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My ignition got left in the "ON" position for a week so when I tried to go fishing Saturday morning I had no cranking power. I put the battery on charge and dropped another one in its place, then went ahead with my trip.

When I first connected the dead battery to a charger, it showed as fully charged. So I unplugged the charger, reconnected the charging clips to the posts and plugged the charger back in. After that, it appeared to charge correctly. Yesterday after church, I took it off the charger and checked the voltage with a meter that showed 13.1 volts. Since that time with no additional charge it has dropped to 12.7 volts.

This is an 800 cranking amp Interstate battery that is about a year and a half old. Do any of you know how many volts it should hold when not on charge?

Thanks - Steve Stinson

I have seen the chart above from time to time.

Real-world experience, my 12v batteries hold about 12.85 to 12.9 volts when new and at least 24 hours after charging has stopped.

My Pathfinder trolling motor batteries are soon to need replacing. They were at 14.45 volts after sitting in the boat shed for 6 weeks without charging. The charging battery right next to them was at 14.8 and is in much better shape.

The battery in my RV sits at 12.4 all the time and cranks a V-10 Ford engine.

So......did you hurt your battery some? Yes you did. Is it trash now...no. Will it last a while longer, probably. Should you take a chance on it since it's a boat not your lawnmower......that's the question.

I would move it to something around the shop/business that is not as critical and buy a new starting bat for my boat.

Thanks for the replies guys. This is exactly what I needed to know and I appreciate the help.

- Steve Stinson

Yeah, if it won't maintain a full charge better than that I'd replace it. Not worth a risk on your boat.

Okay guys - So the battery saga continues.

Taking the advice above I purchased another 800 cranking amp battery from West Marine. I had some store points there from buying a trolling motor earlier and they had their batteries marked down in a sale that ended yesterday. I took the new battery home and checked the voltage which read 12.59. Hmmm, this battery is weaker than the one I am replacing (12.7 on the old battery). Not knowing how long the battery had been on the shelf, I attached a charger to it and let it charge over night.

This morning I headed back out to the boat shed, detached the charger and checked the volts again. 12.90... So either I have purchased a bad battery or the chart provided above is a bit of a stretch for battery life. Seems to me .20 volts is not much improvement and I am not sure what to do with the new battery. I have it loaded in my truck along with my volt meter and may check it again around noon.

- Steve Stinson

If it's not maintenance free, you can test each individual cell. I bought a lawn mower battery that had a bad cell once and had to take it back. It didn't hold charge worth a flip.

Sounds like you may need to take it back to West Marine, Steve, and exchange it. I'd take the voltmeter with me to check the other ones on the shelf. West Marine may not have the turnover on batteries that other outlets do, so it may be an age issue.

12.9 is pretty good, I think that table above is not accurate for regular starting batteries. According to Advance Auto website:

Measuring your car battery’s voltage can be a great way to determine how charged your battery might be. The ideal car battery voltage range will provide you with a relevant interval according to which you can measure precisely what the voltage says about the battery’s current charge. A perfect voltage with the engine running is between 13.7 and 14.7V. With the engine off, you should get a reading of 12.6 volts. If the battery isn’t fully charged, it will diminish to 12.4V at 75%, 12V when it’s only operating at 25%, and down to 11.9V when it’s completely discharged. This data and the way it relates to the design of your battery should provide you with some useful insight into the functioning capacity of your car battery.

Edit/ that chart is for LFX batteries which are Lithium Iron Phosphate batteries. So that chart should be ignored.

leonreno wrote:12.9 is pretty good, I think that table above is not accurate for regular starting batteries. According to Advance Auto website:

Measuring your car battery’s voltage can be a great way to determine how charged your battery might be. The ideal car battery voltage range will provide you with a relevant interval according to which you can measure precisely what the voltage says about the battery’s current charge. A perfect voltage with the engine running is between 13.7 and 14.7V. With the engine off, you should get a reading of 12.6 volts. If the battery isn’t fully charged, it will diminish to 12.4V at 75%, 12V when it’s only operating at 25%, and down to 11.9V when it’s completely discharged. This data and the way it relates to the design of your battery should provide you with some useful insight into the functioning capacity of your car battery.

Edit/ that chart is for LFX batteries which are Lithium Iron Phosphate batteries. So that chart should be ignored.

This is right on..

If you check your Deep Cycle minutes after a charge you should have a 13+ reading. Days later it should be 12.8 - 12.9 for a perfect Batt. Even a 12.5 -12.7 is still good, just older. Sounds like your good to me

Well then, I guess I will just put the West Marine battery back in the boat, and Andrew has himself a relatively good trolling motor battery for the pond boat. Thankfully, I gave the old one to him and used his for the core charge.

Good deal. Glad you got squared away and are ready to go.

Shields Marina finally figured out my trim tab gremlin that I've been trying to solve all summer. Hard to fully realize the performance of a technical poling skiff without tabs. Boats are so much fun and so kind on the wallet.

A perfect voltage with the engine running is between 13.7 and 14.7V.

Is that not the alternator?

I have never owned a battery that I have tested that has been at 13 V or over when sitting idle, but I am probably not typically testing brand new batteries.

A lead acid battery produces 2.1V dc per cell. The math says that a 6 cell battery will produce 12.6V dc. I am not sure where the other figures are coming from, whether they are voltages measured at the alternator with the engine running or at the battery disconnected from the cables, but I am pretty sure you will not get more than 12.9v from a battery tested immediately when the charger has been removed.

onefishtwofish wrote:

A perfect voltage with the engine running is between 13.7 and 14.7V.

Is that not the alternator?

Yes, that is while the battery is being charged by the alternator. I was confused by the chart above because I've never seen a lead acid battery put out more that 13 volts unless it's being charged.

I was gonna ask if you wanted to get rid of the old battery, but I see you have found a new home for it.