I just upgraded my bass boat to a Minn Kota 80# trolling motor with 2 optima 12 V (D27M) trolling motor batteries. These babies are expensive so I want to make them last as long as possible. In some cases, I'll be taking this boat where my outboard motor can't be used so there is no onboard recharging. There are two numbers that I would like to know: 1) what is the voltage below which I shouldn't go in order to avoid damaging these batteries? and 2) what is the voltage at which the batteries won't turn the trolling motor?

I just upgraded my bass boat to a Minn Kota 80# trolling motor with 2 optima 12 V (D27M) trolling motor batteries. These babies are expensive so I want to make them last as long as possible. In some cases, I'll be taking this boat where my outboard motor can't be used so there is no onboard recharging. There are two numbers that I would like to know: 1) what is the voltage below which I shouldn't go in order to avoid damaging these batteries? and 2) what is the voltage at which the batteries won't turn the trolling motor?

Depending on the type of motor, 5 speed or variable, the motor will turn all the way down to below 0% charge. You may not get any movement from it, but a 5 speed motor, on high, will turn all the way down to 2 or 3 volts. A variable speed motor may have some kind of cut off if the voltage gets to low to run the electronics.

Thanks for the help although I'm still not sure exactly what voltage I should aim to stay above.

Kinda confused on what your trying to accomplish. Charge the batteries fully after every outing and "top" off the charge monthly when not in use.

Batteries have a finite life and a maximum number of times/cycles they can be used and recharged. To obtain the maximum number of cycles requires proper charging as soon as possible. Leaving batteries partially discharged results in sulfation, which in turn reduces battery life and capacity. Keeping the batteries maintained by charging as soon as possible after use, keeping the batteries clean and maintaining proper electrolyte levels go a long way to getting the most from your battery.

The other day after fishing for six hours in a strong wind, the trolling motor was pushing my canoe with less oomph. Voltmeter when I got home showed the battery was just under six volts.

Get a good charger which is designed for deep cycle batteries. When the battery is fully charged, it will switch to maintenance mode. A two year battery with little use, but properly maintained may last for four or more years. A four year battery properly maintained, but frequently used may not last two years.

The deeper a battery is discharged, more of its life is consumed.

There are some battery pros on here who can give you a better explanation.

How long do you want the batteries to last? As mentioned they do have a life span based on cycle counts, (one discharge and recharge is considered one cycle). This cycle count is greatly influenced by how deep you discharge the battery. Run it down to 20/30% each time and you may get 200 cycles, run it down to 50% and you may get 400 cycles. Only run it down to 75% each time and you will even get a whole lot more. Oh, if you run it down to where the motor is barely pulling each time, you may only get 100 or so cycles. That's why it wise to buy big enough batteries that at the end of the day, your batteries are still at 50% or more. Yep, bigger batteries cost more up front, but they get a lot cheaper when they last twice as long.

As for charging you Optima batteries. I strongly recommend you get a "GOOD" AGM charger. DO NOT use one of those el cheapo automatic or manual chargers on them. They do not like have voltages gearter than 14.6 - 14.8 VDC hooked to them. It does damage them and will shorten their life. They also have a different maintenance voltage requirement so the charger should be selectable for AGM. If you are going to try and use a cheap or standard battery charger on them, charge them very slow so as the limit the chargers output voltage. Probably only a couple of amps, and it will take many hours to charge them that slow. Use a DVM and measure your charger voltage and make sure it never gets above 14.8 and better at 14.6.

Oh, and for those that have read me saying never charge at two amps, that's for flooded cell batteries, it does not bother an AGM to be charged at very low current rates.

I would also use a different voltage chart. I don't think many AGMs or any other deep cycle battery is only going to be at 12.6 volts when fully charge. It's more like 12.8 to even 13.0 so this will raise the minimum voltage that you should discharge to. I just always say if a 12 volt battery is at 12.0 volts, it's time to recharge it.

I'm using a ProSport12 2-bank, onboard charger, which I can also plug in when I store the boat in my garage. This charger is what was already on the boat and it has been adjusted for use with AGM batteries. Before I was charging the night before I went out, but with the Optimas I've switched to charging as soon as I get home and I just leave the charger plugged in all the time.

I'm using a ProSport12 2-bank, onboard charger, which I can also plug in when I store the boat in my garage. This charger is what was already on the boat and it has been adjusted for use with AGM batteries. Before I was charging the night before I went out, but with the Optimas I've switched to charging as soon as I get home and I just leave the charger plugged in all the time.

Good thing you changed. Now you are doing it the way it's suppose to be done if you want your batteries to last.