I keep killing one of my start batteries

[workmaster2n]

I’ve got a cat with 2 independent start batteries. The alternators are hooked up to their respective start batteries. The start batteries are hooked up via an isolation switch for “bridging” to the house (for emergency start or charging the house from the alternator). Each starter battery is also hooked up to the house battery via a solar charger, which acts as a cheap DC-DC charger keeping the start battery topped off.

My port engine has never given me issues. Fingers crossed that continues.

The starboard engine will often fail to start. I checked the connections, replaced the start battery, replaced the ignition switch, and replaced the starter. The starter was the only thing that gave any noticeable improvement. But now the engine struggles to start. Especially on mornings after I’ve been on board the night before.

The battery is reading 12 and some bit volts at the instrument panel, but I think there is some parasitic draw on the starboard start battery. This morning, after using 150 amp hours overnight (720 Ah AGM bank), the starboard motor would not start. The first attempt you could hear the starter kick over, but then nothing. Attempts 2-4 resulted in nothing from the starboard side.

After I started the port engine and connected all the bypasses and let things run for 15 minutes (to possibly charge the starboard side), the starboard engine kicked on straight away.

In terms of confirming this suspicion, the next time I’m on the boat and the starboard doesn’t start, I’ll swap the batteries and see if I can recreate the problem on the port side. I could also buy a lithium battery jumper and try that out.

If I’m able to identify a battery issue - how do I go about tracking down the parasitic draw? The isolators are off (open), so the only things connected to the start battery should be the windlass (fused open), the solar charger, the isolated house bank, and the engine.

I’m leaning towards battery issue because we haven’t yet had a problem getting the engine started when returning to the boat after a couple days (solar keeps us topped off) and the afternoon also seems to be a perfectly fine time for the engine (again because the solar has been pumping in all day). The previous owner also replaced this start battery in 2020.

[s/v Jedi]

Instead of these experiments and theorizing you should simply get a good meter that includes a DC Amps clamp function and measure what is going on.

[Scubaseas]

Test the battery. Either with a good tester or swapping it to the other side and see if the fault follows it.

To check for a draw and if the battery is OK (and rarely but sometimes even a new one can be bad) then you need a decent ammeter capable of reading down to tenths or better hundredths of an amp. Hook up the ammeter (most amp clamps work but not all. I'm old school enough to wire in an ammeter) see if there's a draw. Bilge pump switches are common problems. There may be a draw from the radio memory circuit in the low 100ths which is not problematic overnight. If you find a draw start shutting down breakers one by one until there is no draw or it drops. If no result and there are wires to the battery start taking those off one at a time. You can rule out the alternator (which has diodes in it) by disconnecting the heavy gauge wire on the back of the alternator (usually red or gray wire) being mindful it's powered and you might consider disconnecting the negative post of the battery before putting any tools on it. If you lose power over night look for something about .2amp or higher draw. Does you anchor light turn off automatically?

You can only deep cycle a start type battery a couple of times before it's trash, just FYI.

[S/V Illusion]

I stopped reading at the point where you wrote “isolation” switch. Presuming you mean battery isolator, anything else you do without first trashing that in lieu of a proper alternative is an exercise in futility.

[ranger58sb]

Originally Posted by workmaster2n

The battery is reading 12 and some bit volts at the instrument panel, but I think there is some parasitic draw on the starboard start battery.

If I’m able to identify a battery issue - how do I go about tracking down the parasitic draw? The isolators are off (open), so the only things connected to the start battery should be the windlass (fused open), the solar charger, the isolated house bank, and the engine.

This morning, after using 150 amp hours overnight (720 Ah AGM bank), the starboard motor would not start. The first attempt you could hear the starter kick over, but then nothing. Attempts 2-4 resulted in nothing from the starboard side.

After I started the port engine and connected all the bypasses and let things run for 15 minutes (to possibly charge the starboard side), the starboard engine kicked on straight away.

The previous owner also replaced this start battery in 2020.

"12 and some bit volts" could well mean the batteries are just too low. 12.4 or 12.5 or higher would indicate better (temporary) condition.

Parasitic draw could be part of the explanation. CO detectors? Stereo memory? Shower sumps? Those aren't uncommon. Could also be your anchor light, especially if that's not LED.

