Battery issue?
 

I'm having an issue with batteries. Twin 500 efi's and brand new batteries that are fully charged. Battery voltage is 12.7 volts. I put a volt meter directly on each battery and when the engine cranks the voltage drops to between 9 and 10 volts. Stop cranking and the voltage is right back to 12.7 volts. The same thing is happening on both motors. The batteries are Duracell size 29 deep cycle with 810 cca @ 32*. Anyone have any ideas on where to start looking? Thanks.

Not a battery expert, but do have some knowledge, if your battery is a true "deep cycle" battery, then you bought the wrong battery. Deep cycle batteries are for trolling motors and house loads.

I would suggest you need at the least group 31 (staying with Duracell and Sam's club AGM) , or better yet get Lifeline or Trojan brand

I've run the same size and type batteries for the past 3 sesons with no issues until the end of last season. Had one battery go bad my last day out last season so decided to replace them both.

I'm just looking at the Duracell site and it shows the 29 as a deep cycle, and the 27 and 31 as starters. That said, start looking at your ground cables, but pulling the bat down to 9V is in most cases the battery or starter. Try cranking and if it cranks for a few seconds, quickly touch the cables and see if they are hot. If their getting hot then it may be the cable connections

There are also 2 battery switches wired in - could one of the switches be bad and causing an issue with both batteries?

Doubt it, because unless your connecting them together there is no path for current flow. To follow on the statement, I don't know how your wiring is setup, but since this is happening to both engines, and it didn't happen before, it has to be wiring or batteries

How well does the boat start as it sits now with the new batteries ? Or does it not at all ?

Its starts very hard. A long hesitation when you turn the key and then it cranks slowly. Seems to start much mote normal when battery switch is set to "both."

Ya know you might just need new starters, but still think this is a wiring or bat thing.

I just changed out this year from original Merc to permanent magnet starter and boy does it spin over fast now

Good point. Seems odd both sides but had stranger things happen. Not sure what kinda switch you have but does it make a difference on all position or try jumper cables from battery to batttery. If your not noticing much of a diff I would have to agree with potential starter issues. Boat starters see a lot of moisture and condensation. I recently bought a battery load tester from NAPA. Can't believe I went all these years without one. As mentioned double check engine grounds from battery which I'm sure you've done already.

The switches are Perko switches. Yes it does start much better with battery switch set to ALL. I'm sure the issue is that the voltage is dropping to 9+ volts under a load but what would be causing such a drop?

Reread the Duracell battery specs then buy starting batteries to start your BBC's and use the deep cycle batteries to run
your stereo.

http://www.driveduracell.com/assets/...image/1853.pdf

Sorry but your logic is Bass Ackwards.

I would start by replacing the heavy leads from starter to switch and from battery to ground. Had that problem in my old checkmate, unless batteries were completely fresh, engine would crank slow and not have enough voltage to fire MEFI1 computer. Replaced the leads and made sure I had a good clean ground. Spun right up good as new. The heavy leads even while insulated will get corroded over time, especially depending how the crimps are made. The factory crimp ends can be improved by sweating some solder into the ends before you install them.

Chances are it is a failing crimp or dirty ground somewhere.

Thanks. I was thinking that was where I might start. Not looking forward to trying to get at the starter cable though. One other note - I did have to replace the onboard charger this spring. Would only put out 11.5 volts to one battery bank but that shouldn't have anything to do with what i'm experiencing now.

Use cranking batteries for your starter and deep cycle batteries for your house loads. Deep cycle batteries are not designed for high amperage / short duration demands as in your starter.

First I would not call 12.7 fully charged. should be 13.2 at a minimum. I would check your charger and or alternators . Second make sure you have a ground wire coming off each battery going to the block. Not just a jumper black wire going from one neg terminal to the other neg terminal then to the block.

Chris

Boat only has room for two batteries. I've always run deep cycles and never had an issue.

I'm having the same issue as you. I have fresh batteries, 1 is an Interstate cranking that I bought last May, 1000 CCA. The 2nd is a Duracell AGM dual purpose, 800 CCA. I load tested both and they are good. But I get a bad hesitation when I start my 496 Mag. I'm worried that it's the started because it's hell to get to if I have to replace it. None of my cables are corroded and I use dielectric grease on the battery posts.

Dielectric grease is non conductive. I wouldn't put it on a battery post.

https://www.w8ji.com/dielectric_grea...ive_grease.htm.

