Interesting battery fact, discharge rates, trolling motor

[fishrdan]

I dug up an interesting fact about battery discharge rates. As the rate of discharge goes up, the battery's capacity goes down,,, and as the discharge rate goes down the battery capacity goes up (see chart below).

Battery Capacity - Hours of Discharge
100------------------------ 20
90------------------------- 10
87-------------------------- 8
83-------------------------- 6
80-------------------------- 5
70-------------------------- 3
60-------------------------- 2
50-------------------------- 1

Article

When I was running an old speed coil trolling motor I would only get about 5 hours of run time out of a group 29 battery, then would have to switch over to a second battery. From the chart, it "looks" like I could have extended the run time 10% by paralleling the batteries and depleting both of the batteries at the same time, at a slower rate.

I actually found the opposite to be true as I got longer run times by running the batteries separate, but I attribute that to having 1 good battery and 1 marginal battery. Marginal battery was discharging and dragging down the good battery.

I now run a digital trolling motor which has far longer run times than the old speed coil trolling motors. From looking a the chart, the slower discharge of a digital trolling motor, could actually extend the capacity of the battery.

An interesting aside, I run a 24V TM on my 18'er and only get 5-6 hours of run time out of it constantly trolling. When I use that same trolling motor on my 14' jon boat I get, well I don't know as I have never run the batteries down in the jon boat, even when trolling constant for 12-16 hours. Of course here is less current being drawn when in the jon boat, but I think the batteries are delivering their full capacity since they are being discharged slower.

What do you think?

 

[Silvertip]

Re: Interesting battery fact, discharge rates, trolling motor

It is obvious that speed coild motors draw more current across their entire speed range than a digital (pulsed) motor. That's because speed coils are nothing more than resistors that turn excess energy into heat so current consumption on those motors is fairly even regardless what speed is selected. However, pulsed motors and speed coil motors will discharge a battery at nearly the same rate when run close to at full speed settings. That's because the motors are essentially being fed directly rather than through any speed control circuitry. In the chart, hours of discharge and battery capacity really don't mean much unless a "discharge rate" is included. Most deep cycle batteries are spec'ed with Reserve Minutes at either a 23 or 25 amp draw. If that battery has 180 reserve minutes at a 25 amp draw, a troller drawing 50 amps would run for roughly 90 minutes before the battery can no longer provide 25 amps. It's all a numbers game but without a "current draw number" run time can't be calculated.

 

[fishrdan]

Re: Interesting battery fact, discharge rates, trolling motor

I agree, but found it interesting that how fast or slow a battery is discharged, impacts it's capacity. 2 batteries discharged independantly in 10 hours would only deliver 80% of their capacity, while 2 batteries discharged together in 10 hours together would deliver 90% of their capacity. Running the batteries together (series or parallel) would discharge each of them at a slower rate, and provide more power from the battery.

If the article is correct, the lower current draw of digital TM's (run at moderate speeds) would lower the battery's discharge rate and allow the battery to deliver more of it's potential capacity. This would throw the AH or Reserve Minute rating out the window as the battery could deliver more of it's capacity, since it's being discharged at a slower rate.

Also, since a faster rate of discharge reduces the battery's capacity, a 180 Reserve Minute battery might not deliver 50 amps for 90 minutes, it might be something closer to 80 minutes or less.

I'm not saying this is gospel truth, just food for thought...

 

[dwparker99]

Re: Interesting battery fact, discharge rates, trolling motor

I’m not a battery expert but I have done considerable reading about the subject. I still have a lot of unanswered questions but here is how I understand it.

How slow or fast a battery is discharged has little effect on actual capacity but has a significant effect on rated capacity. The error lies in the way batteries are rated. We discharge at a certain rate, industry standard is 25 amps, until the terminal voltages drops below 10.5v. The problem with this is that a battery discharged at 1 amp will be almost depleted of energy when it reaches 10.5v while a battery discharged at 50 amps will still have considerable useable energy left when it reaches 10.5v. Since we were able to use more of the energy at the 1 amp discharge rate, it makes the battery seem to have more capacity.

I believe I read that this state of charge (SOC) difference is caused by the inefficiencies in the electro-chemical process. At the higher current loads, the process can’t keep up with the demand so the terminal voltage drops at a much faster pace.

You probably couldn’t say that a battery with an RC of 180 minutes will stay above 10.5v if discharged at 50 amps for 90 minutes. However, I believe it would be safe to say that a battery with an RC of 180 minutes will provide 50 amps for 90 minutes.

One other thing to consider is that much more damage is done to a battery that is discharged down to 10.5v at the 1 amp rate than is done to a battery that is discharged down to 10.5v at the 50 amps rate. The difference is the SOC of the battery when it reaches 10.5v. At the 1 amp rate, the SOC will be less than 5%. At the 50 amp rate, the SOC will be greater than 25%.