Your overnight house bank usage seems to be maybe drawing down your starboard start battery. And then being able to start the starboard engine after you've got the port engine going... could perhaps indicate your start bank -- at least your starboard start battery -- isn't really as isolated as you thought.

I think I didn't see mention of battery size. Could also be it's not really big enough in the first place.

You do mention a solar charger. Maybe it doesn't really charge start batteries? Or maybe it's not working properly...

As others have said, appropriate meters can tell you stuff.

-Chris

[Locquatious]

Leaving the engine start key in the wrong position will cause a constant drain on the battery through the alternator field coil. It is a common mistake to pull the engine fuel cut-off, shutting down the engine, and forget to turn off the start/run key.

[cr180]

Two thoughts:
1. For parasitic loads, look for wires attached to the alternator and at the ignition switch or a terminal block either near the ignition switch or somewhere in the circuit route. Ignition circuit power typically wired from alternator not starter.
2. There is likely a neutral position switch on the transmission wired in series in your start circuit. Another failure mode you may not have checked.

[Lepke]

Disconnect the + cable from the battery and measure amp flow between the two. Disconnect things until it reads zero.
Some 12v radios, appliances, etc., will draw small amps when off and wired backwards.

[Captain Bill]

It might not be a parasitic draw at all, but a bad engine ground. I once had a similar problem. I was measuring Voltage between the + cable and the battery and everything was good. I measured it between the cable on the engine ground and it was still good. I measured it between the + cable and the block and only had 4 volts. The engine ground connection was so corroded that while the cable lug was showing 12.x volts as it was directly connected to the - post of the battery the effective voltage as far as the starter and alternator was concerned was much less. Note that when there was no load the block voltage was measuring 12.x volts, but when I tried to start it the ground connection could not carry the current and that is when the voltage dropped to 4v. It also meant that the alternator could not effectively put amps into the battery, so it both failed to charge the battery and was hard to start. I cleaned up the ground connection and all was well. Just a possibility you might look at.

[smac999]

Originally Posted by S/V Illusion

I stopped reading at the point where you wrote “isolation” switch. Presuming you mean battery isolator, anything else you do without first trashing that in lieu of a proper alternative is an exercise in futility.

Pretty sure he means switch.

Considering this is what a “battery isolator” is.

https://www.partsengine.ca/952-D-vp-sure-power-2344.aspx?gclid=CjwKCAiA_6yfBhBNEiwAkmXy50oUlhSuC-yl7j7IU5--jqIL8fLu-MLDEalM6BeRW5TRv1Jv2e2nEhoC4nkQAvD_BwE

[workmaster2n]

Originally Posted by s/v Jedi

Instead of these experiments and theorizing you should simply get a good meter that includes a DC Amps clamp function and measure what is going on.

I've got a clamp meter from the local electronics shop. It wasn't reading anything, but I'll see if one of my electronic hobbyist friends has a higher quality unit that might read smaller currents.

Thanks for the idea.

[workmaster2n]

Originally Posted by Scubaseas

Test the battery. Either with a good tester or swapping it to the other side and see if the fault follows it.

I hate to admit I didn't even think of this. I'll bring it in to the battery store next time I'm on the boat and make sure it's still showing healthy.

Thanks!

[workmaster2n]

Originally Posted by S/V Illusion

“isolation” switch. Presuming you mean battery isolator, anything else you do without first trashing that in lieu of a proper alternative is an exercise in futility.

Sorry for the confusion, I meant a battery isolation switch, like this (not the exact brand, but similar) https://www.roadtechmarine.com.au/2-...h-afd/p/SF2249

[workmaster2n]

Originally Posted by Lepke

Disconnect the + cable from the battery and measure amp flow between the two. Disconnect things until it reads zero.
Some 12v radios, appliances, etc., will draw small amps when off and wired backwards.

Sorry, what 2 am I measuring the current between?

Thanks!

[workmaster2n]

Originally Posted by Captain Bill

It might not be a parasitic draw at all, but a bad engine ground.

This is an excellent idea! I'll check it out after I get the battery tested to make sure it hasn't gone bad.

Thanks!