Voltage drop when cranking

[bobk]

How far do you expect the voltage to drop when cranking. I have 6V92's and just noticed the bridge gages go to nine or ten volts when cranking. The 8D batteries are five or six years old, but use little water and show 12.7 volts at rest.

Bob

 

[luckydave215]

How did you measure your rest voltage? If the answer is "With a high quality VOM" you aren't geting a full charge, or there's a dead cell. Rest voltage should be ~13.2 at full charge, 12.7 is pretty far discharged.
Of course, dirty/high resistance connections will cause high voltage drop under (cranking) load, and will inhibit charging as well, so it might not be weak batteries or a feeble charger.

 

[TedZ]

Connections...especially ground on engine and/or mount.

12.7 (12.6) is a fully charged lead acid battery. 13.2 is ok when the engine is running.

Ted

 

[TedZ]

Actually 12.7 is fully charged! I thought the math came to 12.6.

http://www.batterystuff.com/tutorial_battery.html

Ted

 

[Will]

The voltage while cranking doesn't seem unusual. If everything else seems OK, I'd not be concerned.
Will

 

[Brian Degulis]

Cranking amperage on a 12V system is very high (over 600 A). With that kind of amperage being used in a burst the voltage is going to drop maybe even lower than 9. If it cranks strong I wouldn't worry about it. If you want to be sure then you need to have a load test done.

Brian

 

[luckydave215]

Brain fart

I hadn't had my coffee yet (it was 6 am)...that's my excuse, and I'm sticking with it.
What I should have said, is FLOAT, NOT REST voltage with a smart charger should be 13.2 ~ 13.4
I will now shock myself with 12.7 volts two hundred times to absolve my sins.

 

[MikeP]

The voltage drop you are referring to is not really important! What is important is the DIFFERENCE between the voltage drop measured at the battery when the starter is engaged and the voltage drop measured at the starter when the starter is engaged.

If there is more than .2v difference at the batt and at the starter when engaged, THAT is a problem that needs to be addressed. The actual voltage measurement you are describing will change depending on engine temp, outside temp, and the draw of the particular starter. Only the difference between batt/starter is important.

 

[bobk]

Well, the starboard pair of 8D's went bad at Wrightsville Beach on Sunday. I could not start the genny to lift the dink. I arranged for two new pair of NAPA 'commercial batteries when I got to Southport through a friend. Fair price and great service from American Fish Company. Everything starts great now.

Bob

 

[REBrueckner]

Batteries these days seem to die overnight.

One way to partially avoid such surprises is to start each engine only from its own start bank (not paralleled) and alternate such starts by paralleling. The paralleled arrangement will always crank faster and will insure your parallel circuit is working. Starting each engine only from it's own battery bank lets you estimate crank speed....and battery bank condituion....when cranking slows, check batteries with a hydrometer...it's usually a signal that you'll need to replace the batteries before long.

If I detect a weak battery, I usually leave it in place over the winter dockside, and install new replacements in the spring....fresh batteries and longer effective warranty period for the new season....

 

[Brian Degulis]

The best way to check the condition of a battery is with a load tester. It gives plenty of warning when a battery is starting to go bad. I always did this untill I got a boat with a 32V system. I haven't been able to find a load tester for 8V batteries.

Brian

 

[REBrueckner]

Batteries these days seem to die overnight.

One way to partially avoid such surprises is to start each engine only from its own start bank (not paralleled) and alternate such starts by paralleling. The paralleled arrangement will always crank faster and will insure your parallel circuit is working. Starting each engine only from it's own battery bank lets you estimate crank speed....and battery bank condituion....when cranking slows, check batteries with a hydrometer...it's usually a signal that you'll need to replace the batteries before long.

If I detect a weak battery, I usually leave it in place over the winter dockside, and install new replacements in the spring....fresh batteries and longer effective warranty period for the new season....

 

[bobk]

I should have mentioned that the voltage when starting now only drops to about 11V, maybe a bit higher. Seems like watching the voltmeter when starting is a good load test.

Interestingly the batteries were only three years old when I bought the boat. I had two load tests during the surveys of engine and hull and both showed good, but the engines never jumped to life like they do now.

PS, we are now down in Beaufort SC, headed to Savanah for a couple of days then on to Port St. Lucie and Tarpon bay YC.

Bob

 

[TopHattandTails]

For load testing 32v systems / 8 volt batteries, would it be correct in thinking to set your load tester to 12v and when the battery shows 8v - or good? I assume that you would want to disconnect all batteries / cables so that your only testing 1 battery at a time... My first tip off to a bad battery is the smell, then lots of water being consumed, then the dead crank... I had 2 new sets of batteries that were about a year old when I purchased the boat (per the PO). I replaced the one bank about 18 months into their life cycle and the other side needs to be replaced ASAP (next spring). I think the culprit was the battery charger - set a little too high, burning the batteries up. Once I replace the last bank next Spring, I will set the charger an ohm or half ohm lower on the charge. Make it act a little more of a trickle charge...

 

[Brian Degulis]

That doesn't work 8Vs is to low to put a load on a 12V load tester. I tried it with a known good battery I thought if I could get some kind of result I could use it to compare. But it just doesn't work. I wish I could find an 8V load tester?

Brian

 

[SKYCHENEY]

How about a 24v load tester and just test 3 at the same time in series? You could switch the cables around and through several combinations you could find the one bad battery in the bank.

 

[Boatsb]

Use the starter as the load and put a meter on the batteries. just crank it with the shut offs held to get a bit of current draw. 10 seconds will not hurt anything and will get a decent reading.

 

[Brian Degulis]

Yeah that would work to some degree. I thought of using the 12V load tester I have and just connecting it on the jumpers at 12V. In other words checking 1 1/2 batteries at a time?

Brian