run direct from the battery terminals, then the only voltage drop when cranking is the resistance of the battery terminal to the battery post which should reduce the severity of the drop compared to connecting elsewhere in the boats wiring.
 

crutschow

Well-Known Member
Most Helpful Member
.................................

Also I agree with people about the wiring, it's best to have critical electronics run direct from the battery terminals, then the only voltage drop when cranking is the resistance of the battery terminal to the battery post which should reduce the severity of the drop compared to connecting elsewhere in the boats wiring.
That may be true to some extent but any voltage drop by connecting to "elsewhere" would only be from current draw by any other devices connected to the same wire and operating at the time. That wire goes directly to the battery already. The starter always has its own large cable connected directly to the battery with nothing else connected to it.
 

MikeMl

Well-Known Member
Most Helpful Member
I have run into this issue with cars, boats, and aircraft. Even with a fresh battery, and clean battery terminals, the voltage during cranking can drop to less than 8V on a 12V system when cranking a large engine.

I have used the following trick: a 2Ah 12V SLA isolated with a Schottky rectifier between the starting battery and the load that must be continuously powered > 12V. While the engine is running, the alternator gets the terminal voltage of the starting battery up to ~14.2V. The forward drop of the Schottky means that the SLA is "floated" at ~13.7V while the engine is running, which is about right for keeping it charged. I used a small relay to automatically disconnect the load (GPS, engine computer) when the master/key switch is finally turned off. The next time the key/master switch is turned on, the (GPS, engine computer is powered from the SLA/Schottky/Starting Battery. During cranking, the starting battery can be pulled down to 8V, while the SLA keeps the loads above 12V...
 
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