Voltage Sag When Starting Restarts Electronics

[CaptTom]

First, I'll admit it. I'm lazy. I can (and often do) avoid this problem. But I want a way to eliminate it completely. Just wondering what others have done...

I often start my port engine off my house bank. That's the engine with the bigger alternator, so leaving it connected to the house bank all the time is simpler than switching back and forth before and after anchoring.

But of course the house bank might be getting low, and the voltage sag from starting often causes anything electronic to reboot. Probably the most inconvenient is the battery monitor itself; I lose the calculated SOC. Again, no big deal, I can see by the amperage going in that it's charging, and when it's done. There are also the radios and chartplotters, but a bit of fiddling gets them back pretty quickly. And, if I really want, I can climb down into the ER and flip a few switches, before and after starting. So this problem has never made it to the top of my list.

Still, it would be nice to press the Start button without having to think about where my switches are set. One thought was a dedicated electronics battery, maybe isolated with a diode or DC-DC charger or something. I know there are combiners and isolators, but those aren't exactly what I'm after, and introduce new problems of their own.

Suggestions?

 

[JDCAVE]

I have a dedicated starter bank. Two group 24’s. It is always fully charged. Engine starts without hesitation. Navigation Electronics are on the house bank. I usually start the Navigation systems once the boat is started. The house and start banks have an ACR between them. I manually combine the start bank with the house bank and alternator when pulling the anchor. The windlass is otherwise powered by the starter bank. Both banks are lead acid.

Jim

 

[dhays]

Rather than dedicated electronics battery, I would use a dedicated start battery like Jim mentioned.

My Cummins QSB is very sensitive to any electronic interference. So my start battery is not connected to anything but the engine. It is charged via a simple Xantrix Echo Charger off the house battery. It always is fully charged with no risk of draining. I do have a cross connect wired in with the house bank in case of some unforseen failure of that battery.

This is a very inexpensive and bulletproof setup. Rod Collins (aka "CMS", aka "Mainsail") has a returned Echo Charger for sail on his website.

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[Nick F]

I had exactly the same problem - very annoying.

I have just installed a small dedicated battery fed from the main battery via a diode. Seems to have cured the problem. I used a tiny lead acid gel cell (7 A-h) and a 10 amp silicon diode

There is just one problem - if I run the electronics at anchor for a longer period without the engine (or generator) running, the 0.7 volts loss across the diode means that the small battery preferentially supplies the electronics load and goes to a low state of charge before the main battery fully takes over via the diode. The electronics then keep working until I crank the main engine, at which point the small battery fails to keep the electronics working.

I am going to source some Schottky diodes which should reduce the volt drop to about 0.3V. In the meantime, I installed a switch which short-circuits the diode for when I am not going to start the main engine. I thought about incorporate this switching into the starter button(s) but decided against this due to the extra wiring complexity.

 

[mvweebles]

This one is simple. As others have said, you need a dedicated start battery. Starting off the house should be reservered for emergency. Starting an engine takes a LOT of short-duration power. That's not the skillset of a house battery. My Perkins 4.236 probably draws 600A for 2-seconds to start so total of 3-5 AH of battery capacity. Big surge/low duration - that's a much different use-case than a house bank that is designed for 250A draw (3000W).

You're bring a knife to a gun fight. Give your engine what it needs.

Peter

 

[Nick F]

In contraposition to the respected Peter of Weebles, I make the following (possibly heretical) alternate suggestion:

My house bank has such large capacity that, even though these deep cycle batteries are not optimised for cranking type duty, the cranking load is actually quite low compared to the battery's capacity. So, I can easily crank up the mains using my large house battery - even when partly discharged.

My redundancy is then supplied by the generator, which has its own dedicated starting battery.

The big win, for me, is simplicity. I love simplicity . . .

 

[SteveK]

Yes to the dedicated start battery for engines and use for windlass and davit motor too. Have crossover for an emergency should it be needed.

ETA: Nick you rebel contrarian.

 

[mvweebles]

In contraposition to the respected Peter of Weebles, I make the following (possibly heretical) alternate suggestion:

My house bank has such large capacity that, even though these deep cycle batteries are not optimised for cranking type duty, the cranking load is actually quite low compared to the battery's capacity. So, I can easily crank up the mains using my large house battery - even when partly discharged.

My redundancy is then supplied by the generator, which has its own dedicated starting battery.

The big win, for me, is simplicity. I love simplicity . . .

Well, I'm not the best at this but I will still admit I'm confused. You state in Post #4 you have the same problem as the OP - electronics drop-out when you hit start on engine. But you also say you have plenty of reserve in your house bank so there should be no issue. Now I have no idea what a Schottky diode might be (related to a Flux Capacitor?), and generally am lame with stuff like this anyway, so if I could indulge you to explain why your massive house bank works fine until it doesn't, and describe what a Schottky diode is and why it solves a problem a dedicated start battery will not.

Peter

 

[Nick F]

Peter - your question is valid (why, if the house battery is so massive, I get voltage dip when cranking).

I suspect that I also have some other wiring problems which contribute to this problem. I am still somewhat new to this (old) boat and am still working through the electrical system.

Also, my older generation Raymarine MFD is extremely sensitive to supply voltage dips, such that the cranking load does, in fact, cause it to reinitiate (only one of the two MFDs - the other one does not seem so sensitive.

I won't go into the Schottky diode thing - this could become another anchoring subject.

 

[rslifkin]

Dedicated start battery is the answer. My setup will have to change a bit when I go to an LFP house bank, but for now, each engine starts off its start battery and there's an ACR (ignition interlocked so it can only engage with that engine running) to link it up to the house bank for charging after startup. It works well and keeps the start surge away from the house bank but allows both alternators to charge the house bank.

In your case, you could do it that way, or for the port engine, just wire the alternator right to the house bank and then leave the engine selected to start off another battery (2 engines can share a start battery with no issue). In my case I only have 2 start batteries, so the generator shares with one of the engines (all 3 can be independently selected to use either of the start batteries).

The one big load I do have on the house bank is the windlass, but the startup spike from that isn't as severe as the engine starters, so it's not enough to reset any electronics even if the house bank is drawn down a decent bit.

 

[jleonard]

I learned on youtube how to do a proper voltage drop test at the starter. I found i had too much drop and changed one old starter cable. My intermittent electronics drop out was cured.