I'm thinking of making my own electric bicycle, and I need help determining what the amp hour rating of my battery pack would be. I'm going for 36v and 8 or 9 amp hours.
So, with 10 batteries in series, I can get a single, low capacity pack with 36v. If I were to make three of these, and put those in parallel, I'd have three times the capacity. But, with each battery providing 2500mah, that's 75 amp hours, and I know that's not correct. What am I forgetting? image according to my knowledge and to the picture, series doubles the voltage and the capacity stays the same
Determine the amp hour rating of a home made battery pack
ampere-hourcapacity
Related Solutions
It is bad practice to connect batteries in series when they don't have the same capacity. The battery with the smaller capacity will be empty before the larger one, resulting in a lower voltage for the smaller battery. At that point things will start to get interesting as the larger battery will start to charge the smaller one through the connected circuit and with reversed voltage. The cell is not designed for being reversed and charged and bad things may happen a.o.: leaking acid and exploding. Neither of these situations are desirable. This is also the reason why most manuals of battery operated devices urge to replace all batteries at the same time.
Your 75W fan required 220 Volts, it will not operate at typical battery voltages such as 12V.
A 75W fan operating (indirectly) from a 12V supply would draw 6.25 A not 0.34 A.
Even with a theoretical 100% efficient invertor, the current on the 12V side is greater than the current on the 220V side to deliver the same power across the system from battery to Fan.
Note that the reason that power is delivered through the grid at high voltage is to keep currents low. losses and cable sizes are proportional to (a function of) current. High V are used to keep currents low.
You convert 12V DC into 220 V AC using your inverter. Depending on capacity and utilisation the efficiency of an inverter might be the 80% you estimate or it may be worse - 50% Ref So your 6.25A may become 13A drawn from the 12V battery.
Therefore your 300 Ah battery-set might power a 75W fan for less than 24 hours.
Update: for 2 x nominal 12V SLA battery, the following diagram may clarify what is happening.
simulate this circuit – Schematic created using CircuitLab
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Best Answer
If you series connect the batteries the amp-hour rating stays the same. The voltage has increased though, so the power available has increased. That's why using watt-hours is more useful than amp-hours when comparing battery packs of different voltages.
So ...
Now here's where watt-hours are so much more useful. Let's say you're going to need 135 W average power for your motor. Your battery duration will be 270 Wh / 135 W = 2 hours duration.
My electric bike uses about 150 W in normal cycling mode at almost 30 kph. I reckon my legs are putting in another 150 W.
Little technical point: In the SI system of units it's capital letters for abbreviated units named after someone but lower-case when spelled out: V for volt, A for amp, W for watt, etc. Yes, there will be exceptions ...