When do you know it's time for new trolling motor batt's????

For starters, how old are the ones you have now?

Testing your batteries can be done in a few different ways. The simplest and cheapest way is to measure the specific gravity using a hydrometer, another option is measuring the batteries voltage. To measure the specific gravity buy a temperature compensating hydrometer that you can buy just about anywhere, or you can measure the voltage using a good DC voltmeter. You could run out and buy a good load tester if you need to test sealed batteries, but that's a little more pricey.

To test your battery you have to first make sure the battery has been fully charged. The surface charge should be removed before you test it. in other words if you just finished charging your battery, you should wait at least a few hours before testing otherwise you will get a false reading.

You could also do a load test on your batteries. A load test will remove amps from a battery just as running a trolling motor or outboard starter would.  

If you take readings using a hydrometer, the difference shouldn't vary more than .05  between cells.

A sealed AGM or Gel batteries voltage should be a bit higher around 12.8volt to 12.9volt range. If you have voltage readings around the 10.5volt range on a charged battery most likely you have a shorted cell.

Sulfation of a battery starts when the specific gravity falls below 1.225 or the voltage measures less than 12.4 on a 12v Battery, or 6.2 on 6 volt batteries. Sulfation will harden the battery plates reducing and eventually destroying the ability of the battery to store and or generate either volts and amps. This is why it is important to recharge your batteries as soon as possible after use.

If you have maintenance free wet cell batteries, your only way to test are using a voltmeter or a load test.

Of course there is one other method and is used by many. Charge them and if you run out of juice in the first hour out, there no good. >