# Battery design problems

Discussion in 'Homework Help' started by Benpeters, Mar 12, 2016.

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1. ### BenpetersNew Member

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Hi, I'm working on a battery to power my continuous 6kw DC motor. The max amp for the motor is 200 and it's max voltage is 60. I want to use 14 3.7v 15ah cells to build a battery, but I'm confused about wiring in series or parallel! I'm unsure how to wire it because either way makes my calculations wrong.
14x3.7v gives a 44.2v output.
15ah x 14 gives 220ah. Thanks anyway

2. ### audioguruWell-Known MemberMost Helpful Member

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Ah is how long a charge lasts, it is not the maximum allowed current. You need to look on the datasheet of the lithium battery cell to see the recommended maximum discharge current, it might be 300A or more (but only for maybe a few minutes). Do you want to abuse the motor by running it at its maximum rated current and voltage?

A rechargeable Lithium battery needs a balanced charger that senses the voltage of each cell to avoid overcharging a weak cell that might cause an explosion or nasty fore. Your circuit must detect when any cell's voltage drops below 3.2V then shuts off the load.
You said 3.7V but that is an average voltage because a rechargeable lithium cell is 3.2V when almost dead and is 4.2V when fully charged.

3. ### BenpetersNew Member

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Thanks, but could you suggest a combination that would sustain the motor at 6kw? The cells max discharge rate is 3C, does this mean the can discharge anything up to 3C or just 3C?

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5. ### audioguruWell-Known MemberMost Helpful Member

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Your battery cells are very weak. Their maximum discharge current is 3 x 15Ah which is only 45A. Max means maximum. A cell can power something that uses 0.000001A or 45A or any current in between.
The current used by an electric motor depends on how voltage you give it and much work it is doing. How many volts will you give it? 14 Cells produce only 44.8V when they are almost dead and they produce 58.8V when fully charged. Maybe you will give the motor the average which is 51.8V? Then the current must be 6000/51.8V= 116A. You need 116/45= 3 strings of those weak battery cells which is a total of 42 cells. A string is 14 cells in series. You need 3 strings in parallel.

Did you calculate how long one charge will last? With a load of 6kW the charge in 42 cells might last for 15 minutes.

6. ### BenpetersNew Member

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Flipping heck 42 cells isn't viable. Thanks man, I didn't even consider the battery life either. This isn't gonna work, any suggestions?

7. ### BenpetersNew Member

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Are there any alternatives to this massive battery pack? It's not gonna work like this

8. ### audioguruWell-Known MemberMost Helpful Member

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I can't think of anything that uses a 6kW electric motor except an electric streetcar (trolley or tram). They do not use batteries. What will you use it for?

9. ### MikeMlWell-Known MemberMost Helpful Member

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6000W/(746W/HP) = 8HP, not so remarkable. The starter motor in your car is approaching that power level; they use a 12V battery...

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