Corroded/resistence wiring someplace. Deep cycle vs start type battery makes absolutely no difference.

Thanks. I agree - I've always used deep cycle batteries for starting and never had an issue.

A voltage drop to 10 Volts at the battery while starting is completely normal, especially on a starter that's spinning slowly. The slower the starter is turning the more current it pulls and the higher the voltage drop across the battery's internal resistance. If you feel the starter is spinning abnormally slow check the voltage at the starter terminals while turning. Any excessive voltage drop in your wiring will show up there, not at the battery.

Those EFIs don't need some specialty 1000 cca starting battery and your deep cycles should do fine.

The probability of two separate engine's wiring systems going bad at the same time is small. Stating there are no differences in the batteries is just plain
wrong.

Some guys had a happy outcome when putting their boners through a hole in a porta-potti. Others got a rat trap!
It doesn't mean you should take the chance.

Your saving grace has been your on board charger. A fully charged deep cycle battery will start an engine but it is not ideal. At what point does a
standard deep cycle battery not start a high compression/high horsepower engine? I don't know. Ask Mercury or one of the pro engine builders.
Now that your on board charger is not performing you are experiencing what most of us that have tried these things have found out. Your engines
alternator will not fully charge a deep cycle or dual purpose battery when just tooling around in the boat going to the beach, bar, or your GF's dock.
These batteries need many hours on the charger to get a proper charge.

Dual purpose batteries would be a better alternative for you.
The best scenario is what I described before.

Replace your on board charger and use it every day and you will probably be O.K.
Do things the way I first described and you will be O.K.

I have used all three types of batteries on my boats and found that using 1 starting battery and 1 deep cycle is the best way.(single engine)
I also remove my batteries and take them home to be put on the charger on a regular schedule.

If I'm not mistaken these are the specs for the OP's engine;
HP500EFI Specifications

Propshaft Horsepower: 470/350
Displacement (Liter/CID): 8.2/502
Cylinders: V-8
Bore (in/mm): 4.47/113
Stroke (in/mm): 4.00/102
Compression Ratio: 8.75:1
Fuel Requirements: 87 Posted Octane (R+M)/2
Full Throttle RPM Range: 4800-5200
Ignition Type: Digital
Alternator (amps/watts): 60/847

Not exactly 'high compression'. That shouldn't require anything more than an average 600 CCA battery. Although I agree, the dual purpose AGM batteries made today are the best choice for almost everyone.

Are these 500s still turning belt driven air cleaners? When I had M3s on my 500s if the battery was in any way down or bad connection it did the same. To be honest, I bought a spool of OO gauge wire this winter and built new cables. My BT was originally setup with a shared ground between the engines then back to 1 battery. Now both motors go to an individual battery. It made a difference.

:)

I think Chris means hair dryer's lol

I wouldn't like that shared ground but very well could common but an easy fix also.

WHAT ?? never mind

12.6 v is fully charged. Check for voltage drop between battery and starter.

Just commenting on what endeavor1 had "originally" with ground from engine to engine and back to one battery. Personally I've always had each engine ground to battery with a strap between engines as well as battery to battery. Grounds can be troublesome leading to failures on who knows what.

Agree ^^^^ I couldn't figure out the wording

I hate grounds!!! They always seem to cause me the most issues and are tough to track down. Over the years I'm getting better though. Ohm meters are my friend along with a wire wheel to clean up contact areas:ernaehrung004:

kinda like an older rusty Michigan pick up truck with a snow plow issue. I just end up running grounds everywhere and problem solved. Lol.

Can you please explain the permanent magnet starters and whether or not they are plug and play? My SB engine spins slower than port and I plan on replacing the starter as a result when doing mani's/risers. . . Maybe a PM if you are kinda enough and don't want to hijack the thread.

I have 1 G27 Deka Deep and per Deka the full charge voltage is 12.5. Just bought a Duracell G27 Combo., full charged volts is 12.7. You have to check with the manufacturers data sheet for your batteries, they're not all the same.

I love them, much more cost efficient than replacing expensive components!

Read up on how to charge agm batteries,,,,,there different than lead acid

Yes these are still "belt driven air cleaners." Think I found my problem - had a ground that wasnt connected (my bad when I put in the new charger). I also rewired the battery switches - not quite sure what BT had in mind when they wired them . . .. thanks for all the help everyone!