Why would a fully charged battery, completely unhooked lose it's charge over the winter? The battery is a Sears 31M Platinum, and is only 3 years old. The last thing I want is to have a battery problem, but also don't want to replace the $250 battery if a simple re-charge will fix it up.

Any comments? It is on the charger now.

charge it up .... just the S.G. and put a load test on it ...



6. Battery Testing can be done in more than one way. The most accurate method is measurement of specific gravity and battery voltage. To measure specific gravity buy a temperature compensating hydrometer, to measure voltage use a digital D.C. Voltmeter. A quality load tester may be a good purchase if you need to test sealed batteries.

For any of these methods, you must first fully charge the battery and then remove the surface charge. If the battery has been sitting at least several hours (I prefer at least 12 hours) you may begin testing. To remove surface charge the battery must be discharged for several minutes. Using a headlight (high beam) will do the trick. After turning off the light you are ready to test the battery.


State of Charge Specific Gravity Voltage
12V 6V
100% 1.265 12.7 6.3
*75% 1.225 12.4 6.2
50% 1.190 12.2 6.1
25% 1.155 12.0 6.0
Discharged 1.120 11.9 6.0


*Sulfation of Batteries starts when specific gravity falls below 1.225 or voltage measures less than 12.4 for a 12v battery, or 6.2 for a 6 volt battery. Sulfation hardens on the battery plates reducing and eventually destroying the ability of the battery to generate Volts and Amps.

Load testing is yet another way of testing a battery. Load test removes amps from a battery much like starting an engine would. A load tester can be purchased at most auto parts stores. Some battery companies label their battery with the amp load for testing. This number is usually 1/2 of the CCA rating. For instance, a 500CCA battery would load test at 250 amps for 15 seconds. A load test can only be performed if the battery is near or at full charge.

The results of your testing should be as follows:

Hydrometer readings should not vary more than .05 differences between cells.

Digital Voltmeters should read as the voltage is shown in this document. The sealed AGM and Gel-Cell battery voltage (full charged) will be slightly higher in the 12.8 to 12.9 ranges. If you have voltage readings in the 10.5 volts range on a charged battery, that typically indicates a shorted cell.

If you have a maintenance free wet cell, the only ways to test are voltmeter and load test. Any of the maintenance free type batteries that have a built in hydrometer(black/green window) will tell you the condition of 1 cell of 6. You may get a good reading from 1 cell but have a problem with other cells in the battery.

You should disconnect all leads from the battery when storing for the winter. Even if everything is turned off there are still very small drains from your connections that wear a battery down after several months.

Also cold weather reduces the capacity of the battery and further brings the charge down. Any battery, even brand new, should be put on a trickle charger every month or two to keep it topped off

I put mine on a trickle charger ever day that i am not useing the boat battery always good to go

Batteries also self-discharge. Different battery chemistries discharge at different rates. typical flooded batteries (the standard type, add water periodically) self discharge at 6-7% a month. AGM type only self-discharge at 3% per month. I do not have the self discharge rate for Gel batteries. Leaving most batteries in a discharged state for a long period will cause permenant damage.

As stated a trickle charge ....or really not trickle ..."smart" charger is best .....I know you want the best ...but 250.00 ? over the years I have learned to buy a brand name battery andI try to maintain it but then just replace them about every 3 years .....In my vehicles I go to walmart .....34.95 and they still last ~ 3 years .....
Revspan .....notice your in Michigan ...do you store the batteries inside ? Cold can kill also ......m

all batteries will self discharge, some types faster than others.

If yours sat uncharged for any length of time it's partially sulphated and will never be reliable again.

Just got a new battery under warranty. Even though I completely disconnect the batteries before storage, I will pull them and bring them inside and make sure that they stay charged next year.

Thanks for the comments. BTW, Sears was great about giving my a brand new $239 battery. I was in and out in 15 minutes. Didn't cost me a dime.

I've always left my batteries in the boat, turned on, plugged in all winter to an automatic tender. Never an issue.

My current 342 with shore power is even easier.

Normal.