 

[Silvertip]

Re: Interesting battery fact, discharge rates, trolling motor

I’m not a battery expert but I have done considerable reading about the subject. I still have a lot of unanswered questions but here is how I understand it.

How slow or fast a battery is discharged has little effect on actual capacity but has a significant effect on rated capacity. The error lies in the way batteries are rated. We discharge at a certain rate, industry standard is 25 amps, until the terminal voltages drops below 10.5v. The problem with this is that a battery discharged at 1 amp will be almost depleted of energy when it reaches 10.5v while a battery discharged at 50 amps will still have considerable useable energy left when it reaches 10.5v. Since we were able to use more of the energy at the 1 amp discharge rate, it makes the battery seem to have more capacity.

I believe I read that this state of charge (SOC) difference is caused by the inefficiencies in the electro-chemical process. At the higher current loads, the process can’t keep up with the demand so the terminal voltage drops at a much faster pace.

You probably couldn’t say that a battery with an RC of 180 minutes will stay above 10.5v if discharged at 50 amps for 90 minutes. However, I believe it would be safe to say that a battery with an RC of 180 minutes will provide 50 amps for 90 minutes.

One other thing to consider is that much more damage is done to a battery that is discharged down to 10.5v at the 1 amp rate than is done to a battery that is discharged down to 10.5v at the 50 amps rate. The difference is the SOC of the battery when it reaches 10.5v. At the 1 amp rate, the SOC will be less than 5%. At the 50 amp rate, the SOC will be greater than 25%.

10.5 volts in your examples IS the state of charge. It is simply another way of expressing battery condition instead of pecentage. In other words, 10.5 volts cannot be 5% AND 25%. If a battery measures 0 volts it is stone dead and its state of charge is 0% if it is fully charged and reading 12.6 - 12.8 volts it is fully charged and therefore state of charge is 100%. One might also use 0% SOC since that represents the 10.5 volt reading (using your example again). So state of charge depends on what point you consider the battery to be discharged. That can be anything from stone dead to anything less than 12.6. Do not confuse "state of charge" (the condition of the battery) with "discharge rate" which is a measure of how rapidly the battery is being discharged.

 

[dwparker99]

Re: Interesting battery fact, discharge rates, trolling motor

10.5 volts in your examples IS the state of charge. It is simply another way of expressing battery condition instead of pecentage. In other words, 10.5 volts cannot be 5% AND 25%. If a battery measures 0 volts it is stone dead and its state of charge is 0% if it is fully charged and reading 12.6 - 12.8 volts it is fully charged and therefore state of charge is 100%. One might also use 0% SOC since that represents the 10.5 volt reading (using your example again). So state of charge depends on what point you consider the battery to be discharged. That can be anything from stone dead to anything less than 12.6. Do not confuse "state of charge" (the condition of the battery) with "discharge rate" which is a measure of how rapidly the battery is being discharged.

We use state of charge as a fuel gauge for out battery. In my example, 10.5v is not the SOC. You cannot use a voltage reading to determine SOC when the battery is being charged or discharged. A battery must be at rest for at least 4 hours and preferably 24 hours before you can use open terminal voltage to determine SOC.

When a battery has a load on it, 10.5v can represent 5% and 25% SOC according to the size of the load. If you take two fully charged identical 100Ah batteries and discharge one at 5 amps for five hours and the other at 25 amps for one hour the SOC will be the same for each. However, if you take a voltage reading at the end of the period for each, the one discharged at 5 amps will have a considerably higher voltage reading leading you to believe it has a higher SOC.

 

[Jlawsen]

Re: Interesting battery fact, discharge rates, trolling motor

We use state of charge as a fuel gauge for out battery. In my example, 10.5v is not the SOC. You cannot use a voltage reading to determine SOC when the battery is being charged or discharged. A battery must be at rest for at least 4 hours and preferably 24 hours before you can use open terminal voltage to determine SOC.

When a battery has a load on it, 10.5v can represent 5% and 25% SOC according to the size of the load. If you take two fully charged identical 100Ah batteries and discharge one at 5 amps for five hours and the other at 25 amps for one hour the SOC will be the same for each. However, if you take a voltage reading at the end of the period for each, the one discharged at 5 amps will have a considerably higher voltage reading leading you to believe it has a higher SOC.

Exactly... Battery discharge rates are based on a standard 25 amp load when the batttery is in new condition. I first discovered the at rest voltage differences a few years ago. Then we all thought wow, look, if I let my battery sit for a while it will come back to life. That was sort of true from a non-scientific viewpoint since the voltage had equalized at whatever the new average was for all the plates. The remaining battery charge did remain the same however and the battery was completely dead shortly after a nominal load was applied. It was confusing unitl I learned a little more about batteries.