Trolling battery reserve capacity

[manfishing]

When looking for a trolling battery it's a 29 group and a 31 group the both have 210 reserve capacity I do not remember any of the specifications, but is the reserve capacity the main specs that I should be looking at. Eather size will fit into the battery tray. $40.00 difference between the two sizes. Is the reserve capacity the main concern. Thanks

[Ozark Bob]

Originally Posted by manfishing

When looking for a trolling battery it's a 29 group and a 31 group the both have 210 reserve capacity I do not remember any of the specifications, but is the reserve capacity the main specs that I should be looking at. Eather size will fit into the battery tray. $40.00 difference between the two sizes. Is the reserve capacity the main concern. Thanks

If you go only with that spec make sure they did the test at the same rate(25 amps). Bob

[Karas3434]

Originally Posted by manfishing

When looking for a trolling battery it's a 29 group and a 31 group the both have 210 reserve capacity I do not remember any of the specifications, but is the reserve capacity the main specs that I should be looking at. Eather size will fit into the battery tray. $40.00 difference between the two sizes. Is the reserve capacity the main concern. Thanks

I have never been concerned with reserve capacity when choosing a battery. I always look for marine cranking amps. The biggest difference between a 29 and 31 is going to be battery life duration. In theory a larger battery will last longer, now this can depend on the type of battery (cranking vs. dual purpose vs deep cycle) but holds true for the most part in a L/A battery.

[GBS]

For a trolling motor, you only care about Reserve Capacity. How many minutes will it last, if the load is fixed.
Cold Cranking Amps is a measure of how much of the charge can be used in 30 sec, at 0 degrees - how much initial burst of power it can supply. It is generally one over the other. More RC, less CCA, and vice versa.

If you are going to start an engine, higher CCA will get that thing turning when the oil is half frozen. If you want to troll all day, then go with RC. Or try to find a happy medium if the battery will be used for both.

As Bob noted, some mfgs play games with the RC. Standard is a 25 amp load. Some use a 20 amp, which can make the RC seem greater....so dig into the specs to see what load they used, so you can compare apples to apples.
You can take RC times the load used, and then divide by 25 to get an actual comparison number, if you want....

[Ozark Bob]

Originally Posted by Karas3434

I have never been concerned with reserve capacity when choosing a battery. I always look for marine cranking amps. The biggest difference between a 29 and 31 is going to be battery life duration. In theory a larger battery will last longer, now this can depend on the type of battery (cranking vs. dual purpose vs deep cycle) but holds true for the most part in a L/A battery.

By size, if you are talking weight then I agree. Battery sizes can be misleading, sometimes intentionally. Go by the spec's and look at the spec's closely. Amp hours , CCA's and Reserve capacity. The battery's weight can sometimes tip you off. With Lead-acid heavier usually means more capacity. Bob
Note: for trolling the reserve capacity is king. A few one way or the other is not important, just make sure they did it with a 25 amp rate. Have seen them use 23 or 20 to make it look equal to better batteries. As always let the buyer beware.

[GBS]

And, just to make your head spin - some batteries are rated in amp-hours. A different way to measure "how long". You'll see it especially in lithium batteries. To convert from Reserve Capacity:

Ampere-hours = 0.4167*(Reserve Capacity)

[Ozark Bob]

Originally Posted by GBS

And, just to make your head spin - some batteries are rated in amp-hours. A different way to measure "how long". You'll see it especially in lithium batteries. To convert from Reserve Capacity:

Ampere-hours = 0.4167*(Reserve Capacity)

Just noticed this. Reserve capacity in the length of time a fully charged battery will stay above 10.5 volts with a 25 amp drain. The above conversion might convert reserve minutes to Amp hours not the other way around.

[GBS]

Originally Posted by Ozark Bob

Just noticed this. Reserve capacity in the length of time a fully charged battery will stay above 10.5 volts with a 25 amp drain. The above conversion might convert reserve minutes to Amp hours not the other way around.

It's just math. So to go the other way: AH/.4167 = RC

If you get into the details, due to the way the test are actually done and the peculiarities of how batteries discharge, you might find the math doesn't quite agree with the actual specs. But it's close enough, if trying to compare one battery only rated in AH, against another with 25 amp Reserve Capacity. Don't get mired down in the "this has 2-3 min. more RC than that if I do the math..."

[Ozark Bob]

Originally Posted by GBS

It's just math. So to go the other way: AH/.4167 = RC

If you get into the details, due to the way the test are actually done and the peculiarities of how batteries discharge, you might find the math doesn't quite agree with the actual specs. But it's close enough, if trying to compare one battery only rated in AH, against another with 25 amp Reserve Capacity. Don't get mired down in the "this has 2-3 min. more RC than that if I do the math..."

Not a math major, I didn't understand the equation. Sometimes if you dig into battery spec's if they don't have all the numbers there are probably some "mistakes" in the pile. Bob