How do you really know the capacity of a marine battery? Today, the Reserve Capacity (RC) rating has virtually replaced the older Amp Hour (AH) rating as a measure of marine battery’s capacity. AH is simple and easy to understand, but RC describes a battery’s capacity more accurately, I believe, especially if that battery is powering one of today’s big trolling motors.

Under the AH measurement system, a battery rated at 100 AH could theoretically power a 1 amp load for 100 hours, a 10-amp load for 10 hours or a 50 amp load for 2 hours. That isn’t exactly true in real life.

The AH rating for a battery is calculated by measuring an amp load drawn over a period of time and, unless stated otherwise, that time period for marine batteries is usually 20 hours. If a 12-volt battery can supply 5 amps for 20 hours before its voltage falls to 10.5 volts, it gets rated at 100 AH. This rating is pretty accurate if you are powering lights, bilge pumps and electronics, but less accurate when you power a trolling motor.

PEUKERT’S LAW
When you pull power from a battery at more than the 20-hour rate (more than 5 amps in this case) you can run afoul of Peukert’s Law. German scientist W. Peukert established back in 1897 that as the rate of discharge increases, a battery’s available capacity decreases. A fisherman’s take on this law says that while you can probably pull 5 amps from a 100 AH battery for 20 hours before it falls to 10.5 volts, your trolling motor probably cannot pull 40 amps from that same battery for a full 2.5 hours and stay above the same voltage floor, even though both loads add up to 100 AH.

The RC rating indicates the number of minutes that a new, fully-charged battery at 80 degrees Fahrenheit can deliver a 25 amp load before battery voltage drops to 10.5 volts. Peukert’s Law doesn’t directly apply because this rating is based on a constant amp load, and since that load is close to the average load a trolling motor draws over a normal fishing day (without having to fight much wind or current) it has more real-world relevance than an AH measurement.

Most marine batteries no longer carry an AH rating on their labels, but boaters who prefer to think in terms of amp hours can roughly calculate an average AH rating using the formula RC x 0.6 = AH.

DEFINITION OF DEAD
You may wonder why manufacturers consider their batteries “dead” at 10.5 volts instead of some other number. It equals 1.75 volts per cell, and taking a battery any lower can cause extra internal heat, strain the chemical discharge process, speed plate corrosion and just generally shorten the life of a deep cycle battery.

Manufacturers often suggest selecting batteries large enough to accomplish their purpose without being discharged more than 50% before being recharged, and this makes good sense for several reasons. First, the more capacity your batteries start with, the smaller the percentage of that capacity your trolling motor load equals and the less trouble you will have with Peukert. The less your deep cycle batteries are discharged each fishing day, the more discharge/recharge cycles they will last and you won’t have to replace them as often.

Until I hear a fisherman gripe about having too much battery power left at the end of a hard day on the water, I will continue to recommend buying trolling motor batteries with the largest AH or RC rating that will fit in your battery compartment without unbalancing your boat.