Battery Questions
There are some folks here that seem to know a lot about batteries and I have a question, but first an explanation of my intended use. My friend and I do a lot of river kayaking and fishing, sometim...
Forum Discussion
To figure how long a battery will last you will use the following formula:
Battery amp hours X .85% divided by motor amp draw = approx. running time.
I agree, they mean 85% not .85% BUT I don't know what they mean by running time. Minutes? 34 minutes seems a bit low for a 40AH battery drawing 8 amps. but ???
I don't think they mean 85% discharge which will mortally wound a lead acid battery which for practical purposes should not be discharged more than 50%. And that is for a deep cycle or trolling battery. A starting battery will not do well with repeated deep discharges. Essentially that means that a deep cycle battery of 40AH capacity should not have more than 20 Amp hours taken from it.
So: At position 1, drawing 8 Amps, your 40 AH battery should theoretically give you 2.5 hours until it reaches 50%. It will run the motor longer but at the cost of battery lifetime.
Here it gets a bit complicated. There are various ways to rate Amp hours, which is just Amps drawn for H number of hours. ie 8 Amps over 5 Hours is 20 Amp Hours.
BUT (I told you it gets complicated) The lower the current (amps) drawn over a longer time results in more amp hours. Higher current draw will give you less amp hours available, even if the basic A times H math works.
The most common rating of AH is taken over 20 hours of current draw until the battery is DEAD. For your battery, that would be 4 Amps for 10 Hours = 40 AH
BUT you will be drawing more than double that current so you cannot expect to pull 40 AH from that battery. (And remember you should NEVER pull more than 50% or 20 AH anyway)
Yours is listed as a "dual purpose" which is to say the very commonly though not very accurately labeled "Marine Deep Cycle" battery. If you keep above the 50% mark it should do you just fine. How long? You will have to experiment.
So what is 50%? Roughly 12.1 to 12.2 volts with the battery at rest, ie no load or charging happening, and has been resting for a while. An inexpensive multimeter is needed to check this. What is the real AH capacity of your battery at 8A current draw? Less than 40 for sure, but not a huge lot less. (I don't have the math for that calculation.)
Since batteries don't like too long periods of heavy current draw, as someone mentioned earlier, not more than C20 (20% of AH rating) 8 amps in your case is right on the line for sustained draw. You can exceed it for short periods. (I'll pull 150 amps from my 480 AH of battery running the microwave via the inverter but just for few minutes at a time.)
Make sure you FULLY recharge the battery after each use as soon as possible. Storing a discharged lead acid battery will kill it. Note you shouldn't recharge really at more than C20 either but you can get away with it for a while.
How long will the battery last for you? You'll just have to experiment a bit. I would start with at an assumption of the motor maker's suggestion of about half an hour (85%AH or 85% of 40=34) and hope you'll do better. You might get a couple of hours out of it.
Enjoy.
Addendum: I have an electric bike with a 350 Watt 36V electric motor, drawing from a Lithium Ion battery pack and on the flat I can run about over an hour. If I can find the specs I'll post them here later.
Battery amp hours X .85% divided by motor amp draw = approx. running time.
I agree, they mean 85% not .85% BUT I don't know what they mean by running time. Minutes? 34 minutes seems a bit low for a 40AH battery drawing 8 amps. but ???
I don't think they mean 85% discharge which will mortally wound a lead acid battery which for practical purposes should not be discharged more than 50%. And that is for a deep cycle or trolling battery. A starting battery will not do well with repeated deep discharges. Essentially that means that a deep cycle battery of 40AH capacity should not have more than 20 Amp hours taken from it.
So: At position 1, drawing 8 Amps, your 40 AH battery should theoretically give you 2.5 hours until it reaches 50%. It will run the motor longer but at the cost of battery lifetime.
Here it gets a bit complicated. There are various ways to rate Amp hours, which is just Amps drawn for H number of hours. ie 8 Amps over 5 Hours is 20 Amp Hours.
BUT (I told you it gets complicated) The lower the current (amps) drawn over a longer time results in more amp hours. Higher current draw will give you less amp hours available, even if the basic A times H math works.
The most common rating of AH is taken over 20 hours of current draw until the battery is DEAD. For your battery, that would be 4 Amps for 10 Hours = 40 AH
BUT you will be drawing more than double that current so you cannot expect to pull 40 AH from that battery. (And remember you should NEVER pull more than 50% or 20 AH anyway)
Yours is listed as a "dual purpose" which is to say the very commonly though not very accurately labeled "Marine Deep Cycle" battery. If you keep above the 50% mark it should do you just fine. How long? You will have to experiment.
So what is 50%? Roughly 12.1 to 12.2 volts with the battery at rest, ie no load or charging happening, and has been resting for a while. An inexpensive multimeter is needed to check this. What is the real AH capacity of your battery at 8A current draw? Less than 40 for sure, but not a huge lot less. (I don't have the math for that calculation.)
Since batteries don't like too long periods of heavy current draw, as someone mentioned earlier, not more than C20 (20% of AH rating) 8 amps in your case is right on the line for sustained draw. You can exceed it for short periods. (I'll pull 150 amps from my 480 AH of battery running the microwave via the inverter but just for few minutes at a time.)
Make sure you FULLY recharge the battery after each use as soon as possible. Storing a discharged lead acid battery will kill it. Note you shouldn't recharge really at more than C20 either but you can get away with it for a while.
How long will the battery last for you? You'll just have to experiment a bit. I would start with at an assumption of the motor maker's suggestion of about half an hour (85%AH or 85% of 40=34) and hope you'll do better. You might get a couple of hours out of it.
Enjoy.
Addendum: I have an electric bike with a 350 Watt 36V electric motor, drawing from a Lithium Ion battery pack and on the flat I can run about over an hour. If I can find the specs I'll post them here later.
