Question about batteries - Page 1
Forum section: electronics / beginners

author: amadeok
author_rank: Contributor
timestamp: March 21, 2020, 07:27:17 pm
content: Hello,I was thinking about adding a 250w 10A dc motor to a bicycle, so i needed a battery pack. I was wondering if it would be possible to get a 12v 15A battery pack, and add a step up converter to make 24v? Would it work? What would be the difference from having a 24v 15A battery pack?thanks

author: bob91343
author_rank: Super Contributor
timestamp: March 21, 2020, 08:32:25 pm
content: There are many solutions to your requirement. A 12V with boost or a 24V will both work.If you are interested in the lowest cost or the highest efficiency or the lightest weight, etc., you need to design a few different systems and compare them with regard to your goals. Further, within each prospective solution there are usually several alternatives. For instance, to boost 12V to 24V you would likely get higher efficiency and lighter weight and lower cost by generating a floating 12V and adding it to the 12V source to get 24V, rather than converting to the entire 24V.

author: Tom45
author_rank: Frequent Contributor
timestamp: March 21, 2020, 08:42:38 pm
content: Given the cost and complexity of the converter to step 12 vdc up to 24 vdc, you would probably be better off with two smaller 12 volt batteries in series. While the motor is rated for 10 amps, it would most likely use a lot less than that in normal use.

author: IanB
author_rank: Super Contributor
timestamp: March 21, 2020, 08:49:32 pm
content: Quote from: amadeok on March 21, 2020, 07:27:17 pmIf your proposed motor is 250 W with a 10 A current draw, then presumably it is expecting 25 V (since 250 W / 10 A = 25 V).If you draw 10 A from a 12 V battery you will have 120 W (since 12 V x 10 A = 120 W). This is about half the power needed. If you had a perfect converter that could step the voltage up to 24 V without losses, you would have 24 V and 5 A available (since 120 W / 24 V = 5 A). To get 10 A you would need 12 V and 20 A.Since a 12 V, 20 A battery will weigh exactly the same as a 24 V, 10 A battery there is nothing to be gained from the step up converter, other than adding extra weight and increased power losses.You should look for a battery pack of the right voltage for the motor. Put two 12 V, 10 A batteries in series if this works out to be more convenient.How big the batteries need to be depends on the expected running time and range of the bicycle. Battery capacity is measured in amp-hours. So for instance a 20 Ah battery could supply 1 A for 20 hours, or 5 A for 4 hours, or 10 A for 2 hours (theoretically).A convenient source of suitable batteries might be the lithium battery packs for hand-held power tools. They come nicely packaged in a robust plastic enclosure with all necessary safety features and matching chargers off the shelf. The motors from such power tools might even be good motors for electric bicycle applications. I even wonder if anyone has built such a thing? You could just plug a freshly charged battery into your bicycle and off you go, while another battery sits at home recharging.

author: Zero999
author_rank: Super Contributor
timestamp: March 21, 2020, 09:08:15 pm
content: Yes, it's generally better to avoid converting one voltage to another, whenever possible, because it increases losses and complexity.

author: amadeok
author_rank: Contributor
timestamp: March 21, 2020, 10:56:31 pm
content: I see, thanks for replies.Is this battery pack fake?https://it.aliexpress.com/item/4000530930819.html?spm=a2g0o.cart.0.0.3e103c003X7edk&mp=1Given the size of the pack, i don't think it's 18000mh

author: IanB
author_rank: Super Contributor
timestamp: March 22, 2020, 12:15:35 am
content: Quote from: amadeok on March 21, 2020, 10:56:31 pmNo, of course it's not 18 000 mAhHowever, there are two measures of battery performance. One would be the maximum current the battery can supply for either a short burst, or for a sustained period. This measure is really important for power tools, since the more current the battery can supply, the more power the tool can generate.Another would be the capacity, or how long the battery can last before draining. The capacity is strongly related to the weight of a battery. If two lithium ion batteries of the same nominal voltage claim the same capacity, but one is half the weight of the other, then the lighter battery is certainly suspect.It might be that these measures get confused. For example, a battery that can sustain a maximum load of 18 A is not the same as an 18 Ah battery. It could be a 2500 mAh battery with a high power rating.

author: Rick Law
author_rank: Super Contributor
timestamp: March 22, 2020, 02:42:24 am
content: Quote from: IanB on March 22, 2020, 12:15:35 am[RL - bold added to quote]My guess is, it is probably 6x 18650 cells each rated 3000mAH. So they have total from 6 cells of 18,000mAH at 3.75V (there about) rather than 18000mAH at 12 V.When I was shopping for power banks, I was initially confused. I found many of the power banks out there just add the raw cell capacity instead of what the power bank can output in mAH at their designed voltage (typically 5V).Back to this pack... If I am right, the math shows 18000*3.75/12=5625mAH @ 12 Volt. Then again, may be this pack is NOT equip it to the max with new 3000mAH cells but instead use recycled 2000-2600mAH cells (as I found in some power banks). If so, you'd be lucky to come out with 3500mAH@12 Volt

author: Sam Spastic
author_rank: Regular Contributor
timestamp: March 22, 2020, 07:13:07 pm
content: Quote from: IanB on March 22, 2020, 12:15:35 amAlso the 22 awg wire will get hot at 18A.

author: David Hess
author_rank: Super Contributor
timestamp: March 23, 2020, 06:45:59 pm
content: Using a step-up converter is certainly feasible although I suspect it would be better to buy a second battery. If the converter is not required to regulate the power to the motor, which will be the case if a motor controller is already present, then an inverter to double the battery output would be a better option.