The Boating Forum - Battery Question for you EXPERTS

Let's say that one battery shows 12 volts and the other one shows 12.4V, so they're both close to the end of their service lives. You want to start a big engine.


Are you better off using just the 12.4V battery (even though it's weak) or will you get more CCA by placing the selector on BOTH?

The average voltage will drop to 12.2V, but will you gain more cranking amps this way?

Yes, more current will be available.

Battery open circuit voltage isn't a great way to judge a battery's condition. Testing it under load after it has been charged is much more reliable.

I would think so yes, I would also replace them. I just bought a new battery from Trojen and it reads 12.9 Anything below 12.4 is weak.

No because the 12.0 battery will take voltage away from the 12.4 battery. They will both try to start the engine but as long as there is a voltage potential between them, the better will charge the worse, and just one of them should have the cca to crank the engine as ling as it is sized appropriately.

Miike is right, don't join the batteries. First, charge both batteries, after charging, if they dont hold a reading about 12.6volts with no power draw, they need to be replaced. I recommend replacing them with AGM style batteries. But in your case, combine the 2 would case the lower battery to draw down the better battery and make starting the engine even more difficult.

Experts?? You want expert advice here on THT?? :grin:

Jeeez, Rick..far fetched answers here already..

"A reading of 12.66 volts indicates a fully charged battery; 12.45 volts is 75% charged, 12.24 volts is 50% charged, and 12.06 volts is 25% charged."

The above was referenced years ago and saved.

Soooo, that battery reading 12.4 volts is really not in that bad of condition as some here may think...

This pertains to lead-acid batteries only..

Are you testing voltage with an 800 dollar calibrated fluke or a 5 dollR eBay dmm?

Load test before relying on gauge.

Also test with hydrometer to check individual cells.

But for a boat, the best advice I can offer is replace every 3 years.

All too often I'll charge a batt, remove the charger, and it's "rest" voltage is around 12.6...not too bad, but I'd like to see around 12.8-12.9. Then I put a load on (trying to start a big O/B), and the voltage drops to less than 10...

The load's the deal...

When you jump start your car with the battery in another car, you are connecting the batteries in parallel. The "weak" battery and the good one in the other car. Both are contributing current to the starter.

Think about it.

The weak battery is making it harder for you to start the car. That's why the other car has to be running and sometimes you have to wait a bit to charge the weak battery before you can start the car. If you took the bad battery out of the car and replaced it with the good one you wouldn't need to wait or have another engine running.

voltage will go from whatever Has a higher voltage to whereever it is lower. If one battery is too low, the good batteries voltage will go to charging it, but when you connect both to the starter, both batteries have voltage avg of 12.2 that is higher than the voltage at the starter, so both will contribute. You are better off, however, not including the weAk battery because the 12.4 should have the juice to crank it, and if it doesn't, you won't start the engine anyways.

My question is theoretical only; our present boat has only one battery and it's new. If we put it into a fishing context, let's say you left both batteries on during a day of fishing and you have maybe one chance to start the motor...do you try it with one or both batteries if one battery tests more voltage?

If we combine two batteries, the volts will drop (to an average)....but, will the addition of another battery (albeit a weaker one) contribute more to cranking power via the Amps?

I try separate first if I don't think I have the juice, but I usually would move the switch to one battery for extended house load with no engine running to keep one topped off, and then switch to the good one to crank the engine, then switch back to the other to charge it back up to tops.

Theoretically, you should never have the switch in the BOTH position. Use one battery or the other. Batt 1 on the ride out and 2 on the way in.

Larger boats have emergency crossovers which are the equivalent of connecting the starting batteries to each other even though each motor has its own starting battery and there is a house bank in addition to those. Emergency crossover would be like cranking off the BOTH position, even though it would be better to use one battery instead of the other rather than in addition to.

Theoretically, you should never have the switch in the BOTH position. Use one battery or the other. Batt 1 on the ride out and 2 on the way in.
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The big word there is, "Theoretically", and we all sometimes make those, "Danggit!!", type of mistakes; usually at the worst possible time. :banghead:

...so your idea is that no more CCA would be gained by using both batteries? That the king is Voltage? That combining two iffy batteries won't be as effective as using just the best of the lot?

All too often I'll charge a batt, remove the charger, and it's "rest" voltage is around 12.6...not too bad, but I'd like to see around 12.8-12.9. Then I put a load on (trying to start a big O/B), and the voltage drops to less than 10...

The load's the deal...

autozone will load test for free, but a voltmeter is easy as Bob says,

I just went through that with my volvo. I put the volvo meter right on the terminals, showing 12+, hit the ignition key and it dropped to 10v.
Bad battery? nope.

Turned out I had a v drain, we jump started it and charged the battery up, then I dis-connected the battery and left it like that,
it was still charged the next morning, cranked right up.

