6BTA 370 Voltage drop while starting

[Chuck Rich]

How much will your voltage drop when you engage the starter ? I have 2 relatively new 31 east Penn gels in parallel and my voltage drops from 13.4 to around 9.7 is that normal ? I just installed some smx voltage gauges and that is the reading I get. 

Topic

[Clark Leighs]

Wow,  I was just writing this reply and finally realized this thread is 6 months old.  Opps.

Oh well, I will leave it as it may help someone else.

 

If one battery is failing the parallel connections will show a good average voltage high enough to make things look OK.   The meter will likely show a good average voltage.

However as soon as you try to crank the engine the failing battery will cause that average to drop big time.

 

I have found a failing battery in a parallel setup by fully charging the system   then disconnecting each battery from one another and allowing the batteries to rest for a few hours to overnight.    The failing battery will show a very different voltage from the good one usually a precipitous drop of its voltage.

Just be sure any disconnected cable cannot get into something you don’t want it to.

It’s not as quick as a good tester , just needs the time to do the test.

 

The last time I did this it didn’t actually take long.     An hour or so separated from each other and I knew which battery was the problem.  Not only did the others hold voltage but they were virtually the same voltage.  Maybe a couple hundredths of a volt different.

2 users thanked author for this post.

blindhog, Kyle Routh

[Chuck Rich]

Yes batteries are parallel, I will check the cables and connections. I will check on the charger question but I think the shore power was unhooked. Also was thinking about isolating each battery and see if I get a different reading.

thanks

[Gene Fuller]

If you are measuring directly at the terminals then a likely culprit is the battery “parallel” wires. A single group 31 is a bit light for that engine. If the batteries are connected so the positive comes from one battery and the negative from the other (generally recommended), then again the parallel wires could be at fault.

Another thing to consider is that “relatively new” does not mean the batteries are in good condition.

Finally, 13.4 volts is rather high for a resting voltage. Are these batteries on a charger when you are trying to start? That could mask underlying battery problems.

[Chuck Rich]

Yes directly from batt. to pos and neg to the gauge.

[Tony Athens]

DVM’s—————-Dedicated POS & NEG direct to the BAT terminals, correct?

[Rob Schepis]

That’s the way. Your drop is a tad much. Clean and reset all connections (positive and negative) starting at the battery terminals and see if you can improve it. If not do drop tests along each circuit until you find the culprit. 

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Chuck Rich

[Chuck Rich]

Hi Tony, It is measured with one of your voltage gauges hooked directly to the start battery bank. 

[Tony Athens]

Two part answer ————————Exactly where and how are you measuring this voltage?

 

Tony

[Author]

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