Battery mystery

[richardsn9]

I would appreciate any thoughts on a problem that I have had with my battery electrics.

My boat has a typical set up of 2 x 105ah house batteries and 1 x 80ah engine battery on a 1/both/2 switch. All batteries are about two years old.

I had had problems starting the engine with the engine battery, as it would not turn over and the other load items such as GPS and depth sounder would trip out when the key was turned, and assumed the battery itself was dead or dying, despite being charged regularly.
However, when I checked, the battery holds a charge ok, and the charging voltage is almost 14v with the engine running. The last time I did this was today, when I started the engine with the house batteries, and switched it over to the engine battery to check its charging voltage, which was 14v as normal. However, as soon as the engine was switched off, the engine battery does not even light up the switch panel.

I moved the battery over and connected it to the house side, it seemed to work fine.

I am now puzzled as to how the battery can take a 14v charge, but seems to have no output on the engine side, despite being to all intents and purposes being healthy? Any suggestions as to where to start looking would be welcomed.

 

[alan17]

I would probably start by checking all of the connections. I had "battery" problems some years ago. I eventually found that the main earth lead going to the engine had a very poor connection between the lead and the ring terminal bolted to the engine block.

 

[Sandyman]

I would probably start by checking all of the connections. I had "battery" problems some years ago. I eventually found that the main earth lead going to the engine had a very poor connection between the lead and the ring terminal bolted to the engine block.

Yes I would agree with that. Sounds like a bad connection to me.

 

[vyv_cox]

Agree with the others, sounds like a connection is poor. Don't check only the terminals, give the cables a thorough inspection. I remember having a similar problem on a car, couldn't find it until I checked in the dark and could see a corroded earth cable glowing.

 

[boguing]

until I checked in the dark and could see a corroded earth cable glowing.

That gives me an idea Vyv. I might try my infrared thermometer when trying to locate bad connections. I do realize that in some situations leaving power on might do more damage.

Really just trying to justify buying it! It has proved that extra cold Guinness is only half a degree C colder than normal, my Land Rover engine runs consistently hot on all cylinders and did diagnose which component had failed in an expensive amplifier...

 

[VicS]

Another recommendation to check connections....... i reckon from what you say it will be one of the engine battery connections or the 1,2,both switch itself.

Not going to be anything common to both batteries so I'd think that rules out the engine negative connection this particular time.

If the problem is so bad that it wont even light the panel its not going to be difficult to find.

BUT beware of checking with a digital voltmeter. They take so little current that they will give a "normal" reading through very bad connections. A test lamp on a couple of leads will be better, or a bulb in parallel with the meter to draw some current.

 

[halcyon]

Easy way to check bad connection is get say 5 metres of 1 sq mm cable, strip one end back, twist tight around negative probe on multimeter, rap some tape around for security. Strip back other of cable around 1 inch, undo a battery negative terminal, slide bare end of cable in and reclamp. This now gives you a fixed negative to measure to.
Turn on cabin lights to put a load on the battery, you can now put the positive probe on any negative terminal or connection, voltages above zero indicates a negative fault, bigger the voltage bigger the fault. Equally, putting the probe on positive connections you are looking for a falling voltage based on battery voltage, say battery is 12.5 volt, if you get 12.0 volt that would indicate a fault.

Brian

 

[richardsn9]

Many thanks for your help and suggestions! I will get my head torch on and go delving!

 

[prv]

BUT beware of checking with a digital voltmeter. They take so little current that they will give a "normal" reading through very bad connections. A test lamp on a couple of leads will be better, or a bulb in parallel with the meter to draw some current.

+1 - I spent £100 on unnecessary fridge parts due to making this mistake

Pete

 

[MM5AHO]

+1 for bad connection.

The reason you see the right voltage is that the poor connection works Ok at low amperes, but at high it doesn't and so you get lower voltage (GPS etc stops).
The high load comes from the starter motor. That high current through the bad connection drops the voltage.

Tighten everything. Clean everything.

 

[gordmac]

Probably a bad connection but I did have an old battery that would charge up and hold near 13V but wouldn't start an engine.