You may want to test it like that before throwing new batteries at a problem.

Theoretically, you should never have the switch in the BOTH position. Use one battery or the other. Batt 1 on the ride out and 2 on the way in. .

The "Both" position allows both batteries to be charged when the engine is running. This is exactly what battery combiners do, but automatically.

The reasons there is a "Both" option are:

1. Allow both batteries to be charged at the same time by the engine or single bank charger.

2. Combine both batteries to start the engine if neither has the remaining capacity to start it on its own.

The "Both" position allows both batteries to be charged when the engine is running. This is exactly what battery combiners do, but automatically.


2. Combine both batteries to start the engine if neither has the remaining capacity to start it on its own.


HONEST, Guys, this is a Theory question; nothing more or less.

Ron, what you're saying is that.....IF you feel like you only have One Try to start the engine before the batteries are too weak to do anything....then you'd try the strongest one by itself (instead of combining both for one last try)?

P.S. I think that both batteries get charged while underway regardless of the selector setting...but that's another thread.

Theoretically, if someone used the switch properly, one should never have two dead batteries that both need charging. If the person used the switch properly, he would only have one dead battery and would only charge that one. If one runs both of his batteries down in the BOTH position, he is not using proper power management.



Again, THEORETICALLY, you should never need to use both, and if you practice proper power management you won't need to. If you are running both batteries for an extended period of time with the engine off, then you are being foolish and you will get two dead batteries and your boat is not going to start one day. Use one battery or the other. Also, if neither has the capacity to start on its own, then it is likely that both will not either. So don't plan on running two batteries dead and using them to start.

P.S. I think that both batteries get charged while underway regardless of the selector setting...but that's another thread.

Not on any boat I've ever worked on.

P.S. I think that both batteries get charged while underway regardless of the selector setting...but that's another thread.

Only if there is a battery combiner (voltage sensitive relay) or a battery isolator installed on the boat.

Ahh yes, that is correct. Most boats do not have charging leads from the alternator to both batteries but if they have a combiner then they would have that, or if they had an onboard battery charger, but that would be from shorepower not from the motor.

......Ron, what you're saying is that.....IF you feel like you only have One Try to start the engine before the batteries are too weak to do anything....then you'd try the strongest one by itself (instead of combining both for one last try)?

In the real world, you will not know which is the strongest battery when it comes time to start the engine. In the real world, you will turn the key and get clicks or a slow engine turnover. You will probably switch to the other battery and try again. If thast doesn't work, you would try "Both". Save enough power to call for assistance on your VHF.

What is important here, theory wise, is not the battery's no load voltage, but its ability to provide the required current while maintaining a high enough voltage to operate the starter.

For example, let's say your boat's battery voltage is 12.8 volts and it drops to 12.0 when starting thr engine. If it starts, that's fine.

Now, just for example, connect nine 1.5 "D" cells in series. You will measure a little over 13.5 volts. Connect them to start the engine and what happens? The voltage will drop to ner zero and the starter will not turn over. Why? Because the "D" cells cannot provide the necessary current to operate the starter motor

...and your last sentence is the basis for my OP. Assuming that you had noted the error of your ways and both batteries were tired...and you learned that one was a bit stronger than the other....

Would you try to start with the strongest single battery, or with a Combined set.

Remember, you only get ONE try in this scenario ;) Which method would you choose?

...and your last sentence is the basis for my OP. Assuming that you had noted the error of your ways and both batteries were tired...and you learned that one was a bit stronger than the other....

Would you try to start with the strongest single battery, or with a Combined set?

Remember, you only get ONE try in this scenario ;) Which method would you choose?

...and your last sentence is the basis for my OP. Assuming that you had noted the error of your ways and both batteries were tired...and you learned that one was a bit stronger than the other....

Would you try to start with the strongest single battery, or with a Combined set.

Remember, you only get ONE try in this scenario ;) Which method would you choose?

turn to both and try to start as fast as possible....;cool;

I understand how a battery switch works and I understand how a combiner (ACR) works.

With outboard motors which have a secondary alternator output, how would that be integrated into a battery management scheme, or do most people just ignore the secondary output, or just use it for a dedicated battery like for a trolling motor?

I believe Yamaha offers the feature on 115hp and above...not sure about other motor brands.
Also, I assume that the secondary alternator output is much less current than the primary output?

...and your last sentence is the basis for my OP. Assuming that you had noted the error of your ways and both batteries were tired...and you learned that one was a bit stronger than the other....

Would you try to start with the strongest single battery, or with a Combined set.

Remember, you only get ONE try in this scenario ;) Which method would you choose?
There's now way to answer that question in theory or in practice because you have no way of knowing the remaining capacity of either battery. If one battery was so low it wouldnt work a light or radio, I would not switch them together. If both batteries were low, I would combine them.