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Old 10-07-2018, 06:15   #1
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Electric Propulustion

I am thinking on putting a 100 hp in my boat, and having some math problems. If the motor draws 236 ah @ 80 %. If I have 7320 ah batteries does this make sense. If I divide by 2, which is 50% left in the batteries equals 3660 amp hrs. 3600 divided by 236 ah equals 15 hrs? Does this look right, or am I missing some thing or a lot. I really appreciate your time on this, and thank you very much.
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Old 10-07-2018, 06:22   #2
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Re: Electric Propulustion

Is the motor rated at 12V?

If not work it in kWh.

100HP is a lot.

Also how much does that bank weigh?
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Old 10-07-2018, 06:25   #3
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Re: Electric Propulustion

Hello, and thanks for your reply, it is 144 volts and the weight of the batteries is 2400 lbs
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Old 10-07-2018, 06:32   #4
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Re: Electric Propulustion

144V * 236Ah = 33884Wh or 34kWh ( I guess per hour, so 34kW ), it translates to 46hp.
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Old 10-07-2018, 06:33   #5
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Re: Electric Propulustion

> If the motor draws 236 ah @ 80 %


Motors don't draw Ah, they draw Amps.


100Hp = 75kW (approx). 80% = 60000W.
If your motor draws 236 Amps @ 80%, that means it is running on a nominal 240 Volts. (236 A x 240 V = 56640 Watts).


You don't say what voltage your 7320Ah of batteries is at, so we have no idea how much energy you have stored. But it is very doubtful that it is at 240 Volts. If it is only a 12V system, you have about 88000 Watt hours. So 50% discharge would be 44000 Wh or about 3/4 of an hour running. (44000/56640)



BUT! the big problem is that to get 236Ah @ 240V, if your battery bank is 12V it will need to put out 4720 amps!


So what voltage is your 7320 Ah battery bank?
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Old 10-07-2018, 06:38   #6
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Re: Electric Propulustion

Ah, I see you have posted your battery voltage while I was posting.


144V makes a big difference.


144V * 7320 Ah = 1054080 Wh. 50% is 527040 Wh. So you can run for about 9.3 hours. But you are still going to be drawing at almost 400 Amps.



Hope you've got some very heavy duty wiring there :
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Old 10-07-2018, 06:46   #7
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Re: Electric Propulustion

Wha battery is that? I have 12V / 1000Ah LFP, 15kWh Energy stored.
527kWh is 35 times more. It must be a beast. Is this a old submarine or what?

An 35 times bigger LFP would weight 1.6t or 3200lb, yours will be 2500lb, so probably no lead acid. But why then only 50% usable?
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Old 10-07-2018, 06:51   #8
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Re: Electric Propulustion

Please describe your battery bank in more detaiil. Type and number of batteries.


I find it impossible to believe that you have 1 MWh of storage in 2400lb of batteries.



If it's LiFePO4 (in which case, why only 50% DOD?) a 12V 200Ah (2.5kWh) battery weighs about 55lb. To get your 1MWh you would need 400 of them weighing about 22,000 lbs (ten times as much).

There is clearly something VERY wrong with your numbers.
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Old 10-07-2018, 06:53   #9
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Re: Electric Propulustion

Just noticed "40ft fibreglass trawler style".


No way do you have anywhere near that battery capacity.
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Old 10-07-2018, 06:58   #10
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Re: Electric Propulustion

Quote:
Originally Posted by StuM View Post
Just noticed "40ft fibreglass trawler style".


No way do you have anywhere near that battery capacity.
Stu, its doable, you need a tender for the batteries - like a cabuse... [emoji12]
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Old 10-07-2018, 06:58   #11
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Re: Electric Propulustion

Quote:
Originally Posted by StuM View Post
> If the motor draws 236 ah @ 80 %


Motors don't draw Ah, they draw Amps.


100Hp = 75kW (approx). 80% = 60000W.
If your motor draws 236 Amps @ 80%, that means it is running on a nominal 240 Volts. (236 A x 240 V = 56640 Watts).


You don't say what voltage your 7320Ah of batteries is at, so we have no idea how much energy you have stored. But it is very doubtful that it is at 240 Volts. If it is only a 12V system, you have about 88000 Watt hours. So 50% discharge would be 44000 Wh or about 3/4 of an hour running. (44000/56640)



BUT! the big problem is that to get 236Ah @ 240V, if your battery bank is 12V it will need to put out 4720 amps!


So what voltage is your 7320 Ah battery bank?
24- 6 volt batteries, with 305 amp hrs
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Old 10-07-2018, 07:04   #12
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Re: Electric Propulustion

Just re-read your earlier post it seems that the motor is 144V, not the battery bank That doesn't make sense with your initial 236 Amps @ 80% of 100HP.


Bank voltage still unknown, but based on looking at 50%, I assumeL ead Acid and based on 2400 lbs, that looks like about 24 x 200Ah 12V batteries. or 48 x 200Ah 6V batteries. (I'm guessing you may have 6V batteries, because that would be in the ballpark for 7320 Ah.



Please explain exactly the specifications of your motor and battery bank so that we can try to make sense of your numbers.
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Old 10-07-2018, 07:08   #13
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Re: Electric Propulustion

> 24- 6 volt batteries, with 305 amp hrs


OK, now we are getting somewhere. You have a total of 43920 Watt hours (24 x 6 x 305). With 50% max DOD, you have about 22,000 Wh available.



Your 144V motor @ 236 Amps uses 33984 Wh (144 x 236) per hour.


So you have less than 40 minutes running time available.




What you were missing is Watts = Volts x Amps.
You can't use Amps and Amp hours directly when you have different voltages in the system.
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Old 10-07-2018, 07:12   #14
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Re: Electric Propulustion

Quote:
Originally Posted by StuM View Post
Just re-read your earlier post it seems that the motor is 144V, not the battery bank That doesn't make sense with your initial 236 Amps @ 80% of 100HP.


Bank voltage still unknown, but based on looking at 50%, I assumeL ead Acid and based on 2400 lbs, that looks like about 24 x 200Ah 12V batteries. or 48 x 200Ah 6V batteries. (I'm guessing you may have 6V batteries, because that would be in the ballpark for 7320 Ah.


Please explain exactly the specifications of your motor and battery bank so that we can try to make sense of your numbers.
24 6 volt batteries @305 amp hrs. Lead acid. I am thinking on a Elco electric and it draws 295 amp hr @100 percent. At 80 percent it draws 236 amps Thank you
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Old 10-07-2018, 07:14   #15
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Re: Electric Propulustion

Also, if the 144V motor draws 236 Amps (33kW), that's only 44HP, certainly not 80% of 100HP


Added: 144V x 295A = 42480W = 57HP, not 100
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