# Will a 42000mah portable power bank charge an electric bike battery?

The electric bike I have has a 48v , 15.4ah battery.

The charger for the bike reads a 120v-240v ac input , with a 2a output.
The portable powerbank I am looking at is here, https://www.amazon.com/dp/B06Y3NF7N2/ref=sspa_dk_detail_0?psc=1&pd_rd_i=B06Y3NF7N2&pd_rd_wg=xjXEP&pd_rd_r=RPT4RMBVXEH370Z8P1AQ&pd_rd_w=vuzsq
42k mah capacity with 250w/110v output
Would this be sufficient in charging this battery?

Also if I were to keep the powerbank charging the battery in a bike mount , while the bike was in operation, would that cause any problems?
Below are pictures of the battery and the weight. It's a dinosaur.

That is 12lbs.
(I bought the bike from a previous owner who seems to have shorted the battery , because the bike does not seem to go as far as I would like it to , and the battery charges very fast , which lead me to the idea of a portable charger.)

• Please put the powerbank specifications into your question so that it is still understandable when the Amazon link dies. – Transistor May 23 '18 at 22:09

The powerbank can (apparently) output enough volts and watts to power your charger, so yeah, sure, it could charge your ebike battery. The real question is: would it be a good idea to do that?

Most likely the ebike battery manufacturer has not foreseen the use of a charger on a moving bike, so I would not bet on cell balancing and other "details" to work properly. But that's a minor detail, if the battery bursts into flame, the brakes will still work.

The electric bike I have has a 48v , 15.4ah battery.

That's about 740 Wh, which is rather huge for an ebike.

Note:

current * voltage = power

power * time = energy

current * voltage * time = energy

Ah = current * time (ie, amperes * hours)

Ah * volts = current * time * volts = energy

(if current is in A, time in hours, and voltage in volts, then energy is in watts*hours)

The powerbank weighs about 1 kilo, so let's say it contains about 800 grams of batteries (I'm being generous) if the cells are good quality, at 200 Wh/kg, then it will hold about 160 Wh of energy. This is small compared to the main ebike battery, so even if the thing works, it won't make a big difference.

Note:

Ah is the unit for the capacity of a battery, but it doesn't say how much energy it contains. Most likely the monkey who wrote the marketing blurb for your powerbank went like so:

"It contains 14x 3.0Ah 3.6V LiIon cells, so its capacity is 42Ah, so 42000 mAh! More zeros is more good! Yay!!!!!"

Note 42 Ah x 3.6V = 150 Wh which is pretty close to my 160 Wh wet-finger-in-the-wind estimation by weight.

This is slightly different from what we engineers call a "specification". "150 Wh" is a specification, it tells you how much energy is stored in the box. "42Ah" does not tell you how much energy is stored, only the total battery capacity. If you want to know the energy you have to multiply by the battery voltage, which is not mentioned. The output voltage is mentioned, but it has no relation to the voltage on the internal battery, since there will be a converter.

TL/DR:

1) This will not extend the range of your ebike by enough km to be worth the headache of making it work.

2) Purchasing a spare battery compatible with your ebike (even one of lower capacity) is a much better idea, because it will work. Also you'r only paying for battery, not for voltage conversion electronics.

• i've included pics of the battery! – Pacified May 24 '18 at 1:05

Go by weight.

The PLUG weighs 2lbs 3oz with dimensions of 5.6in x 1.6in x 8.6in.

That's 1 kg. Let's guess that 75% of that is battery and that they're Li-Ion.

Wikipedia's Lithium-ion battery states that the specific energy is in the range of 100–265 W·h/kg (0.36–0.875 MJ/kg). We'll go with the mid-range of 180 Wh/kg: The powerbank capacity is 180 x 75% = 145 Wh.

The electric bike I have has a 48 V, 15.4 Ah battery.

So your bike has 740 Wh battery.

If you were able to DC-DC convert from that pack to 48 V at an efficiency of 80% then the useful power would be 145 x 80% = 116 Wh or a best case of 15% reserve.

Would this be sufficient in charging this battery?

No.

Also if I were to keep the powerbank charging the battery in a bike mount , while the bike was in operation, would that cause any problems?

You can't put current into and take it out of a battery simultaneously. If the batteries were properly matched they would both discharge. If not then the one with the higher voltage will drive the bike and feed current into the other.

Can you weigh your bike battery and estimate the energy density in Wh/kg based on the calculated capacity of 740 Wh? Please post it in the comments. I'd be interested to know where it lies on the Wikipedia energy density specifications.

[The battery] is 12 lb.

That's 5.4 kg.

If we do a simple weight ratio between the two packs we get $\frac {1}{5.4} = 18.5%$ which is very close to the 15% I calculated using the specific energy figure.

Note: the seller says it's 42k mAh. No engineer would write it that way. It would be 42 Ah. This is a clue that the batterypack is low quality. Avoid.

• I have included pictures of the battery in the post to make sure I provided the right information – Pacified May 24 '18 at 1:05
• Why didn't you work out the energy density as I suggested? See the update. – Transistor May 24 '18 at 6:20